Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas - AMA | November 2020

Episode Date: November 20, 2020

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Hello everyone. Welcome to the November 2020 Ask Me Anything edition of the Mindscape Podcast. I'm your host, Sean Carroll. Let me explain what's going on here. As some of you know, we have a Patreon for people who want to support Mindscape financially. You can go to patreon.com slash Sean M. Carroll. And there's two benefits for actually contributing to the Patreon. One is you get ad-free versions of the podcast. And the other is every month we do one of these AMA episodes, Ask Me Anything Episodes. So near the end of the month, I put up a post on Patreon, and anyone who's a Patreon supporter can ask a question. And I try, don't always succeed, but I try my best to answer every question that is asked. And this episode represents that. So these episodes, these AMA recordings are published on Patreon near the end of the month. In fact, this one that you're about to listen to, if you're going to listen to it, came out at the beginning of November just before the presidential election, which seems like years ago now. But what happened is a lot of the Patreon supporters, by their own initiative, said it would be better if we could share the results of the AMA widely, not just to Patreon supporters. So Patreon supporters will continue to get the benefit of being able to ask questions on the AMA.

But going forward, anyone will be able to listen to the answers. So we're going to make it just a regular part of the Minescape feed. It won't replace the regular episode. So it won't get a number. We're going to release these episodes in the middle of the month. So the Patreon supporters get a week or two after the AMA came out to listen to it and know in their hearts that only they have been able to hear it. And then we're going to send it out to everybody. So as I always say here, no pressure to join on Patreon or anything like that.

We're the happy community whether or not money is exchanging hands. But I would like to convey my thanks to the Patreon community for suggesting that we share these AMA episodes. more widely. So these words I'm saying right now are replacing the intro I did for the actual AMA that the Patreon supporters heard because I was only talking about the election that was going to come up in a day or two, not really relevant anymore. Going forward, this is going to be the same intro that you get for the wide thing and for the Patreon supporters. Usually I babble on for a couple minutes about something that has been going on in the last week and then we dive into the questions. So let's do that. Let's go.

Manuel Bavond says, what is an opinion you used to have that you completely changed your mind about? I mean, I hesitate on this one just because there are so many. And I guess it depends a little bit on what you mean by opinion. If we stick just to science, I definitely had the opinion that the cosmological constant was probably zero. And then we discovered it in 1998, which made the universe a lot more interesting place. So that's a very clear cut case where everyone changed their mind, I think. You know, on more wishy-washy things where your mind is not changed by vivid, startling data that comes in. You know, I used to be someone who philosophically thought that was more, I guess, what you would call,

scientific about morals and ethics and things like that. someone who thought that, you know, we're human beings, we're living organisms, we developed where we are by a biological evolution, and you could use those features of who we are as human beings to derive clear philosophical guidelines for what is right and what is wrong. I see the fallacies in that now, right? So I changed my mind about. There you go. There's many, many other opinions, but that's one of them. Elias Borgensen, sorry, No end. Sorry, Elias. Do you use Lotech or Microsoft Word for your publications? This is an excellent question. For those of you who are not working scientists or in that field, scientists or anyone

who uses equations a lot in their writing tends to use this typesetting program called Laotech. It looks like it's spelled latex, but it's pronounced Lotech. Trust me on that. And so certainly for research publications, I'm going to use Lotech. There's lots of equations in them. There's really just not even a choice. I know Microsoft Word has some equation capability, but it's not in the same league as LATEC is. Now, for articles or something like that, that you would put, like if I send something to New York Times, for example, it has to be Microsoft Word. That's the only format that they will accept. So I was dragged kicking and screaming into using Microsoft Word. For long form publications, well, for a textbook, you still get a

got to use Lotech because there's a lot of equations in it. But for other books, for popular books, I use something called Scrivener. The publishers will only accept Microsoft Word. But if you have a book that's 100,000 words long, either you have one giant document, which is very big and it's hard to find things in it and you don't know what you're doing. Or you have, in Microsoft Word, you know, one document per chapter. That's how I wrote from Eternity to Here. But Scrivener is an example of a program that is meant for long-term, long-form writing. So it has one big file, but separate files within the big file, within the document, as they call it, or the project or whatever it's called. Within the project, there's different files for chapters, and also you can import research, like you can import PDF files or figures or whatever into the one project.

and it sort of looks like a book. And so if you suddenly say, well, actually, I want to move chapter eight and make it chapter six. Trivial to do something like that in Scrivener. And you can search the whole document all at once. So it's a very good thing. A lot of book writers use that. Aaron DeSario says,

in April 2020, the U.S. Department of Defense released a statement about three unclassified Navy videos that have been circulating on the Internet for a few years. Then Aaron goes on to add more details about UFO sightings and other things like that. And he says, can you be convinced that we are being visited right now?

So to me, this has a very similar feeling to, is there anything that could convince you, that could convince you that God exists? Because the answer is, of course there is. If God existed, it would be no problem

for him to convince me that he existed, right? He just has to come visit. Tell me he exists. You know, manifest in such a way that it would be almost impossible to fake it. And the more,

he did things that only God could do, the more I would believe that it was God. I'm a good Bayesian, more evidence I collect, the more likely it is. But in fact, the world we live in looks exactly like you would expect it to look if God doesn't exist, right? You can argue that God should exist, but you never see God in the everyday world. And this is a long argument, obviously, not everyone agrees with me, but that's how I see it. And the same thing is true with aliens visiting us in UFOs. As a good Bayesian, what you're trying to do here is to compare two different comprehensive views of the world, one in which aliens, intelligent, extraterrestrials,

highly technologically advanced, come to visit us and they hide, right? Like, they don't make it easy. They don't come and announce themselves. They sort of have enough advanced technology that they can fly light years across the galaxy and come visit us, and they kind of hide, but not very effectively, right? So you can see some flashing lights and people in their backyards see things, and Navy pilots make recordings of things that seem to be moving very fast, because the aliens had the technology to fly many light years across the galaxy, but number one, for some reason they want

to stay hidden. Number two, they're not very good at it. And number three, the, you know, there's the, we should trust or we should, we should have credence in the views of all these people who claim that the only way that they can explain what they've seen is UFOs. That's one theory. That's one possible hypothesis. The other is people make mistakes. People are wrong. People see things. They misinterpret them. They make videos of them. There are all sorts of errors in the videos, whether from lens flares or, you know, optical illusions or whatever. And there are no aliens visiting us. In my mind, given what we currently know,

including all the unclassified videos, blah, blah, blah, blah, it is overwhelmingly more likely that aliens have not been visiting us. Because if aliens had not been visiting us, I would still expect some people to claim that they were. I mean, that's what people do. People claim all sorts of crazy things. I would still expect that people would take sort of very vague, sort of interpretable in various ways, pieces of evidence and spin them. I would also expect that that people just lie and fabricate, right? Because it's kind of cool. And some people can even run industries, you know,

get paid off of this kind of belief. Whereas, if the aliens really existed and were visiting us, I don't think they would hide at all. They would just say hi with overwhelming probability. So given the current evidence, I have very, very close to zero credence that aliens are visiting us. But if you want to say,

is there anything that could convince me? Sure. The aliens just got to come down and introduce themselves. It's not that hard. Like, the amount of mental gymnastics you have to go through to imagine the aliens are visiting us, but we don't really know it very well. Seems to me to be a little bit implausible. Okay, Peter Gerdes says, when you argue for the Bourne Rule in terms of what would be rational to expect, what's the connection between what's rational to expect and what we've actually observed?

In other words, do you take your analysis to explain why it's unsurprising we seem to actually seem to have evidence that validates the born rule, or does it only give us reason to place forward-looking vets in accordance with the born rule? I don't know if there's some subtext here, but the answer is no. It goes both ways. I think that all of the reasoning that says you should place credences on what branch of the weight function you're on according to the born rule works both forward and backward in time. I'm not sure why it shouldn't. So, and that, I mean, we have a whole paper, I mean, the whole section in the paper that I wrote with Chip Sibbins on deriving the Bourne Rule on theory confirmation. You know, how is it that you can sort of simultaneously confirm the specific ideas you have about the wave function of a certain system and the broader framework in which you're working in which the Bourne Rule gives the right statistics?

But it's pretty straightforward. It's just Bayesian reasoning. It works. It works both directions of time. No problem. Dave Stern says, I remember you saying that it is easier to do some calculations assuming many worlds than using more traditional methods of calculation. Is my memory correct? I guess that depends what you mean by the word calculations. I don't think it's necessarily easier to do calculations assuming many worlds, but it depends on exactly what you're trying to calculate. There's two things that I definitely have said. One is that it is easier to conceptually understand some purported paradoxes of quantum mechanics. from a many world's perspective or some puzzles or some weird features of quantum mechanics. For example, best example would be the delayed choice quantum eraser, which doesn't seem at all mysterious to someone who believes in many worlds.

And slightly relatedly but slightly differently, I've also said that there are legitimate puzzles in theoretical physics, like quantum gravity, for example, which you might have a new and productive angle on if you believe in many worlds, versus other theories of quantum gravity. So it's less about doing calculations than about understanding, I would say. Matt Barber says, I just went through the something deeply hidden audiobook. I have a question about the Alice and Bob entanglement thought experiment,

where Bob flies to Alpha Centauri, Alice measures spin up, and then Bob measures spin up because of entanglement, action at a distance, et cetera. What I'm used to hearing from popular explanation of relativity is that the notion of before and after depends on the frame of reference. So is there a fact of the matter

whether Alice actually measures spin-up before Bob. No, there's not. You're exactly right. That's what relativity says. Relativity says that what happens before and what happens after for two events that are separated by a large distance in space will be different, can be different if they're space like separated, is the technical term that we say. Can be different.

Can depend on the frame of reference or more generally the coordinate system that someone uses in space time. So the rules of quantum mechanics have to be that if you have spins entangled, two spins entangled, in the sense that they're both spin up or both spin down, let's say, then it doesn't matter who measures first. There's just one fact, one prediction of quantum mechanics, that they're both going to measure the same spin. And that's true whether Bob measures first or whether Alice does.

Jacob Schneider says, Is creating emergent theories too easy? I feel like Hilbert space is so large that it becomes almost infinitely flexible, Like maybe it's possible to create any emerging classical theory you want as long as you make the right definitions in coarse graining or at least to approximate any theory that you want. No, I think it's exactly the opposite, to be honest. I think that it's miraculous that immersion theories exist at all. It's not at all easy to make them because remember what you need for a theory to be properly emergent is an ability to make somewhat reliable predictions using that theory on the basis of a very, very, very tiny amount of the initial data. for the actual system.

So when you have the air in the room and you say, well, really, we know it's a bunch of atoms and molecules, and every atom and molecule has a position in a velocity, but what we're going to do is give you just the fluid dynamical degrees of freedom. So, you know, maybe in every cubic millimeter

of the room, I'll tell you the temperature and velocity and pressure and things like that. So that temperature velocity pressure data is way, way, way less data than giving you all the positions in bowl, velocities of all the atoms, but it suffices to make really, really accurate predictions for what

the air in the room is going to do. That is a highly non-generic situation. The example I've used very often before is what you did in that sort of emergence is you averaged over what was going on in every cubic centimeter of space. You could also take averages, for example, in momentum space, right, invent a quantity, which is the average position for all the molecules with a certain momentum, rather than the average momentum for all molecules with a certain position. So you get a whole bunch of data, and from it you can do nothing. You can't make any predictions at all. And that's the much more generic situation.

If you don't have some structure in your theory that allows emergence to happen, you generally need to know almost all of the initial data to make precise predictions about what's going to happen next. So I would not say that creating emergent theories is easy at all. Joshua Hillerop says, You've said before that many worlds is just the wave function of the universe evolving as per the Schrodinger equation. Do you actually mean the non-relativistic Schrodinger equation here, or are using that to mean whatever method best describes quantum physics? So I encourage you to read my book, Joshua, something deeply hidden.

The Schrodinger equation is not the same as the non-relativistic Schrodinger equation, which kind of should be obvious, since otherwise you wouldn't need something called the non-relativistic Schrodinger equation. The existence of something called the non-relativistic Schrodinger equation implies the existence of a more general Schrodinger equation. So when I say the Schrodinger equation, I mean the most general form of it, which physicists would write as H-Sai equals I, D by D-T-T-Sye,

where si is the wave function, H is what's called the Hamiltonian operator, and D-by-D-T is the time derivative, and I is a square root of minus 1. So that equation, that general form of the Schrodinger equation, completely true in any version of quantum mechanics. Doesn't matter whether it's relativistic, non-relativistic, whether it's the standard model or QED,

or loop quantum gravity, or super string theory, or whatever. This is the fundamental equation of all forms of quantum mechanics. Robert Ruxendreskew says, I'm wondering what's your take on the ontology of the universe. My own opinion is that as you get deeper on the scale of reductionism, what you end up with is the abstract laws of nature that are immutable and exist without the need of a creator, since you can't really grasp them and destroy them.

They don't have a choice but to exist. Therefore, I feel like the ontology of the universe might be math itself like Max Teigmark is proposing. Well, I think this is going to sound like a cop-out maybe, but I think that the universe, and what I call a reality realist, the universe is what exists. And there's this natural tendency that people have

to say, okay, but what is the universe? and the response to that would be, you know, what could possibly qualify as an answer to that question? The universe is not an example of a set that we're otherwise familiar with outside our experience of the universe, right? It's like, is Fred a mathematician or a physicist? Well, we know what mathematicians are and physicists are. We've met more than one of them before, and we can ask what Fred does. But the universe is not like that. It's kind of unique by construction.

So I think that you can describe the universe mathematically, and I think as we'll get to in a later question, I think the best current description we have of it is as a vector in Hilbert space. That is the representation of the universe. But as for what the universe is, it's the universe. There's really nothing else that is a completely adequate way to describe it. Nicholas Walker says, do you think the dark matter and or energy will be discovered at the quantum level? I'm not sure what that means discovered at the quantum level. You know, I think that dark energy is overwhelmingly likely to be the cosmological constant. I think we know what it is to a pretty good approximation with 90% confidence, let's say.

