Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas - AMA | August 2023

Episode Date: August 7, 2023

Welcome to the August 2023 Ask Me Anything episode of Mindscape! These monthly excursions are funded by Patreon supporters (who are also the ones asking the questions). We take questions asked by Patreons, whittle them down to a more manageable number -- based primarily on whether I have anything interesting to say about them, not whether the questions themselves are good -- and sometimes group them together if they are about a similar topic. Enjoy! Blog post with questions and transcript: https://www.preposterousuniverse.com/podcast/2023/08/07/ama-august-2023/

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For plans starting at just $4.99 a month, go to homeserve.com. That's homeserv.com. Not available everywhere. Most plans range between $499 to $11.99 a month your first year. Terms apply on covered repairs. Hello, everyone. Welcome to the August 2023. Ask Me Anything Edition of the Mindscape podcast. I'm your host, Sean Carroll. This is the week after I released my solo episode on The Crisis in Physics. I just wanted to say that I'm quite gratified at the reception that it has gotten. If you ask what I mean by the reception, there are various places where people can talk about the podcast episodes. Of course, Patreon supporters, who are the one asking the questions for this. these Ask Me Anything episodes can do so on Patreon. We have a somewhat active group there. There is a Sean Carroll subreddit, S-A-N-C-A-R-R-O-L where people could talk about podcast episodes but also other

things that I do and so forth. There's also comments that come up on Twitter and YouTube and what have you. And people seem to like the solo episode and they took it in the spirit, I think, that it is offered, which is it's not primarily polemical, right? It's not primarily there. It wasn't primarily there to persuade you to have a certain point of view. I have a point of view, and I'm trying to explain to you what my point of view is, but also it's mostly to help you understand why physicists do the things they do. You're allowed to disagree with how physicists go about their business and what problems they choose to tackle and how they choose to do so, but you should do so, everyone should do so, from a situation from a standpoint of understanding why

they're doing the things they do. And that was really the primary point of the episode and why it was so long, because I got to sort of expound on the whole history of 20th century and now 21st century physics. There are a couple of people, of course, who just snarked about it. But if you look carefully at their snark, it was also completely clear they didn't listen to the episode. So the lesson I'm taking from this is that all of my episodes should be four hours long. And then the people who want to snark about them will clearly be not very relevant to the actual discussion. Probably not going to happen. Four hours a lot of time. I don't know if it's clear that it was not a single sitting that I did that episode in, but sometimes you have a lot to say and you got to keep talking about it. I also wanted to express appreciation because the recent episode with Katie Elliott was also very well received. And you never know. It's a many episodes are slightly different from each other. Katie was kind enough to come on and rather than just talk about her specific work, help us give a general chat about metaphysics and what it is and what it's for, and people really liked it, which I thought was great. And so kudos to Katie for that. And also, she was very good at asking me questions and having it really be a conversation. That's hard to do for many guests, you know, depending on what their thing is that they're doing for a living. Why would they have any questions for me? But hopefully it came through why I think that the physics, science, I should say, slash philosophy boundary is such an incredibly fertile area to think about things.

Philosophy does have something to offer, and physics absolutely has some questions that they have some answers too that will help the philosophers, but they have some questions that they can use philosophical help with. I should also say that I've been on a couple of other podcasts lately. Most notably a couple of episodes of Robinson's podcast. Robinson Earhart does a very nice podcast where he gets a lot of philosophers and other thinkers on. So recently I was on with David Albert talking about quantum mechanics and Boltzmann brains and things like that. I was also on an episode of his podcast with Slavoy Zhijek, the famous, in certain circles, extremely well-known psychoanalytic philosopher, Slovenian, and quite a character also. And Slavoy apparently

is thinking very hard about quantum mechanics these days. So poor Robinson got to say very little in the podcast, because it was basically Slavoy was the host, and he was giving his opinion and then asking me questions. So you can look that one up on the internet, sits on YouTube and elsewhere. Otherwise, remember, these Ask Me Anything

episodes are sponsored by Patreon supporters of the Mindscape podcast, and you too could be a Patreon supporter. Go to patreon.com slash Sean M. Carroll and pledge your $1 per episode or whatever you want per episode. No one has yet

pledged $1,000 per episode, but I don't discourage it. You're allowed to. I hope that there's no one out there who's really been wanting to pledge $1,000 per episode and thought that I would be insulted. I would not be. Pledge whatever you want. Or pledge nothing at all. We love all Minescape listeners equally, but we appreciate the support from Patrions. So with that, let's go. There's a few hot topics out there in the sciencey, physicsy discourse going on. So, naturally I get some questions about them. And so I'm going to start by grouping together some questions on these various topics and sort of giving you a collective answer. I don't have anything very deep to say about any of them, but I'll explain why I don't have deep things to say about them.

So the first one is two questions I'm going to group together. One is from Anonymous who says, can you weigh in on the plausibility of the room temperature atmospheric pressure superconductor papers released to archive last week? I realize it's paper reading, but I would be surprised if you haven't brought it up before. If you just have judgment without reading, it would be interesting as well. And Supendu Harsh says, are you satisfied with the evidence for room temperature superconductor LK99, or do you think extraordinary claims require extraordinary proof? So for those of you who have a very different social media feed than I do, there have been a couple papers.

There were a couple of papers, I don't know, a couple weeks ago now, maybe, from a group in Korea. It's a little South Korea. There's a little bit of confusion about why there are two papers and they don't have exactly the same author lists. And there's one person who's on one paper but not the other. And one of the papers has six authors and one of the papers says three authors. And some people are saying, well, the three author paper was trying to get their names out there so that they could be the three Nobel Prize winners. If this is true, I have no idea. So I've not actually read the papers. No, I've glanced at the papers. But there's another level of reading if you want to really understand things at a physics level,

and I have not done that. But the claim is that they have a room temperature, atmospheric pressure, superconductor. For those of you who are not super familiar here, superconductors are just materials that have no electrical resistance. You can send a current down them without losing any of that current or without even really putting any effort into it at all. It would be a very big deal, technologically, if you had easily makeable room temperature, atmospheric pressure superconductors. You have to say atmospheric pressure there because you can create higher temperature superconductors by putting enormous amount of pressure on them, but that kind of removes the whole benefit of just having something that you can put in your computer. No problem, right?

We have high temperature superconductors, which are a very, very big deal in physics, but high temperature just means high compared to absolute zero Kelvin. These are still quite low temperature compared to room temperature. And the more this new claim is that you actually have room temperature superconductors, which are things that people have hoped for and looked for for quite a while. So I will say that I'm a little skeptical. I am not on the bandwagon saying that this is probably going to be right. If I had to bet at even money, I would say it's going to go away. There's various pieces of evidence that are given here. One of them is that you can sort of float a little piece of material above the superconductor,

and that is evidence that the purported superconductor is expelling magnetic field lines, the Meisner effect, and this is evidence for superconductivity. But there's also, as it turns out, other ways to get that kind of behavior that don't necessarily rely on ordinary superconductivity. And the other thing about these papers, and all I know is from reading people, who are actually experts. I'm a physicist, but I'm not a condensed matter physicist, not a superconducting expert. As I've said before, though, I am an expert on sort of knowing how reasonable physics claims sound ab initio or a priori or whatever you want to call it. So it's just a

little shaky, you know. The plot in the paper, as I understand it, that purports to establish superconductivity, number one, doesn't look like superconductivity in the sense that when you have a superconducting transition, it's a phase transition, and you plot the electrical resistance as a function of temperature, and it tends to have a discontinuity near the superconducting phase transition, and in these papers it doesn't. It sort of gradually goes down to the lower level. And of course, you can't measure a number that is exactly zero, so you have some measurement uncertainty when you're measuring the superconductivity or the conductivity or resistivity, which is one over the conductivity. And in, as I understand it, again, from what other people are saying and, you know, reproducing

plots and things like that, there's basically two different samples that the original papers looked at. One is a relatively thick slab of the stuff, and it doesn't even conduct as well as copper does. It doesn't, it's definitely no evidence for superconducting in this thing. The other sample is a thinner film and it conducts better because maybe because it's thinner, right? So it doesn't smell like real to me. I am not averse to their being room temperature superconductors. There's no law of physics. It says that can't be true. There's nothing special about room temperature, right? It's just where we happen to live. So I'm very open-minded about the possibility of room temperature superconductors being discovered. And maybe there's something

interesting going on with this particular object, LK99, this particular substance. But it is hard, you know, doing physics at a very precise high level is just hard. So it's very, very easy to trick yourself if all the bases are not covered, et cetera. And the people who are experts who have looked at this are like, well, you know, maybe it's true, but it doesn't, you know, kind of fit together, it doesn't kind of seem like what you would expect. So I would say that it's absolutely possible. I certainly, let me put it this way, hope that it is. It would be awesome if it is. So I'm kind of rooting for it. But there's a whole other aspect to talk about, which is the fact that it's actually not that hard to synthesize this material, LK99, following the instructions in the

papers. So this is not like searching for supersymmetry. the large Hadron Collider where you need a $10 billion particle accelerator, if you have various lab equipment pieces already in place, you can just do it yourself. And what this has led to is a whole bunch of people, some of them are at established labs, others are, it's a little unclear where they are, trying in real time to reproduce these results and see whether or not they can check it out. I have strongly mixed feelings about this whole thing. On the one hand, I feel very positively about the fact that if you can do it, if you have the technology to participate in the science search, then great. The more people, the better. That is wonderful. On the other hand,

the fact that so much of it is being done in real time in public might be fun for the spectators, but it does not lead to the best science. You know, science has as its product, something that should be widely shared and understood by everybody. But there is a moment in the doing of science when you need to be careful and quiet and by yourself, you know, and really concentrate on getting it right. And having people like following your every move and cheering you on while you're doing it, that is not conducive to the most careful scientific practice. I think that in the modern world, it's so easy to put things out there in public, right? to put various work that you're doing or ideas that you have. And overall, that is a good thing,

I think. But I do notice that, you know, when I talk about a workshop I was at or a talk I went to or am planning for the future, people will always ask, you know, is it online? Is it being streamed so we can see it? And sometimes the answer is yes, and sometimes the answer is no. It's perfectly legitimate question. Is it going to be online? But then sometimes when the answer is, no, I will get responses like, well, that's not how science should be done. You should put everything online. And I just don't think that's true. I don't think you should put everything online. There has to be a place for people just to talk without the whole world looking over their shoulder. And that's just for theorists who were relatively low stakes in this game, right? For experimentalists who are trying to

do very, very careful testing and measuring of quantities and there's expectations. And it would be world-shattering if this were true. Well, not shattering, but changing. It would have an effect on the world if you had a room temperature superconductor that was easily synthesized. So the pressure is on, and that's not the kind of situation that under which we do our best science. Let's put it that way. So I have very mixed feelings about the public nature of people trying to reproduce this. And also just the impatience, like, you know, the day-to-day, oh, this person did this, this person did that, that's also not really conducive to good scientific practice. It's kind of like if you are very good at playing the piano, you can have a concert and people

can come and listen to your concert, and that's great. But you have to practice. There's some parts of your mastery of playing a musical instrument that are private by construction. And likewise for science, there should be a public aspect and a private aspect. And I worry that this kind of thing is a little bit too messy and out there and it's getting people too excited too quickly. Like sometimes science has to be careful and slow, and I'm not sure whether this is going to be the best example of that. Okay, the next set of questions is about Oppenheimer. So what did you think of, AJ says, so what did you think of Oppenheimer, the movie? Carlos Nunes says, what is your opinion of the new movie about a Robert Oppenheimer by Christopher

Nolan, do you think that the physics is represented accurately? And if not, what would you have done differently? I did see Oppenheimer, also saw Barbie, which is the other part of the pair of movies. People are very excited about these days. I thought Oppenheimer was brilliant, honestly. I was pleasantly surprised at how much I liked it. I am generally a fan of Christopher Nolan, but I think that he's uneven. I thought that Memento, his very first movie, was genius, one of my favorite movies of all time. His subsequent movies, I've seen good bits in, but, you know, every one of them kind of had its ups and downs. I think Inception was really, really good. Some other movies were not as good. So Oppenheimer just blew me away. It was a three-hour-long biopic of

J. Robert Oppenheimer. And I think that biopics are just intrinsically very difficult to pull off, because human beings' life stories are not structured like a three-act Hollywood movie, right? Or not structured dramatically, more generally. There are ups and downs, and there's no, you know, culmination, et cetera. And so Nolan, who wrote the script, you know, based on a book called American Prometheus, which was a biography of Oppenheimer, but he pulled out all the stops in terms of storytelling. He had two parallel tracks going, one of which was sort of up to, you know, from Oppenheimer's young days as a physics graduate student, up to through the Los Alamos Manhattan Project,

and then up to his trial, not really trial, I should say, but if you know Oppenheimer's story, he was at one point the head of the Manhattan Project, and then not long after his security clearance was taken away. That's a very controversial moment in the history of American politics and its relationship to science because, you know, Oppenheimer would hang out with known communists, his brother and I think his wife also were pretty much admitted to communists. And Oppenheimer was a complicated guy who, on the one hand, he built the atomic bomb. He was in charge of that project, and he was very proud of that project, and he was very much in favor of it.

He wasn't ambiguous about that. But then he also kind of felt bad about the fact that, oh, it killed hundreds of the thousands of people, and he was partly responsible for that. And you can argue back and forth, whether or not, well, he shouldn't have felt bad because he helped end World War II, or he should have felt way more than bad because he killed a lot of people, right? And other people will say, well, he didn't kill a lot of people, the government, you know, so there's a lot to be said about that. It's a complicated thing. So that's one story. And then Nolan inserts another story, which he sort of tells in parallel. It's a more minor story,

but it's about Lewis Strauss, who was a trustee of the Institute for Advanced Studies, who helped hire Oppenheimer as the head of the IAS, but then also had his own political issues. He was nominated ultimately for Secretary of Commerce, I think, and Congress refused to approve him for that, in part because of his betrayal of Oppenheimer in a very real sense. And so I, about the physics in the movie, you know, it was fine. I don't look at movies like this as physics lectures myself. I'm not looking for the physics to be 100% accurate. I'm not even looking for the history to be 100% accurate in the following sense.

I don't want them to dramatically misrepresent history. I don't want them to show things in a movie based on historical events that are really just completely opposite to the sense of what actually happened. But people complaining that this or that figure who was really playing a role at Los Alamos but didn't appear in the movie and whatever, or Oppenheimer was shown talking to Einstein when he's really talking to somebody else, I don't care about that, you know, make a good movie. And I thought that Oppenheimer was a good movie. And it was very much, as far as I could tell, honest and careful about the spirit of what was happening. It was not really dramatically misrepresenting people. or events, it streamlines things to tell a story, and I think that's great. So I encourage people to see it. It's a long movie.

I'm getting, you know, as I get older, it's harder for me to watch three-hour movies and keep riveted the whole time. But this did a pretty darn good job. I was very impressed. And I didn't think that it would be that cinematic, honestly. So I was very happy to see it come out that well. There's also Barbie.

And I love Barbie, too. You know, Barbie is a whole other movie. It was very hilarious that came out at the same time and people got excited about it. Like a couple months ago, someone on Twitter said, you know, yeah, these movies are coming out the same day. And I can't imagine there's a very big group that's going to want to see them both. And I instantly replied like, okay, I guess I'm in a very unusual group because I definitely want to see them both. As it turns out, lots of people wanted to see them both. And I love Barbie too.

It was a great movie. I think Oppenheimer was better, was a higher level achievement. It's really just a memorable movie in terms of my whole history of watching movies. As far as this year is concerned, I think it's Barbie, Oppenheimer, and Into the Spider-Verse, which were my favorite movies this year. And Barbie was just unapologetically feminist and talks about the patriarchy and everything, but it does so in such a lighthearted, loving way. and the, you know, there's no easy resolution to the problem that Barbie is a toy and, you know, it starts in Barbie land and the toys don't really have the kind of agency that you would want to have if you were a functioning human being. And that's a structural problem when you want to make movies about toys, right? I think Toy Story has some philosophical issues with it, for example. But yeah, it was just fun and super well acted. And again, you know, a little bit. A little bit. You know, a little bit. You know, a little. You know, a little.

little bit long, Barbie, but it was threading a needle of, you know, okay, you want to get, you want to support Barbie in her desire to have a more fulfilling life, but you don't want to do that at the expense of anyone else. And, you know, Ryan Gosling, who's playing Ken, does an amazing job. He starts the movie as basically the only character in Barbieland who is not satisfied, right, who is not happy living in Barbieland. And that's telling because Barbie land is a matriarchy, and matriarchies aren't any better than patriarchies, right? He is just a secondary character, and it's completely fair for him to be a little bit dissatisfied. And how do you resolve that? It's all very tricky. I encourage you to go see the movie. It was pretty awesome.

Okay. Final big topic of the last couple weeks, I'm going to group together several questions on it. Robert Ruxendrescue says, what do you make of the recent meeting in Congress on the topic of UAPs slash UFOs. For those of you who don't know, UFOs, of course, unidentified flying objects, but UAPs are the rebranding effort to try to make them seem more respectable, unidentified aerial phenomena, okay?

Stevie CpW says, in a cocktail-infused debate with friends over the recent congressional inquiries about UFOs, I proclaim that the more reports of extraterrestrial events there are, the less likely they were to be true. My argument, sorry, my argument was based on the incredible odds in space and time against even one visit from an extraterrestrial, let alone hundreds. Is my logic flawed? Also, what is your opinion on this topic, and do you think that it could be a subject of a congressional hearing?

Should be a subject of congressional hearing. And then Ali Wright says, I am a natural skeptic, but the recent congressional hearing on UAPs, and in particular the UAP amendment to the National Defense Authorization Act contained stunning claims. My question is, how would you suggest approaching this situation? What is a rational path that avoids both conspiracy theory, woo, but also remaining open-minded in the scientific sense? The Great Deceiver finally says, first off, did you watch any of the hearings into UFOs, even for fun? And if yes, what are your thoughts? So I'll give you my general thoughts, and I feel bad about giving you my general thoughts because they do not line up with some of the people asking the questions.

Sorry about that, but I think it's all complete nonsense. Those are my general thoughts. I do not think that there's any evidence to be taken seriously presented in those hearings that should lead you to believe that aliens are visiting the Earth in their little spaceships and crashing and being captured by the government and kept from us in secret. Okay. Again, it's possible. And again, I kind of hope that I'm wrong.

Like, that would be intriguing but weird, but kind of awesome. But I would put that as way lower probability than the superconducting stuff. I suppose in the superconducting question, I should have been more clear in the answer to Subendu's question. Do you think extraordinary claims require extraordinary proof? Yeah, exactly. Absolutely, I think that. But it's not that extraordinary claims in the sense of it would be really important if this is true, require extraordinary proof. Extraordinary in the sense that your prior is really low, right?

as a good Bayesian, you start with priors on all these propositions, and in order to be persuaded that there is substantial credence in them, you need the data to come in, more information to come in, that you judge the likelihood of that data to be really, really big if this extraordinary claim is true, and really, really small if it's not true, okay? That's the kind of extraordinary proof that you need. And for the superconducting stuff, the proof is not really that extraordinary right now, but, you know, who knows, maybe it will firm up. For the UFO stuff, the proof, the evidence, is just laughable. It is true that this dude, I forget what his name is, went and made claims to a congressional committee that were quite astonishing claims. Let's put it that

way. It's also true that he has all of the vibes of a complete crackpot. I mean, at one point, he was talking about the holographic principle and extra dimensions and stuff like that. If that didn't set up off your alarm bells, I'm not sure what would. So the two things to say are that, first, my prior on the idea that there are aliens buzzing us and UFOs and crashing and being captured by the government is extremely, extremely low. That whole scenario makes no sense. For many reasons, I've talked about it many times before, the whole idea that aliens are so sufficiently technologically advanced, that they can easily fly across interstellar distances, and yet can't, and try to avoid detection, right?

Like, they're not just landing on the lawn of the White House and saying, take me to your leader, right? They're trying to be cagey. But they're not very good at it. So we get these fuzzy pictures of them, right? And then, you know, you probably have seen these maps of sightings of UFOs worldwide. And when, you know, they take a map of the world and they put little dots. where there are sightings of UFOs, and they're basically all in the United States.

That would be weird, right? Why would the aliens so much care about the United States? That doesn't make a lot of sense. Maybe there's something else going on. So I think that it's just not the way that it would happen if there were really technologically advanced aliens out there, that they would buzz us in their spacecraft and then occasionally crash and be captured by the government. That's just a very, very low prior probability on my chart. and then you ask, okay, well, what is the evidence here?

Well, what is the evidence? It's a guy making claims. That's the evidence, right? To change my prior about that, to update my credences, show me a spaceship, show me an alien, you know, let us go touch it and do science on it and stuff like that, then I'll be convinced. This is nowhere close. This is just some dude who likes attention as a bit of a conspiracy theorist. You know, again, as I always say, I could be wrong about this, but if I have to bad, if I have to think about what I'm going to do with my life, I'm not putting a lot of credence on these particular claims. And by the way, these claims happen all the time. It's like every six months or every year, there's some new idea that, okay, this time we're going to get the evidence,

we're going to find out, and you'll be sorry, you know, you being all of us skeptics. And I'm like, okay, call me up after it happens and I'll look forward to being sorry. And by the way, I will look forward to being sorry. I will absolutely very, very quickly repent and say not, and I will not claim, if we actually find out that the government does have alien crash relics, okay? I will not pretend that I was supportive of that idea all along. I will be completely wrong if that turns out to be true, and I'll be very, very quick to admit that I was wrong.

Let's just get that in the record right now. Meanwhile, though, let's move on to other AMA questions. Michael Lesniak says, basketball games, okay, now we're talking. Basketball games tend to drag out with all the timeouts. Curious what your take would be on handling them the way curling does. The team calling timeout can talk to coaches and strategize, but the other team cannot unless they also burn a timeout. Or do you think that the game is fine just the way it is? You know, I think there's two things going on here.

One is that the level of physical exertion involved in basketball and curling are not really comparable, okay? Basketball is really tiring, even though you're only, you know, the clock is only running for 48 minutes, which is not that long. You're running very fast up and down the court that whole time, and you're being smashed into by other players, and there's a high probability of injury. The number of injuries in the NBA in recent years is just astonishingly high and very frustrating to me. I don't remember it being like that back in my day, and I'm not quite sure why they seem to be more prevalent these days. But basketball players, you know, the timeouts are not just to strategize.

They're to catch your breath. That's kind of an important thing. So I think that you shouldn't change the general feeling of timeouts and what they are. Having said that, there's just an obvious problem with NBA games and how they end, right? When the games are close, there are various ways for, coaches to drag out the ending of the game to give themselves some small chance of maybe pulling out a miracle win. You foul the other player, you send them to the free throw line, and then you get the ball back, and they can only shoot twice if you foul them on a two-shot foul, and then you can try to

make a three-pointer, so maybe you can climb back into the game. There have been suggestions for ways to do it better. There's something called the Elam ending, or Elam, I don't know how to pronounce it, but there's also just ways of, you know, punishing the team more harshly if they foul in the last two minutes of a game, right? So I would be entirely in favor of fixing the rules so that the very last couple minutes of an NBA game don't drag out that long. But the general strategy, the general tenor and tone of timeouts in the NBA is fine with me. We all need a commercial break for various reasons, right? Nick C. says, what do you say is the best or most convincing explanation of the Copenhagen interpretation that you've come across, and what is the interpretation of quantum mechanics that you find most compelling besides Copenhagen in many worlds? So let me be clear. I've tried to be clear before. The Copenhagen interpretation is not an interpretation. It's not a theory. It's not at all respectable. It can't be right. It's just not on the table as a competitor, okay? It is what most physicists go along.

with, but that's just because they're not trying to understand it very hard. And the reason why is not because the Copenhagen interpretation is wrong. It's just because it's incomplete. It's fuzzy. It's vague. It's not a theory. It doesn't tell you what happens. You know, you say, I measure a quantum system and its wave function collapses.

But it doesn't say what a measurement is. It doesn't say when things collapse. It doesn't say how closely you have to measure any of those things. So it's just not one of the things that are taken seriously when it comes to. into formulations of quantum mechanics. Now, there is a sort of post-Copenhagen school of epistemic approaches to quantum mechanics, which is to say we don't treat the wave function as representing reality, we treat it as a way to make predictions, and the fundamental real things are the agents, the observers, making those predictions. That's more respectable than Copenhagen. It's sort of a

gussied up, more respectable-looking version of Copenhagen, because Copenhagen also tries to make some of those moves. But I still don't think they make any sense in their current state because you haven't told me what an agent is. Like, what is an agent that has experiences? How do I model that mathematically? That's just not what I do in physics or in science more generally. I need to be a little bit more specific, a little bit more rigorous, et cetera. So I don't think that Copenhagen is in the running, but the epistemic approaches are, even though I think they have very, very deep problems themselves. In terms of other interpretations, probably something like Bomeen Mechanics is the next one after many worlds, but honestly, it's so far behind in my personal ranking that I don't

spend any time thinking about the alternatives, to be honest. Bob Zanelli asks a priority question. So remember that AMA questioners, that is to say, Patreon supporters, have have in their pocket as soon as they join Patreon one opportunity in their life to ask a priority question. And a priority question is one that I will do my best to answer. The idea that you can only do it once in your life is not very harshly enforced. If you want to sneak in another one, it's hard for me not to, it's hard for me to notice because I'm not really keeping track. So I'm relying on the honor of the questioners to only ask one question in their life. lives. We're at the point now where I cannot answer every question. I feel bad because some of these

priority questions, people clearly very much wanted to ask the question. Maybe they did ask it before and I didn't answer it. And usually when I don't answer a question, it's because I don't have anything interesting to say. And then they ask it as a priority question and it still remains true that I don't have anything interesting to say, so I feel sorry about that. Sorry. Anyway, Bob's question is, assuming an extremeal black hole is possible, in very general terms, how would its gravity field be described? Yeah, not that much differently than a regular black hole, to be perfectly honest. The idea of an extremal black hole has to do with the fact that black holes are characterized

by their mass, charge, and spin, right? And in some sense, not a very exact sense, but in some sense, the effect of charge and spin counteracts the effect of mass on the gravitational field of the black hole. So it's easiest if we think about electrically charged black holes. So spin is a whole other thing because then you're violating spherical symmetry and you have to be careful and you can be, but let's think about electrical charge. Okay. So the thing that happens when you have an extremal electrically charged black hole is imagine

that you have two electrically charged black holes. And they're sitting at some distance away from each other, right? So they have a gravitational pull because they're pulling. black holes, so they have gravitational pull, but they also have electrostatic repulsion, if they're both positively charged. For an extremal black hole, those two forces exactly cancel. The two black holes can just sit there at a constant distance from each other because they are pulling each other gravitationally, pushing each other apart, electromagnetically, and therefore they don't move. But if you're an uncharged particle, if you are not yourself electrically charged, then you will just fall into the black hole. It's just a black hole as far as you're concerned. You're not being pushed away from it by any force if you're just not electrically charged yourself. There's a whole very, very interesting discourse about extremal black holes because the interior space-time structure of extremal black holes is quite different.

I once wrote a paper about this with Matt Johnson and Lisa Randall because there's a sort of discontinuity in the internal space-time structure as you go from a regular black hole to an extremal one. Basically, what you're trying to do is imagine taking a regular black hole and then throwing electrical charges into it to charge it up, right? But the thing is, when you're throwing electrical charges

into the black hole, those electrical charges are massive particles. There's no massless electrical charges. So you're adding both mass and charge to the black hole. And as far as we know, you can't make an extremal black hole because you are always adding more mass than charge in some sense in which you can generally compare them. So if you were able to go super extremal, if you were able to imagine a black hole that in some units, once again,

had more charge than mass, then you could have a naked singularity out there in the universe. And so that's a super extremal black hole. So an extremal black hole is kind of on the boundary. It's an unstable thing. It's right barely at the point where there might be naked singularity, but there isn't, really. So it's thought that in nature you're not going to get any extremal black holes for that reason, but they're interesting things for theorists to think about.

In particular, if you're doing supergravity or super string theory, there are configurations of super symmetric extremal black holes where you can really talk about some of their properties analytically very nicely. So they're very fun things for a theorist to think about. They're not really out there in nature as far as we know. And if you were just standing next to one, you'd fall into it, and then you could not come out because it's a black hole. Leon B. says,

loved the conversation with Katie Elliott. In the last half an hour or so of your discussion, it became clear to me that she was fishing for something she found unclear in your philosophy and was probing to uncover exactly what it was. after her parting shot, that's the saddest thing I've ever heard. It occurred to me that maybe what's missing is humanity. You have a very well-defined philosophy of particles, but doesn't philosophy have to say something about people?

Well, yeah, sure. Philosophy has something to say about people. If you read my book, The Big Picture, I talk about people an awful lot. It's a big part of things. I think it's got to be compatible with your philosophy of particles because people are made of particles at some simple level, right? And I actually don't think that that, I agree with your diagnosis that there is something that

Katie found unclear or unsatisfying in my perspective, but I don't think it's that people were not there. I think it's more metaphysical, ontological. And it's a very common difference. Like on some spectrum of philosophical views, there's various different questions you can ask yourself philosophically, right? So there's a particular question you can ask yourself about what is the world made of, how many things, how many kinds of things are there out there in the world? How rich is your ontology versus how sparse and barren is your ontology? There's one point of view which says, like, there's lots of different kinds of things in the world that have some reality, and they all

interact together in certain ways to make up the world we know. There's another point of view that says the world is basically one kind of thing, and that for our own personal convenience, we subdivide it into pieces and talk about those pieces. In that particular continuum, I am far on one side, where I think that the world is just one simple kind of thing, and all the subdivision into pieces, et cetera, is apparent and effective and useful to us at a higher level. It is emergent. It is not fundamental. And I think that that is what I think, you know, we'd have to ask Katie, but I think that is the difference that she was trying to pinpoint because I'm really in favor of removing almost everything from the ontology.

and then showing how the stuff that we see in the world, whether it's causes and effects or tables and chairs or free will or choice or anything like that, emerges at some higher level. And I like to say it doesn't mean it's not real. It's still real. Tables and chairs are real. So is free will.

So are causes and purposes. But they're approximate higher level things that we explain in terms of some deeper level unified thing. I think that's what she was aiming at. but, you know, we'd already gone on long enough, so maybe she'll write a paper about it and then, you know, refute all of my opinions. Liam McCarty says, I saw you on Robinson's podcast with David Albert. David disagreed with you about the many world's interpretations, saying it wasn't clear how probability could apply to branching of the wave function since each branching event happens only once. You said he needed to get to let go of the frequentest view of probability. After all, we speak of the probability of a candidate winning a election, even though the election will only happen once. But I've never understood how such a

probability, say, of a candidate winning an election, makes any sense. Sure, it expresses a degree of confidence in the election result, but there's no way to ever know if that confidence was appropriate. Since the election happens only once, can you explain how a non-frequentist view of probability makes sense? Yes, I can, or at least I can try, and then you can decide for yourself whether it works or not. I think that your attitude or your perspective on this, Liam is a common one. I think that a lot of people come across the idea of probability. It gets explained in terms of a frequency. You know, what do you mean when you say the coin is 50% chance of being heads, 50% chance of being tails? What you mean is if you did it more and more times,

that would converge on 50-50, right? And that is the objective frequentist view of probability. The subjectivist says something that is very, very different than that. And you can agree with it or disagree with it. It's going to completely match the coin example in the simple examples, but it is fundamentally a different kind of thing. And so what I would just say is that you can't just secretly or not secretly assume that what is meant by probability is an objective frequency. That is one possible thing. It could be, but other people mean something else by it. What they mean is a degree of confidence that something is going to happen. When I have a coin and I'm going to flip it,

I don't know anything more than to say that it's 50-50 heads and tails, okay? And I can't flip it an infinite number of times. So the subjectivist says, look, all this talk about infinite number of times is just something you made up to make yourself feel better. In the real world, you're going to flip the coin. It's going to be either heads or tails. and the statement, I give it a 50% chance of being heads and a 50% chance of being tails, is nothing more than a statement of your ignorance as to which the actual future is going to be.

It fits well with the sort of eternalist perspective on time, as Katie and I talked about a little bit. If you think there is actually some fact about the future and you just don't yet know it, that would be the relevant feature of the world in the case of the flipping coin, and a subjectivist says, what I mean by saying this is a 50-50 chance is that I will give equal confidence, equal credence to either one of those two things happening. And I think that the thing to pinpoint is where you say, where is it? Oh, yeah. This probability expresses a degree of confidence in the election result, but there's no way to ever know if that confidence was appropriate.

Aha. So basically you're showing us that you have a pre-existing view of probability where it must be objective. That's what appropriate is doing here. Like, what do you mean appropriate? You mean the right probability. But a subjectivist about probability doesn't believe that. There's no such thing as the appropriate confidence, right? There's the best we can do. That's all there is. There is a fact that in some way, the probability of the probability of the probability. coin being heads is either one or zero, but we don't know which one. That's the only appropriate thing to do. So we subjectively say it's 50-50. And the subjectivist is going to say that's no different than saying who's going to win the election, who's going to win the NBA championship, which branch of the wave function I'm going to end up on. There's a bunch of things that happen. And this is why I like which branch of the wave function I'm going to end up on, right? because that's clearly a misstated idea, because I'm not going to end up on one branch of the wave function or another. There will be a version of me that ends up on one and a version of me that ends up on the other.

So there's no objective probability. There's no frequentest probability that you can assign to that. All you can do is say once I'm in that state, that, you know, I measured a particle, it's either spin up or spin down. I don't know which one it is yet. What is the probability? What is the credence that I put on being one branch or the other? That's an intrinsically subjective thing, and I can make arguments based on rational, pushing around of those credences that says I'd better use the Bourne rule to do it. But this is a fundamental psychological barrier.

People who think that probabilities must be objective are not going to be happy with the many worlds' interpretation of quantum mechanics. All I'm saying is that there's a whole bunch of things that they're not going to be happy with in which we use probabilities all the time. I go to the internet, you can find the odds about who's going to win the Super Bowl. That's a statement of probability, even though it's only going to happen once. Paul Torek says, oh, another one about the same podcast. When you and David Albert went on Robinson's podcast, David mentioned self-locating uncertainty as a guide of quantum probabilities. He gave a thought experiment involving Captain Kirk using a transporter that splits him into two people, one wearing green and one blue, and before Kirk can see his clothing, he attaches some probability.

to green. You then talked about betting odds and wound up supposing that the two resulting Kirk's share one bank account. Do they have to share some reward like a bank account? Isn't the point, rather, that the one Kirk anticipating having transported and the two after must all share the same probabilities since the brain processes that lead to their credences are identical, and the only probability that makes sense in that context is 50-50? Well, I think that you've stated the issue clearly and correctly, but I think that there's two different issues going on here, okay? One is the one that you're getting at is what could the probability possibly be other than 50-50? And there's no sensible thing that it could be other than 50-50.

There's no rational decision procedure or way of thinking that says in this experiment, by the way. So the experiment is, you know, Captain Kirk is put in the transporter machine, but it's a malfunctioning transporter machine that makes two copies of you, right? So it's just the duplication machine. And in one copy, you know, Kirk is wearing a blue shirt, the other one, some green shirt. And so what should the probabilities be? Like you say, it's exactly identical. It should be 50-50, right?

But there's no possible way that you would ever get to 90-10 and really defend. Oh, yes, it's definitely 90% green. That makes no sense. But that's not David Albert's objection. His objection is not that it should be 90-10 rather than 50-50. His objection is that you shouldn't assign probabilities at all. You should just remain silent. You should follow Professor Wittgenstein's advice.

This is something about which we cannot speak, so we should just remain silent about it. I don't agree with that at all. I think that that's not a good way to go through life. If you are going to wake up after this experiment and either have the green church or the blue shirt or whatever, it is part of your attempt to cope with the world around you to assign credences to the different possibilities. You know, I can say if I climb out the window and jump, I just don't know whether I will fall up or float in the air. Who knows? That's asking me a question about how the fundamental laws of physics work, and I'm not sure, so I can't possibly

be expected to assign a credence, right? No one believes that, by the way. No one actually thinks that way. My point is, you should think that way. You logically are bound to think that way if you think that you cannot assign probabilities in these duplication processes. It's in both cases, a subjective probability about a case where there's, you know, objectively something's going to happen. You don't know what it is for sure. You are basically required as a functioning human being to have a credence in the different possibilities. You have a credence in whether or not you're going to float in the air or fall down if you jump out a window, right? That credence is it not a frequency. We're not going to, you know, change the laws of physics many, many times.

Laws of physics are what they are. That particular event only happens once. But you can get evidence for it by doing other experiments. It's a tricky thing. I don't want to, you know, I disagree with David about this. This is a central point of our disagreements, even though we agree with many, many things. But it's not a silly argument. His argument makes perfect sense. It's one that I don't agree with, but I can see why someone would feel that way because the situation is quite different than what we're used to. We're used to situations. We feel comfortable in situations where what we mean by probability is some borrowed version of a frequency, right? When you're flipping coins or playing dice or playing cards, then you can really imagine doing the experiment in

infinite number of times get a frequency. These kinds of things where you can't imagine doing the experiment in the number times and get a frequency, therefore bother people. My response is that you never really were doing it an infinite number of times. You were always attaching a credence to it. You had to do your best to come up with a credence that makes logical sense to you. Anonymous says, if we look at the global economy as a complex system, can we see it functioning so that each individual person could spend continually less working time to meet his or her needs, say according to Maslow? Well, this sounds like a very complicated question about the function of the global economy, but I do think personally that, yeah, I mean, it's pretty darn

clear that we are in a situation where we generate a lot of wealth in the world. And and it is not spread anything like equally, right, or evenly. We could lift a whole bunch of people out of poverty. The question is, sorry, by spreading around the wealth more equally, I think that's perfectly clear. The question is whether or not that is a net good thing to do because the argument, the counter argument is twofold. Number one, there is a sort of results-oriented argument

that says that the existences of inequality are important, for making progress. Yes, not everyone benefits, but the people who do benefit benefits so much, and there's so many of them that it's still worth it. That's one attitude. The other is just a more moral-based argument that says that, you know, people have the right to keep what they earn and things like that. But anyway, I do, this is, you're, you're generating answers to a different question than what you asked, so I should come back to the question you're asking. I do think that it makes no sense that we have so many people in the world who are so poor. Let's put it that way. I think we could do much better than we actually do. Could each individual person

continually spend less working time to meet his or her needs? I think that just depends a lot on what you mean by the needs, right? I didn't need to have the internet a hundred years ago. Do I need to have the internet now? Well, I don't need it. You don't need to be listening to this podcast. What do you mean by need, right? As the world changes, the things that we want and the things that we might reasonably aspire to change with them. Life expectancies were a lot shorter 100 years ago, and now we might reasonably expect to get decent health care that carries us through ordinary medical circumstances. We don't always achieve that, right? I do think that we're just building computers, building robots, building manufacturing that lets us produce a lot more

stuff. So if you keep the amount of stuff everyone has constant, the amount of work everyone has to do, will go down. But almost no one wants to do that, right? Maybe some people do, but most people want more stuff. And that doesn't sound, I mean, that sounds worse than it is. Some of the stuff we want is just like healthy food and, you know, long life and health care and the ability to talk to our friends and have fun and stuff like that. These are good things to want. Let me put it this way. I think that the world is changing very rapidly. And our cognitive set up, the way that we think as human beings, is trained on conditions from a thousand years ago or five thousand years ago. And I don't think that we have done a very good job

in setting up the global economy in the best possible way, given our current capacities. I don't know necessarily what the best way to do it is, but I think that this is something where it's just hard to do the right thing because a lot of people don't know what the right thing is. They don't even agree on what it is if they do think that they know it. They can't figure out ways to work together to make it happen. Some people don't want the right thing to happen because they benefit from the wrong thing happening. It's a complicated mess. So I would like to move toward – we did a podcast with John Danahir about this,

the possibility of an automated utopia where people could just stop working and enjoy life and be creative and so forth. And I don't think that's what will happen in the medium-term future. but it's something that we can at least talk about now in a way that we couldn't talk about 200 years ago. Brent Meeker says, Roy Kerr claims that Penrose's singularity theorem for black holes doesn't actually apply to the real world. He says that there's a spinning object

at the center of a black hole. He points out that the solutions that describe black holes are vacuum solutions and so a vacuum is just assumed. They are not solutions for collapse of real bodies. Are there any solutions, even computational ones, for the collapse of matter,

into a spinning black hole? Do they treat the matter quantum mechanically? I think that Penrose's singularity theorems are the wrong thing to talk about in this context. The whole point of the singularity theorems is they don't assume spherical symmetry, they don't assume vacuum or anything like that. They're quite generic. They assume what are called energy conditions, which means that there are certain kinds of energy and momentum that you're not allowed to consider. Basically, negative energy energy, right? Negative mass, repulsive gravity. could have repulsive gravity, negative masses, then you would not need to have a singularity.