Dark matter is probably some kind of particle. So I'm not sure what do you mean at the quantum level. The only level is the quantum level in some sense. All of the laws of nature are fundamentally quantum mechanical. But I don't think that. So usually, let's back up a little bit. Usually when we say the quantum level, we are contrasting it with a classical description. The quantum description is more comprehensive.

We never have to use the classical description, but there might be a set of a set of conditions under which a classical description is a good approximation. So everything is at the quantum level, but in everyday language, we use the words quantum level to mean outside of the domain of applicability of the classical description. I don't see any reason why dark matter or dark energy need to be outside the domain of applicability of the classical description. Tyler Whitmer says, if you weren't allowed to have cats, would you choose another animal as a pet, and if so what? I grew up with all sorts of pets, you know, with dogs and with birds and fish and a whole bunch of things. I think that dogs and cats are the only animals that I really enjoy having as pets.

You know, they're a little bit smarter. They have more personality. You know, you can play with them in a different way. But dogs are too much responsibility, honestly. Like, you have to take them for walks all the time. Cats have a litter box, as fans of pretty litter, a former advertiser on the podcast will know. So, and I like the cats personality-wise also.

So I think, although it's hard to say, but I think my choices would be I would have cats or I would just go pet-free entirely. Jose Ignacio Alcantara says, Do you think the truth and beauty are related with regards to mathematical, descriptions of the physical world. If it came to a crunch, which would you choose? I mean, obviously, you got to choose truth over beauty. Like, I would much rather believe in ugly truth than a beautiful

falsehood. But they seem to be related. I mean, in our historical experience, as we discover more and more about the world, the descriptions that we cook up for those things we discover, most often work out to be beautiful when viewed from a certain light. And I think that that's, you know, a very reasonable thing to say, a very truthful thing to say about our search for truth so far. So the way that I think about it is imagining that future, as yet untested, physical theories will also be beautiful, is a good starting point. It has worked in the past. So by all means, use it to make guesses about the future. But of course, you choose the ones that fit the data better.

Beauty is a guide to searching for physical theories, but it's not a guide to choosing between theories, some of which fit the data and some of which don't. Victor Alejandro Beiner or Weiner says, I keep hearing about research on room temperature superconductivity. This has to date been unsuccessful. The whole enterprise seems to me as impossible as perpetual motion or alchemy.

Doesn't the notion of acquiring room temperature superconductivity violate the second law of thermodynamics, or is it a linguistic issue, and in reality, they're looking for near room temperature or near superconductivity. So I'm not sure what is bothering you here, Victor. You know, I could imagine a principled belief

that superconductivity could only happen at zero temperature, but we've known for a long time that that's not true. There are temperatures above zero where things are superconducting, and it's not nearly superconducting. In fact, this is one of the features of superconductivity, that basically it's on or off, right? It's not like you creep up on it.

There's a phase transition, and you are now superconducting versus you're not. And if that seems weird to you, right, like why don't you creep up on superconductivity? You know, a metaphor that may or may not be useful, analogies, you know, they're always a little bit dangerous to use. But imagine, you know, you're packing little cubes into a box which fits exactly a certain number of cubes, right?

And so there's going to be one way to pack them. Forget about the interchanging individual cubes, but you better line them up, you know, facing the flat side of each other because you don't have any other room to fit the cubes in, right? So if you have one cube that is at an angle or something like that, then you're not going to fit all the cubes in. So the question of, do you fit all the cubes in the box or not,

is a yes or no question. And superconductivity is kind of like that. There's a phase transition into a state where there's zero resistance as things move down. And if you are willing to believe that that's possible at 10 degrees Kelvin, then it's just an engineering problem to say, well, can we do it at 200 or 300 Kelvin, right? I mean, there's no phase transition there. There's no big difference between those two things. It's really kind of a matter of degree.

And of course, I'm not quite sure how much you've been following the news, but they have achieved room temperature superconductivity. It is unfortunately not at room pressure. You have to put incredible pressure, like squeeze diamonds together on this material to make it superconductor room temperature, but they can do it. It's not going to change the world technologically anytime soon, but it could happen down the line. Humberto Nani says, are Laplace's conservation of information and quantum mechanics's unitarity meant to traverse the Big Bang event? We don't know. We don't know what happened at the Big Bang. We don't know if there was anything traversing anything at the Big Bang event, the Big Bang event.

Big Bang could be the beginning of time. It could be a moment through which, yes, there is unitary evolution and conservation of information, or it could just be like a fuzzy approximation sitting as a placeholder for our future knowledge that isn't there yet. So I'm very willing to believe and explore personally the idea that quantum mechanics, that there is unitary evolution through the Big Bang, but it's certainly something that we don't know yet, and it's not even clear what it would mean to say it's meant to traverse. is that. We just don't know.

Andrew says, what's your process for preparing for interviews? You're always so up to speed, not only in each guest's work, but also the context of their work, even in disciplines outside your own. You know, it depends on the interview, obviously. Like when I have Adam Reese on the podcast talking about cosmology, I don't do any reparation whatsoever, roughly speaking. Whereas if I have a historian or an economist, I'm going to have to work harder than that. So, you know, part of the inspiration for starting the podcast in the first place was that I had a long pile of books, tall pile of books by my bed that I wanted to read and kept buying and couldn't find the time to read.

So I kind of nudge myself into reading some of these books by having their authors appear as guests on the podcast. Now, I'd be very, very honest. I do not always read every word of every book, right? You know, sometimes you've got to skim. Sometimes time is short. It's very useful to just look at talks that people have given. already or articles they've written or whatever. So yeah, it's different. But I do, you know, it's actually a good, it's an interesting philosophical issue, how prepared you want to be for a podcast. You know, I definitely, I think in early days sometimes made the mistake of being overprepared. Like I had a view in my mind of exactly how the conversation was going to go. And maybe I still do that to some extent. But I'm trying to do that less and less because I think that

you really should be responding to what the person is actually saying, right? And my role as the interviewer is to play the role of someone who hasn't read the book, right? So you need to read the book to know enough about the field to ask some questions, but you need to know not too much so you don't try to guide the answers very much either. Hi, my name is Lisa, and I'm a licensed psychotherapist, which means my work doesn't magically end when the session does. There are notes to write, appointments to manage, billing, insurance follow-ups, and somehow all that admin used to creep into my nights and weekends.

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It's Toyota Electric. We make it easy. Toyota, let's go places. Chris M says, do you have a favorite poker hand that you have played, be it a winning or losing hand, is there one that sort of stands out for you for one reason or another? Ah, I have, you know, I am not very good at remembering this kind of thing. You know, in general, my memory is not very good. And I think this is in part, maybe this is a two-glyb explanation, but I'm a forward-thinking person.

So I care more about the future than the past. So I'm terrible at remembering all sorts of things, plots of novels or movies and so forth, and poker hands include it. So the really good poker players will tell you, you know, every hand in a tournament that they played five years ago. And I'm lucky if I remember many at all. There are some that stick out, you know, for both positive and negative reasons. Like one that I will never forget. I was in Caesar's Palace in Las Vegas.

And I had just busted out of a tournament. So I figured I would play a ring game, as it's called. So, you know, for those of you who are not poker players, there's tournaments. where a lot of people come in and gradually you eliminate until you get a winner. And there's ring games where, you know, nine or ten people sit at a table and just play against each other. People come in and out as spaces open up. So I entered a ring game. You know, I bought, I forget what the buy-in was, a couple hundred dollars.

I sat down and the very first hand, I got pocket queens, which is an excellent hand to get. And I raised, and I got re-raised, and I re-raised again. and we were all in before the flop came, before the extra cards came. And I ended up getting, I think it was three of a kind, another queen came down, and the other guy got a straight. So literally on my first hand,

I lost all my money on a very good hand that I had. So again, I'm not remembering this because I either played it well or badly. It sort of played itself. That's another thing the poker players sometimes say. Like you didn't have a lot of choices. If you have a hand that good, you're going to play it, and then sometimes the cards are against you.

I have done other things that were good. I've had some success, but I don't really remember, you know, big hands in the biggest successful tournaments I've done. It's more like just not making mistakes is more important than doing brilliant hands. I do once have a happy event where, I was in a hand. It was a ring game.

It was not a tournament. And it was one of those speculative hands where, like, you know, suited connectors. So like five of diamonds, six of diamonds. And I had not been doing that well. And two other players just kept betting against each other. I just kept calling.

And I made a flush on the last card on the river. And they just kept betting. And my flush was completely disguised because only one other diamond had fallen on the flop, and I sort of got two more. And when I won the hand, both of them were outraged. They're like, oh, man, we thought you were a good player. Why were you hanging in there with such a crappy hand?

But because there were two other people in there, and because even though my hand was intrinsically crappy, there were many ways for it to improve, I still had the odds to stay in the whole way along. So it's a special pleasure you get when you do the right thing, but the other people at the table think you've done the real. wrong thing, and you come away with all the money. Samuel Berry says, you are both an actively researching professor and do so much work

writing about and communicating the ideas that you love. Do you have any advice for young scientists who would love to get more involved in science writing, but aren't sure how to both be a practicing scientist and a writer-educator? Well, the simple advice is if you want to do that, you have to prioritize being a practicing scientist first and foremost. My short advice would be do very little outside outreach until you have tenure. If, if, and it's a big if, if being a tenured professor is the most important thing to you. Because being tenured professor is hard.

I'm not, by the way, right? So I'm not a good role model here. But I'm a research professor at Caltech. So it counts as senior faculty, but not technically tenured. But the point is, getting a tenure professor job is hard. Along the way, there are many bottlenecks, right? There are many more people who want to be graduate students than can be. There are many more grad students who want to be postdocs than can be.

Many more people who are postdocs who want to be faculty members who can be. Many more people who are faculty members want to get tenure than can be, okay? So they're at every step looking for reasons to reject you, to not hire you, to not promote you. And the fact that you put a lot of effort into something that is not research is one of those ways, One of those reasons that they will cease upon to not promote you or hire you. That is not to say it can't be done, right? Of course, there are wonderful examples of people who have been very, very successful at both. But almost always, they get tenure first and they start writing.

People like, you know, Stephen Hawking or Lisa Randall or Stephen Weinberg, these people were famous physicists before they started having a big public profile. So if you're very, very, very, very very. dedicated to the idea of writing and communicating about the ideas you love, to the extent that you are willing to decrease your chances of becoming a tenure professor, then do that. But do it with open eyes. That's my only bit of advice. There's no certainties in this game. You know, if you're a genius, if you're revolutionizing the field, then you can do whatever you want. They will still hire you. But for most of us, we have to maximize our chances. Shion says, I was wondering

if you do an episode with my friend and former boss Wei G. Ma, Professor of Psychology and neuroscience at NYU. Maybe, you know, happy to take suggestions. Thanks for the suggestion. Eric Skoglund says, do you have a top five philosophy books everyone interested in it should read? So let me give it some preliminary waffling answer here first. I'm not that fond of this kind of question. I think I'm going to get it again later in the in the Q&A, like top five or, you know, biggest best of list or whatever. That's just not how I think about these things. You know, my own education, whether it's physics or philosophy,

doesn't come from a top set of books. It comes from all over the place. I'm very, you know, haphazard and grab bits of information here and there kind of person. And like I said before, my memory is not that good. So it's very possible I read a book and it was very influential on me and I've forgotten that I read it. But having said that, that's all the preliminary throat clearing. You know, I'm not, sorry, that's not all the preliminary throat clearing. Let me do some more preliminary throat clearing.

There was a recent kerfuffle on philosophy Twitter about the importance of reading the great works of philosophy in the original, right? Primary sources rather than secondary sources. Do you need to read Kant and Hegel and so forth? or can you get the best of their ideas from secondary sources? And it was a legitimate conversation because there's one side that says, look, you have to be able to read the primary sources, otherwise you're not a real philosopher. Otherwise, you're not engaging with the best opinions and ideas that we have in their original form.

There's another side, which I guess I'm more sympathetic to as a scientist, which says if what you care about are the ideas, then those ideas should be expressable by people other than the people who came up with them. Indeed, in science, generally, and certainly in physics,

it's almost always the case that we get better and better at expressing ideas as we understand them more and more. So reading, like, nobody in physics reads Galilei or Newton or Maxwell, right? That's just, if you have an interest in the history of physics,

you can do that, but no, working physicist thinks that that's their job. Whereas in philosophy, is very much the other side, all of which is to say, my favorite philosophy books are not the classics, right? I mean, I like some of them, you know, David Hume, I like, Lucretius, you know, Aristotle is always interesting to read for very different reasons. People like Nietzsche are interesting to read, but I've never really read Kant or Hegel, as those two examples would be. Plato, of course, is a lot of fun to read.

But most of my philosophical education for what it's worth comes from more modern sources, one way or the other. In that world, more modern sources, again, it depends on do you want a work of original philosophy, or do you want something more pedagogical? You know, in what I do, a book like David Albert's Time and Chance is what I would consider to be a classic. David Albert, former podcast guest where we talked about quantum mechanics, but he also, indeed, probably his most important contribution is in thermodynamics and the arrow of time. And his book, Time and Chance is the classic statement of that. It's the kind of book where 20 years later you have a series of essays devoted to the legacy of time and chance, right? Which I know because I just contributed to such a volume.

And David, of course, if you listen to the podcast, he has a very particular specific way of speaking that some people love and some people can't stand. and I'm here to tell you that his way of speaking is 100% carried over to his writing. It sounds exactly like him speaking. And so if you like that, you will love it. And if you don't, you won't. On the more pedagogical side of things, I've recently read a wonderful book by David Papineau, who is a philosopher, I think King's College London. He's in London anyway, called Philosophical Devices.

And if you're interested in sort of modern philosophy, in a digestible form. I can't do better than to recommend philosophical devices because basically he talks about the big ideas in analytical philosophy, analytic philosophy,

in a very clear but very, very concise form. So he talks about set theory and numbers, but he also talks about probability, and he talks about things like causation and necessity

and reference and naming, and all the big ideas in modern analytic philosophy. hopefully, maybe there's a similar book in continental philosophy, but I wouldn't know specifically what. Daniel Fox says, given that predation is sort of baked into evolution

as a more efficient means of creating genetic diversity and energetic organisms, do you think that perhaps with the advent of advanced gene editing, we can slowly move away from the aggression and violence that predation engenders? Well, we can. Yeah, if your question is, can we? which it technically was, then yes, probably we could.

You know, we could mess with people's with the genome. Remember, this is what everyone who comes on the show, I've had people come on the show like Fyodor Ornove or Karl Zimmer who thought about these things. They're all very much against what is called germline editing, right? So fixing or improving, if you want, parts of the human genome to be passed on to future generations, as opposed to just, you know, removing diseases or something like that.

that. But it might happen, and I think, you know, as Ornov said, probably will happen. Whether we like it or not, it's just going to become too easy to do it. So my thought, if the hidden question that you're asking is, are we going to do this? I think that that's not the right way to think about it. I think that it's going to become common to edit people's genomes, and different people will do it in different ways. Some people will edit so the people who are born are more aggressive and violent. Some will do it who are less. And natural selection will still play its games, still work its wheels, turn the wheels of selection on those people, whoever they are. So I do think that it's really going to have a huge impact on the future of humanity. That much I'm clear, but I think it's very, very difficult to predict exactly what the direction of that effect will be. Steve Pelling says it could be a bit of a stupid question, but could there maybe be some, link between ADS-C-F-T, the holographic principle, and brain theory, B-R-A-N-E. All of these seem to share a common theme of reality being constructed in a space based on a

fundamentally different number of spatial dimensions. So yes, that is not a stupid question. In fact, the answer is clearly yes. There's 100% a link between ADS-C-F-T, the holographic principle, and brain theory. And the very brief history of it is the holographic principle came first, late 80s, early 90s, maybe early 90s, it tuft and suskind from different ways of thinking about black holes

suggested the holographic principle that we could think about the information that encodes what's happening in a black hole as being distributed on its two-dimensional event horizon rather than its three-dimensional volume. Then brain theory came along.

People like Jobolczynski and others realized that there were these things called D-Brains and you could be stuck on the brain, right? You know, the fields that make up the standard model or whatever, some other model you were just inventing, would be confined to a brain, even though it was embedded in extra dimensions. And then the ADSC of T correspondence

came about because Juan Maldesana was thinking about stacks of brains that sort of created a black hole. And he realized that if you go to, if you do this in a certain way, you stack a bunch of brains on top of each other, and you go take a certain limit of a certain region of the resulting space time, it looks like anti-de-sitter space. And so he invented. the ADS-CFT correspondence on the basis of using brain theory. And then people realized, in fact, Ed Witten made it very, very super explicit, that this was obviously the holographic principle at work. Witten's famous paper right after Maldesanis is called ADS anti-desider space and holography.

I mean, Maldesanist paper has the worst title in the history of titles. It's something like the large end limit of N equals four supergravity or something like that. I'm not going to remember exactly. what it was called, but it gives you no clue that it's about ADS-C-F-T in the modern way of thinking about things. And it went on to be the most highly cited paper of all time, so that tells you how important it is to get a good title, namely, maybe not at all if you have a brilliant idea. Stefan Lyon says, at age 25, I find myself having a quarter-life crisis about the Faustian bargain of working in corporate America. Do you think that there are opportunities in the academic

research side of physics for people who do not have graduate degrees in physics, but bring separate technical skill sets like data modeling and visualization to the table. Well, there absolutely are, but you know, you have to think carefully about what that would mean. So in theoretical physics, in the part that I know about and do myself, there aren't, roughly speaking. In order to do the kind of theoretical physics that I like doing, you need essentially a graduate level education in the very specific field that you're working in. I mean, 25, you're not dead yet.

You still have time. You could get a graduate level education in theoretical physics or anything else if you wanted to. But theoretical physicists don't need big teams. They don't need a lot of data modeling or visualization. They just need pencil and paper. Now, there are, on the other hand, experimental physicists and observational astrophysicists that work on very big data sets, on huge experiments. And there, they have not only a bigger team, but the team is not all PhD physicists. There's plenty of other people who work on those experiments.

And so in that kind of thing, in places where you have big data involved, you can absolutely make a contribution to physics, astrophysics, bio physics, kinesmatter physics, particle physics, gravitational wave physics, all of those things are definitely ways that you could in principle make a career out of doing that. Beyond that, in more specifics, I'm not the person to ask because I don't do it myself. But yes, those opportunities are there. Gabor Peter Sir says,

These days there is a loud group of anti-vaxxers, pandemic skeptics, fat earthers, et cetera. According to you, what is the best way to strengthen trust in science? Short answer is I'm not sure. I don't have a simple plan to put forward in that direction. I have opinions, though, which I tend to do. I think that a lack of trust in science

usually doesn't have that much to do with science. It usually has a lot more to do with trust. I think that it's one aspect of a lack of trust in elite institutions and opinions. And science is one of those, but it's not the only one. And I think that, so the question to ask is, why don't people trust institutions more generally than that, right?

What leads people to conspiracy theories and denial of scientific consensus and things like that. And, you know, that's a good question, and I don't know the answer. I mean, economics has something to do with it, political power has something to do with it, the world is changing, very fast and scary

has something to do with it. Values in our educational system have something to do with it. So I don't think there is a simple answer, but, you know, we can all do our little part to make science seem more trustworthy. Justin Bailey says, would you describe a paradox that would occur

if you could receive information faster than light? I can see how I would have knowledge of the future, but I don't see how I could create a paradox, such as preventing my grandparents, from meeting or similar. Well, I think, you know, the thing you're referring to is the idea that if you can send information faster than light, then according to the rules of relativity, that is equivalent to being able to send it backward in time.

There's no simple division of the universe into past, present, and future. There's past, future, and space-like separated. So if you can move in space-like ways, then you can travel into the future just as well as into the past. So if that's true, it's not just that you can receive information from the future,

you can send information to the past. So you can send information to your grandmother saying, don't marry that guy. And if for whatever reason you're convincing, then you could cause a paradox. I'm not sure with that's what you're asking, but I think that's all I can come up with

for an answer. Doug C says, do we all travel through space time at the speed of light? And if so, can you explain this in an intuitive way? No, I don't think that's right. I don't think we travel through space time

at the speed of light. I need to give a nitpicky answer to this. When you talk about traveling and speeds, you're specifically already talking about travel through space, not through space time, right? What is a speed? A certain number of kilometers per hour

or kilometers per second or whatever, right? So that means, just by the definition of what a speed is, It means a certain amount of space that you traverse in a certain amount of time. And in a certain reference frame, or with respect to some standard of observation, you can measure your speed. You're not moving. You're moving at 1% the speed of light. You're moving at the speed of light.

Whatever. There's no such thing as the speed through which you travel through space time. Okay? It's just not a sensible concept. The closest you could come is to ask how much time is elapsed or what, is the space time interval that is traversed along some path that you might take through space time. And because we all move slower than the speed of light through space, the space time interval that we traverse will be time-like. It will be measured in units of time, not of space.

So really you're asking how much time passes as we move through space time. And the answer is always one second per second, which is not the speed of light, right? The speed of light is one light year per year or one light second per second because that is literally a speed through space. There's no such thing as a speed through time. And if there was, it would be one second per second, which would be one, which is just silly. Adam Baz says, do you see any issues with the treatment of the general concepts of religion as an emergent phenomenon in the sense that they are real, but only to the degree that they allow humans to talk about or make predictions about their behavior and orient themselves in ways that are evolutionarily favorable over long time spans? I don't think that's the right way of thinking about it, to be honest. Like, if you think about, I'm not sure what you mean by the general concepts of religion.

Let's say the existence of God or the idea that Jesus died for our sins or that there's an afterlife, okay? To me, these are factual claims about reality. They could be true or they could be false. They don't explain anything unless they are actually existent. It's not like there is a difference between fundamental and emergent. But both fundamental things and emergent things are real. But there's also a distinction between real and false, truly existing and real and not existing, right?

Unicorns, in the usual way of thinking about things, are not real. They're not emergent. They just don't exist. And to me, the concepts of religion are likewise like that. Now, you can say, but wait a minute, unicorns are used in part of our explanation for certain human behaviors, you know, like putting up stickers of unicorns on our lunchboxes. That doesn't make them real. That makes the concept of a unicorn real, but doesn't make unicorns real.

So the concept of an afterlife or God or Jesus or whatever, the concepts are all real in the sense that there exist people who have those concepts in their heads, and you absolutely need to take account of the existence of those concepts to make sense of their behavior, right? I did this because I think that Jesus would have liked me to do it. That is no bearing whatsoever on whether Jesus actually was real or Jesus is really the Son of God or anything like that. You have to distinguish between those two things. John Bach says, what is your take on model-dependent realism as described by Hawking and Bodnob in the grand design? Is this a popular view among physicists?

You know, I don't think, so for those of you who haven't read, Stephen Hawking and Leonard Maudnav had this idea in their book, The Grand Design, called Model-dependent realism, which, you know, says, well, what we call really existing depends on the model we use to describe the world. So, for example, I'm not completely sure this is an example, because I'm not completely sure that I understand what they meant. In ADS-C-F-T, right, when you have two different quantum mechanical theories, one is a field theory in four dimensions, and the other is a theory of quantum gravity in five dimensions. And you say, but they're really the same theory. They really describe the same physical situation.

So if you ask what is real, is it really the field theory, is it really the gravity theory? There's no answer to that. Either one is perfectly okay. So I think that the basic idea of this is pretty amenable, pretty acceptable to most physicists who think about foundational physics, you know, quantum field theory, space time, kind of things like that. I don't, you know, philosophers have put a lot of work into this, and Halking Mladenov did not read any of them or anything like that. So the idea of model dependent realism has some aspects of truth to it. It's not a very sophisticated way of asking these questions, to be honest, which doesn't mean it's wrong, right? I mean, it can be mostly true, but a more sophisticated version of it would sort of fix up the ways in which it might fail,

you know, make a little bit more precise and rigorous and stuff like that. So I would not turn to the grand design for my best way of thinking about what is real and what is not real. Let's put it that way. Paul Cooper says, how do physicists know that the Hubble expansion and black hole event horizons and so forth are the same for all observers? As I understand it, time moves more slowly as you go more deeply into the well of a gravitational field. A clock on the surface of the Earth runs slow relative to a clock in orbit. And there's more to that. I'm just condensing the question a little bit. Yeah, and I think we've answered questions like this before, or at least I have, maybe not in AMA form.

But, you know, there's very, very often in a set of assumptions that physicists are making when they talk about things in cosmology or in relativity that they don't always make explicit. So when we say something like the age of the universe, right, the universe is approximately 13.8 billion years old since the Big Bang. And people always want to say, well, but wait a minute, relativity tells me that the elapsed time depends on how I move through the universe. And the answer is, sure, it does. And in fact, when we say the universe is approximately 14 billion years old, we mean as measured in the reference frame of someone who is essentially stationary with respect to galaxies or the microwave background. There is a reference frame being picked out.

The Hubble expansion rate is a real physical thing, you know. So I'm not exactly sure which of these questions you're asking. So I'm going to give a whole bunch of different answers. The Hubble expansion rate is a real physical thing. thing. The light we receive from distinct galaxies is observed at a different wavelength than it was emitted because then we know what it was emitted at because that's a question of atomic physics, not of cosmology. So there is something that is universal called the Hubble expansion rate. There is something that is universal called the Age of the Universe. These are maybe measured

with respect to some universally understood reference frame, but they're really things. Likewise, the black hole event horizon. A black hole event horizon is a point of no return. If you're inside the event horizon, you cannot escape to the outside world. If you're outside, you can. That is absolutely true for all observers. Anonymous says, suppose I understand functions, RN, S. S.3, 1, you know, a bunch of mathematical things, but not tensors or differential geometry.

Do I have the tools to understand the mathematical structures of a gauge theory in Minkowski space? I could understand 4D space time calling boosts the global phase shifts equivalent to something like R4, Modulo, the Lorenz Group or something like that. But that doesn't seem local or gaugy enough. Well, so again, I struggle to know exactly what it is you mean. If you understand that much math, then you can pick up the other math you need. You know, you absolutely need tensors to understand the conventional presentations of gauge theories in Mkowski space. Yes. Why? Well, forget about, you know, fancy things like the strong interactions QCD or whatever. Electromagnetism. Electromagnetism is invented by Maxwell back in the 1850s. Actually, when did he put the finishing touches? I don't know.

But the modern way we understand electromagnetism takes the electric field and the magnetic field and combines them in something called the electromagnetic field strength tensor, F. Mu Nu. So you need to know a little bit about tensors to do that. But so what? It's easy to pick up. It's really not that hard. If you know these other things, you can just learn what you need to know. So I think that a lot of people who haven't yet studied advanced physics have this idea. They need to learn the math first, and then they'll have no trouble just going through the physics. But almost everyone who understands these things just picks up the math as they need it, because the purpose is different. If you're a mathematician, you're learning the math in order to prove theorems. But physicists don't care about proving theorems. They just want to know what's true. which, you know, I'm joking about it, but is way easier than proving theorems. So the kinds of math you need to understand gauge theories or particle physics, you can absolutely pick up as you learn them, and almost any good field theory textbook will teach them to you. Wes Clyburn says,

if any, what novels have you reread the most during your life? So probably, I was trying to think about that, caveats about memory taken and read onto the record here. Probably the ones I've read most of my life are the ones that I started reading a long time ago, right, when I was a kid, when I first read when I was young, when I was reading mostly science fiction. And so it's probably either going to be Robert Heinlein's The Moon is a Harsh Mistress or Rogers Zelazine's Lord of Light. I think that these are clearly two books that I've read multiple times, many times. slightly later in life, I stumbled across Pride and Prejudice by Jane Austen, and I've read that many times since then,

so it's possible that Pride and Prejudice has taken the lead, but certainly it started out behind. Very different, obviously, pride and prejudice than Lord of Light or Muzza' Harsh Mistress, but they all have their charms in different ways. Simon Carter says, I recently read about David Deutsch and his theory about Shor's algorithm, seemingly proving many worlds, by the fact that it can factor a number greater than the amount of atoms in our universe.