You could just throw a bunch of negative energy into the black hole and prevent the singularity from happening. But the whole point of the singularity theorems otherwise is that they are otherwise completely generic. They do not assume any symmetries or anything like that. Before the singularity theorems came along, you could very well have worried that singularities you predict in gravitational collapse. By the way, Robert Oppenheimer's most famous physics work was exactly on this. But anyway, those gravitational collapses that we could understand analytically were all spherically symmetric and maybe the singularities were just an artifact of that. The whole point of the singularity theorem is to say no, even if you look at more general

situations, different things happen. Now, if you want to know exactly what does happen, that's harder. Okay? So the singularity theorem say that as long as you have certain energy conditions being obeyed, then you will get a singularity. They don't tell you how you get a singularity. They don't tell you what kind of singularity you have. The Kerr solution, Roy Kerr, for those who don't know, was the guy who wrote down the first exact solution to a spinning black hole, the Kerr metric. And outside of the spinning black hole, the Kerr metric is very close to being exactly right. Inside, it predicts singularities, but weird singularities. It also predicts like wormholes that go to other universes and things like that, things that almost know physicists think actually

are there because they're not realistic solutions describing the formation of a black hole. They are solutions describing a black hole that was always there with no matter in it, okay? When you do have spinning matter, matter with non-zero angular momentum, the collapses to a black hole, the situation is way more complicated. There are computational attempts at understanding it, but you don't know. Things go crazy, and it's hard to know how reliable they are. Do they treat the matter quantum mechanically? Well, what do you mean by quantum mechanically?

Do you mean you have a wave function or you're doing quantum gravity? Or do you just mean that the energy and pressure are compatible with more quantum mechanical constraints? I'm not sure. I'm not a super expert on this. I do know that the quest to understand exactly what happens when realistic things collapse to make black holes is an ongoing one. it's not a settled field right now. There is a sort of consensus as to what happens, which is that you collapse to a singularity

in a real world spinning black hole. But, you know, until you know for sure, you don't know for sure. Paul Conti says, assuming that university-level textbooks in general will reach a smaller number of readers, do you think it would be better to write more popular science books, which I imagine reach a larger audience slash readership? Do you prefer writing popular?

science or textbooks for students. You know, you're not going to be surprised at my answer here, which is that you should do both. I am always a believer in a wide, pluralistic ecosystem of attempts to talk about science, to teach science, to educate people about science. There's absolutely a very obvious good reason to write textbooks, which is that even if you reach a smaller readership, that readership is very important. It's the readership that will grow up to become professional physicists and do new physics, right? So that's a important thing you can do for the world. I'm always very, very tickled when I see people who are now doing great things to, you know, finding gravitational waves or something like that. And they say they learned general relativity

from my textbook. That, you know, that tells me I did something worthwhile for the world. But it's also very, very important to write popular-level things. Like you say, you reach a larger readership. That readership might be mostly interested in simply, understanding things or increasing their knowledge about the universe for their personal reasons rather than for future contributions to science. But that's entirely okay. I'm very much in favor of that. I think that the same thing goes for different media, whether it's books or podcasts or blog posts or lectures or TV shows or movies or whatever. Do it every single possible way that you can do it. I think they're all different ways of reaching.

different kinds of audiences. In terms of preference, you know, I don't know about preference. You get different rewards from writing a good textbook versus writing a good popular book. So it's a tie. Let me cop out by saying that. Kevin Herang says, I learned a lot from your recent discussion about artificial intelligence with Raphael Millier. Thank you.

But I'm still wondering where you personally come down on the question of whether current or future iterations of AI or something to be worried about, like Einstein warned about nuclear technology, either intrinsically. or just in the wrong hands. Yeah, sure. You should definitely be worried. The question is, what is the form that that worry takes? I thought that, you know, the open letter that said,

stop doing research on AI for six months while we figure out what to do was as a practical matter completely unrealistic for various reasons. Number one, how are you going to get everyone in the world to agree to stop doing that? What's to stop people from actually continuing AI research? just not tell anybody about it, right? Especially people in other countries or whatever. Number two, what in the world makes you think that in six months you'll figure out what to do about it? I think it's a misunderstanding of how these things work.

These things are processes that are ongoing, both the process of developing AI and the process of developing safeguards and figuring out what to do about it. The open letter did a lot of good in the sense that it, you know, sort of got people talking about the issue, which is great. but I don't think it actually what it was asking for was the right thing to ask for. The right thing to ask for is very accelerated, hard thinking, and policy implementation

of safeguards that would protect us from the worst aspects of AI. What those aspects could be, there's obvious many choices. Like if you turn over important technological or industrial features to control by computers

rather than humans, you open up the possibility of disastrous errors. Humans using AI to spread misinformation and fake news and so forth is an obvious big problem. So I think it's very, very important to put safeguards in, to think about what the safeguards are that we need. I didn't really like the idea of just pausing research for six months. I also don't like the idea of emphasizing the possibility of existential risk, where you kill every human being on Earth.

that it's absolutely possible, but it's sufficiently low probability that I don't think that it's the right way to think about it. I think that if you will actually increase the chances of bad things happening, if that's the kind of risk you worry about, rather than the real very, very imminent risks that we have from AI. And furthermore, if you worry about the imminent risks, you're much more likely to also ameliorate the existential risks. So that's what I would be in favor of doing. I'm going to group two questions together. One is from Yaspir Nagra, saying, leaving aside the provocative name,

does Conway's free will theorem prove anything stronger about hidden variable theories than bells? And then Kyle Hicks says, I've heard Roger Penrose state something to the effect that the superposition of the wave function makes the geometry of space time unstable, which causes collapse of the wave function, even proposing that the time of collapse can be predicted by the formula

T equals H, Planx constant, divided by the gravitational energy. Could you explain why he would believe that spacetime could become unstable and your general thoughts on the Diochi Penrose model? You might be wondering why I'm grouping these two questions together. They're both about quantum mechanics, but otherwise they're very different. It's because I wanted to say something. I've said it before, but maybe, you know, I got to keep saying it. Different people are listening.

I am not an expert on other theories of quantum mechanics and the foundations of quantum mechanics in general. You know, the foundations of quantum mechanics is one of my specialties. If you, like, look up my CV. It's one of the things I do for a living. But what I mean by that is I investigate the many worlds interpretation of quantum mechanics. And in particular, what that has to say about other areas of physics, whether it's quantum gravity or field theory or whatever.

Because I personally think that many worlds is much more likely to be true than any of the other ones. As I always say, I would love to be wrong. It'd be cool. And then I would change my mind. that would be great, but yet only have a finite number of heartbeats, right? Every human being has, on average, 3 billion heartbeats in their life. You have to allocate your heartbeats. You have to allocate the time you have on Earth to think about these questions.

If you're a scientist, if you're a researcher, there are literally an infinite number of research questions that you could devote yourself to. You have to pick the ones you think are going to be most productive. So I actually don't spend any serious time thinking about other approaches to quantum mechanics. And this is all by way of preamble of saying that I'm not going to give very satisfying answers to these two questions. So Yossphere's questions is about Conway's free will theorem. The one thing I can say about this is a terrible name. Everyone agrees that a terrible name has nothing to do with free will.

If you're not, if you don't know what this is, this is a theorem that was proved a couple years ago, perfectly valid result in quantum foundations. And it has to do with the fact that, you know, it's one of these theorems, like Bell's theorem, that tries to make statements that are independent of a particular picture of quantum foundations. So rather than saying, let's work within this picture or some other picture, it says, imagine the following axioms hold. Then what can we conclude about that? Right. So Bell's theorem assumes that you have definite measurement outcomes, various other kinds of things. The free will theorem is basically that says under certain very reasonable assumptions,

there will be a case where you cannot predict the spin of the electron. It is indeterministic. It is not deterministic, okay? Under certain very reasonable sounding assumptions, you know, no signals traveling faster than the speed of light and so forth. So this seems very, very unsurprising. know about quantum mechanics. Quantum mechanics already says that you can't predict the outcome of an experiment completely deterministically. The free will theorem, which is Conway and also Cochin, I think,

they say that, well, okay, we don't need to assume quantum mechanics. We're assuming these weaker principles, and you can derive the fact that physics will not be deterministic. They called it free will rather than indeterminism just for PR purposes. It has literally nothing to do with free will, it's just saying that certain aspects of physics are indeterministic, which if you believe in quantum mechanics already, you already knew is true. So just like Bell's theorem, these kinds of theorems are constraints on alternatives you could maybe imagine someday developing to conventional quantum mechanics. But for me, and I'm not looking for an alternative to conventional quantum mechanics. I like conventional quantum mechanics. I'm sticking with that. Likewise, for Kyle's question about Penrose's model, Penrose's, model. violates conventional quantum mechanics.

He has an objective collapse model. The most popular objective collapse models in quantum foundation circles are the spontaneous collapse models where wave functions just have a certain probability of collapsing, all by themselves for no good reason, whereas Penrose's model is in the vein of what you might call triggered or induced collapses

when a certain condition is met the wave function collapses, and that condition in Penrose's version has something due with gravity. that's all I know about it. I don't know anything else about it for all the reasons that are conventionally put forward. I don't love the violence being done

to the Schrodinger equation by saying that wave functions truly collapse for occasional purposes, nor do I think that gravity should play a role in the understanding of quantum mechanics at a deep level.

Now, I understand, I don't hold that latter point of view quite as strongly as some of my friends because some people are like, well, look, when I talk about the simple harmonic oscillator or two spins or whatever. Gravity is not involved. I don't need gravity to talk about quantum mechanics. And I do think it's perfectly fair for Penrose to respond to that by saying, what I care about is the real world. And in the real world, there is both gravity and

quantum mechanics. So I'm perfectly allowed to use gravity to help me understand the mysteries of quantum mechanics. I'm sympathetic to that prospective argument that Penrose could put forward, but I don't think that as a matter of fact, gravity has anything to do with quantum mechanics at the fundamental level. Aaron Bowden says, I found Joe Silk's moon science argument compelling. Would you go to the moon if you had an opportunity to work there for, say, a six-month mission? You know, I think this depends a lot on what my credence was that it would be safe to go to the moon. I would love to go. You know, that would be a lot of fun, right? Just for a touristy mission, not for a one-way trip, if I thought I was going to come back. The question right now is, that space travel is certainly not safe. There's a certain fraction of the time when you have a disaster and people die,

and that fraction is uncomfortably large. I think, you know, one way to put it is, if I thought that the chance of a fatal accident was 10 to the minus 4, then I would go. If I thought it was 10 to the minus 1, then I would not. Somewhere in between, it would be a difficult decision. But, of course, it would depend a lot, therefore, on exactly who was flying, what their track record was, and so forth. Oleg Ravinsky says,

I've recently read your new paper on discretizing quantum mechanics. Yeah, so by the way, I put a couple of papers on the archive lately. One was one on, they're very completely different. One was with my old grad school advisor, George Field, on making primordial magnetic fields from axion dynamics. And the other one was from just me on discretizing quantum mechanics. So the idea here is that we talk a lot about, discrete space time, right? But even if space time is discrete, if you believe in quantum mechanics,

the theory, the fundamental scientific theory that you're working with is not discrete because it's quantum mechanics. Quantum mechanics is not a discrete theory. The elements of the theory are vectors in Hilbert space, and vector spaces like Hilbert space are smooth. They are not discrete. The space of states is in no sense discrete in quantum mechanics. And furthermore, there is smooth time evolution, according to the Schrodinger equation. So I asked myself the question, is it possible to discretize Hilbert space in a way that is naturally compatible with the Schrodinger equation? A couple other people have already asked about whether you could discretize Hilbert space at all, and they said yes, but I specifically said, you know, could you do it in a way compatible with

the Schrodinger equation? And I said, yes, you can, but there's problems with doing it. It does get rid of infinity in the sense of an infinite number of states, but the use. The universe still lasts eternally, even if time steps are discrete, according to the evolution, and so you get recurrences. There's only a finite number of things that can happen, but they happen infinitely often, and so you get all the problems that you normally get with Boltzmann brains and things like that. So anyway, you can look it up on Archive if you want. So Oleg asks, I also recently read another paper, the Bordeaux-Felan theorem in extended decider spaces, which, if I understand correctly, states that the universe is geodesically complete,

i.e. had to have a beginning. I wonder whether there's a contradiction here between these models, or are they compatible. So I'm using this. I have not read this paper that Oleg is pointing to, the Bordeaux-Welan-Ther-Lanth theorem and extended to sitterspaces, but I'll use it as an excuse to talk about the Bordeaux-A-Guth-Welan theorem in general. Bordeaux-A-Guth-Velanken are top-notch theoretical cosmologists. You should believe what they say. But what they do is they prove a theorem about certain kinds of classical space time. And all the words here matter. They make some assumptions. They say if the following things are true, then the following things follow. And the thing that they're proving a theorem about is classical space time. Now, some people want to

pretend that they're not proving a theorem about classical space time, because when you think about the difference between quantum mechanics and classical mechanics, there's two differences. There's many differences, but two are relevant here. One is that you are talking about a different kind of thing, right? If you have a point particle in classical mechanics that has a location and a momentum, and that's its state, and you can predict its future. If you're doing quantum mechanics, you're talking about a wave function of that particle. That's a completely different kind of thing. Likewise, if you're doing gravity, you have classical space time. If you do quantum gravity, you have a wave function of space time, a completely different.

different kind of thing. The second ingredient is you have specific equations of motion that tell us how those things that you're studying evolve with time. Newton's equations or Hamilton's equations or the Oil or the Orange equations, there's a separate set of equivalent ways of talking about classical equations of motion, and likewise there is the Schrodinger equation or the Heisenberg equation of motion or the Feynman-Path integral. These are distinct equivalent ways of talking about the quantum motion. And the Bordeaux-Guth-Felenkin do not assume classical equations of motion. So some people say, well, they're not really doing classical space-time. But the space-time is completely classical.

They've thrown out the equations. They're not assuming any certain equations about it, but they're still looking at classical space-time. And what they're saying is certain kinds of classical space-time, under certain assumptions, have a singularity in the past. So what do we learn from saying? that? Can we say that the universe therefore had a beginning? No, of course not. There's no version of the Borde Guth-Velan theorem that implies the universe had a beginning. I'm not sure how to say this more clearly. For two obvious reasons. Number one, their assumptions might be wrong. There are plenty of examples on the market of universes that don't have a beginning, models of universes that don't have a beginning.

They're all completely compatible with the Bordeaux-Welan-the-Ther. because they don't fit the assumptions that it makes. But the second and more conceptually important thing is, space time is not a classical thing. We have quantum gravity. So you cannot possibly learn anything fundamentally necessarily true about the real world by looking at the Bordeaux-Belan theorem or any of its generalizations.

What you learn when you look at a theorem, whether it's Penrose's singularity theorems or the BGV theorem, is that there is a singularity in this classical space time, and how to interpret that is to say that our understanding gives out. Our ability to think of space time as classical ends at this point. It says nothing at all about the universe ending or coming into existence or having a beginning or anything like that. It's just about the applicability of the classical or prognosis.

So, therefore, no, there's no incompatibility whatsoever. My paper on quantum mechanics was about quantum states that evolve eternally with time, that is completely compatible with anything that you might want to argue about classical space times. I mean, the question is, does a certain model of quantum mechanics, that is to say, a certain Hamiltonian, et cetera, reproduce this particular behavior or not? That I don't know. But there's no, in principle, incompatibility there. By the way, I said the BGV theorem is perfectly respectable scientific theorem. If you look at who cites it in the literature, it's almost all religious people. It's almost all people arguing for the beginning of the universe because they want to prove the existence of God.

Scientists know that it's an interesting theorem, but just not that central because the universe is fundamentally quantum, not classical. Craig Hooper says, what do people mean when they say the universe is infinite? Do they think it's, do you think it's infinite? I'm with Jan 11 in the, we've never encountered any physical quantity that is infinite camp, so it seems unlikely to me that there is one quantity like diameter or volume that is the exception. Well, I don't know about that particular argument. We've never encountered any physical quantity that is infinite. How would you know whether you did? On the real number line, there's an infinite number of points between zero and one, but if you actually draw a line segment on a piece of paper in front of you,

you can't tell whether there's some discretization and there's really only a finite number of points or whether there's an infinite number of points. So my attitude is, I don't know, we let it be either way. We don't have any way of telling right now. There are, I don't want to underplay the importance of the question. That's why I wrote this paper I was just talking about because maybe you can help resolve some conceptual, mathematical, philosophical puzzles

if you don't need to invoke infinity when you talk about the physical universe. And in quantum mechanics, standard quantum mechanics, you do need to invoke infinity. So I wrote a little paper saying, here's a version of quantum mechanics where you don't. But again, it's all toward the purpose of keeping an open mind about whether infinity really does exist or not. I just don't know. Georgio says, the professor I work for to achieve my master's degree has high ambitions and a broad spectrum of knowledge. He chooses his students based on potential and their motivation, and he gives us complete freedom in our research. Do you also prefer this long-leash approach with your students, or do you prefer a short of more hand-holdy approach?

Well, I think something in between. I'm not very hand-holdy. That is true. Just because I'm stretched too thinly. I'm doing too many things. I'm working with several different students. I have a podcast that you might know about. I write books. I teach. I do various different things. So I am not the kind of advisor who is working on the student's problem just as hard as they are. I know that there are such advisors and I admire them,

but I just don't have time to do that. And I try to tell prospective students exactly that, that I'm very good at coming up with ideas. The ideas aren't always very good ones. I'm very good at coming up with them. So they'll never be a point in our relationship where I say, yeah, geez, I don't know.

I'm not sure what to work on. I have no good ideas. I have too many things. things I want to work on, and I'm very happy to share them with students. But when I do, the student is going to be much better off if they can take some initiative. We'll talk, you know, regularly, and we'll try to bounce ideas back and forth, and we'll both do calculations, but the student is going to do much of the heavy lifting. If a student wants to do just work on their own ideas, that's great. I'm perfectly happy to do that, but most students don't want to

do that. At least they don't want to exclusively do that, or they don't want to do that in the beginning. When you just start out as a graduate student, one of the most useful things that an advisor can do for you is not just give you ideas to work on, but help you understand which ideas are promising, worth following up on, et cetera. It's a real art form, choosing not just to do some projects, some calculation or whatever, but also to do the right ones, to do the ones that are worth doing. So, you know, generally speaking, graduate students appreciate this, either implicitly or explicitly, so they want to talk to the advisor about what are good things to work on, and I'm very happy to do that.

You know, I'm still new here at Johns Hopkins, so I haven't built up a group like I used to have at Chicago, and then at Caltech where I was having four or five grad students at once or more, and they could work together and talk to each other, but we're going to get there. We're working toward that goal. Jan or Jan, I'm not sure, M, says, in the past, you gave several talks about religious beliefs and even had public debates with apologists, which I find very interesting. However, in the last few years, talks and debates from you about this topic became quite rare, as far as I can tell. Do you still think that engaging in these debates is a worthwhile activity for you? And if not, what has

changed your mind? Well, I think two things have changed. Number one, I've kind of said most of what I have to say, you know, that there's a segment of people who enjoy and participate in either debates or public speaking more broadly for the intrinsic thrill of it, right? Because they actually enjoy that. They enjoy getting up there and matching wits with someone they disagree with. That's really not what I'm about, to be honest. That's why on the podcast I have people who I disagree with sometimes, but I'm really about learning from them. I'm not really about debating them. It's just a personal style thing. Sometimes debates are useful. I know that there's other people on the other side that I don't quite agree with who say like, ah, debates are a waste of time because that's not how real

intellectual work gets done. And it's certainly true that real intellectual work is not getting done in the debates. They're more about entertainment, but also about inspiration and information. You can reach audiences by doing a debate that maybe you couldn't reach otherwise. But I've done them. You know, I've said what I have to say, they can find those debates on YouTube. I think I would just be repeating myself. But the other thing is, there's just not that many of them. Like, I have never been the person to suggest doing a debate with a religious person or about some topic like that. They've always come to me, or some intermediary has come to me to set it up, and that doesn't happen that much anymore. I think just because that whole genre has faded away

a little bit. It's not just me. Most people have said what they've wanted to say. It's not, doesn't seem to be a lot of hankering as far as I can tell for new material along those lines. Perfectly okay with me. I have plenty of other things to talk about. I'm not running out anytime soon. Yohonathan Peretz says, we know we don't live in antide-sitter space, or so I understand. Assuming that's correct, can you help shed light on what we hope to gain from ADS-CFT theory? You know, sure, very briefly, it's a lamp post. You know the old joke about looking for your keys under the lamp post, even though it's not why you drop them. It's supposed to be a joke, because of course, if you dropped your keys, keys someplace, you should look for your keys there. But if you have a lamp post, you at least

have a chance of finding your keys, because you can see where they are, right? So it's not a completely insane strategy. There's something to be said for the drunk guy looking for his keys under the lamp post. ADS-CFT is not the real world, but it is a model that we understand much better than any model that does approximate the real world. So I personally don't do a lot on ADS-CFT. I've written a couple papers along those lines, but I prefer to spend my time working on ideas that are closer to the real world, even if they're less well-defined, more speculative, and so forth. Other people are going to say, no, let me work on a model that I understand. Maybe I will gain some insight that down the road will be of applicability to the real world. This does intersect with something I said

in the solo episode on the crisis in physics. One of the very legitimate critiques of modern theoretical physics, especially in the string theory side of things, is they're too willing to say, someday this will be relevant to the real world, even though not now. You know, that's a perfectly good status, but tell me when you're going to get there, or at least, you know, remember that getting there is the important thing. It's the real world that ultimately matters. And I think that it's kind of sad that you can get as far as you can and make a whole career without making much contact with the real world. So I think it's perfectly legitimate reason to work on ADS-CFT.

I just think that it should be part of a broader portfolio. It can't be what everyone focuses on. Girolamo, sorry Girolamo, I'm not pronouncing your name very well. Girolamo Castaldo says, I heard you mention at least a couple of times the Geiger counter as an example of quantum effects influencing the macroscopic world. Would you mind elaborating on that? Sure, the Geyer counter is a quantum measurement device because it measures the decay of a radioactive particle.

It clicks when a particle has been emitted by a radioactive substance. That nucleus or whatever the particle was that was doing the decaying has a wave function. That wave function is typically in a superposition of, I've decayed and I've not decayed, and the Geyger counter performs a quantum measurement, so it collapses that wave function, or at least apparently collapses it. And so if you do anything in response to what the Geiger counter is doing, then in some very real sense you are responding to a measurement of a quantum state. You have amplified a quantum superposition to a real-life superposition. The real-life superposition is the Geiger counter clicked and it has not clicked. Right. So that's why Schrodinger, when he invented the Schrodinger's thought experiment, had a Geiger-counter-like thing as part of it. It's just a way of measuring a quantum system in a very straightforward, familiar way. Andrew Goldstein says, Consequences of climate change

continue outpacing computer-generated weather forecasts. Wouldn't computer models be more accurate if fudge factors were included to account for consistent underestimates? Well, I want to say yes and no here. I mean, the first thing I want to say, the zero thing is I'm not a climate scientist. I'm not going to second-guess what they're doing.

It's a very, very hard problem to work on, very non-linear, many, many aspects coming in and the measurements are hard to do, et cetera, et cetera. Okay, so you should really talk to a real climate scientist, not to me, about this. But there's two things going on. You're right that in the last few decades, the amount of temperature variation that we've seen, et cetera, has generally been more than was predicted by typical models.

So there's two things that you can try to do. One is you can just say, okay, if every time that I try to make a prediction, the real result is 1.2 times the prediction that I'm going to make, I'm just going to multiply my prediction by 1.2 and make that my prediction. I get why you would want to do that. The problem is that these models aren't just best guesses. They're models.

They're trying to actually model the underlying dynamics. You can't just multiply by some fudge factor and declare a victory. You have to understand why, you release your goals to understand why

the actual readings are coming out on top of your predictions ahead of time. So it would be much more respectable to dig into the simulations and figure out why the effects are bigger than what you predicted. The other thing is, though, you might want to, as a policymaker rather than as a scientist, you might want to plan for the future, and then you might want to take into account that over the last several years or whatever, the models have been underestimating the effects.

In that case, it's perfectly legit to say, you know, the models have been traditionally underp predicting the size of the effect, therefore I'm going to play it safe and boost a little bit. So I think it's the difference between being a modeler who's doing their best to understanding what all the effects are and to put them into their computer code or whatever and make the best prediction they can versus a policymaker or a predictor of the future where you can be more phenomenological about it and say, well, given the track record so far, I'm going to predict the following thing in the future. There's no reason why you couldn't do both, depending on what hat you were wearing at any one particular time.

Bits Plus Adams says, was it a difficult decision to go onto the Joe Rogan show? Do you worry that having a reputable scientist like yourself on the program Casahalo over other less reputable guests? Is the audience reached too good to pass up? You know, it's a complicated question. I was on the Joe Rogan show. You have to remember, it was years ago.

It was pre-pendemic, okay? I have not been on in a long time. and I do think that the show has steered much more towards, even more, let's put it that way, towards conspiracy theories and things like that since the pandemic has hit. So I think it would be a much more difficult decision now. But in general, even if that's true, it's still a difficult decision. It's a little bit easy to be overly moralistic about it, but I think that there's a lot of factors that should go into thinking about something like this. one is it's really important to reach audiences that you can't otherwise reach. It's not just the audience is big.

It's that it's a different audience. And if you say, well, you know, they're just hearing from all these crackpots. Isn't if you're really on the side of these people who are listening, getting the best information, shouldn't some good people go on to? I don't know. I mean, I'm just raising the issue. I'm not giving you the answer because I do think that, yeah, you can also lend your credibility to an operation, if it's not clear what is going on. But maybe you can combat

that by being very clear where you stand. When I was on Rogan, we talked about pretty straightforward questions about quantum mechanics and things like that. Nothing controversial or conspiracy-centered. If I ever went on again, I think that I would, which I don't know, I'm not been invited. I'm not contemplating it anytime soon. So it's completely hypothetical, but I would absolutely want to talk about what I consider to be an anti-science attitude in the conspiracy theories, the anti-vaccination stances, things like that that we've seen on the show since then. Having said that, I'm not a listener of the show, so maybe there's a whole bunch of things going on that I'm just not aware of. Moshe Fader says, Imagine that your novel definitive proof of many worlds has won a Nobel Prize.

You decide to indulge yourself by using a fraction of the new spare cash for three timepieces, a dress watch, a sports watch, and an over-the-top weird or technically dazzling watch. Which three watches do you pick and why? Oh, this is a good question, because it's never going to happen. I am neither the Nobel Prize nor the spending all that money on watches. I am a fan of mechanical wrist watches. This is an outgrowth of the first trade book that I wrote from eternity here about the arrow of time where, you know, when you write your first book, you never know whether you're going to write another one. You try to pack everything in. So I tried to talk a little bit about everything having to do with the idea of time in one book, which that's a lot of stuff. So there's a lot of stuff in the book. And I discovered I hadn't really followed the whole thing. Many people haven't. But, you know, the idea of making a functioning wristwatch is quite a technological challenge, right? You know, there's the whole latitude issue that was the subject of prizes back in the day of sailing and so forth,

and John Harrison eventually won the prize. So a mechanical wristwatch, that is to say, not a quartz battery-powered wristwatch, but a little machine with gears and springs and things like that that accurately keeps time is quite a marvelous kind of thing. And when you dig into it, you realize there's a lot of history and artisan ship. going on. And plus there is this whole historical event called the quartz crisis where, you know, like in the 1950s, the only watches were mechanical watches. And they figured out how to make quartz wrist watches. And basically the whole Swiss mechanical watch industry essentially collapsed.

And it's not mostly, I mean, to some extent it is, but most of these watches are not like mass produced. You know, there's a lot of actual human effort that, goes into them and know how that is easily lost. Like, just like we used to be able to put people on the moon, and now we've forgotten how to do that. A lot of this wristwatch making knowledge was not really passed down from generation to generation. So there's a famous story, for example, Zenith, I guess.

It looks like zenith, but it's a Swiss company. So pronounce it differently. They had a very famous movement, as it is called, for a chronograph, the El Pramero movement, the first automatic, that is to say, self-winding, mass-produced, or popularly produced chronograph. And they had, you know, a particular machine that was clearly necessary in the construction of these chronographs, which were, you know, famous and historic and so forth. And in the 70s, the word came down from the owners of the company. Yeah, just throw all that stuff out. We don't use that anymore. We're making Chris Quartz watches from now on. And one of the

old timers basically hid the plans and the machine, you know, in a room where they couldn't be found. And then, what, 10, 20 years later, whenever it was, the whole Swiss watch industry realized, oh, my goodness, we can rebrand ourselves as a luxury good. And now we want to recover all this knowledge we had, and they were able to pull the machines out of the mothballs. So I love all that stuff. I think it's great. And I love, you know, wearing real wristwatches. I don't wear like a smart watch or anything like that.

the bad news is, and I should say it's like quite a hobby, right? So there are people, they get together, they have message boards on the internet and magazines and whatever. Okay. So it's fun and I like it. The problem is it costs money and you quickly realize that it costs too much money. And so like I'm not going to be a player in this game really. It's much more sensible to look at pictures of wristwatches on the internet than to actually buy them all. So I have a collection of several different kinds, but they're not like the super-duper fancy kinds, because those are beyond my budget.

So I like this idea that I would get the Nobel Prize and blow all the money on wristwatches. But I don't have good answers to – well, I don't have a good answer to the – the question is dress watch, sports watch, and over-the-top weird or technically dazzling watch. I don't have a good answer for sports watch. Well, actually, maybe – no, there is a famous Pattec Philippe, which is. is a super duper hoity tooity watch company. They make, what's it called, the aquanaut, which is like a very kind of weird. It was the 70s, you know. It was a, it's a very, you would never, if you were not a watch person, you would not see it on someone's wrist and think it was anything special. But it's like, you know, the good watch companies are

able to like just do all the details exactly right. And so it's a very functional fun sports watch. For dress watch, I'm a huge fan of Lange and Zerna, which is this German company. The whole German watchmaking industry was devastated, not just because of the quartz crisis, but because of the Iron Curtain and the Berlin Wall, et cetera. But now Glacuta, Germany has become second to only Switzerland in terms of watchmaking capabilities.

They've come back to life. And Lange and Zerna is a German company that makes some of the most beautiful dress watches in the world. I don't have a specific one that I would want, but I think that company way outside of my price range now. So yes, my future Nobel Prize winning self might splurge on that. In fact, I would not buy three watches. I would just buy one and probably that would be it. For technically dazzling watches, again, I don't have a particular favorite, but there are some amazing ones out there. People

really have fun because it is, it's purely, what can I say, it's purely frivolous. I think that's the thing to say. There's no, nothing about, I remember, you know, speaking of, religious debates when I debated William Lane Craig. One of the comparisons he made was to say that something was he compared a Rolex to a Timex and the Rolex being

much fancier and therefore more accurate and the Timex being cheaper and therefore less accurate. I forget what the purpose of the analogy was but I pointed out no, a Timex is a quartz watch. Those are going to be much more accurate than even the best mechanical watches if you do a halfway decent jobs. Or Rolex is a perfectly

decent mechanical watch, but it's not going to be as accurate as a good, cheap quartz watch. So the mechanical watches are not made for any technical purpose except to kind of have fun with it. And I kind of like that. I like the search for having fun. I'm in favor of that. So anyway, I don't remember any like the companies or anything that do this, but there are some astronomical watches that almost look like a little miniature solar system on your wrist that you can see all the planets moving around, like you have a little aurory down there. I have no idea whether that would be practical to wear. But again, since it's not happening, I'm happy to say that would be my choice.

Robert Antonucci says, a priority question once again, is dark energy the only thing that affects other things without being itself affected? No back reaction. Well, I can see why you would ask that, but I think that if you're bothered, there's an implicit thing here that it bugs you, that dark energy affects other things. without itself being affected. So I think there's two ways to ameliorate that bothersomeness. One is, in some sense, it is affected because, as spacetime expands, there's more and more

dark energy, right? There's the same amount of dark energy per cubic centimeter. There are more cubic centimeters, therefore there's more dark energy. So in some sense, it is definitely being affected. But the other one is the amount of dark energy per cubic centimeter is just a constant of nature. It's like the mass of the electron or the fine structure constant, et cetera. It's a parameter. It's a value. So, you know, the fine structure constant is not affected by things around it, but it does affect things around it in some sense. You know,

I think you have to be a little bit more precise about what is a thing that does affecting and so forth. The, you know, this is one of those questions where the completely technically correct but unsatisfying answer is, all the equations work out fine. If you write down the theory of quantum gravity, you know, in the weak energy regime with the standard model of particle physics, with some cosmological constant, with some dark energy, everything works out fine. Energy conservation is obeyed in the covariant sense, that is to say, in the sense of expanding space time, et cetera. There's nothing weird or special about the dark energy. It's just the same kind of thing you see elsewhere in the equation's emotion of the universe, except it's a constant

rather than a variable. Dave Whip says, I recently heard of Brown and Suskin's proposal for the second law of quantum complexity, intentionally evocative of entropy, but it continues to grow even past thermodynamic equilibrium. If this understanding is correct, would that make it a more fundamental arrow of time than entropy, or is there some characteristic of entropy that is lost? I'm not going to give you a satisfying answer to this one. Sorry, there is this idea, the Second Law of Quantum Complexity,

that Lenny Suskind, Adam Brown, his collaborator, and others have talked about. But there's two things about it. Number one, it's not supposed to be a general principle like the second law is. It's supposed to have something to do with black holes, or at least horizons or quantum gravitational systems looked at in a certain way. And the second thing is, I'm not really sure. haven't looked at it very closely. You know, I've read the abstract, but I've not dug into the paper and tried to reproduce the results myself. But I'm a little skeptical of things like this because what they do is they define complexity by imagining that you have a bunch of cubits, okay, you know, two-dimensional quantum states that can be zeros or ones or superpositions thereof. And you think of the universe as a quantum computer, or at least some part of the universe, as a quantum computer. So you take your qubits defining the state of the universe and you run them through some gates. you know, and gates and not gates and stuff like that, just like a regular computer,

except the quantum mechanical equivalent of it. And then you can define the complexity of a state as some measure of the number of gates you have to run it through to go from an unentangled state to whatever state you're looking at. I'm very leery about the choice of gates that you have to do that. You know, you can always take any state and turn into any other state in one step, okay? But that one step might be non-local or something like that. So I'm not quite sure where the justification comes from for using some gates and not others. There might very well be on page 38 of the paper a perfectly good justification that I just haven't seen yet. So I don't think that I'm not completely sold on the proposal yet, but I haven't tried to be sold on it, so I'm still open-minded

about that. I'm going to group together a bunch of questions. I'm sorry, they're long questions. hopefully we can keep them in mind, but you'll see there vaguely along the same topic. So David Rabinovitt says, when you interviewed Judea Pearl, he seemed to argue he saw macroscopic states such as billiard ball arrangements as a fundamentally subjective concept. You have generally leaned against this idea with a counter that macro states are not arbitrary, but rather are constrained by the laws of physics.

But when Stephen Wolfram claimed that broad conceptions of possible agents leave an enormous scope for what a macro state might be, you seem to sympathize in part. So the question is, do you have more precise views on what constitutes the most relevant and useful physical constraints upon observers choosing micro-states? Maybe you meant macro-states, I'm not sure. And how important is it to understand these constraints to understand entropy and emergence? Then Henry Jacob says, I recently listened to the Judea-Purl interview on causality, and he asked you about time reversibility and billiards. You mentioned coarse-graining in stat-mec to illustrate how an arrow of time emerges.

Pearl felt that the way course-graining was done introduces a choice and is therefore not fundamental. However, you held yourself back from responding other than saying you felt as if the arrow was fundamental. Can you elaborate here? Okay, so I think these are more or less the same question. But then there's another question from Jim Murphy. I was listening to old AMA episodes, and you had a good response to my question about complexity and cellular automata. You explained that the automata would need to start with some low entropy state in order to see complexity emerge. What's interesting is that, for any particular set of rules, the states that constitute

low entropy may look very different. Has anyone done research into determining the entropy of different states for various CA rule sets, or should I start this research fresh? So, okay, maybe the third question is sufficiently different. I should address the first two first. The general idea, in case you're lost by the questions, is that when we talk about entropy, it's always a macroscopic, I'm thinking classical entropy, not quantum mechanical entanglement entropy or anything like that, but classically, entropy is a macroscopic coarse-grained idea. On Boltzmann's tombstone, it says S equals K-log, W,

what W means is the number of microstates within a certain macro-state. And a macro-state is defined as the set of all microstates that are observationalally, macroscopically the same-looking to you. So in a box of gas, if you have macroscopic access to the temperature and density and velocity of the gas at every point, how many different arrangements do you have of the microscopic atoms that have that set of macroscopic variables? That's the macro state. And from that understanding, if you start in a small entropy macro state where there's not that many states that look like that, it's very natural for entropy to increase and you go to larger entropy states.

So once you have defined your macro states, the fact that you have entropy for any particular state of the universe is an objective fact. It's conditionalized on the particular choice of macro states that you have, but it's not something that has anything to do with your knowledge of the system. There's a whole other way of thinking about entropy, which is more gibson or Shannonian in spirit, where you say you have some distribution over possibilities and the width of the distribution is, is roughly telling you the entropy. This is not that. The Boltzmanian way of doing it is how many other ways are there counterfactually

that I could rearrange my microstates to look the same macroscopically. So it does depend absolutely on how you coarse grain into those macro states. And there are various people, including Judea Pearl, who have sort of objected to that,

saying, you know, who says that I should coarse grain in that particular way? But I think that those objections are misplaced. I think that people are imagining incorrectly that it's kind of innocent and okay to imagine that we have perfect information about the system. The reason why that's not okay, not only that we don't ever have perfect information about the system, but if we did, the world would be an entirely different seeming place.

There wouldn't be an arrow of time if you had perfect information about the system. You would just say what the system does. again, we're thinking classically here. You would be Laplace's demon. There's no entropy for Laplace's demon. So this introduction of subjectivity in the definition of your macro states, or for that matter, if you want to be more gibson information theoretic about it, in your distribution over possible microstates, these are both subjective quantities. That's absolutely necessary. That's not, you know, optional when you have, when you're talking about entropy and the arrow of time. The good news is that even though it's subjective, it's not arbitrary, we basically agree on what the

right way to coarse-grained systems is, or at least we agree enough that the differences don't make any difference to how you define entropy or the fact that it's going up. So I might coarse-grained by taking a box of gas and dividing up into one cubic-millimeter cubes and then averaging over the number of atoms and so forth, and you might divide it up into one cubic centimeter cubes and do the same thing. So we have a different course graining, but they're completely compatible with each other, right? Why do we all agree on how to do it? Because again, we define our macro states by what we can observe about the system. Things like temperature and velocity and pressure and density or things that we can observe with a measuring apparatus. Even if we don't actually do it, we could do it, whereas the

position and velocity of every individual atom is not something. that we can observe even in principle. So I think that, you know, this is something that people who want to help themselves to perfect microscopic information about the world have to get over. You can't do that. You don't have that information, and it's crucially important that you don't have that information. That is really where the arrow of time comes from. So I think that answers, at least as much as I can do for the first two questions. For the third question, I shouldn't have lumped this question in. Sorry about that, Jim. But the question is, has anyone done research into determining the entropy of different states or various cellular

automata rule sets? So I think this is a different thing, because now we're talking about cellular automata rather than statistical mechanics. And there is a huge, huge difference between the two because the whole thing about statistical mechanics and classical physics is the microscopic rules of engagement are perfectly reversible. That is what allows for the existence of Lepas's demon, et cetera, who from the current information about the world can perceive both the future and the past. The typical cellular automata that people talk about aren't irreversible, even at the microscopic level.