What is your take on this? You know, I think it's clearly true that nothing about quantum computation proves many worlds. Why? What is the proof? Well, because you can derive all the results of quantum computation without many worlds being true. The results of quantum computation are just as true in pilot wave theories or dynamical collapse theories or epistemic theories as they are in many worlds. I think that the quantum computers are an example of something, as we mentioned earlier in the AMA, maybe you can make the case that they make more sense from a many world's perspective, but calculationally you get the same answer, no matter what choice you're using.

Bill Goss says, I'm 67 years old. Let's assume I'm lucid for 25 more years. Which, if any of the following do you think are likely in that time period? A, confirmation of alien life elsewhere in the solar system. B, experimental proof of the source of dark matter. C, an apparently credible theory of quantum gravity. Yeah, these are all interesting questions because you could make a case either way for the next 25 years.

It's a reasonable time scale. You know, I mean, I think that if there exists life elsewhere in the solar system, I presume you mean not life from other planets that came, life from other star systems that came here, but like life on Mars or Europa or Titan or whatever, Enceladus, if it's there, I think there's a very good chance we'll find it in 25 years, but I don't know if it's there. That's something that I truly have no strong opinion about. Experimental proof of the source of dark matter, I would have said is the most promising, if you asked me 10 years ago, but, you know, we've been looking for it and haven't found it, which means that it's more likely now than then

that the dark matter is one of the things that is hard to find. Axions would be hard to find, but probably we could find them in 25 years, but it also raises the credence on some other candidate that is very, very difficult to find even in 25 years. So that is probably less likely now that it used to be. Empirically credible theory of quantum gravity. That is, I want to guess that one. It seems a little bit audacious, but they're all audacious.

and, you know, I think that we're getting some progress. I really do think that. We're making some progress in quantum gravity in ways that are a little bit more tangible than what we've seen in looking for dark matter. Of course, we're making great progress in looking for life in the solar system, but we don't know if it's there or not. We know that there is some theory of quantum gravity. So I'm going to guess that that's the most likely.

Philip Lachshus says, if there was no decoherence, how would conscious entities like humans perceive the world? Alternatively, what would the world look like to us if our consciousness emerges from a handful of elementary particles such that decoherence plays no significant role? I don't know the actual answer to this. I don't even have an intelligent answer to guess at, but let me point out that I am completely convinced

that consciousness cannot arise just from a small number of particles. What I don't know is whether or not you could get something you would call consciousness if you didn't have a classical limit. If the world were fundamentally quantum mechanical in a way that could never be helpfully described classically, I'm not sure if you could get what you and I would recognize as conscious behavior.

Maybe I would lean towards saying yes, but I don't know enough about how that would even work to say with great confidence one way or the other. Certainly it would be very, very, very different. Let's put it that way. Yochim says, can you summarize Jeremy England's thermodynamic argument for evolution of complex adaptive systems in a tweet? So again, short answer is no. In fact, I'm not even sure what precisely you're referring to.

Thermodynamic argument for evolution of complex adaptive systems. You know, in some sense, it's a little bit weird. You know, the ideas that Jeremy has had that have garnered the most public attention. are the ideas that, to me, are the most obviously true. Like, of course, entropy is increasing, and of course, nevertheless, complex structures arrives. We know that. It's pretty obvious that those complex structures make use of the entropy gradients. You know, Roger Penrose wrote about that in his book, and he's just a physicist. The biologists have certainly known this for a long time. What's interesting about Jeremy's

arguments is that he has some specific mechanisms by which structures can, you know, hold together either by absorbing energy or by emitting energy or fail to hold together and just reflect the energy and some other arguments about the efficiency of thermodynamic engines and things like that. So in my mind, you know, the important thing that he's done is the detailed work more than some sweeping tweet-length argument. Paul Hess says, what does the term geometrical degeneracy mean in the context of talking about the CMB and determining whether or not we lived in curved or flat space. It's just a way of saying that if all you measure, if I'm understanding it correctly,

if all you measure the CMB, just like if you measure any other single cosmontical observable, you're trying to determine many parameters from one data set, right? You're trying to constrain the Hubble constant and the geometry of space and the amount of dark energy and the amount of dark matter and all these different things. A degeneracy in that is, well, I can change. the geometry of space, but I could change other things like the Hubble constant and the dark energy to compensate so that the CMB would look the same. There's no guarantee, in other words, you can measure one thing like the CMB, and from that pin down all the cosmological observables

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Michael Will says, who do you think would win in a fight between Leonard Suskin and Lee Smollin? MMA rules. Not sure, never seen either one of them fight. I'm going to guess Lenny Susskind, just because Lee seems like such a soft-spoken guy, whereas Lenny seems like a little more rough and tumble by nature. But, you know, these are the kind of things you have to do the experiment. I don't know. Set it up.

Marcello Legory says, would you ever consider having an objectivist? I presume that's in the Enrandian sense, as a guest to discuss proper role of government. You know, you never know, but I go by actual people. You name some names, but I didn't recognize either one of those names. So, you know, when a good name comes along, who has some... something that I think would be interesting to hear about, then I will always be open to have them on the podcast. Jared Kusulish says,

if someone were starting their journey into physics, how would you recommend they start? Are there areas that would provide a high return on their investment in the short term with regard to understanding the world? Are there areas that, with enough effort, someone might be able to make a contribution to the field in a five to 10-year time frame? Well, it depends on where you're starting your journey from. You know, most working physicists spent four years as undergraduates and roughly five or six years as graduate students.

And while graduate students, they can make contributions to the field. So most working physicists start from a high school education and make contributions in a 10-year time frame. Yeah, there you go. I'm not quite sure where to start. I mean, there's a wonderful web page by Gerard de Tufth where he goes through the whole curriculum that you need. to be a theoretical physicist. In fact, it's a very, very ambitious version of the curriculum. I've often thought about, like, making a stripped down version.

But I think it's pretty basic stuff. You know, you start with classical mechanics, and then you do E&M and relativity and statistical mechanics, and quantum mechanics and field theory. You know, I think it's a pretty standardized curriculum you can find just about anywhere. Duncan Palmer says, after your podcast with Ned Hall,

you seem to be reconsidering your position along the humian-antihuman-humian spectrum. Have you resolved your position since that conversation? Not really. I'm still, you know, if you put a gun to my head, still pretty humian about things. I do think there's certain kinds of questions where if you're anti-humian, the answers seem more natural. But I'm not convinced that those are good questions, right? I mean, sometimes we can trick ourselves into asking questions that make sort of grammatical sense and seem to make sense and as an ordinary natural language thing, but we realize aren't good questions you should be asking about the world. As I said before, last month, I'm thinking about the philosophy of mathematics right now. So that is forcing me to really confront these issues. So ask me again in a couple months.

We'll see where I land. John Eastman says, could atoms be shrinking rather than space expanding? And then some more elaboration there. So sure, they could be. But you have to realize what it would require, right? I mean, if you watch my biggest ideas in the universe videos, I did a video on scale where I explain why atoms have the size they do. You know, it's not just a random number. It's set by other constants of nature, the mass of the electron, the charge of the electron, and the fine

structure constant in particular. And the fine structure constant, you know, can also be thought of as the speed of light plus the strength of electromagnetism, etc. So if you want to use a sort of system of measures in which atoms are shrinking rather than the universe is expanding, you have to maintain consistency with the fact that the sizes of atoms are set by particle physics. So what you need to do is assume that all those other constants of nature,

speed of light, fine structure, constant, etc., are also changing in such a way that the relative distances of spectral lines and stuff like that remains the same. So it's overwhelmingly more convenient to say that space is expanding than to say that atoms are shrinking. Will S says,

believe Laplace's demon can precisely calculate the future in the past, is that because you are many worlds adherent? Laplace's demon is a thought experiment. Laplace's demon doesn't exist. Just footnote, just so everyone knows that. So this is basically equivalent to saying, is the universe deterministic in the sense that, based on information about its current state, its future and past are both determined. My favorite theory of quantum mechanics is the many worlds interpretation, and in many worlds

the evolution is deterministic. So yes, in that sense. There are also other versions of quantum mechanics that are also deterministic, like pilot wave theories and so forth. But until we know for sure, you know, we should be a little bit open-minded about that, but my view is it's probably deterministic. Lothian 53 says,

what do you think of Roger Penrose's Eon concept? So Roger Penrose, as we discussed very briefly in my podcast within a while ago, he has what is called a conformal cyclic cosmology model. and in that model the universe expands for a long time, very, very, very long time, which he calls it eon, and then some magic happens, a miracle occurs, and the universe turns back into the Big Bang, and so then it cycles through eons after eons after eons.

I'm not a fan, honestly. I don't think it solves the arrow of time problem. I don't like that magic has to occur, that there's no physical mechanism with equations of motion that tells you how this transition happens, et cetera. So all in favor of clever people, pursuing ambitious new ideas, but that one is not very compelling to me. I think we can do better than that. Phil Aloria says, curious about your view on the anti-realist argument, the pessimistic

meta-induction of science. I believe based on past comments, you're a realist about science. You seem to think that physics is telling us something about the real world. Do you find the argument to be a good one? I'll be super duper honest. I've heard of this argument. I think Larry Loudon was a proponent of it. But I'm not super familiar with it, so I'll probably botch it. The idea, roughly as I recall, is that, you know, our past attempts at understanding the world, Newtonian mechanics, classical mechanics, Aristotle, Maxwell, whatever, have all eventually been superseded by better theories.

In other words, in some sense, they've been wrong, and therefore our present theories are probably wrong. But that might be a wrong remembrance of the argument, but to the extent that it's accurate, that's completely unconvincing to me, because those things. theories are not wrong. They're right in a certain regime of applicability, a certain domain, just like I expect our current theories to be right in a certain domain of applicability. And then that domain of applicability will come to an end and probably does not include all of reality, but it captures something true about reality. So I'm quite optimistic in that sense about realism.

Mike Briggs says, how much overlap is there between these three professions, astronomer, astrophysicist, and cosmologist? Well, you know, astronomer and astrophysicists are terms that are used more or less interchangeably. Sometimes some people try to draw a distinction between them. Astronomers are a little bit more observational, astrophysicists a little bit more theoretical. But, you know, there are departments, there are astronomy departments that turn out theorists and experimenters and observers, as well as physics department. So I kind of don't make a big deal out of the difference between those two words. Cosmology is a subfield within astrophysics.

And maybe you could say a subfield within physics, but definitely at the overlap of those two things. So cosmology is specifically the whole universe at once, right? Or parts of the universe that are big enough to be comparable to the size of the universe. So maybe galaxies or clusters of galaxies and above is cosmology. If you study star formation, then you certainly are an astronomer or astrophysicist, but you are not a cosmologist. David Lang says, what do you think of John Wheeler's geometro-dynamics and the idea that space-time is the only existent of the world? So I usually think of the word geometric dynamics as just a synonym for general relativity. Now, Wheeler did, in which case I think is great.

But Wheeler did, as you allude to, want to be more ambitious than that and imagine that things like elementary particles could be modeled by wormholes in space time. And I think that doesn't quite work. You know, we know better now. It's quantum field theory that is correct, not wormholes in space time. That's not to say there's not a role for wormholes in space time, both in the podcast with Neda Englehart and with Lenny Suskin. We talked about different ways in which wormholes might be relevant to the real world. And so it's one of those things where there might be something of the argument that is accurate and useful

or something of the picture, I guess, even though the literal idea that he's, had wasn't quite right. Kirk Briggs says, according to Hume's doctrine, the edifice of geometry is nothing but sophistry and illusion. As a humian, how do you reconcile or view geometry? So I don't think that's quite right, actually. You know, David Hume has this famous quote, again, I'm not going to get it exactly right, but he basically says, is a statement about abstract reasoning, like numbers and quantities? Or is it empirical? Like, do you actually go do experiments and view what you see out there in the world, either one of those two things is okay. Roughly, the first is math slash logic, and the second is science, right? And he says,

besides those two things, you're just whistling in the dark. You're just making stuff up, right? Then that's the sophistry and illusion. But geometry is part of math, right? I mean, geometry absolutely would not be labeled by David Hume as sophistry and illusion. There you go. Joy Colbeck says, are Higgs bosons or other large particles created in a black hole? So first, a little footnote, when you say Higgs bosons are large particles, you know, Higgs bosons are massive particles, but again, as I think I did in the same exact video, the scale video, and the biggest ideas, massive in quantum field theory particles are actually smaller. Okay, so they're not large in the extent of distance across, right?

The Compton wavelength of a massive particle is smaller than that, of a low-mass particle. Okay, so having said that, the Higgs boson is a massive particle. Are they created in a black hole? So then I'm not quite sure what you mean. I mean, inside a black hole, there's no particles being created. It's just sort of ordinary physics, so probably you don't mean that. There is, of course, hawking radiation from near the event horizon of a black hole. and usually we think of hawking radiation

as taking the form of photons, gravitons, neutrinos, very low-mass particles. But in principle, there will be some high-mass particles in that radiation that is given off. But the way to think of it is, there's a temperature for the radiation, right? And if the mass of your particle is much bigger in natural units than the temperature,

then a very, very, very small fraction of your particles are going to be that massive particle. So a black hole that is anywhere near astrophysical size has a very, very low temperature of hawking radiation. So basically, zero mass particles can be emitted, but it won't even be emitting electrons and positrons in any noticeable numbers, much less very heavy particles like top quarks or Higgs bosons. But in principle, they could be there. Sandro Stooky says, why is it that we get to assume that every microstate compatible with a given macro state is equally probable? I kind of agree with David Albert in your conversation with him that invoking the principle of indifference seems dubious. The way he put it in the debate, David and I had a debate that you can find online about ever-ready in quantum mechanics.