So you don't need to start in a special state necessarily to see complexity emerge. It's an open question. It depends on the kind of cellular automaton you're working about. you're working on. So in the microscopically reversible case, because information is conserved, at the micro level, no new complexity can ever arise that wasn't implicit in the state all along. Whereas in the cellular automata, irreversible case, you can start with just not only low entropy, but almost no information in the initial state, and it can evolve into a very high information density state. That can't happen in classical mechanics, but it can happen in cellular automata.

So it's just a very different question. I'm not giving you the answer to the question, because I'm not quite sure what it is, but it's a very different kind of setup with, therefore, different approaches are going to be necessary. John Stout says, I recently bought David Albert's book Time and Chance. I also bought The Probability Map of the Universe essays on David Albert's Time and Chance. The latter makes reference to your book from eternity to here. Yes, and in fact, I think I have an essay in that book, so I hope that there's more than just my book appearing there. I think my name appears in there. Anyway, John goes on. The latter makes reference to your, sorry, can you provide some context around this issue of the arrow of time and what the disagreements are,

or alternative theories, what the alternative theories are in terms of the arrow of time, if not driven by entropy? You know, in principle, one could imagine that, that our current best theories of fundamental physics are very wrong. I say that because our current best theories of fundamental physics are fundamentally reversible at the microscopic level, as we just talked about. Macroscopically, we see a world where there's lots of irreversible things happening, ice cubes melting into water, breaking eggs, what have you, mixing cream into coffee. And we have a very good reconciliation of those two facts based on the fact that we're coarse-graining and we have entropy and the entropy started low, etc.

There's a set of people, and Tim Maldon might actually count himself among them, who think that actually the fundamental laws are just not reversible, that there just is an arrow of time built in. But I'll tell you, that's a very minority point of view these days. I think the majority of physicists fall in the two camps. One, what you might call the conventional picture that I just sketched out, where you have reversible fundamental laws, but some boundary condition, the past hypothesis that says entropy grows and the arrow of time stems

from that. And two, people who haven't thought about it that much. There's going to be plenty of those people. Bless their hearts, but they just haven't really worried about this question. I think that the overwhelming number of people who have really thought deeply about the arrow of time think it comes down in one way or another to entropy. And the reason why the alternative is not that attractive is just because it's really putting a huge amount of effort into pushing against a completely open door, because the fact is there is entropy, right? There is microstates. We can coarse grain. We can calculate the entropy. That entropy was low, whether or not you have fundamentally reversible or irreversible laws, and that entropy will go

up. So whatever you think are the fundamental dynamics of the world, we have an arrow of time from increasing entropy. And there's a project that is ongoing, and I can you know, play my little part in it and so do other people, but connecting the growth of entropy to other manifestations of the arrow of time, causality, memory, aging, whatever. But, I mean, we already have an arrow. Let's just work on understanding that arrow versus inventing an entirely new arrow and then trying to give that credit for everything. That's what I think the others are trying to do, so it's not that attractive a proposition to me. Jameson says, gravity is sometimes described as the bending of space time and other times described as a force from gravitons.

I've even heard that in string theory, if we are living in a three brain that gravity is the only force that could escape into extra dimensions. If it's just carried by gravitons, why is gravity still so different than the other three forces? Well, there's particles and there are particles. Particles are different, is one very simple answer. but there's this sort of more subtle answer. So let me do the first answer first. Gravity is special because it's universal. And what that means is that the particles, the gravitons, coupled to everything. They couple to all of energy and momentum in the universe.

Photons and gluons and W and Z bosons interact with some kinds of particles. They don't interact directly with other kinds of particles. Gravitons interact with everything. fundamentally a consequence of the principle of equivalence, as Einstein dawned on Einstein back in the 1910s or whatever. So the particles interact differently is the short answer to the question. The other answer is that, you know, when you say that gravity is due to graviton particles,

that's supposed to be, there's all sorts of hidden footnotes there and caveats that maybe you're not aware of. It's really a field, right? really the gravitational metric tensor field, just like photons come from the electromagnetic field and so forth. Gluons come from the chromodynamic field. And there is a regime in which the excitations of that field look like particles. So in particular, when you're very close to the vacuum state, empty space, nothing else going on, and you just perturb the field a little bit,

those perturbations look like particles. But in other regimes, they don't look very particle-like at all, right? Inside a proton, the gluon fields do not look like individual gluons because they're not small perturbations of the vacuum. And maybe in a black hole near the singularity, there's really no regime in which the gravitational field looks like a collection of gravitons. So the particle language is a little bit less general than the field language. And the particular way that the field work in general relativity, of course, is the very shape of space time itself. That's the explanation in Einstein's way of thinking about it for why gravity is universal and couples to everything,

as opposed to other forces which are fields living within space time rather than fields defining a feature of space time itself. So the gluon fields, the electromagnetic fields, and so forth are allowed to interact with some particles and not others. Gravity has to interact with everything. Frederick Apollo says, why do particles decay into other particles? Is it really just the second law? Well, it's not just the second law, but the second law is absolutely involved. In particle physics, any interaction, like let's say you have a muon, and muons can decay

into electrons and a neutrino-anti-neutrino pair. That is an allowed interaction in particle physics. the reverse interaction is always allowed. You can always imagine taking an electron, a neutrino, and an anti-nutrino, pointing them exactly at each other in exactly the right way to make a muon, at least temporarily. But, as you might guess, it's harder to do that. And in fact, it is just a little toy version of unmixing the cream from the coffee or unbreaking the egg.

There's a smaller number of ways to be a single muon than there are ways to be. an electron, a neutrino, and an antineutrino. So the second law is not the explanation for why particles decay into other particles, period, but it's the explanation for why that's a much more common event than several particles coming together to make a small number of particles. David Maxwell says, my favorite modern sci-fi is called Bobaverse. It tells of a near-future IT engineer who pays to have his brain frozen on death, but wakes up much later as the sentience of a von Neumann probe

able to harvest resources with drones, 3D print anything, and replicate himself. He cannot travel or communicate faster than light, but he does have fusion engines. Being artificial, he's also able to live in any speedy chooses and meet with other bobs in real time, etc., etc. His choices are around time and resources, replicate, civilize, or explore.

Would you choose this option for life after death, and how do you think you might prioritize your choices very broadly? Would you be more an explorer, a world builder, or a multitude? So I'm once again not going to get a very satisfying answer to this question because I kind of question one of the implicit assumptions of even asking the question where it's not implicit, it's very explicit, you say, would you choose this option for life after death? So I think that I would no longer exist.

There's no sense in which I can continue my life as a von Neumann probe. And there's a philosophical point here that we talked about with Katie Elliott a little bit. I think that, and we talked about with David Albert also when I talk with him about Captain Kirk and things like that, strictly speaking, you know, why do you think you're the same person that you were five? minutes or five years ago. Well, you're not exactly the same person, right? You're a little bit different. You've aged a little bit. You have a memory that you didn't have back then. Even your constituents' atoms are a little bit different now than they were back then. But there are reasons to psychologically as well as physically identify some continuity over time between yourself now and yourself some time ago. And we can sit down and debate how accurate or how fundamental

that identification is, but it is convenient. It serves some purpose to us because there is continuity there. This kind of thought experiment, which I'm not arguing against this kind of thought experiment, it's a lot of fun, by all means. Let's contemplate it. But it is doing something that is enormously far outside our experience, and it's asking us to make decisions based on our experience, which is very hard to do. It might even be fundamentally no rational way to do it, which is something we talked about with Lori Paul back on the podcast some time ago about transformative experiences. In other words, unlike me now and me five minutes ago or me five years ago, there's relatively little continuity between me now and some uploaded version of me

in some interstellar probe that is completely artificial, right? I mean, maybe what you mean is that there are some patterns in my neurons that you try to replicate inside some silicon substrate or something like that and do your best. But it's not going to be me anymore. It'll have some relationship to me, but the direct relationship that I have with my five-year-old self, five years ago's self, is just not there.

It's something much more tenuous than that. In particular, I would imagine that all of my motivations and drives and so forth would be completely different in that context than they are now. So I don't think that the question, would I choose that option and how would I behave if that option were chosen, makes any sense. You can ask a more specific question if someone takes this and that neuronal pathway pattern from your brain and reproduces it in a machine and launches it into space. What do you predict that thing would do? I have no idea what the answer is, but I don't think it's me in any real sense. says, of the many ways I could buy or borrow your books, which would you recommend? I'd like to vote with my wallet, and I have my own thoughts about the relative merits of borrowing from the library, buying a book used, buying an e-book, buying a book from the publisher, etc. But I'd like to hear yours. I appreciate the question. Thank you, John. But, you know, honestly, it doesn't matter

that much. In terms of me making money, I think that you buying it from a bookstore or from Amazon is probably the best. Maybe buying from the publisher. I honestly don't know. I haven't really kept track. I don't think it matters from those three options. There's an absolutely sentimental part of me, which would like to see people buy from bookstores, from good old independent bookstores. Or you can go to bookshop.org, and that's an internet way of buying from independent bookstores. That's usually how I try to buy books if I can. But if you want to buy the e-book or if you want to get it from the library. I don't make any money if you get it from the library, but I have no objections to that whatsoever. I'm a huge fan of libraries, a huge fan of people

taking my books out in libraries. That's great. Whatever is most convenient for you. You know, whatever makes you most likely to get the book and read it, do that. That would be my advice. Masterwork Tools says, I was explaining the new gravity wave results to my father, and he has a question for me that I couldn't answer. What other types of wave are there and can we expect to be able to eventually detect them? As I understand it, we have mechanical waves, electromagnetic waves, and gravity waves. Am I correct on these? And are there any others we know of that we can expect will be useful for investigating the universe? Well, I think there's not quite a parallel between some of the examples you're giving, mechanical waves, electromagnetic waves, and gravity waves. Those are all

waves, but mechanical waves is a huge category. I mean, there are sound waves, right? That's a kind of mechanical wave, but there's also vibrations. There's waves on water. These are all kinds of waves that are propagating in some particular medium, and there's potentially huge number of different kinds of them. I think maybe what you're thinking about are more fundamental particle physics-y notions of waves. So there are electromagnetic waves and gravity waves right there in the basic laws of physics, unlike sound waves, which are higher-level emergent kinds of things. So if what you're asking is, are there other kinds of fundamental waves that we might measure astrophysically, the short answer is no. It does depend on whether or not you count matter particles

as well as force-carrying particles, right? Like in some sense, because of quantum mechanics, when you detect cosmic rays or neutrinos or high-energy electrons or whatever from space, those are kind of wavy in some sense because of quantum mechanics, but they're what we call fermions in physics. So they're actually particle-like waves. You cannot pile them on top of each other to make bigger and bigger waves like you can with electromagnetic waves and gravitational waves. There are also other forces in nature.

There's the weak nuclear force and the strong nuclear force, but they're both short range. So electromagnetism and gravity are the only two fundamental force-carrying waves that are long-range and can therefore propagate over astrophysical distances. At least those are the only ones we know of. We have zero reason to expect there are any others besides that, but they could be there, right? That's the job of theoretical businesses to invent models,

in which that's the case. I've done it myself, so I think it's a respectable thing to do. But short answer to your question, in terms of wave-like things that we might find by doing astrophysics, it's one version or another of electromagnetism and gravity. Chris Murray says, it is said that barring proton decay, quantum tunneling will eventually turn black dwarfs into iron stars, then into black holes. It's been pointed out that such a black hole would come from a small fraction of the iron star, which then swallows the rest, since quantum tunneling is overwhelmingly more likely to form a smaller black hole before a larger,

one. But doesn't this mean there would be an abundance of the very smallest black holes first, some being on the surface, whose instant evaporation would gradually eat away the whole star before any long-lasting black hole could be formed? I'm not sure who is saying these things. You keep saying it is said that. I'm not sure who is saying that and pointing it out. This is a true-ish kind of scenario, but weirdly specific. It's weirdly specific because it's a thing that can happen, but other things can happen also. It's much more likely that stars or planets fall into big black holes than that they quantum tunnel into being black holes in their own right. But it could happen. But when it happens, if you, so you're correct, I think, that when there's a quantum

tunneling event that makes a new black hole, that black hole is going to be very small. But it will eat, right? It will accrete other things, or it will just evaporate away right away. It's very possible it evaporates away right away, in which case it's just not there anymore. But if it's big enough that it can survive by growing by eating, then it will generally eat up whatever star or planet it's in long, long before that star or planet has a chance to quantum tunnel into another black hole. So there's no regime that I know about where you will accumulate many small quantum tunnel black holes before the whole thing just collapses to one big black hole. Gregory Kusnik says, if you were education emperor, how would you reform college admissions policies?

Yeah, I'm not going to be education emperor anytime soon. I'm not quite sure who counts as that. It's not something that I put a tremendous amount of thought into, very honestly. I do have, like, vague feelings about it. I think that we stress it too much college admissions. You know, I think that the undergraduate college experience is just not. not nearly as affected. Well, sorry, I should say this. The educational side of the undergraduate college experience is not nearly as affected by where you go as parents and high school students tend to think it is. I was a grad student at Harvard. I know plenty of undergraduates who did not

get a great education while they were there at Harvard because they chose to focus on doing other things. I was an undergraduate at Villanova, and I know that there were students there who got a wonderful education. As long as you're at a relatively good school and you put in the effort as the student, you can absolutely get a wonderful education almost anywhere. There are slight differences that can be very important. For example, I definitely suffered as a Villanova undergraduate from not having a graduate program in physics that I could sort of sneak into most undergraduates who want to grow up to go to grad school. If they're at a place that already has a graduate program, then they will take some classes ahead of time.

It helps preparation and things like that. But that's a very specific thing. If you want to eventually go to grad school, it helps you go to an undergraduate school that has a grad school there. It's not necessary. If you're at a smaller school, you can get more attention. So, you know, there's pluses and minuses. But the idea that, you know, you have to go to the top school

to get a good education, I think, is entirely wrong. In fact, it's very often the case that the top schools are the ones where, well, again, this is not. necessarily a correlation, but there's plenty of professors at top schools who are not really interested in undergraduate education. What makes the top schools, the top schools, is their research, right? Their scholarship, their intellectual output. It makes a big difference for graduate school where you go because there you're working with an advisor and that advisor really has a huge effect on your life. For undergraduate education, it's honestly probably the biggest

effect is who your peers are, who your other students. students are going there, and the benefit of going to a place like Harvard or Princeton or Stanford is that you're hanging out with people who are going to go on to rule the world, and that can be beneficial to you. So I don't know how to reform it, but I would lower the temperature on discussions about it. You know, honestly, I could see an argument for just making it random. You know, having some lower cutoff, like if you're, okay, you're good enough to get into the basic schools, and then we'll just randomize who gets in what I think that would be fine, you know, in many ways.

Not necessarily advocating that, but it might not be worse than the current system. Let's put it that way. There is, of course, a lot of discussion about this issue right now because of affirmative action and the Supreme Court coming down against it, which is just, I think, the reaction against affirmative action is just weird and bad, I think, and it's also intellectually misplaced. You know, affirmative action is not a great system. For those of you who are not in the U.S., affirmative action is a system according to which black people and other underrepresented minorities get preferential treatment when being accepted into colleges or other things. You know,

you could define the term affirmative action differently. It's not a great system because, you know, you would like, in principle, to very carefully judge each individual case, and if someone really was discriminated against personally, then, okay, you can make up for that in your admissions decisions because they're fighting against this extra optical, et cetera, et cetera. But all that takes effort and time and resources that places don't have. The thing about affirmative action is, it's cheap and easy. Just say, okay, we're going to let in more members from this particular applicant pool to increase the diversity of the school. That's it, right? It's just not a big deal. It's cheap and easy, and yet super effective. Studies have shown that affirmative action has absolutely

changed the demographics of the United States by contributing greatly to the growth of a black middle class because people go to these schools they wouldn't otherwise have gotten into. They get to know friends and whatever. They're exposed to a whole different world. And that helps them later in life. So I think that, you know, there's various things to be said. I don't think that schools should discriminate against Asian Americans like they sometimes do, because otherwise Asian Americans disproportionately do very well on tests and would get into schools. On the other hand, I don't think that, you know, test scores should be the overwhelming thing either. I think that it's perfectly okay to take plenty of different considerations under your advisement when you're

letting people into schools. So overall, all, I don't care that much, but I'm absolutely in favor of doing things to make a diverse and interesting student body. Peter Kane says, are there any fun and interesting ideas about dark matter that could be used for science fiction novels, TV shows, or films? You know, it's very hard because one of the things we know about dark matter is it doesn't interact with itself very strongly. If it did, then it would be more like ordinary matter, and the distribution of dark matter would be very similar to that of ordinary matter, but that's not true. Dark matter, halos in a galaxy or cluster tend to be big and puffy and diffuse, whereas the

ordinary matter, because it interacts electromagnetically, can give off photons, lose energy, dissipate, and fall to the center of halos, as we discussed in the solo episode recently. So the zero's approximation, your first guess is that dark matter just doesn't interact with anything else, at least not very much, and therefore not very interesting for science fiction novels, etc. Maybe you can try to be more creative. You know, I did write a paper with Lottie Ackerman, Matt Buckley, Mark Kamienkowski about dark radiation. We imagine that there could be a version of electromagnetism that only interacted with dark matter. And once that's true, and we show that it is allowed, at least it seems to be allowed. So you can then imagine dark atoms,

dark chemistry, dark molecules, etc. But it has to be different than ordinary matter because of just what I said, because the dynamics of dark matter are different. So I'm not sure if there's a regime where you can do interesting things like that. But even that would still be, you know, the dynamics of things going on in the dark matter by itself, not interacting with ordinary matter. If dark matter interacted noticeably with ordinary matter, we would have detected it a long time ago. So in terms of like being useful to we people made of ordinary matter, I don't really see how dark matter can be very helpful. Simon Carter says,

I was beginning to be convinced by Tim Malden's argument for pilot waves, but in the reflections, you mentioned that making it compatible with quantum field theory is a hurdle too far. Could you expand on that, please? I can't expand very much for the reasons that I've said earlier that I have not actually dug into the details about these alternatives to Everettian quantum mechanics. But there's a difference in philosophy here. In BOMian mechanics, pilot wave theories,

they were first developed in the context of non-relativistic quantum mechanics of point particles. And the motto was, you know, why do you see wave-like behavior sometimes and particle-like behavior sometimes in quantum mechanics? In pilot wave theories, the answer is because there are both waves and particles, right? And that seems good. But then you realize, well, actually, in modern physics, the world isn't made of particles. The particles are just quantized excitations of fields. So now, what do you do?

And the BOMian people don't agree yet on what to do. There are different strategies going on. The point is that one way or another, you need quantum wave functions and something else and some hidden variables. So constructing a quantum theory in BOMI mechanics is more difficult. than doing so in Everettian quantum mechanics because it's more complicated. There are more ingredients and more equations. So there are choices you need to make, things you need to think about, and quantum field theory doesn't naturally fit in.

Maybe you can squeeze it in some way. Whereas with Everett, you just tell me what the Hamiltonian is, which runs the dynamics, and you're done. That's all you've got to do. Everything else is supposed to follow from that underlying equation. And the same thing goes true, but even more, You get to something like quantum gravity, where you're not going to assume ahead of time there is anything called the metric tensor. It might be emergent and so forth.

So again, forever it, this is all plug and play. It's all very easy. Every time you invent a new kind of quantum theory, you have to reinvent your pilot wave theory. But you don't have to do that in many worlds. One of the reasons why it's so much more attractive. Proitos says, I read Thomas Hurtog's book, thanks to your recent podcast. Thomas admits in the book that Stephen Hawking's favorite technique of rotating time into an imaginary direction to create Euclidean quantum gravity has long been dismissed as a Cambridge eccentricity. There's a difference between mathematical calculational tools like WIC rotation and Stephen's belief that imaginary time is in some sense physically real.

How do other professionals view Euclidean quantum gravity as nothing more than a dubious parlor trick? Well, it's an interesting question. So for those of you who don't know the background here, when you learn, Relativity, when you learn special relativity first and then general relativity, you learn that basically the idea that space-time is a unified thing means that you can calculate distances or intervals in space-time, just like you can in regular space. In regular space, if you have X, Y, Z coordinates, you use Pythagoras' theorem to do that. X-squared plus Y-squared plus Z-squared equals the distance squared, okay? The difference is in space time that you do the same thing with not only space but also time,

so it's T, X, Y, Z. The difference is that there's a minus sign in front of the new version of Pythagoras' theorem for the time direction. This is all explained in grisly detail in the biggest ideas in the universe, volume one, space, time, and motion. So it's minus T squared plus X squared, plus Y squared, plus Z squared. And that's, you know, fine, it fits the data, and you can test it, et cetera. but it's a little bit mathematically inconvenient. So what you can do is, if you want a little bit more mathematical convenience,

make a change of variables from T, the time coordinate, to tau, where the relationship is T equals I times tau. Okay, so I is a square of minus one. We're just defining a variable. No one can stop you from doing that. But in terms of tau, which is minus I times T, if you want to put it that way, minus t squared is plus tau squared, right? So in terms of the tau coordinate, the metric, the distance is plus tau squared, plus x squared, plus y squared, plus z squared.

That's a Euclidean kind of metric, which is why we call it Euclidean space or when we do gravity, Euclidean quantum gravity. Now, this trick turns out to be super useful in ordinary quantum field theory. It's just a trick in quantum field theory. That's the way that we usually think about it. WIC rotation is sometimes the word that is used to describe it because we're thinking of time as being a complex number, T plus I-Tau, and rotating from the T-axis to the I-axis to the tau-axis is rotating from a real number to an imaginary number. So we also call it imaginary time. And it doesn't make that much of a conceptual difference in quantum field theory. It makes a big

conceptual difference in gravity because you're integrating over the space of all possible metrics, and now these metrics are Euclidean metrics rather than space-time metrics. So you guess or posit that the answer is the same, that it doesn't matter whether you integrate over Euclidean metrics or Lorentzian metrics, as we say, space-time metrics. Nobody really knows for sure, but it's certainly easier to deal with the Euclidean calculation, so you cross your fingers and you hope it works. That is kind of half of the particular program that Hawking and others have called Euclidean quantum gravity.

Half of it is this rotation from real time to imaginary time, but the other half is in the context of quantum cosmology dramatically cutting down on the number of degrees of freedom that you're considering by just assuming that the universe is homogeneous and isotropic to start, right? So-called mini-superspatism. models. And maybe then you perturb it a little bit by adding some perturbations on top of that. But you're still treating the metric as the fundamental thing, and it's not very holographic or emergent or anything like that. So I think that, you know, people respect this tool very, very much as a tool

for doing calculations. The hardle-hawking vacuum state in the quantum field theory version of this really just is the vacuum state, right? You just calculated a vacuum state, which is an interesting, useful thing to do. So I think that most people think that it has some use. I think that most people think that it's not going to be the definitive final way to construct a full theory of quantum gravity. Paul Hess says, if you could meet a well-versed philosopher from 100 years in our future, what would you ask them about? And do you think their answer would be any more valuable than asking a well-versed contemporary philosopher? I am trying to get at whether you feel that philosophy is an area that actually advances like physics does.

Well, I absolutely think that philosophy advances, whether advances like physics does is a harder question because it's a very different field. Physics is about creating models and fitting them to the data, right? And philosophy is not about that. Partly, you know, sometimes that happens at the boundaries, of course, between physics and philosophy, but philosophy is about the conceptual, logical, logical analysis

of the underlying arguments, not about creating models and fitting them to the data. So it's a different kind of advancement. Obviously, in my mind, philosophy advances, you know, if you've ever heard me say, thinking about morality, for example, well, you can consider consequentialist theories or deontological theories or virtue ethics theories, right? So there's a kind of a roadmap for different possibilities. As soon as you say those words, you are borrowing advances that were made by previous philosophers. Even if the advance is not of the form, here's the once and for all final answer, understanding the space of possibilities is absolutely an advance.

Some areas of philosophy are more rigorous and logical, and you prove theorems in mathematical logic, for example, right? There's no question that, you know, once you've proven some theorems, you've made a little bit of an advance. What I would want to ask a future philosopher, there's lots of questions I would like to ask. I would like to ask what they're thinking these days

about morality and ethics, meta-ethics, and the grounding for ethics, the role of consciousness and AI and emergence and things like that, there's a whole bunch of things I would like to ask, not even to mention what I do for a living, the philosophy of cosmology and quantum mechanics and the arrow of time. There's a whole bunch of things I'd be very interested to see what philosophers have learned over the course of a century. Ned Grady says, in your most recent solo podcast, I heard, Gravity is non-local, along with some quick examples of the non-locality, e.g. black hole information. That's the first I've heard of it after listening to plenty of your podcast. Can you explain in any more detail the examples and their consequences?

Well, this is not something that is very well understood. Let's be very clear. And it's a purely quantum gravity statement that gravity is non-local. Classically, gravity is perfectly local. It's a local field theory. And what that means is you have an equation of motion, Einstein's equation, that holds separately at every point in space time. Okay? That's what it means. What happens at one point in space. time is being affected by its derivatives or its very, very nearest neighbors, not affected directly or immediately by things that are happening far away. In quantum mechanics, there's my personal point of view here, and there are things that are well recognized by others. In my personal point of view, locality is something that we need to explain why it's a good approximation at all, because quantum mechanics doesn't look local, it doesn't have space built into it, much less locality. You have to get that as an emergent thing. So to me, it's completely unsurprising that there might be some non-locality. The question is what exactly is the kind of non-locality that arises.

The evidence that we have that this is an important thing comes, like you say, from black hole information, but also just from the holographic principle, right? If you think about ADS-CFT, the claim is that there are two theories that are equivalent to each other. One is defined on the boundary. It's a theory without gravity, and one is defined in the bulk. It's a theory with gravity in one higher dimension. So the relationship between those theories is going to have to be non-local. They're not even the same number of spatial dimensions, right? So holography, if you believe that it's a good approximation or valid in any case at all, is going to be a very vivid example of non-locality and quantum gravity. The black hole information puzzle is also seemingly such an example because the

whole reason it's a puzzle is because once you send the information into the black hole, it's behind a horizon, and it's hard to see how it gets out. How does it get out? Well, through something non-local, but very, very subtle, okay? We don't know the details about that. I don't know the details. There are people who are working on it and have made a lot of progress. I don't think the situation is quite settled yet. But there's evidence that, you know, at the quantum mechanical level, there's little tiny influences that share entanglement over non-local distances, but it's not something that is perfectly well understood right now. Brian says,

I find your work in discussions of position and momentum as emergent properties from the state vector in Hilbert space fascinating. Do you have similar lines of thinking about the emergence of fields and the internal spaces they inhabit? Well, not really. I mean, I have some aspirations, some vague ideas, some scribbles, maybe some notes in a notebook somewhere. I have not put a lot of effort into thinking about that.

in part because it's supposed to be easier. You know, getting fields or whatever to emerge from quantum mechanics already shouldn't be that hard. But then it becomes hard when you want to get the right fields, right? The fields that we actually have in the standard model of particle physics, etc. And also when you want that to play nicely with the idea of space and maybe space time, themselves emerging. So there are people who thought about that, but I haven't put a lot of effort into it myself quite yet. Christopher Humble says, my question concerns Cormac McCarthy. Did your paths ever cross at the Santa Fe Institute?

And if so, what were those conversations like? Also, more generally, can you talk about the interdisciplinary collaboration at the Institute and the kinds of conversations that go on? You know, I almost didn't answer this question because the answer is no. I never ran across Cormac McCarthy. The time that I started spending more time at the Santa Fe Institute was about the time he started spending less time, just because he was not feeling as well.

But when I was just there, most recently, there was a little memorial celebration for Kormick McCarthy. He was, and so I was there for that, and it did make an impression. He was a very big presence at the Institute. You know, he had a lot to do with the physical design of the place. You know, he had opinions. He helped redo the entire library. He donated wood for the stairs, you know, from his farm and things like that. I was what was claimed, and I have no reason to doubt it, is that he never really liked Santa Fe.

You know, he was not a New Mexican. He was a Texan originally. And the reason why he moved to Santa Fe was to be close to the Santa Fe Institute and to interact with people there. If people don't know, his two most recent novels, his last two novels, the passenger in St. Lamareris are very much based on conversations that Cormick McCarthy had with physics. especially at the Santa Fe Institute. He helped SFI rewrite their publicity materials, things like that. He would help people edit their books. You know, you get a feeling that he had a certain grumpiness about him in some cases, but also a really touching warmth about him in other cases. So I'm sorry that I didn't get to interact with him much more. In terms of the

interdisciplinary collaborations and conversations that go on, you know, a lot of the effort or a lot of the secret to success at a place like SFI is just making sure the people you invite are into the spirit of the place right away. So, you know, the thing about SFI is you go to lunch and everyone goes to lunch, it's served, and everyone sits in the same place, and you can sit down next to whoever you want, and there might be an architect and a historian and a linguist and a computer scientist, but the thing is they're not randomly chosen architects and historians, etc. are ones who want to come to the Santa Fe Institute. They are the ones who are interested in talking

to physicists and to biologists and to whoever, right? And so that's what makes it great. And there's plenty of people out there who are interested in this kind of interdisciplinary collaboration as a matter of finding them and getting them into the right place. You know, not every physicist wants to talk to a historian or an economist or et cetera. But SFI attracts those people. Nalita S. says, I really enjoyed listening to your episode with Katie Elliott. Would you kindly expand on the connection you postulated in Newcomb's paradox between eternalism and the many world's interpretation of quantum mechanics? I'm going to have to apologize for not remembering exactly what I said. I was thinking about, in the case of Newcomb's paradox,

You know, if you believe in quantum mechanics, then the existence of this kind of spooky being who knows exactly what the future is going to hold is much harder to understand in quantum mechanics where there's randomness in the world, right? So if you think that there's some quantum fluctuationness that will prevent you from getting the right reward, then it's harder to even set up Newcomb's paradox. I don't think that many worlds either implies eternalism or is against it or anything like that. I do think that there's an overall philosophy of physics that says, you know, the physical laws describe what happens in reality as convenient summaries of the actual motion dynamics of the physical world. And the physical world comes first. and the world includes various moments of time and described by different states of the wave function.

So it all kind of fits together into a neat package that includes many worlds and eternalism, but I wouldn't want to say that either one of them nudges you in the direction of the other. You can be classical eternalist perfectly well or a bo-mean eternalist or whatever. All right, I'm going to group two questions together. Doorbell Jeff says,

I'm a PhD student in observational cosmology, and I don't get the cosmological constant problem. To my limited understanding of quantum field theory, at a certain energy scale, we measure the renormalized value of a parameter of the theory, and thanks to renormalization, we can then predict how it scales with energy. That is correct.

Cosmologists measure the renormalized value of the cosmological constant to be small, and this is often claimed to be a mystery, but we can obtain that value through a renormalization procedure. To me, this seems to solve the problem, like it solves the problem for any other parameter we measure in physics, What am I missing? Whereas Bill Quirk says, I enjoyed episode 245, Crisis in Physics.

You mentioned the difficulty of understanding the small but non-zero number for the cosmological constant. What are some of the ways that people have tried to explain why the number is so small but not zero? I believe the calculation depends on the plank scale. What would the plank scale have to be for the number to be as small as the observed cosmological constant? So it is completely true that the cosmological constant at the end of the day is something you go. out and measure. And you can always fit that into your theory as a renormalized parameter that you measure, just like you can fit the mass of the Higgs boson or the fine structure constant or whatever. That's not the mystery. There's no mathematical obstacle to just dealing with whatever cosmontal

constant you measure. The mystery is that the value of it seems unnatural to us. This whole procedure of renormalization is not just a way to fit the final answer into your calculations, but it also offers a natural scale for these answers to be at, especially when you have numbers that are dimensionful. They are compared to other numbers in some way, and there's a set of contributions that you add up. I think that the real impact of the cosmological constant problem comes from imagining that you treat it as the sum of many different contributions, right?

So there's a vacuum energy in a different quantum field, but there's a separate contribution to the vacuum energy from the electromagnetic field and from the neutrino fields and from the quark fields and all these different fields, right? Some of these contributions are positive, some of them are negative. There could even just be a classical bear contribution, right? And the mystery is, why in the world do all these positive and negative numbers add up to give you such a tiny number? That's the mystery.

And, you know, it is very important to emphasize that it is clear. clearly a misunderstanding, right? So you can't be too adamant about what I just said, adding up all these contributions and getting the tiny number, as the right thing to do. It seems to be the right thing to do from our current understanding of effective quantum field theory, but it doesn't give you the right answer. So clearly something's wrong. That's easy. The hard part is what is wrong? So to Bill's question, I don't have any good theories to explain why the cosmological constant is small. I proposed a couple, but they're not great. I wouldn't say that they're on the right track. I will mention two ideas that are out there that may or may not develop

into the right answer. The best idea we have right now is the anthropic principle, right? There are small numbers in physics, and the cosmotial constant has the special property that it is small, but if it were bigger, we wouldn't be here to talk about it. Okay, it would not allow for the existence of galaxies and stars, et cetera. So therefore, if you have a multiverse with different values of the cosmological constant, you can account for its smallness by saying that we only see those regions in which the value is small. This was argued by Linday and Weinberg and others all the way back in the 80s, maybe even before that. Who knows? So that's absolutely a possibility. It would be a sad possibility in the sense that it would remove the ability to create a clever theory that actually predicted the value of the cosmological constant. The other perspective, which Tom Banks and others have pushed for a while, and I'm actually quite fond of, is in the case of the cosmological constant, the entire paradigm of effective field theory is just not the right way to think about it. He has a paper, Tom has a paper, the cosmontical constant, maybe with Willie Fisler, the cosmological constant as a boundary

condition. And the idea is the following, that if you live in decider space, which is the empty space solution to general relativity with a positive cosmontial constant, then you have a horizon around us in all directions. It's sort of like we're in the center of the horizon, rather than outside the horizon, as we would be in the case of a black hole. And there's an entropy associated with that horizon, and it's the area of the horizon in blank units, just like the entropy of a black hole, calculated by Hawking way back when. And you can make the case that it's a high entropy state, so the dimensionality of Hilbert space is E to that entropy. So E to the 10 to the 122, roughly speaking, okay? So it's a very big number, the dimensionality of

Hilbert space. But the point is, what Tom would say, and what I would completely agree with, is if that's the right way to think about it, that's not something you get by renormalizing of parameter in your effective field theory, that's the dimensionality of Hilbert space. That's just a fixed number. That's just given to you by God, okay? It's not the sum of various contributions. And then, so this doesn't solve the cosmological constant problem. It just says you're thinking about it in the wrong way. The cosmological constant is not supposed to be thought of under the rubric of effective field theory, okay? It doesn't tell you why it's so small, but it tells you that it's not the kind of thing you should think of as being the sum of many different contributions, some positive and some negative. Thomas Pronti says, I've heard you describe gauge theories a couple times, like in the crisis in physics episode, and while I think I understand the words I'm still missing something, if all field configurations related by a gauge transformation are physically the same, then how can that transformation also generate a force which does not have real effects? It seems like the symmetry should keep things the same rather than generate something new.

Yeah, so I think that there's an ingredient missing here, which is that the gauge transfer itself does not generate a force. What happens is you would like your theory to be invariant under gauge transformations, and that means you have to be able to compare what is going on at different locations in space. How have you rotated your axes in some charge space or color space or whatever at one point compared to another one? What that means, as you crank through the math and figure that out, means that you have to introduce another field.

A field in addition to the electron or the quark and whatever you had, there has to be another field that helps you compare what is going on at one point in space to what's going on at another point in space. That field is called the connection field or the gauge potential. And how that field changes from place to place, if the field is constant, if the connection is just constant everywhere, then there is no force. then there's no thing that is physically pulling particles together or pushing them apart. But that field contains energy, the connection field, if it has twists and turns in it,

those cost energy to make. And we call those twists and turns the electric field and the magnetic field. And so the force carrying fields are related to the changes through space time of the connection field that helps you implement those gauge transformations. So it's not that the gauge symmetry itself gives rise to a force, but mathematically making it all work out introduces another field which gives rise to the force. Sandro Stuckey says, I love the episode with Katie Elliott. During your discussion of the principle of sufficient reason, I picked up on this statement of yours which resonated with me.

How confident are we that when we're reasoning about things that are very different from our universe, that we can say things that are not overly tainted by our real world experience? I feel like that's a big issue that goes right down to the foundations of philosophy and logic itself, but maybe that's not what you meant. Can you elaborate a bit on that thought and share some more of your ideas on the topic? Well, I don't think I can give a once-and-for-all dependent answer to it, but I do agree with you, I think, that this is a central concern when you're doing philosophy or, for that matter, when you're doing science. We have certain intuitions, feelings about how the world works. We are not blank slates, we human beings. beings, right? So when we do the practice of science, which is to propose theoretical models that might explain the world and then experimentally test them against what the world is actually doing,

we naturally tend to gravitate to things that make sense to us, that make intuitive sense to us. Sometimes the data or some other feature of the world is just so unmistakable, so impossible to miss, that it pushes us away from our intuitions, like in relativity and quantum mechanics and so forth, even in natural selection in some ways. But other times it doesn't, or other times its influence is a little bit weaker, so we have to work harder.

And there's no magic pill. There's no algorithm that says, here is how to avoid being tainted by your everyday experience of the single universe that you find yourself in. You have to work at it. So it's just a matter of, you know, practice and concentration to be able to take a step back and say, like, okay, am I trying to force the

universe into my preferences, or am I taking the universe on its own terms? Dirk Schmithofer says, what is your sleep schedule roughly hours per day? Do you take naps, or are you one of those people who only need three hours a night? I am not one of those people who only three need three hours a night. I would love to get eight hours a night. probably it's more like six usually. Big fan of naps when I can get them. I can't always get them,

especially after a big meal or something like that. But, you know, the sleep schedule changes around depending on what I'm doing, how I'm traveling, things like that. I'm just not a very organized person when it comes right down to it. I don't know whether that's useful information or not. Dan Inch says, do you have any plans to incorporate the cats into your reflections videos?

The spectacle of cats crawling over a famous physicist would be fun, even if they are not in a superposition. You know, I would love it if the cats wanted to participate, Ariel and Caliban, but I think that they don't. You know, I've noticed they don't like it when I'm sitting alone in my office talking into the microphone. They think that's weird. I know other cats are different. You know, every cat is different.

They have their own personalities. That's part of the charm. So they're very happy to climb over me if I'm just sitting and reading. But if I'm talking into the computer, they are not interested in that. They will go search for other fun. Maybe just because they can tell that I'm constantly. concentrating on something else. They're not as interested in bugging me. They're like most cats. If you have a book, oh, they love that. They love sitting on the book. But if I'm at the computer, they're less interested.

Okay, I'm going to group two questions together. Aaron Munger says, shouldn't Boltzman brains become less likely over enormous amounts of time as particles become more spread out in space and interact less often? And Linneu Miziarah says, how can a Boltzman brain pop up in the very, very distant future? if in this very distant future there will be nothing to interact with the quantum fields and produce particles. Well, this is actually a contentious issue, okay? Let me tell you the conventional wisdom, and then I'll tell you a little spin on it. The conventional wisdom is that as we just said a little bit ago, if you have a positive cosmological constant and you approach a dissitter phase of the universe, then you have a horizon around you and that horizon has an entropy.