The way David put in the debate was something like this. If you tell me there are five boxes out of which one contains a marble and you ask me what the chances are that the marble is in the first box, I really don't have any reason to say 20% unless I know something about how the marbles got into the boxes. Yeah, and I think that David's wrong about that. I disagree, but let's be a little bit clear because there's the general question of how you reason under situations of incomplete information and the specific question about how you reason in statistical mechanics. In statistical mechanics where you have, you know, microstates and macrostates, and you derive thermodynamics and so forth, then we make, when you say your question is,

why is it we get to assume that every microstate is equally probable? Well, we get to assume whatever we want. And the question is, what works? You know, what assumptions pay off, right? And so, as a matter of fact, in statistical mechanics, assuming that every microstate is equally probable, works. So it's one of these things where it's part of science. You make an assumption, then you test it against your data. More generally, like the conditions, so, you know, the thought experiment where you have the boxes,

I think that David is just not playing fair because you need to do something. He's invented a very contrived circumstance. Someone hands you five boxes and a marble is in one of the boxes. And so someone else might say, well, but what if they held a gun to your head and insisted that you put a probability on it

or insisted that you bet, right? But if you were forced to bet money, okay, would you accept four to one odds against the marble being in one box. And that's a way to sort of give some heft to your credences about these different things. And David would say, well, I think it's just very rude for people to hold guns to my head

or to force me to bet. And I think that's a little cheating, like I said, because it's very much like, it's essentially exactly analogous to the question of how do we think about the correct physical theory of the world? Right. How do we think about,

science, you know, what is the dark matter? Okay, so is it an axione or is a weakly interacting mass particle or whatever? We put credences on that. We don't know which one it is, but we say, well, I think there's probably 50% chance of this, 60% chance of that, right? And you might think that the cases are not quite analogous because in one case, there's literally boxes in front of you and someone put the marble in the box. And in another case, we're talking about the law of physics, but in both cases, there's something you don't know about the world, right? And I would argue that in any case where there's something you don't know about the world, if you're interested in getting along in the world, in living your life, you are inevitably, irresistibly,

unavoidably putting credences on things. When you walk down a flight of stairs, you have a credence that the stairs will not collapse, that gravity will not reverse its direction and push you away, that the stairs are really there, that you're not a brain and a bat, you have credences on all these things, right? You need to, to act and to live in the world. Now, the importance of having these credences is going to be very different from question to question. Most people can get by perfectly well without having any credences on what the dark matter is, whereas you do need to have credences on whether gravity will reverse or not. So I think the same thing is true in these cases of physics questions.

I mean, we're asking questions that we care about. If by assumption you care about what box the marble is in, then you have no choice, but to put credences on the different possibilities. And I would say, if you have no other information, then there are five boxes, then why in the world would you do anything but put equal credences on them? You're allowed to do whatever you want. just like you're allowed to think that gravity will reverse its direction at any moment. You know, it's a free country. But you, but you, and if you're actually sort of agreeing to play the game and say, well, what, what do I think is the most sensible thing to do, then in cases like that, it's pretty obvious what the sensible thing to do is. Ben Schwer says, thoughts on Tannone's integrated information theory.

For those of you don't know, Giulio Ternone has an idea about the nature of consciousness based on integrative. information. I'm not an expert myself, but I am skeptical of it. You can look up online, Scott Aronson, who was a previous Mindscape guest, has given pretty dramatic takedowns in my mind of the integrated information theory. You know, roughly speaking, the argument is the good thing that Tannone has done and his collaborators is they have provided a very clear, quantitative criterion for when something is conscious or not. That's good, right? They're not weaseling out of answering the question. The bad news is that if you just naively apply that criterion, then a whole bunch of things are suddenly conscious. And in fact, you know, Tannone and Christoph Kock,

also who has sympathies along these lines, tend towards panpsychism for that very reason. If you listen to the panpsychist interview I did with Philip Gough, you'll learn about that. And to many of us, you know, some of these things that qualify as conscious by integrated information theory standards are clearly not conscious. And again, the good thing that Tannone does is bite the bullet. He says, nope, they are conscious. You know, this book just sitting there on the shelf is actually conscious because it is by my theory. But to me, it's not capturing what we're trying to understand about consciousness, as far as I can tell. Seamus Maxwell says, would you consider recording a solo podcast while in a really bad mood and or

drunk. I realize this would be a radical departure, but I have a feeling it would be incredibly compelling. Nope, not going to do that. It may or may not be compelling, but it would be, you know, look, I'm not objecting to the idea. You know, I would in principle do something for entertainment purposes, but it's not this podcast, right? Mindscape has a purpose, which is not just to have some laughs. We'd like to have laughs along the way, but we like to tackle big issues and work hard at understanding them and being in a really bad mood and or drunk are not conducive to that particular project that we have here on the podcast. Fran Plaas says, I'm really fond of your idea that baby universe is pinch off from a multiverse. It's both cool and helps make sense

of the arrow of time and entropy. My question is, can the process of creating baby universes be eternal and infinite? Yeah, the answer is yes. There's no problem. That's one of the good things about this model. And it relies on the fact that in general relativity, the total conserved quantity, whether it's energy or charge

or angular momentum or whatever, the total conserved quantity of a closed universe, a universe that is sort of closed in on itself, like a three-dimensional sphere, conserved quantities are all exactly zero for closed universes,

whether it's energy or charge or anything else. So it doesn't cost anything to make baby universes. You don't run out of resources because it costs zero resources to make them. So that is very much part of the charm of that kind of model that it can go on forever. Peter Humble says, I would like to hear your thoughts on how well established from a

scientific perspective is the theory of evolution. I understand it's leaning much closer to fact, but will we ever be able to ditch the noun theory when using the term evolution? So there's two points here. One is the direct answer to your question. It's true, the theory of evolution. In broad strokes, what Darwin said was correct. Now, in detail, it was not correct. He didn't even know about DNA, much less genes or, you know, Mendelian inheritance or anything like that. He certainly didn't know about things like horizontal gene transfer or genetic drift or a million other things. So we have now what we call the modern synthesis of genetics with Darwinian evolution. And now there are some people, as we mentioned, uh, with Sean B. Carroll on that podcast, there's some people who even want to say, we need to, a new synthesis beyond the modern synthesis, but they're all updates and extension of Darwin's theory of natural selection. It's just true, okay? But then you also say, will we be ever able to ditch the noun theory when using the term evolution? We're not going to ditch that word, because that's just not a thing we do. We don't ditch words just because theories become

better established. The idea, and I know it's very popular out there, the idea there is some hierarchy of truth between models and hypotheses and theories and laws is just wrong. There is no such hierarchy. I mean, there are different levels of credibility and truthfulness to different ideas, but we do not in any systematic way attach words to those ideas based on how credible or true they are. The standard model of particle physics is like the best confirmed thing we have, but we still call it a model. We're never going to call it the standard theory of particle physics or the standard law of particle physics, right? It's called the standard model.

It's just its name. Whereas Newton's Law of Gravity is wrong if you get to strong gravitational fields. It doesn't get the orbit of Mercury right. But we're not going to stop calling it Newton's Law of Gravity. Okay. So we don't update these words. You know, just don't care about, just don't fool yourself into thinking that the word we use to describe a scientific idea is meant to convey how accurate or truthful that idea is. It's not how scientists talk.

Ashley Hyatt says, will any particles of matter exist in the far future of the universe? And if so, doesn't this imply that maximum entropy won't ever be reached? So this is a tricky sort of subtle question. you know, we're assuming here, let's assume that the universe will expand forever as if we have a cosmological constant. Because if it collapses, then the far future doesn't get reached, right? If the universe expands forever and lasts forever,

then there's a theorem. Bob Wald proved what we call the cosmic no-hair theorem that the universe empties out and asymptotically approaches empty space. Okay. Now, so what that means is if you have some set of particles that have a, you know, there's a fixed number of particles, right? like protons in the world or something like that,

which is not quite fixed because you can combine with an electron to make a neutron and maybe protons decay and whatever. But let's imagine that protons live forever. Okay. The number of protons in the universe is roughly constant, but the universe is expanding. And the size of the observable universe in light years, or megaparsecs or whatever unit you want to use,

remains constant even though the universe expands. The size of the horizon in an accelerating universe in a universe with a cosmological constant, the size of the part of the universe you can observe attains a fixed value in real units, in light years or parsecs. And what that means is, if space is expanding with a fixed number of particles,

and observers in the universe only have access to a fixed volume of that universe, the probability that there's even, even one particle in any one particular observable patch of the universe goes to zero. So particles will exist in this particular, very specific scenario we've sketched out, but the density of particles approaches zero, the probability you'll bump into a particle, approaches zero. So for all intents and purposes, at that point you've reached maximum entropy.

Nathan Simmons says, you get to assemble the genius team to work on whatever problem you want, say quantum gravity. What historical figures would you choose to work on that problem with? So this is like, is why I said before, that we're going to get top ten lists in this AMA. This is kind of like, you know, the favorite books question, you know, favorite physicists working on quantum gravity. You know, I think that it stems from, I want to say a misapprehension, but let's just say this isn't the way I think. Okay. I do not think that Isaac Newton, for example, who I do think,

accomplished more than any other physicist in history would necessarily be the best person to think about quantum gravity. He was the best person at that time to think about classical mechanics for a whole bunch of reasons. I have no idea whether he'd be any good at thinking about quantum gravity. I mean, he certainly had some mathematical and physical talent. He was also a little loopy in other ways. So I just don't know. You know, I think that honestly, in 100% honesty, if you, chose the best five physicists in history and brought them to the present day and educated them about

modern physics and said work on quantum gravity. And I got to pick an all-star team of five people working on quantum gravity right now who are alive. I would place heavy odds that my team would make more progress than yours. Because my team is made of people who've been selected to be good at working on quantum gravity, right? So I think that's a more important criterion. Santiago Torres says, do you foresee your holiday season plans being different this year due to COVID-19? Yeah, we're not traveled anywhere. We haven't gone anywhere. Remember, I did a little speaking tour in Australia and New Zealand. That was just before the lockdown, or the pandemic really hit. And I haven't been on an airplane or an airport since then. And so usually I go to

visit my mom in Florida. And we're not doing that this year. I think that the holidays are going to be bad. I think that, you know, as I record this on November 1st, numbers are already ticking upward. Thanksgiving and the Christmas holiday is going to make things much worse. And if you don't have to, I would not intentionally make a choice to spend a lot of time in airports and airplanes. It's not the worst possible thing you can do, but it's just an extra little bit of risk that you can. You should avoid if you can. It would be my thought. Saraj Raj Raj Rajan says, quantum experiments that suggest time travel was achieved, reported the media.

Do they actually do a time travel as in particles traveling to their past states, or do they just appear so because of the math? I'm not exactly sure which reports you're talking about, but nothing has actually traveled backward in time, in any reasonable sense of the word. Maybe they appear so because of math. More likely someone tried to write a very provocative headline or press release. What can I say?

Gregory Kusnik says, Imagine a touring complete cellular automaton implemented by a population of enzymes that bind to specific sites on the free end of a vast bundle of protein strands and attached new amino acids in accordance with the rules of the cellular automaton. Now let's run Bostrom's ancestor simulation

on that computer. The protein crystal we end with is the block universe of the simulated world with each layer embodying the state of the world at some moment in time in all moments equally real. How should we think about the simulated people embedded in that crystal? Is there some sense in which their subjective experience remains real even as the crystal sits passively on the shelf? This is, I can tell you've worked hard on this question and good.

You did a good job. This is in the vicinity of what I consider to be a very, very puzzling, difficult question, which is, Let me phrase it as a puzzle rather than as a question. On the one hand, everything that I would think about as being characteristic of consciousness or experience depends very, very importantly, on time passing, right? It is not a static thing. What it means to be conscious is a whole bunch of criteria, all of which in one way or the other heavily depend on the time. evolution of the system. Okay, that's point one. Point two is, you can imagine time being emergent, right? So this is not exactly your thought experiment, but I'm mentioning a very similar one.

You can imagine in the fundamental laws of physics, the time is not there, and the time is emergent in some sense, so it's not there in the fundamental level, but it's there at some emergent level. And the way it emerges is that the overall state of the universe is static, but there's a way of conceptualizing that static universe as a set of clocks that are in tight correlation with a set of other things, and we can then treat those other things just as if they were evolving in time.

Okay? So, and this is what most people who think about quantum gravity think might be true, right? As I'm not saying it is true, but there's a lot of people who think the time is emergent in exactly this way. So in that sense, time is not real, but it's apparent and emergent,

but what's really there is something that is timeless and not evolving. So there seems to be a dilemma there. You're saying on the one hand, I'm saying on the one hand, everything that is necessary for consciousness to be in existence depends on time passing. And on the other hand,

time is just emergent, if this scenario is true. So you might want to say, okay, well, I buy both of those, and time is emergent, and it's still, it's emergent, enough to count as creating consciousness. But then on the third hand, you say, well, then you do your thought experiment, Gregory.

You say, well, then why can't I just make something, which even though time is passing for me, it is just sitting there statically. And it has the structure of what you would call a static system where time emerges within it. Isn't there consciousness going on in there in some sense, right? I think that's my version of your question. So I think it's a very good question. And I'm not quite sure the best way to answer it. The intuition I have is that there is a way of thinking about this static crystal that you've constructed in which conscious experiences are real.

But the trick is that that way is orthogonal, literally, to our experience of time and consciousness, right? Because our experience of time and consciousness is in the time very. that you and I all use that we measure on our clocks, our wristwatches. Whereas if you constructed this static thing, the sense in which conscious creatures exist and live out their lives in that static thing is only an emergent sense within that thing. It is not the sense of time that you and I have. So I think they can be compatible both at the same time, but they're in some sense incommensurable. What the creatures in the crystal have as an emergent experience of consciousness is just inexpressible and inexperienable. I just made up that word.

Unexperienable by us in our world. I'm not sure if that idea that I just sketched out holds together. I haven't thought about it very deeply. It does go back to this question we just talked about with Giulio Tannone's integrated information theory. Because one of the sets of examples that Scott Aronson brings up are all these states. static things, you know, decks of cards and things like that, that in principle could satisfy his criteria for being conscious. So, short answer, it's a good question.

I'm not quite sure what the answer is. Mike Dillingham says, which book or books have the most profound impact on your view of the world and what was it that changed for you? I'm going to punt this question, too. Sorry about that, Mike. You know, for the reasons already mentioned, you know, I just don't attribute profound changes in my view of the world to individual books. And I know that sometimes something can crystallize in you when you read a certain book or hear a certain argument or read a certain article or watch a certain talk or whatever.

But in my view, those changes are almost always predicated on a whole bunch of other changes that were less visible that happened earlier, right? It's like a phase transition. It's like, you know, you boil the pot of water and eventually you see bubbles. But all that invisible work heating up the water before you see the bubbles is just as important as the moment the moment the bottom. bubbles form. So, there you go. Completely punting on this question. I can't even, you know, I would like to fake it.

I would like to offer some books that I could say, well, this made a big impact on me, but I'm not sure in good conscience. I really can. There's plenty of books that I like and they contribute to my worldview. But profound changes are something that are harder for me to pinpoint there. Neil Glew says, in one of the recent podcasts, you were asking the guest about whether university staff or students should be able to prevent a campus group. from inviting a controversial speaker to campus

for an event private to that group. It wasn't clear to me what your own position is on this matter. Could you say what you think is the policy of university should use in this situation? Yes, so that was with Teresa Bejohn. We talked about that. And my own view is that universities, or even better, subsets of universities

should have very little veto power over what other subsets of universities do in terms of inviting speakers. Okay. Even if the speakers are really bad, I would like it to be the case that we have a very, very strong bias in favor of letting people do all the stupid stuff that they want to do if it doesn't bother me. It doesn't bother us. If it doesn't bother anybody else, right? But I don't want to make that an absolute prohibition because I don't think there's there are any absolutes in this game at all, to be honest. You can imagine situations in which even though you were just inviting a speaker to talk. to your group, there were circumstances under which doing that would bring harm to other people, even who are not in your group, right? And I think that that's a good reason to prevent it. So I think that in practice, you know, if someone is a Nazi or a racist or whatever and some group on campus wants to listen to a talk by them, they should be allowed to do that. But I don't want to

put a blanket prohibition on appreciating the nuances of individual situations. Hi, my name is Lisa, and I'm a licensed psychotherapist. which means my work doesn't magically end when the session does. There are notes to write, appointments to manage, billing, insurance follow-ups, and somehow all that admin used to creep into my nights and weekends. That's why I switched to simple practice. Simple practice is an all-in-one electronic health record built specifically for therapists with HIPAA compliant tools and high trust certification,

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Toyota, let's go places. Chris Shaw says, I was wondering, does the Big Bang start the process or even before during inflation? I think that there's some words missing in that sentence, Chris, but it might be my misreading. Or does it even matter? Do the wave function Hamiltonian vector even need a cosmological start? Yeah, I mean, you're going to guess my answer to this, which is nobody knows. We don't know. As I said, already once in this podcast, the Big Bang may or may not have been the Bexmong of the universe. Inflation was almost certainly not the beginning.

Inflation is literally a physical process that goes on but needs to start somewhere. The Big Bang could there could be, so forget about the phrase the Big Bang, which might come laden with preconceptions, there may or may not be a beginning to the universe. We don't know. And if there is, it could be a sharp beginning or a fuzzy beginning. In other words, the idea of space and time might be approximations that sort of gradually kick in rather than instantly starting. We just don't know. Sorry.

Linneumiziarah says, according to black hole complementarity, if an astronaut passes through the event horizon, he would not feel anything special. However, according to a distant observer, the infinite time dilation would cause hawking radiation to kill the same astronaut. What is going on?

So there's no contradiction here. This is the whole point of black hole complementarity is that everything is ultimately consistent even though different observers use different languages to describe it. So it is not true in particular that to a distant observer, hawking radiation would appear

to kill the astronaut. It's a tricky thing because, you know, general relativity is tricky and the whole bit is tricky, but when you are a distant observer, you see hawking radiation come from the black hole, and it's at some temperature, right? There's some typical energy,

and the photons you see coming from the black hole. And if you just use classical general relativity and ask, what would that radiation have looked like if I hovered just above the black hole, right? So if I put on my rocket chip, my rocket engines, and I went right to the eventorizing, but not quite to cross it, and I fired my rocket engine

so I could just hover above the eventorizon, what would that same hawking radiation look like? And the answer is super energetic, super high temperature. radiation that would immolate you, okay? But if you just fall through, if you just freely fall, if you do not accelerate to see, to stay above the event horizon, if you let yourself cross over it, you don't see any hawking radiation. There's, it basically turns off as you approach the black holes eventorize,

and it just looks like empty space. And that is what the distant observer would see. The distant observer sees radiation, but they don't see high intensity radiation killing you, because they don't see you hovering above the event horizon. What they would see is you crossing the event horizon, well, what they would see is you coming closer and closer to crossing the event horizon. Time dilation means they never quite see you cross it. But also remember, and this is what people have heard that you never see people, the infalling observer, cross the event horizon.

Because of time dilation, what they either have not heard or have chosen to forget is the radiation that you see that would illuminate that in-falling person. I'm not talking about hawking radiation. I'm just talking about like if you have a flashlight or whatever. That gets time dilated too. So the radiation that you would use to see the person falling in is number one, redshifted. Every photon gets redder and redder and redder and redder. And therefore, if you have visible light eyeballs, you don't get to see them because they become infrared or much longer. And number two, the number of photons goes down because the time in between photons gets longer and longer, attending towards infinitely long.

So in practice, you see the person disappear, even though in principle in the classical GR limit as a point test particle, there you would never not have them in your past light cone, but you never actually literally see them cross the event horizon. Fred Alexander says, you're fond of saying such and such is not one of the top ten things that keep you up at night? What are the top two or three things that actually do keep you up at night? Another top ten list. You know, I'm not very clever about this particular question. You know, I worry about politics right now, not just who's going to win the election, but the very idea of democracy and the fact that so many people don't value it as much as I do. That bothers me,

not just the U.S., but worldwide. I worry about climate change. I worry about a whole set ways in which rapid technological progress could shake things up. So under that, I include genetic engineering, but also artificial intelligence, brain computer interfaces, things like that, biological weapons or, you know, pandemics that are either natural, like the one we're fighting now or artificial, in which case they could be much, much deadlier. There's a whole bunch of ways, you know, in some sense, the invention of technology has given humanity enormous leverage. long before it gave us the wisdom to deal with that leverage. We somehow escaped large-scale nuclear war in the 20th century. There's no guarantee that we won't have large-scale nuclear war, even small-scale nuclear war any day now. We can't guarantee that. There's thousands of nuclear weapons out there ready to be fired.

And with every passing day, it becomes easier for someone who is not a relatively responsible state actor to build a nuclear weapon. So plenty of things keep me up at night. I have trouble sleeping. Happily, I have two cats that keep me comforted and they purr a lot and it feels good. Brendan Hall says, I know you're busy with this great chunky chat of questions, but I was wondering if you would wish my friend Nadia a happy birthday. And he goes on to mention that Nadia accepts the Everett interpretation of quantum mechanics. Hoof, I'm glad you added that, Brendan. In which case, happy birthday, Nadia. I hope you have an exceptionally good birthday. I'm not quite sure what day it is, but whatever day it is. I hope you have a happy birthday this year.

all the other years. Having happy birthday is a good thing. It's a little day when you can, you know, pat yourself on the back for making you through another year. Jonathan Park says, let's say we have two quantum entangled particles with the up-down spin of each undefined until measured. We measure the spin of one and get the result spin up. This means when the spin of the other is measured, it must be down.

Because the spin of both particles is undefined before measuring, is there a way to fool the universe by measuring the spin of both particles at exactly the same time? So first, let me just parenthetically note, there are different ways for particles to be entangled. So you've chosen something where they're entangled with opposite spins, right? One is up, the other's down, or vice versa. But as we already had earlier in the AMA, you could also entangle them, so that both of them had the same spin. That's a different way to be entangled. So there's nothing about the idea of entanglement that says that spins need to be opposite.

That's just a little aside. But the answer is no. You cannot fool the universe by measuring them at the same time. The rules of quantum mechanics are perfectly clear. If you measure them at the same time, you'll get two answers that are compatible. It will never be the case. If you put the two particles into a superposition, which is entangled, in the sense that the two spins are oppositely aligned, then that is always what you will measure, no matter

if you do one first or the other first or both at the same time. James says, rather than describe our universe as expanding, can't we describe it as a fixed size with a matter within it shrinking? I have an easier time to visualize it this way. I already answered that one earlier in this AMA. The answer is you can, but be very aware of what you're giving up and the hoops you have to jump through to make that kind of thing make sense. Bendy Bruce says, when I learned about the concept of quantum entanglement, it made me sad because we could not exploit it to transfer information. However, I recently saw a video about a thing called quantum teleportation. I wonder if you know anything about this and whether you're willing to try to explain it in a way that is somewhat accessible to an armchair science fan.

I can, but believe me, it does not let you transfer information faster than light. The thing, I don't know why people don't explain this better, to be perfectly honest, but the thing about quantum teleportation is, it is not better than classical teleportation. The point is that there are reasons to think that it should be harder to move quantum information from one point to another. Namely, that when you measure a quantum system, you destroy the quantum information in it, right? If a single spin is in a superposition of spin up and spin down, you can't directly measure that. You can prepare it in such a way that you know it's true, but there's not a measurement you can do that says, yep, just like I suspected, it's in a superposition. of spin up or spin down. When you measure the spin, you get either up or down. Those are the rules of quantum mechanics, right? So, classically, it hasn't you do with teleporting. It's just like

moving or let's say faxing something, right? You have some information on a piece of paper, you put it in a machine and you fax it. It reads the information. It sends some data, and that information is reconstituted somewhere else. That's really what we mean. You know, it's nothing more elaborate than that. And the problem is that you can't do that. quantum mechanically. You can't read the information, send it somewhere else because you destroy the quantum state in the process of reading it. So the whole thing about quantum teleportation is it's a very, very clever setup that allows you to nevertheless move quantum information from one point to another. But it's not instantaneous. It is not faster than light, and it is less effective

than classical teleportation. What you need to be able to do it is to establish an entanglement between where you start, some entangled particles shared between where you start and where you want the information to go. And then you send some classical information also,

and you can use that set of information to basically move the quantum state from where it starts to where you want it to end. And you can't duplicate it. This is a corollary of this thing that when you look at a quantum system, you collapse its wave function,

you can't make another copy of it. There's literally a theorem, the quantum no cloning theorem. You cannot clone quantum information. But the clever thing that quantum teleportation lets you do is move quantum information. Okay, if you have some superposition of a particle right here, I can turn that into a superposition

of a different particle very far away without knowing what the superposition is, without ruining it by measuring it. That's quantum teleportation. Ricardo Rosera says, at the end of a recent Veritasium video, Derek proposes a question relating general relativity with accelerating charges. The question is, does a freely falling particle emit radiation because it is accelerated in the coordinate frame of someone else at rest on Earth, or does a stationary charge emit radiation because it is being accelerated

by the ground, i.e. is not in a geodesic. Yeah, this is a classic question. It's an really old question people have fought over for years. And it's the only reason it even seems hard is because people are a little sloppy about what they mean by the word radiation. Okay. If I have a charge, an electric charge that is just sitting there not moving and has not been moving forever and I ignore gravity, I ignore the earth, I ignore general relativity, okay, just have a single charge. There's an inverse square law electric field around that charge, okay, an electron. let's say. And if I take that charge and now I move it, so I shake it up and down. So I need to accelerate it to shake it up and down. Then far away, there'll be a little wiggle in the

electromagnetic field that arrives at the speed of light, right? And that variation in the electromagnetic field is perceived as radiation. But really, that makes most sense, like when you're literally shaking the electron up and down repeatedly and making a sort of standing. wave that is spreading out in some sort of continuous fashion. What if I just move the electron a little bit and didn't move it back? Well, the electric field around it changes, but it's kind of like a wave that goes up, never comes back down. It just, it was low, and then it goes up, and then it stays up.

Does that count as radiation? Well, who cares is my answer? Like, you know, it's a changing electromagnetic field. Basically, like what I'm trying to say is, the only reason this is an interest, question or seems like an interesting question is because it asks a question that lives on the boundary between static electric field and radiation where the definition is fuzzy. It's not a physics question. It's just a definitional question. So if you have an electron that is sitting on my desk here on Earth, right? Okay. By the rules of general relativity, it is accelerated, right? Because it's not on a freely falling trajectory. But who cares? What I care about is, Is there some time-dependent electric field far away that I would detect in a photomultipler tube

or something like that? If the electron is just sitting there, stationary, right, then I don't detect anything in a distant electromagnetic detector. So I would not call it radiation. So this idea that acceleration and gravity are similar, remember, that's the principle of equivalence which is only supposed to apply

in very small region. of space time, but the idea of radiation is something that's supposed to apply in big regions of space time. So it's just non-overlapping domains of what you're supposed to be talking about here. Hershey Silver says, a quantum computer is exponentially more powerful because each byte is both simultaneously one and a zero. However, under the many worlds theory, the different worlds once actualized do not intersect. Thus, how can a byte in my current realized world on my actual computer be quantum. Put differently, once a byte computes an output, would it not lose its quantum capabilities for future processing? Well, I don't know how much you know about quantum mechanics here,

but you've put your finger on what is the single biggest technological challenge to quantum computers, which is maintaining the bytes, the quantum bytes, or quantum bits, the qubits, in superpositions, right? You are correct that in order to do a truly quantum calculation, you need to manipulate and entangle qubits without measuring them, right? So you're using words like actualized and realized, which are not words that we traditionally use in quantum mechanics. We use words like measure or collapse the wave function or decoher, even better, right? Once you measure or decoher, a quantum byte,

then it's not being quantum anymore. Sorry, it's classical. It's up or down. So you want, while you're doing the calculation, to keep things in superpositions. That, in other words, to keep them coherent, not let them decoher. But once you measure it,

that's how you get the output. And indeed, it's true that once you let the quantum computer do its algorithm, then you measure the state of your cupits, and that destroys all the quantum entanglement. So you need to start again, if you want to run it again.