And as Hawking showed for black holes a long time ago, having a heart horizon with an entropy also implies a temperature. So this is a feature of quantum field theory and curved space time that in the future, even though everything empties out classically, quantum mechanically, there is still a non-zero temperature. And so naively, you expect that at that temperature, and the temperature does not go to zero, okay? It goes to a certain number that depends on the value of the cosmological constant, the higher the cosmological constant, the higher the temperature. So it does not asymptote to zero. It asymptotes to some finite non-zero number. It's a very, very low number, but, you know, much lower than the microwave background temperature today, but still not zero. So the conventional way of thinking is that if you have a non-zero temperature, you have thermal fluctuations. They're very, very rare, very, very mild, but you have infinitely long to wait, so eventually you will get some dramatic, large fluctuations. I think that that's a mistake. I think that conventional wisdom is wrong about that. At least, if it, let's put it this way, it's not necessary. It depends on details of quantum gravity and things

like that. In fact, it depends on the dimensionality of Hilbert space and whether we can consider our observable universe as a closed system or open system, et cetera. So I wrote a paper with Kim Boddy and Jason Pollack a while ago making this point that the word temperature means something a little bit different in quantum mechanics and classical mechanics. In classical mechanics, the temperature of a box of gas or something like that is the average kinetic energy of all the molecules up to some constant factor. In quantum mechanics, it's a little bit different because in the classical box of gas, even if the macro state is constant, right, even if you're in equilibrium, so all of your gas is perfectly smooth everywhere, the microstate is not constant. At a constant

temperature, at a fixed temperature, the molecules are moving around. So the fact that that the state looks like it's static is just a trick, because you're not seeing the micro-state, you're only seeing the macro-state. Whereas in quantum mechanics, a state that is in thermal equilibrium, what we call a thermal state in quantum mechanics, is actually truly static, microscopically as well as macroscopically. So there are no thermal fluctuations that dynamically bring things into existence, like they would in a moving around box of gas. This is a subtle difference, but an obvious one, once you think about it, between what we mean by a thermal state in quantum mechanics

and a thermal state in classical mechanics. In quantum mechanics, there's no actual single, unique micro-state where the particles are moving. There is a unique quantum density matrix, or, yeah, it's a density matrix if you have a thermal state, but it's static because nothing is happening. So Kim and Jason and I already, that in fact, if that's the kind of state that the universe asymptotes to in the future,

which it arguably is, then you will not get fluctuations into Boltzmann brains. So I would say, and not everyone agrees with us, even though they should. So I would say that the question about whether or not Boltzman brains actually do fluctuate into existence in the real world is an open one. We don't know whether that's actually going to be what happens. Not to mention we don't know whether the cosmontral constant will last forever and things like that. Those are separate questions. Mark says, would your views on religion and atheism be substantively different had you not studied and been trained in astronomy and astrophysics and were instead a non-science professional or tradesman? Well, you know, as we just discussed a little bit ago, I would not be me if I were not

someone who studied what I studied. I'd be a different person. So you're not asking what I would think. You're asking what some completely different person would think, and that's impossible to tell. If what you mean is, do I think that my studies in astronomy and astrophysics had a large impact on my opinions about theology and religion? Not a large impact, no. I mean, I think vaguely maybe thinking like a scientist has had a large impact, but I don't know whether if I were a completely different profession. So, I mean, this is why it's a difficult hypothetical question. I don't know why I would be a different kind of profession, whether I would still have the love of science and the interest in science that I have. There's plenty of people out there, of course, who are not in any way experts are knowledgeable about science who are still atheists, so I'm not quite sure if there's going to be a strong connection there. Oh, or OA, says, it's been stated several times in the podcast and elsewhere that an event horizon is not a thing, and you wouldn't notice passing through it if the title forces are sufficiently small.

But once inside the black hole, all future paths must be closer to the singularity. What does that mean for biological functions like pumping blood or sending signals over nerves? Yeah, there's two things going on here. One is the event horizon, which is a location in space time. And again, you're right, it's not a thing. And I therefore say, when I'm careful, I say, if the black hole is big enough, you would not notice when you cross the event horizon. So what does the size of the black hole have to do with anything?

And the answer is that there are tidal gravitational effects, title as in T-I-D-A-L, okay? In other words, when you're near a black hole, depending on its size, again, and the smaller, the more dramatic the tidal effects are, gravity is pulling you in some directions and pushing you in others, just like the tides here on Earth are pulled toward the moon, but that they are also squeezed in the direction perpendicular to the line of size. pointing toward the moon. That's what tides are. So gravity doesn't just pull overall. It also stretches differentially in different directions. That's a very noticeable effect. And in a small black hole where the differences from place to place are large, the noticeability is bigger. This is what leads to the phenomenon known as spaghettification. If you fall into a black hole eventually,

you're going to be stretched in one direction and squeezed in other directions and turn into a little piece of spaghetti. So you will very definitely notice the gravitational field of the black hole. It's just that there's no signpost precisely at the location of what we call the event horizon. Redmond says, with the unambiguous detection of polarized B-mode swirls in the cosmic microwave background constitute case-closed evidence of cosmic inflation. So for those of you who don't know, when you have inflation, inflation in the early universe, there's still quantum fluctuation. There's still quantum fluctuations in the early universe. So inflation tries to make the universe as smooth as possible, but it can't make it perfectly smooth because of these quantum fluctuations, quantum uncertainties,

if you like. And that's what, if inflation is right, gives rise to the density perturbations we see in the cosmic microwave background. That's a very nice picture in inflation, and it's one of the reasons why people are so positive about the idea of inflation. But it's not, you know, knock-down evidence for inflation, because maybe there's some other mechanization. that we haven't invented yet, that accounts for those fluctuations. So people who think about inflation have pointed out that there is another thing that inflation does. It doesn't just lead to fluctuations in densities. It leads to fluctuations in the gravitational field as well, which show up as gravitational waves. And those have a very

specific unique signature in the cosmic microwave background, a certain kind of polarization signature that is called the B mode, as opposed to the E mode. The E and B are the signals for electric field and magnetic field, but they don't mean that in this case. They just mean different mathematical characterizations of the polarization pattern in the CMB. And the E mode is predicted to be there whether or not there are gravitational waves, and it has already been observed. The B mode is predicted to be a special thing predicted by inflation, and it has not been observed yet. To be a little bit more careful about that, a primordial. B mode has not been observed, a B mode from the very early universe, there are other things that

can give rise to B modes in the later universe that don't give us evidence for inflation or against it. So the question is, this is a prediction of inflation. Would it be case-closed evidence? No, because there is never case-closed evidence in science. That's just not how things work, especially when the science is something as speculative and hard to constrain as the super-duper early universe. The question again would be, is there something else that could possibly give rise to the same signature?

Is there another model or another theory? And we don't know of one right now. But what you would do is just you would be a good Bayesian. You would say, what is the chance that I would see the signature if inflation were right versus if it were not right, and I would update my credences appropriately. I would say that it would be strong evidence. A lot of people's minds would be noticeably shifted by observing B modes, but it's not case-closed evidence in any real sense. Amel Rojas says, are particles in an atom or molecule quantum entangled, or in the nucleus for that matter? You know, yes, they are, but it's a subtle kind of thing, and I'm going to try to say things that are all completely true. Let's see if I can succeed here. The reason why they are is because the kind of particles, let's say electrons. Let's take to a specific example.

Electrons in an atom. Okay. Electrons in an atom are identical particles. There's no difference between one electron and another one. You can't say, well, I noticed electron A over here and electron B over there, other than to say where you found them. So if you have, let's say, two electrons that are in two different orbitals in an atom, and the same, it doesn't matter. The same atom could be a different atom very far away, whatever, two different electrons, right? And you measure one of them. You can't say which one you've measured. There's no such thing, and that is a feature of them being entangled with each other. The reason why I have to be very cautious about this is because it

doesn't quite have the same implications that you're used to entanglement having. When I talk about entanglement, when you talk about like spins being up and down, an Alice and Bob and an EPR experiment, then there's a feature that says, you know, if you know the particles are entangled so their spins are oppositely aligned, then we say, when you you measure one and it's spin up, you instantly know the other one is spin down. You've learned something about the particle over there. In the case of electrons entangled in an atom, maybe their spins are also entangled in such a way, but we're ignoring that for right now. We're just talking about the necessary entanglement because they are identical particles. You don't learn anything

about the other electron by measuring one. All you do is that you learn that the other one is the other place. So if you have two different orbitals and you know exactly what they are, and you have exactly two electrons in them. They are in some entangled wave function, but all you learn by measuring one is that the other one is in the other one, okay? So you don't actually get any new information by measuring them, but strictly speaking, they are entangled, yes.

But because of this observational implication, you can very well treat them as if they are not. So when you take your chemistry class, you don't need to learn about entanglement or anything because there's two electrons and two different orbitals, but they're identical. It doesn't matter which one is in which one is in, which one. So if you just want to talk about the electron that is in this orbital and the

electron that is in that orbital, you're fine. All the physics is the same, all the observational consequences are the same, et cetera. Danny Avedan asks a priority question. Last time I asked you who constitutes a moral subject, that is, how should we treat morality? A follow-up, please. Do you think we can understand, analyze, critique, argue, and expand our moral system without a definition of who is and who should be included in it. No, I think that, you know, obviously if you're going to talk about morality, you better talk about who it applies to. I can imagine different moral systems having different answers to that question, right?

Some would be for all conscious creatures or sentient creatures, some would be for human beings, some would have a nuanced in-between kind of view where sufficiently intelligent creatures would count under your moral system and others wouldn't. Some would say that some moral strictures apply to some creatures and others. So I think all of these are on the table as possible things to think about. The other thing to think about is that, you know, when I think about morality, I don't think that there are sharp, bright lines anywhere. I think that morality is kind of a fuzzy thing, and we should be better

at embracing that and accepting it, or at least not living in denial about it. That doesn't mean it's not super important, and you shouldn't work hard to be moral, but you should realize that it's not like a mathematical proof or something like that, right? It's a little bit more subjective, more than a little bit, more subjective than that. Igor Parskin says, in quantum field theory, why is it, why is the invariance of the Lagrangian that we require and not the equations of motion? How do we know our theory requires this exact invariance? So I don't think it's true that we require invariance of the Lagrangian rather than the

equations of motion. I mean, maybe there's a subtlety because sometimes you have an equation of motion and a symmetry. And the thing is that the symmetry acts to change the equation, but it changes every term in exactly the same way. So the underlying solutions to the equations are not changed, whereas you might have a situation where the Lagrangian is simply unchanged. So again, for those of you who don't know, the nice thing about a Lagrangian, the Lagrangian is an expression from which you can derive many different equations of motion at once. So you might have a theory that has different kinds of particles and fields and whatever. In good old

fashion, Newtonian mechanics, classically, you would have to separately have equations of motion. For all of those pieces of your theory, they might involve interactions with each other, but they would have separate equations. The Lagrangian point of view lets you combine all of the things in your theory into one expression, the Lagrangian, and you integrate that overall space time, and you minimize that, and those are your equations of motion. And the great thing about this single expression is that it not only has equations of motion for everything, but that it's very easy to implement the symmetries. You just look at the Lagrangian as a whole rather than all the separate equations of motion.

Okay. So there's various conveniences involved in doing things, the Lagrangian way. But at the classical level, there's no deep difference between the Lagrangian and the equations of motion. You just use one to get the other. Quantum mechanically, there's something extra going on, which is not mentioned in this question, which is that you're not classical anymore, you're quantum mechanical, and neither the Lagrangian nor the equations of motion are the whole story. One nice way of thinking about

that is to think about the path integral. You know, when you have a regular integral of X, you write integral F of X, DX, right? And F of X is the function you're integrating. DX is the measure. It tells you how much counts from each little interval DX. The same thing is true for the path integral formulation of quantum mechanics. You're summing over all the different possible paths or histories or field configurations. And what you're summing is E to the I, S, where S is the action that you get from the Lagrangian. But there's also that measure there in the path integral, and you need to separately have invariance of the Lagrangian end of the measure. If you have a theory, and you can very well have theories, as I remember I talked in the solo episode about anomalies, that was one of the reasons why string theory had a bandwagon that got launched in the 1980s because people realized that anomalies could cancel in string theory.

And an anomaly is precisely a situation where there's an invariance or a symmetry that is there in the Lagrangian, but is not there in the measure that you put in the path and girl. So it is a classical symmetry that is violated by quantum mechanics. So anyway, all of that is to say it's not the Lagrangian that matters. It's the whole theory that matters. Some ways of thinking about the theory might be more convenient than others. Shambles says, after listening to all your podcasts and reading your books, it could be said there's a kind of emergent Sean Carroll that exists in my brain, perhaps to the point where I can imagine your response to certain AMA questions or words or phrases you might use

with a fairly poor degree of accuracy. I can imagine for your spouse or a close friend, their version of you will be much more real. In what ways is the emergent Sean Carroll in their brains the same were different to the emergent Sean Carroll and Sean Carroll's brain? I don't... So, I mean, I presume that by the emergent Sean Carroll and Sean Carroll's brain, you mean myself.

I mean, not just me having the emergent person in my brain. I think the point is that you do model other things in the universe in your own mind. And that includes the physical universe. It also includes people, right? So you have different ideas about how people will act, what they will do. They might be probabilistic or approximate and so forth. They might be more nuanced once you get to know people more. Some people might not need a lot of information to model them very effectively. They're very predictable. Other people will need a lot of information to think about them and they're less predictable. So, you know, I think that it is, I'm not exactly sure what the question is, to be honest, or how best to answer it.

You know, I think that whenever you try to imagine what another person is going to do, you will inevitably oversimplify, right? You don't know all of the details about what is going on in someone's brain to get them from point A to point B. Sometimes you don't have to, right? Sometimes the answer is just obvious, or you know it, or you know it. you've heard it before. Other times the answers can be very tricky. This is what makes human beings so delightful that they're not quite so predictable. So, you know, you don't have enough room in your brain to accurately model many other people because their brains are also pretty complicated, but it will, you know, differ in the details absolutely from person to person. I'm not sure if that's

in any way a satisfactory answer, but it's the best I can do. P. Walder says, you have provided an explanation for how the core theory can fully account for the matter we engage with in our everyday lives, including neurons in brains. Is this now the accepted view in the physics community, or are there gaps in the explanation which would allow for yet to be discovered energy fields to play an explanatory role without having to create a whole new physics paradigm? I think it is overwhelmingly the accepted view among people who have thought about it that much, which is not that many people, right? It's the kind of thing where when you tell a professional physicist, what this claim is, they will think about it and go, yeah, okay, that sounds right.

There are some well-known counter examples. There are absolutely professional physicists who want to think that the fundamental laws of physics, as we currently understand them, will need to be updated even to take account for things that we see in the everyday world. So they're out there, but I think that they are a dramatic minority. I think that the more important thing is that most people haven't even asked themselves the question. Chris V. says, do you have any guidance for folks who work in intellectual fields? Curiosity, learning, and iterating on ideas is wonderfully fun. However, I often find myself lost in thought or living in my head. Have you experienced this yourself? And if so,

do you have any advice for separating that aspect of your life in order to be more present in others? So I'm not exactly sure whether you're focusing in on being a productive, intellectual or being a productive or maybe productive is in the right word, being a successful person who is also an intellectual but working but appearing sometimes in non-intellectual contexts. So there's two issues when it comes to sort of being lost in thought. One is that your job is an intellectual job, you know, to come up with ideas and talk about them and test them and so forth. There are absolutely people who like the part where they get to think and don't like the part where they got to work to explain their ideas

to somebody else or to develop them fully or to write them down or whatever. If that's the issue, then, you know, there's one thing which is being intellectual and thinking. There's another thing which is earning a living at it. And earning a living at it absolutely means you got to do the work. You got to really put in the effort to turn your thinking into something that is productive for other human beings. There's nothing bad about that, you know, by all means, spend your time thinking. But if you want that to be your job, then your job involves also talking to other people. If what you're getting at is you love thinking and you get lost inside your head, and then the person you're having dinner with says, hey, you're not paying attention to me.

That's a very different kind of issue to worry about. I think that's just a matter of politeness. You have to be able to balance the fact that it is fun to think about things. to get lost in your head, to dream about things, with the fact that there's a time and place for doing that. And, you know, we all know charming people who are kind of absent-minded. There's famous stories about intellectually accomplished people

who were kind of unable to get through the day, tying their shoes and so forth. You know, the charm only goes so far. I think that most people, if you're smart enough to be good intellectual work, you're also smart enough to tone it down, in circumstances where it is not appropriate. Josh Charles says, when looking through a telescope at a galaxy,

is it fair to say that those photons are branching their wave functions by interacting with my retina for the first time since being emitted? Or would that have happened when the first lens it encountered? Well, it happens both. So, you know, very often a photon that is emitted from an atom in some astrophysical event has a wave function that moves in more or less a spherical pattern away from its source. And what will happen is it will bump into that spherical wave function as a probability of bumping into various things along the way.

So if you have lots of little atoms around the event, and maybe some of those atoms are in the form of telescopes, some are in the form of eyeballs, but some are just in the forms of rocks and things like that, for each thing that the wave function could bump into, there's a branch where it does. And then the wave function disappears in that branch from all the other directions in which it was moving. But then there's the other branch where it didn't hit that thing you're talking about, and it continues to move on in all of these different directions,

and then it could be absorbed by something else. So that's just to say that the ultimate set of all the branches of the wave functions are ones in which that photon has been absorbed by many different things throughout the universe. So there's a first time it gets absorbed, but in the rest of the wave function, it keeps going, and it will be absorbed and branch further down the road, depending on what it hits. Ram Sashadri says, if I was a lowly patent clerk and came into possession of earth-shattering new physics, how could I go about attributing credit to myself, and is there any chance I can pull it off without getting, You know, I think that this very question really is completely upside down. It shows that you don't, not really familiar with how academia works. The, it is absolutely possible to have your ideas stolen, okay, in academia. But the way that that happens is precisely that you don't tell anybody about your idea, except for maybe one or two people and those one or two people go and scoop you.

Okay, that's how it happens. Once you put a paper into the public domain, whether it's on archive or in a publication or even just on your website, then you have priority. If there's a date on it that is verifiable, then someone else can try their best to write a paper and say, hey, I thought of this, but the whole rest of the world can just say, no, no, it appeared here earlier, right there. There's no issue when you put a paper out into public. That's when you get to claim it. That's it. That's when you get the idea. idea that by putting a paper, a result into public, it will get stolen is precisely backwards. It is by not putting the idea out there into public that it can get stolen, okay?

Especially if it's a true idea, if you actually come up with something correct, you're not going to be the only person who comes up with that idea. Someone else is going to hit on it before too long. So all of the incentive is to get it out there into public as soon as possible. So in terms of if you're not a credentialed scientist, where do you put it, et cetera, the best thing is to put it on the archive, but you need to get some endorsement from some person who knows you and who has talked to you who already has the ability to put it on the archive and then can endorse you to also put it on there. If you find yourself never having talked to a professional scientist about your work, then I think that the chances that your work is really earthshadowed. are probably very small. Certainly that is not the condition that Albert Einstein when he was a patent clerk was in in any sense of the imagination. Rue Phillips says, I am writing a book on prediction. I spent a

couple of years and got a solid foundation but eventually got stuck. I write in a robotic voice, not super interesting, and I'm not especially skilled with grammar, language, etc. Enter chat GPT. I'm doing an experiment where I revise my chapters using GPT4 with a particular tone. It is making great improvements, along with better structure and even providing additional examples to include. It has unblocked me. However, I'm now worried that I'm a fraud or that no lit agent or publisher would want to touch it. What are your feelings about the situation I'm in or what advice can you offer? I think this is a great question. This is a question that is going to become very, very important down the road. And I don't know really what the right thing to do here is.