You can't just run it backwards through the quantum computer anymore. You get an output, and in fact, you might want to check that you do it more than once because you're only going to get the right output with a certain probability at the end of the measurement process.

James Kittick says, okay, since our mental energy is near the vacuum state, I agree with you there, James. Here's an about-you question. What are your eating habits like? Are you a three-square meals a day person, a grazer, midnight snacker,

has COVID-19 affected this? I'm a, you know, two-square meals a day person, mostly with lots of cups of coffee, not lots of cups of coffee, three cups of coffee a day. It's not really too much. But I generally don't eat breakfast. I'll have a decent lunch, a decent dinner. And I, you know, I am someone, you know, different people respond differently to times of stress or anxiety.

And I don't want to overblow my own situation because I don't have it nearly as bad as many people do. But, you know, our pre-election state, pandemic state, various state of the world states that we're in, does cause one's some worry and that affects one's eating habits, as you imply. I'm someone who comfort eats. So I, you know, if I'm, you know, worried or whatever, I will just eat and eat and eat. There's other people who like shut down and don't eat when they're in anxious situations. So my, the thing I need to control in situations like this is that I don't eat too much. You know, happily, I can't go out to restaurants with a the convenience anymore that I could. Unhappily, people will deliver food to me like ice cream.

Jenny's ice cream is very convenient. Pizza, very, very convenient. Wonderful Thai restaurants we have here in L.A. So, yeah, so I need to, you know, be careful not to eat too much. Let's put it that way. Michael Cronin says, Congratulations, you've been gifted 500 Schmeckles and a Star Sprinter 6,000. What celestial object?

you explore first and why? So I'm, you know, I've not read the catalog for the Star Sprinter 6,000. I'm hoping that it goes faster than the speed of light so I can go wherever I want. It's, that's a close one. You know, there's sort of three obvious choices. One obvious choice would be somewhere right here in the solar system where we think it's, it's plausible that life might be, like Europa or Enceladus like we talked about. But if I have such wonderful technology, maybe I want to go further away. I mean, it might be more compelling or more interesting to go to an extra solar planet on which intelligent life might exist.

But the problem there is that I think it's very plausible that the chances that any one such planet has intelligent life are very small. So I might just waste the trip. So part of me says that the real fun thing to do be go visit a black hole. You know, get up close to it without falling in. And I think that, you know, the one black hole that we know about with highest confidence is at the center of our galaxy, Sagittarius A-star. So that's probably where I would go. I would just check it out. And if you want to know, you know, other than getting to tell my friends I visited a black hole, well, the scientific payoff of that be, I think you could measure the curvature of space time around the black hole and see if it lined up with our expectation from general relativity.

That's an important science experiment to do. Peter Bamber says, as an object approaches the event horizon of a black hole, an observer at a safe distance sees the object become ever slower and never sees the object cross the horizon. But that observer sees black holes grow. How can that be when the observer never sees anything

actually cross the event horizon? This is a good question, but there is an answer to it, you know. And I have to blame myself and other scientists for just being sloppy when we explain these things. Like I already alluded to this in an earlier question. But there is a calculation that says, imagine you have a test particle. Okay, so a test particle is something that is moves slower in the speed of light, so it technically has a mass,

but it has zero gravitational field. Okay, so a test particle is the limit of a massive particle as you take the mass to zero, okay? So it's not a real physical thing that we can actually imagine. But we can ask about what the test particle would do in general relativity, and the answer is it follows what we'll actually. it follows what we call a geodesic, right, a path that is curved,

but it's trying its best to move on a straight line. And then you could make a statement that a test particle falling into a black hole would never leave the past light cone of an observer at infinity. That's what it means to say. An observer would never see anything across the event horizon, okay? But in the real world, in the real world,

you don't have test particles. You have real particles that really have mass. And that means they really have a gravitational field. And that means that as that particle approaches the black hole, not only is the black hole pulling it in, but the particle itself is exerting a gravitational pull. It's deforming the space time around it. So it happens, and people have gone through this and checked it, essentially what happens is because of the gravitational field of the infalling observer, which is usually neglected, but you shouldn't, the eventorized and the eventorized. the black hole expands. It reaches out to cover the infalling observer and swallow it up in a finite time, not in an

infinite time. So that's how you see black holes grow because you need to take into account the gravitational field of what's falling into them as well. Peter Humble says, have you ever seen the science fiction series called Devs? What did you think? I did see it. In fact, there's an, I think, an interview with me. Again, my memory is not that good, and it's not getting better over time.

Let's put it that way. I think it's not an article I wrote. I think it's an interview with me in the New York Times about devs, about my opinions of the quantum mechanics in devs. And look, it was a fun series. It was perfectly worth watching as fiction. It is not, in any sense, accurate physics, okay? It's not trying.

I shouldn't say it's not trying. Maybe it was trying. You should certainly not judge it by how accurate its physics is. How about that? Uha Kevaluoma says, I'm troubled by the prospect of infinite many worlds. Do you decrease your credence towards infinite many worlds because of the qualitative, qualitative difference between the infinite and the finite? No, I don't. You know, we have an argument.

I was a co-author on a paper saying that the Hilbert space of quantum gravity is locally finite dimensional, not infinite dimensional. But our argument there is based on physics. It's based on Jacob Beckenstein's argument that there is a maximum amount of entity. you can never fit into a region of space. I have no conceptual problems with infinity. I know that there could be conceptual problems with infinity, especially once you have uncountably large sets. There have become questions that are very difficult to answer

and puzzles and things that seem paradoxical. I'm not sure whether they should be paradoxical or not. I honestly don't know. I appreciate the point of view that says that, well, those puzzle would go away if you just stuck to finite things, But on the other hand, I think once you learn that in the conventional way of thinking about infinity, the number of points on the real line between zero and one is equal to the number of points in the real line between minus infinity and infinity.

Right? The infinity from the real line comes from the smoothness of the real line. It doesn't come from the fact that it's infinitely big. Once you learn that in my view, not in everyone else's, but in my view that reconciles you to, infinity a little bit. Infinity is just a way of talking about continuity and smoothness, not about things that are so far away that I can't see them. So I have no trouble imagining that there could be infinitely many worlds or the universe is infinitely big or anything like that. I try to be humble about questions like that. I'm open-minded. Balkia says, when Trump caught COVID,

you and a lot of other people who think he's an awful person and president still wished him a swift recovery, while a lot of others felt like he got what he deserved and would have enjoyed a good amount of shot in Freuda from him either getting worse or even dying. I struggled a lot with this because it was by far the biggest challenge to my personal philosophy I've encountered yet. I don't think he deserves death, but at the same time he's a deeply awful person who spent the last four years making life miserable for a lot of people and indirectly killed hundreds of thousands, if not through COVID, then through enabling various extremist behaviors. The conclusion I came to after a week of deliberating was, I won't wish him death, but I don't have to wish him well.

Even though I think everyone by default deserves sympathy when bad things happen to them, I also think people can, through their actions, lose that right. So my question is, in your personal philosophy, what is the reasoning that leads you to actively wish him a swift recovery, despite him not extending that courtesy to anyone else? So I'm very sympathetic to the idea these are tough questions. I'm very much, as a moral constructivist, it would be hypocritical of me, not to allow people to have different moral takes on these. And we can talk about why we come in different directions.

So I'm not going to harshly condemn anyone who disagrees with me about this. I'm open to changing my own mind about it down in the future. My own view is, you know, I think the idea that he deserves bad things to happen to him is probably not the right way to think about it. Not that it's wrong or right. that those categories don't apply. I am a big believer in, you know, punishment as something that affects your behavior, right?

You know, whether it's putting people in jail or finding them or whatever or other forms of negative reinforcement to stop people from doing bad things. But I'm not a big believer in punishment because people deserve it. I think that's just sort of a leftover from our lesser instinct.

that we evolved over evolutionary time. So I'm not moved by the argument that, well, he's a bad person, he deserves bad things happening to him. I am moved by something that Hillary Bach, who is a philosopher at Johns Hopkins said on Twitter, and I retweeted, she argues that our attitude towards bad people, what they deserve and what they don't deserve, is not about them and how good or bad they are. about how good or bad we are. I want the world to be better. I want the kinds of behaviors

that Donald Trump has engaged in that have brought misery and death to many, many people to stop. But those are not predicated on him personally suffering, right? If I could make them stop and not make him suffer, I would do that, right?

Unless, like I said, there were some, you know, deterrence, future deterrence kind of thing. Like I'm all in favor of judging presidents after they leave office for breaking the law. You know, I don't think there should be any immunity for presidents just because they once were the president

because we will have future presidents. And we don't want them to get away with anything just because they're the president. But I'm not as a general rule, a thorough consequentialist about morality. I am not someone who thinks that, I'm not a utilitarian. I'm not someone who thinks we can add up good and bad, and we try to maximize the good and minimize the bad.

I think it's more complicated than that. I'm not a strict deontologist either. I'm sort of wishy-washy and in between. Here is an area where I'm mostly consequentialist. So my question to myself is, would Donald Trump suffering and dying from this disease make the world a better place? Or does it just, you know, speak to our worst instincts? And I think that mostly it just speaks to our worst instinct. I want him to lose the election.

That's what I want. I don't want him to die. It doesn't make the world a better place for that to happen. Dan O'Neill says, when I think of the branching, his little whiplash here going from these questions. This is, let me pause to say, this is probably my favorite AMA, because I really love this month that we're doing.

We're in the middle of right now because it's really a diverse, fun, brain-stretching set of questions. So good on you, Patrions out there in the midst of global disaster. You're able to come up with a lot of fun set of questions. Dan O'Neill says, when I think of the branching four-dimensional space times in the many worlds' interpretation of quantum mechanics, it's very hard for me not to picture this as a branching that takes place in some larger pre-existing space time that they would all inhabit. The way the branches of a tree occupy the block of space that is my backyard, but I'm pretty sure this larger framing is not required. Is there

a better way to picture many worlds, or do I need to accept that it's unpicturable? Good. Part of me wants to say it's unpicturable, but of course that depends on how realistic you want your picture to be, right? You know, everyone has seen pictures of little like tree diagrams, like a line goes up and then it branches in two, and then each of the two, they branch into two more themselves. And many worlds is kind of like that. I mean, that's a way of visualizing something, but you're just a visualizing something. but you're just visualizing lines on a piece of paper.

You're not visualizing whole three-dimensional realities. Ultimately, these theories live and die by their strict mathematical descriptions and how those descriptions fit the data. They do not live and die by our ability to visualize them. So what I'd like to say is space time exists within the worlds. The worlds do not exist within spacetime. There's zero reason, conceptuality.

or mathematically to imagine all of the branches living in some bigger space. But it is very, very difficult to imagine visualizing them unless you do. That's why it's hard to accurately visualize them. When you draw that little tree diagram on a piece of paper, there's a piece of paper that is separating the lines, right? It's just almost inevitable. So I'm comfortable saying you shouldn't expect things like this to be pictureable in any standard way. the human brain just wasn't evolved to think about these ideas.

Klausian Runia says, I think it's really cool that you're now also an external professor at the Santa Fe Institute. My question is, what in complexity do you hope to find out, given your quantum mechanical explorations of Hilbert space? You know, there's not necessarily a direct relationship between my quantum mechanical research and my complexity research. They accept for the fact that they both arise from a certain foundational cast of mind. And here I'm sort of invoking terms of art to distinguish foundational from fundamental.

There's something called fundamental physics, which in most people's minds is, roughly speaking, field theory and particles, gravity, you know, the basic fundamental laws of physics. And there's the foundations of physics, which are a little bit more philosophical, a little bit more conceptual. and may or may not refer to the foundational level. So when I work on the arrow of time, for example, why does the past seem different than the future? That's absolutely in the foundations of physics, but whether or not it's fundamental physics depends on your particular viewpoint on what the answer might be.

The paper I'm trying to get done now on causation and the arrow of time has almost nothing to do with fundamental physics, but is absolutely foundational. You know, why do causes precede effects? How do you get more foundational than that, right? So, I mean, maybe the answer is, this is, this maybe is not your question, but this is, you know, I answer the questions that are asked with the answers I want to give whether or not they match up to questions. Sorry about that.

The thing that connects my quantum mechanical research with my complexity research is that it's done by me. And I am interested in these foundational questions. Now, there are people who will ask questions about, well, why do causes precede effects in quantum mechanics, right? Or where is the quantum arrow of time come from? So there's overlap between these questions, and I may or may not be interested in them, but there's no direct connection between these different kinds of things I'm talking about. Johnny says, if you could install your consciousness into an android and effectively extend your life that way indefinitely, would you do it? Assume all hurdles have been overcome in that process and that you retain all your ability for thought, emotion, and feeling. So I'm going to hesitate about this one because I am very, very unsure that this is possible.

In the following sense, I am quite high credence on the idea that we can imagine building conscious creatures in asteroids or in artificial intelligence or something like that. I see no fundamental need to have neurons and wet biology to make consciousness. You can make consciousness in all sorts of different ways. But I am a little bit skeptical that it would be me, that it would be my consciousness, right? It would be some extension of my current self into something different. And, you know, I would go back to the podcast I did with Lori Paul about transformational experiences. Transformative, sorry, transformative experiences. And Laurie's idea is that there are situations of rationality where the way that you would judge something good or bad depends on who you are, right?