I'll give you an impression, which is sort of you should think of it as my first impression subject to being updated later, as I think about it more and we learn how this goes, which is that chat GPT or GPT4 or whatever, these are tools that you're very, very welcome to use. If they help you write, then that's great. But maybe you want to give credit to them. Like maybe you want to say in the acknowledgments of your book, I used this tool to help me write or whatever. I don't think that there's anything tawdry about using this. tool, any more than using a dictionary or a calculator or whatever. Having said that, there's one huge footnote here, which is very, very important, which is that unlike a dictionary, where some people sit down and consciously compile it and then offer it out there into the world for you to buy, these large language models have been trained on other people's writings. So it's completely legit to worry that things that are written by chat. GTPT, et cetera, are just plagiarized in some sense. They're not literally plagiarized because

chat GPT remixes them to be completely unrecognizable in some way, but they were trained on something, and those some things were written by other people. So I would tread very, very carefully. You know, I would use it, you know, I have a friend who uses the E. Ching to make difficult decisions. You know, you cast some stones or some sticks, and then you look up in a book what it's advising you, he doesn't actually think that the E. Chang is predicting the future or anything like that. The point of using the tool is to shake yourself out of your rut, right? To roll some random numbers, get a suggestion and then think about the suggestion. You're like, do I like that suggestion? Do I not like it or whatever? I think that's the best way right now to use these

AI tools, not to do your work, but to inspire you to do better work. If you can come up with a better way of writing, a better metaphor or whatever that is inspired by playing around with chat GPT, I think that's okay. That's no different than being inspired when you're writing a novel by reading other people's novels, right? You're not rewriting the same novel, but you can absolutely get stylistic points or inspiration or things like that. I think that's how you should treat AI models.

But again, all of this is very new and changing rapidly and subject to update without notice. Anonymous says, there is renewed interest in psychedelics for mainstream media and institutions like Johns Hopkins. Have you any thoughts on this that you want to share, either the renewed interest or the compounds effects themselves? Yeah, I'm very much all in favor of it. We did a podcast back a while ago with Robin Carhart Harris, who's one of the world's leaders in this field. I think that research into psychedelics got a bad rap back in the 60s for cultural reasons, not for scientific reasons. And you can use them for pleasure, but you can also absolutely use them for therapeutic reasons. And I'm not an expert, but my vague impression about how to think of it is that there's various syndromes in the human mind that come about because you get stuck.

You get stuck in a vicious cycle or a rut or whatever. It can be PTSD or depression or addiction. You know, there's various ways in which the brain kind of gets stuck in a bad place. And this kind of thing is exactly what psychedelics are pretty good, apparently, in terms of the tentative research that's been done, at breaking us out of. So I think that there is potentially enormous possibility for using psychedelics for good therapeutic reasons. We need to understand what the side effects are, et cetera, and I, like I said, and my, an expert, so I don't, but I'm pretty optimistic, but this is something that's going to be very useful going forward.

Emerge Holographic says, atheists often take the stance that reincarnation is impossible, that we get one life and that's it. But it would seem to me that placing such importance on one life is actually granting exceptions to consciousness. We used to be dead before we came to life, so why can't that happen again? What is the functional difference between pre-life-life- death and post-life death that makes them mutually exclusive. I think that there is an actual simple answer to this, which is that it is true that I can take the raw materials of a person,

you know, the atoms, the hydrogen and carbon and oxygen and what have you, that has died and their body, you know, ceases to be in existence, et cetera, but I can take all of their atoms and I can imagine reassembling them into a different person. The important thing is that there's zero sense in which that person is the same person that you started with because they don't have any continuity of memory or of experience or anything like that. So there's nothing special about the state of your atoms pre-death versus post-death, but there is a cessation of your consciousness that is irretrievable. Once you're dead in that sense, you're not coming back. Sid Huff says, the idea of the multiverse seems to be having its day in the sun, the June 24th

issue of The Economist included in editorial discussing how popular the multiverse concept has become in modern cinema. The article suggests that the popularity of the multiverse might be responding to some deeper yearning, perhaps that reality might be more complicated than previously thought. Theoretical physics aside, do you sense that the growing awareness of the multiverse idea is emerging from a deeper yearning in humanity? Short answer, no. I think that there's nothing new about this deep yearning, right? I think that there's nothing new about this deep yearning, right? I think that, you know, there are ideas that sort of catch fire in popular imagination. And if someone uses the idea of a multiverse in a movie or in a novel or whatever, it's probably

because they saw it in somebody else's movie or novel or something like that. I don't think it's because humanity has developed a new deep learning, yearning. And also, I don't quite agree with the diagnosis that there is a yearning that reality may be more complicated than previously thought. I mean, maybe that's true, but I think there's a much more straightforward reason why the multiverse is so fascinating from a narrative point of view. It's that you and I have in our heads different alternative histories, right? Have you ever done something in the past that you regret? Have you ever thought of how things would be different? Had you done them differently? Or had things happened differently, I think that for 99.999, et cetera, percent of humanity, the answer is yes.

And the multiverse in the narrative sense allows you to explore those possibilities, how your life could have been different. People care about their own lives, basically, right? That's really when it comes down to it, what they're interested in. So, of course, there's alternate histories, and you can tell stories of alternate histories without imagining that they're literally there in the multiverse, but imagining that they are there. in the multiverse puts a little bit more umph to them, right? Imagining that there's a version out there who did ask that person to senior prom or whatever. I'm not sure if it's valid to think that way.

And you just think of these things as completely hypothetical, rather than caring about whether or not they're actually out there in physical reality would be my personal advice. Michael Lacey says, Tim Maudlin mentioned that quantum entanglement might enable faster than like communication under certain conditions. What are your thoughts on the likelihood of this being possible? It seems that it would wreak havoc with relativity

and lead to paradoxes, such as information traveling backward in time and arriving before it was sent. Well, I think it's super-duper unlikely, honestly. You know, Tim was very vague about what exactly the claim was, and it seemed to be in the context of Bomiya mechanics in particular, which I don't think is the right version of quantum mechanics. So one of those things were very unlikely to come true, if it does come true, it'll be big.

I wouldn't rule it out a priori, right? I mean, physics got along pretty well for hundreds of years without realizing that the speed of light was a limit on anything. So we could go back to that. Just because you can go faster in the speed of light doesn't necessarily mean you can go back in time. To be super-duper careful about it, if relativity is true,

and you can go faster than the speed of light, then you can go back in time. But under these theories, maybe relativity just isn't true. Maybe there's some other way of slicing space time so that you can't go backwards in it. So I don't know exactly what the idea is.

I would be open to the possibility of it, but my credence on it ahead of time would be very, very small indeed. Love feels best, says, if I crafted a glass sphere with a perfectly reflective interior mirror surface, and I inserted light into it, along with a tiny camera,

would we see light forever from a device receiving the camera's signal? Nope, you would not, and the reason is because when the camera detects the light, that photon or whatever it was stops existing. So you can't say both that you have a glass sphere with a perfectly reflective interior mirror and that there's a camera in there. You can say one or the other. If you did just have a perfectly reflective interior, and we forget about details like the whole glass sphere, collapsing into a black hole or whatever, then the light would stay inside bouncing around forever. But once you start observing it, you start attenuating it, and it would go away.

Sam Hartzog says, outside of the obvious difference in rate, how does cosmic inflation in the early universe differ from the expansion we observed today? If I had a dial that let me manipulate the value of the cosmological constant, and I cranked it up from 10 to the minus 52 to 11, do any phenomenological distinctions remain? Given the naive similarity. between the two, I was surprised that my perfunctory Googling didn't turn up any relevant results. Well, I'm not quite sure what kind of differences you want. You know, in terms of the difficulty in solving Einstein's equation, it doesn't matter what the size of the cosmological constant is.

You just plug in, you get the solution, et cetera. But the cosmological constant is not a dimensionless number. It's a dimension full number. It has a scale. So one way of thinking about that is if you are in decider space, so again, you you have not, if you idealize your space time by saying you have nothing in the universe other than the cosmological constant, then you would have a horizon and that horizon would have a size. Okay, so you have a physical length scale associated with the value of the cosmological constant.

If the cosmological constant is large, that length scale is small. They are inversely proportional to each other. So you can compare that length scale to other things, to the sizes of atoms or stars or whatever. If your length scale, your Hubble radius, or your decider radius is small compared to the size of a star, then you're not going to have any stars in your universe and so forth. So there will absolutely be quantitative differences, but it will still at the end of the day be the same kind of solution to Einstein's equation underlying the whole thing. Joy Colbeck says, do gravitational waves from separate black hole neutron star mergers

constructively and destructively interfere? Yeah, they do, but not that much. Usually, you know, when you think about observable interference patterns, like in a double-slit experiment, it's kind of important that the slits are nearby each other. Once you move the two sources far away from each other, then it just becomes harder to see any interference patterns. So short answer is yes. Longer answer is yes, but there has no really noticeable effects on anything that we see in these events. Colleen Edwards says, in your academic journey, was there one or maybe a few particular concepts or ideas in quantum mechanics that really challenged you when you were trying to wrap your head around it? I don't know about that one. I think it's a good question because it's very hard for me to remember what was going through my mind when I was an undergraduate learning quantum mechanics for the first time.

Honestly, when you typically, and certainly for me, when you first learn quantum mechanics, it's mostly, about doing the problem sets, right, which mostly involves solving the Schrodinger equation for some potential matching at some boundary, calculating some rate, doing the WKB approximation, thinking about harmonics of Enclepch-Gordon coefficients and atomic orbitals, you know, all that detailed technical stuff. It's really not about deep conceptual things. These days, you know, I'm learning more about quantum mechanics in a different context, when you have quantum information and measurement theory, there's a whole new world out there

of different kinds of measurements you can do and different results you can get from them and information being shared between different quantum subsystems. And all that is fun but hard. So it's hard just because it's hard. I mean, it's not like conceptually difficult. I can't get it. My brain is not big enough.

It's just work. You know, it's work and it's rewarding to do the work and get the answer. So, you know, I taught a course at Caltech several years ago where I taught some of that stuff, it was very rewarding to do it. And I'm going to try to figure out that I can fit some of that stuff into a quantum mechanical textbook, but not other stuff. I don't know.

Whenever you're writing a textbook, you have to take into consideration that you can't do everything, right? You have to kill some of your darlings, and that's going to be something I need to keep my mind on and decide very carefully what deserves to be in and what doesn't. Anonymous says, I really like the conversation with Brian Lowry on the socially constructed nature of the self. Here's a question related to that. It's possible to socially construct an identity that's detached from any physical or tangible living thing. For example, a fictional character, a personified concept like Lady Liberty, a sports mascot. Intuitively, I think that as a human being, my identity has a certain secret sauce that Spider-Man will never have,

no matter how many movies or comics or fan works people make about Spider-Man. Is this secret sauce an innate self that I have after all, or is my intuition wrong? Am I just another Spider-Man? I think it's a great question, and I think that my favorite answer is that neither one of those is quite right. I think that Brian is right to say there is no secret sauce there. There is no separate, immaterial, immortal soul, or even mortal soul, or even immaterial anything. You are a bundle and jumble of different impulses and thoughts and social relations in various ways. And so that gives you some freedom, that gives you some agency to shape your notion of self,

especially in response to the social world around you. But I don't think it's arbitrary or unaffected by physical reality. I think that there are also constraints. Like I cannot identify as a 10-foot-tall person, no matter how much I want to. I can claim that I am, but no one's going to take me seriously. But I could identify as all sorts of different things that do plausibly comport with the physical reality around me. So I think there's a given take and interplay. I think that Brian's point about the social construction of the self is a good one, but it's not the only thing to say.

I do think that physical reality also has a say, and you need to be compatible at the end of the day. I don't have any great ideas about working that out in full. I'm going to leave that to the psychologist. Jeffrey Siegel says, I enjoyed your conversation with Katie Elliott. Regarding the conversation about traveling back in time before the coin is flipped, if the Everettian view is correct, couldn't you have seen the coin come up heads, but then travel back in time and then end up on a branch of the wave function where the coin is tails instead of heads?

Well, the only honest answer to a question like this is, who knows? Nobody has a good theory of time travel in Everettian quantum. mechanics. As we discussed, I think, a little bit in that episode, I certainly have mentioned elsewhere, there are theories of how to construct consistent quantum states in the presence of close time-like curves. David Deutsch, for example, has put such a theory together, but it's not the whole Everettian apparatus of branching in multiple worlds or anything like that. It's just the wave function as a whole that he's talking about. So nobody has a sensible physical picture in which you both have branches of the wave function with different outcomes and time travel

that starts you in one branch and goes back to a different one. So you would have to make such a setup in order to answer these questions. I thought about doing it. I thought about putting effort into trying to make that make sense. And then I realized that, you know, I have a job. I have real work to do and inventing the time travel scenarios is probably not the most productive thing. I can do with my time. But I don't think that you can claim right now that there is a once and for all sensible answer to that until you have some specific scenario in mind. Tara Lumagi says, considering the new nanogramrab results, do you think we may be able to detect a frequency that actually tells us what space time is made of? Any experiments that can help us rule out

quantum gravity versus string theory, maybe even detect an echo of the Big Bang. You know, mostly no is what I want to say here. I mean, by the way, it's not quantum gravity versus string theory. String theory is an example of a quantum gravity theory, one that also involves other things as well, but quantum gravity is part of it. But that's just not what nanograv is doing. You know, nanograv is like all other currently available windows on gravitational waves very, very deep into the classical regime, you know. It's Gravity, sure, it's telling us something about gravity, and so far what's telling us about gravity is that general relativity is doing fine. But it's not telling us about quantum gravity. The whole reason why quantum gravity is hard is because the scales at which you can detect gravitational effects are just different than the scales of which you can detect quantum effects. And that doesn't change with nanograv or anything like that. You're not looking at gravitational waves made by single atoms, etc.

But there is, you know, the one, there's a couple of, you know, caveats there that might be relevant. You know, maybe we do continued data collection and experiments, and we find that we no longer fit the data with classical general relativity. Then you open up a can of worms, but a very delightful can of worms, where you can start saying, well, maybe it's quantum gravity or maybe it's some other classical theory, or who knows, what's going on? This hasn't happened yet, so we don't know what the answer would be. And you can imagine detecting something that you might call the echo of the Big Bang,

but it's only a poetic kind of phrase. That is to say, you can imagine primordial gravitational waves. We've already talked about them, generated by inflation. My impression is that the predicted amplitude of primordial gravitational waves generated by inflation is nowhere close enough to be detected by nanograv. But maybe I'm wrong about that, or maybe everyone's wrong, and maybe there is some particular feature that you could someday detect, or a phase transition in the early universe or something like that. I just don't know. If we did, it would be indirect evidence. It would not be immediately very useful, but it would be a clue to something, and that would be very, very exciting indeed. Jonathan Bird says, besides calculus, what branches of mathematics do you use the most in your work?

What would you recommend for a physics undergrad now that might not be obvious? Is there some little sub-field that has been surprisingly helpful for you? You know, I think it depends a lot on what kind of physics you do. You know, I think that maybe I'm being curmudgeonly about this, but I have the following thought in mind. There's a lot of physics students who catch on pretty quickly that math is kind of important in this game, right, in doing physics. And so they get the idea that they can sort of advance their physics knowledge by learning more math, by taking math classes, et cetera.

And you know what, they're right. It's true. Learning more math, I mean, whether it's analysis or group theory or abstract algebra or topology or whatever, can be very useful to your future physics career. However, all else being equal, it is less useful than learning more physics before worrying too much. before worrying too much about learning more math, don't forget to learn physics. Learn E&M and stat-MEC and classical mechanics and quantum mechanics and quantum field theory and general relativity and condensed matter physics and all those things. There's a lot going on in the world of physics.

And much of it doesn't require super advanced math. It requires calculus, maybe some complex analysis. If you're in the quantum mechanics side of the world, there's a lot of linear algebra. Okay, that's maybe the one thing that they don't teach you enough of. Because everyone learns calculus and differential equations and things like that. Some subfields, you're going to have to spend a lot of your time doing integrals or doing series expansions or what have you, various approximation schemes, WKB and so forth. Others, you're going to be getting exact results or you're going to be doing linear algebra,

diagonalizing matrices. It depends on what kind of physics you want to do. But almost always the kind of physics that you are learning will teach you the math along the way, or at least will indicate to you what kind of math it is you need to know. Every good general relativity book teaches you enough about differential geometry and tensors to get by. There's always more to learn. You can learn the math for its own sake, and that's fun, and maybe you end up being in that small minority of physicists for whom this extra bit of mathematical knowledge is super helpful. But typically, honestly, as a physicist, you pick it up as you need it, as you go along.

You don't spend your time guessing that maybe this is going to be useful to you down the road. Tyler Whitmer says, do you think there is any specific area of inquiry in physics or philosophy that are being held back by institutional barriers to interdisciplinary work that could be advancing dramatically if more institutions took the SFI approach? Well, I don't know about specific areas, but I think that the, the whole boundary of physics and philosophy falls into that category. Likewise, the boundaries of psychology and philosophy, or computer science and philosophy, or biology and philosophy, etc. Or, for that matter, history and economics, or economics and literature. I think that there are various aspects of academia that, for reasons that make sense but are imperfect,

concentrate on specific disciplines to the failure, not to the detriment of interdisciplinary work. People want to know not just that the work is good, but which area it's in, which department in particular it should be in. And so you have to target, if you want to do interdisciplinary work, you have to come up with some justification with this particular kind of interdisciplinary work. You know, the interdisciplinary work of physics and philosophy and the foundation, of physics and so forth, that's a kind of work that is very important. We're trying to build up that effort here at Johns Hopkins. SFI is doing a different kind of thing, right? It's looking at

complex systems, which is a different area, which I also think is interesting, but it's not exactly the same kind of thing. So I think that you have to have a particular area in mind. Every area is going to be different, and then you have to figure out how to make it work, right? Where it could work, who, what kind of people, remember when I was talking about Kormick McCarthy and SFI, I said, the individual people matter a lot. You know, the barrier to physics and philosophy getting along better is partly that there are institutional barriers to it, but also partly that the individual physicists and philosophers aren't interested, right? Some of them are. Many of them have been on this podcast, but most of them are not. And that's sad in some sense,

makes me a little bit sad that they don't see the beauty of it and the intrigue of it that I do. But, you know, that's more work for me, more things for me to do. So I'm, I don't have an overall theory of how to do this. I think you have to do it on a case-by-case basis. We're trying to do it in foundations of physics, you know, philosophy of quantum mechanics, space time, arrow of time, statistical mechanics, cosmology, all of those things. There's plenty of room for great work to be done at those interfaces. Okay, the last question of this overly long AMA is from Nikita Lozovani. Nope, there's no end there.

Lozovoi. Do you sometimes think about the future of humanity, specifically whether we are going to reach other habitable worlds? And if yes, how or if we are going to be confined within Earth and solar system quarters until humanity is extinguished due to the environment becoming uninhabitable or due to lack of natural resources, et cetera. Yeah, I mean, I think about it. You know, I don't want humanity to become extinguished. I think eventually it will happen, right? You know, I don't know if it matters to our current planning strategies, but we're not going to be immortal. We are going to

equilibrate. We're headed toward thermal equilibrium, but that's a long way off, so we have plenty of time. I suspect that humanity will survive and it will expand beyond Earth. It will expand to the solar system and beyond. Some people worry that, you know, the stars are too far away and that it's a science fictiony scenario to imagine traveling to them, but in reality, if you're hard-nosed about it, it would take too long. I think that's wrong because I think it's easy. It's not possible to go faster in the speed of light,

but it is easy to live a very long time. It's much easier to solve the problem of longevity than it is the problem of FTL travel. So the galaxy is some tens of thousands of light years across. You know, give us a few million years. There's no reason at all that we shouldn't be able to fill the galaxy. The word we there is doing a lot of work. Millions of years is a very long time on human evolutionary timescales,

especially now that we're entering a technological era where we have not only computers and AI, but gene editing and synthetic biology. things like that. So whatever it is that ends up traveling to other parts of the galaxy might end up looking very different than we humans look now. But I'm overall optimistic that we will not kill ourselves, and I suspect strongly that if we don't kill ourselves, we will not just stay here on Earth. We are going to travel to the stars. That's an optimistic place to end. Thanks as always for supporting Minescape on Patreon and elsewhere. Hope you have a good month. Talk to you next week. Bye.