And that's an obvious statement. Like some people like tomatoes, some people don't like tomatoes, right? But there are questions that come up where the decision that you have to make happens when you're one person and the consequences of those decisions are felt. when you're another person. That's a transformative experience. So how do you act rationally in that situation? Do you judge whether you want this to happen or not by your current self or by what your future self will be? Standard example is turning into a vampire. Most people don't want to become vampires, but once you're a vampire, you like it. But of course you would. You're a vampire. So I have no idea, given the current state of the art, what the continuation of me would be like.

if that continuation took the form of an android. So plausibly, it would be fun and interesting and worthwhile to imagine extending my life that way. And plausibly, it wouldn't be. Again, I'm just being honest about what I don't know. Jan Smith says, will there be a branch of the wave function which Trump is not reelected? Yes, if you believe many worlds, there absolutely will be many branches like that. The question is, what is their weight, what is their thickness, what is their amplitude squared, what measure do we put on them in the way of the universe.

That requires action on our part to effect. Dan Inch says, what are your views on geopolitical, geopolitical tensions with China and Russia? As a physicist, you must have had a lot of contact with your peers in both countries and had probably visited one or both a couple of times. What is your perspective? Look, I mean, both China and Russia are, well, so number one, obvious caveat, I'm not an expert. I'm really just not. Sorry.

But, you know, like I said before, I had my opinions. China and Russia, in their own ways, in very, very different ways, are big, important countries with rich histories and vibrant cultures. The world would be best off if these two countries as well as others were stable, flourishing democracies that worked with the rest of the world to make us all a better place. Right now, neither China or Russia is anywhere close. to a democracy. The Chinese economy is doing well. Russian economy is not doing that well. Those are things that can change, as we know, over time.

One worries whether or not there are implications of the fact that these two giant countries are not democratic for the stability or fragility of other democracies that already exist. There's no law of nature that says that once democracies form, they stay forever. In fact, if anything, the law of nature is the opposite, right? there are plenty of people here in the United States who are envious or admiring of the Chinese system, less of the Russian system.

The Russian system is more gangstery, but the Chinese system, even though it is absolutely an autocracy where plenty of individuals don't have rights that we would consider to be important rights, it's efficient in some ways. They can get things done, right? I mean, there are people who admire that

and wistfully, wonder what it would be like if we could get things done like that here in our messy democracy. Personally, I am super duper dedicated to the idea of democracy, as flawed as it is, as obvious as the shortcomings are. It's way better than any other possibility. And my question about China in particular is whether or not their system is so resilient that democracy will just never catch on. You know, China, has never historically been democratic in any substantial way, for any substantial period of time.

And so, I mean, I don't think it's genetic or anything like that, but there is culture, there is history, there is, you know, education. And it might just be that people don't have the sort of cultural memory to say, like, yes, we should go back to being a democracy because they never have. And that, I think, is a terrible tragedy, given all the people who live in China. You know, I have visited China. I've never visited Russia. I had a wonderful time visiting China.

I mean, it's such an amazing place. I've had Chinese graduate students, plenty of them, actually. You know, lots of brilliant physicists come from China. There's no doubt about that. Brilliant thinkers in other ways. But it's a big, complicated country. You know, it's the biggest most complicated country on Earth. And, you know, who am I to make any judgments about it?

Let's just say that. Robert Callahan says, how would you now answer question number two of lecture 23 of your dark side of the universe Great Course's book. In that 2007 book, you anticipated the observation of gravitational waves. Now in 2020, what further do you anticipate in the dark side? I haven't looked at the book. I don't know exactly what I said.

It was in 2007, it was a safe bet that we would someday find gravitational waves, and we would someday find the Higgs boson, right? It might have been hard to predict exactly when, but, you know, within a reasonable period of time was a sensible thing to bet. I think it's safe to say that now there are no sure things like that. I mean, it's never a super duper sure thing, but there's nothing where you would say, oh, yeah, 99% chance. There's this hugely important thing that we haven't found yet, but we will find it, right?

You could say that in 2007, but it was the two examples were the Higgs and gravitational waves, and we found both of those. The next obvious thing, the most important, most likely thing to happen is we would directly detect the dark matter. But like I said, the chances of, you know, the ways that we have of doing that haven't yet paid off. And so the chances that we will do it on a short period of time are maybe less rosy than they were a little time ago. Still could happen any day, right? You don't know. But it's up in the air and there's no guarantee.

It's absolutely possible that whatever the dark matter is, it's not something we can detect with human. technology. So, so I don't know. We could find the other thing that we're looking for cosmologically is evidence of inflation in some way, in particular gravitational waves from the early universe. We thought briefly that we had seen those. Remember, Bicep 2, they went away. Some people make mistakes. That always happens. It's okay. But there, that's, I mean, that's not, that's just an idea that may not be right. It's not like dark matter that we have evidence for. So we'll look, but we don't know. That's all I can say, sorry.

Spencer Hargis says, in the many world's interpretation, how do computers behave? Let's assume my computer is not hooked up to peripherals or the internet. Can I assume that a process running on the computer will do exactly the same thing in almost all the branches of reality that result? Yes, in almost all, but not in all, that's exactly what you can assume. The right way to think about it is there's no difference between practical predictions. of the probability of observing one thing or another in many worlds versus any other interpretation of quantum mechanics. Not exactly true, sorry.

There are other interpretations where there's extra weird things, but the standard textbook interpretation of quantum mechanics gives you the same predictions as many worlds does for the kinds of experiments that standard textbook quantum mechanics is good for. So it has nothing to do with many worlds. You could rephrase your question as, what is the chance, given the rules of quantum mechanics,

that if I run my computer in a room all by itself, it will get the right answer. And the chance, hopefully, is very good, unless your computer is broken for some reason. And that's not my fault. Can't help you there. Anders says, I guess you don't write papers with this in mind, but which one of your papers is most likely to win you a Nobel Prize?

Yes, it is true that I do not write papers with that in mind. For a couple of reasons. One is, it's just not a healthy attitude to have, you know, trying to win the Nobel Prize. But another is there are certain kinds of papers for which you could win a Nobel Prize. Like the easiest way, sorry, not the easiest, there's no easy ways. The way that most people win Nobel Prize is to find something experimentally, right? To detect gravitational waves or to find the Higgs boson or whatever.

Although the Higgs boson people didn't win the Nobel Prize because the team was too large. Maybe they will win it. Hopefully they will win it soon. The theorists won it. For theorists who might want to win the Nobel Prize, the kinds of theory that will win it for you are generally writing down what we call a model. So what I mean by this is theorists can do different things. Sometimes theorists will try to, you know, explicate the general structure of some kind of theory, right? other times theorists will try to come up with a specific theory.

And it's that latter kind of thing, which we call model building, which is much more readily Nobel Prize worthy, right? If you build a model and your model turns out to be the right model, like when Stephen Weinberg wrote down the Electra Week theory in 1967, you're going to win the Nobel Prize. Everyone agrees on that. Okay, that's just easy. You got a little piece of reality.

you got it right. The conceptual stuff, it's harder. And I say this, knowing that Roger Penrose just won the Nobel Prize for a very conceptual thing. He didn't invent general relativity. He showed that general relativity, there were singularity theorems, which, if you believe in cosmic censorship, imply the existence of black holes. I would not have bet.

I was surprised that he won the Nobel Prize. I think it's very worthy of it, clearly, especially now that we have so much empirical evidence for black holes, but it's not generally the kind of... thing that nobels are given for. And it is the kind of thing that I like to write papers about, right? So my favorite papers are not model building, but sort of understanding the conceptual bases of things. That's not the Nobel's stock and trade, as it were.

Having said that, I've written models down, you know, it's a good, it's a good, healthy thing to do. I wrote down a theory of dark energy. You want to write down a model that makes very specific experimental predictions. I wrote down a model of dark energy that makes very specific experimental predictions for what is called cosmic birefringence rotating the plane of polarization of photons

they travel through the universe. And probably that is my most likely Nobel Prize winning paper because it's, I wrote the paper, you know, single author, I'm not sharing with 3,000 other people. I made a very specific prediction. Now, it's not a hard prediction. Like, you don't need to be genius.

to write that paper that I wrote. But, you know, if it turns out to be true, then you can get lucky. And I say this, you know, knowing that last week, someone wrote a paper claiming to have detected cosmic birefringes. Well, not to have detected, but to get very faint but plausible evidence for some birefringens. So if that, I might talk about that more in an upcoming podcast, but if that holds up, then that would be incredibly wonderfully important, would teach us a lot about dark energy, most likely,

and might even be the kind of thing that would increase my chances of winning a Nobel Prize from 10 to the minus 6 to 10 to the minus 4, something like that, or even more. But not very much in any event. The other thing that I did that is plausible, I mean, I wrote papers on Lorentz violation. If there was a Lorentz violating vector field,

I could possibly win it. I wrote papers on modifying gravity to explain the acceleration of the universe, but there are other people also wrote kind of similar papers, so it's harder to say. So, you know, I'm not, if you're on predicted or Ladbrokes or whatever betting site you like to hang out at,

I would not put a lot of money on me winning a Nobel Prize anytime soon. Joseph Tongretti says, quantum entanglement is usually described. So, sorry, let me just, the finishing thought on that is, even if my paper with Chip Sieben's on deriving the born rule in the many world's interpretation of quantum mechanics is 100% correct and revolutionizes the field of foundations of quantum mechanics,

zero chance I'm going to win the Nobel Prize for that, right? That's what I mean by this conceptual stuff, not being what the Nobel Prize committee is very interested in. So the kind of work that I care about the most and I'm most proud of is the kind that is least likely to win the Nobel Prize. you go. Okay. Joseph Tangretti says, quantum entanglement is usually described as happening between pairs of particles like spin up and spin down. Are there any circumstances where entanglement can occur among three or more particles all becoming entangled together? Yeah, absolutely. 100%. In fact, if you either read my part in something deeply hidden where I talk about immersion space time,

or if you go back to my solo podcast on emergent space time and gravity coming out of quantum mechanics, the whole scheme relies on the fact that there can be entanglement between an infinite number of degrees of freedom scattered across the universe. So yes, they very easily can be. And also three, not just infinity, three particles can be entangled. If you look up GHZ state, okay, you'll see various thought experiments you can do with three entangled spins.

Eric Coker says, you asked Jeremy England this week on how his work could be applied to our understanding of the anthropic principle and the fine-tuning of the universe. I want to let you answer your own question. Does studying non-equilibrium statistical mechanics and its relationship to life

change your feeling about the anthropic principle and fine-tuning? So it could change it. I mean, it wouldn't, sorry, it wouldn't change my feeling about it. It might change the specific numerical credence that I would attach to. the idea that we live in a multiverse versus we just live in a one-shot universe because that argument

cares a lot about the likelihood, the probability that life arises in a random universe, right? How finely tuned do the laws of physics need to be to allow for the existence of life? If really, really, really, really finely tuned, then that increases the probability of there's a multiverse and we're just seeing the part where life can exist. If life existing is pretty robust, then you don't need, you get less benefit from imagining that there is a multiverse. It's important to remind people that the multiverse was not invented to solve fine-tuning problems. It was invented because, I don't know who invented it, but it was not first pursued by working cosmologists in order to solve fine-tuning problems. It was pursued because theories predicted it, right?

the string theory landscape and inflationary cosmology. It was instantly realized that it might be relevant for fine-tuning problems. In fact, other people had mentioned that before, but it was never popular among cosmologists until the idea became predicted by responsible theories. Brent Meeker says, there is an idea that the universe started very small, but at the maximum possible entropy for its size.

The started small but at maximum entropy story implies that the number of available microstates increased with expansion, which would be irreversible and non-unitary. My question is, why should we, in the absence of a quantum theory of gravity, assume that the expansion of space cannot increase the microscopic degrees of freedom and allow the first story to be true? You can assume whatever you want, again, free country. It's always a matter of what scheme seems more plausible to you.

So for those of you who are not quite following this question, it makes perfect sense. If you think about the world as a box of gas, right? If you forget gravity, a bunch of photons or something like that. And imagine the world, you know, the universe is a sphere. So imagine it's a closed universe just to make our lives easier. So it's finite in size. Then when that sphere is small, there's only a certain number of things that the photons inside can do. And when it's bigger, there are more things that they can do.

And the maximum entropy changes depending on the size of the universe. So that's what Brent is referring to. Maybe when, you know, someone like me says the early universe was low entropy, maybe that's true, but what we really should say is it's high, as high entropy as it could have been, given that it was small. Now, my view is that that's completely nonsense for a whole bunch of reasons. One, it treats the size of the universe as an external parameter. In general relativity, that's not true. The size of the universe is part of the phase space. It's part of the set of degrees of freedom we use to describe the physical system.

You're not allowed to say, if the universe is small, what is its maximum entropy? Because the physical system that is that universe didn't have to be small, right? It could have been big. So the maximum entropy is when it's big. You're not allowed to sort of pre-existingly externally make it be small. The other thing is if you imagine the thought experiment of having universe collapse, okay? And it's exactly, if you believe in reversible laws of physics, then the set of things that can happen as the universe expands is the same set as what can happen as the universe collapses, just run backward in time. And our universe, I even wrote a paper about this.

I wrote a couple of papers, but there's one called, how finely tuned is the early universe or is the universe? If the universe collapses, it does not smooth itself out. It gets lumpier as it collapses. You make black holes, you make regions of empty space, the whole bit. And so you just would never get to this small region of the universe where everything is smooth, because most states don't look like that, right? So Brent is correct in saying, well, if we give up on the assumption of reversibility, then that would change the story, right?

If somehow the laws of physics were irreversible, If somehow there was some fundamental timekeeping in the universe that had the property that at what counted as early times, the space of possible configurations of the universe was just smaller than the space of possible configurations at late times, then that would explain the arrow of time. So sure, that would explain it, but it is entirely incompatible with everything we know about the fundamental laws of physics. So you can pick your poison here, right? I mean, yes, you can imagine throwing out everything we know about the fundamental laws of physics and inventing a solution to the arrow of time problem on the basis of that. But good luck, you know, coming up with a complete and rigorous and sensibly defined alternative to the current laws of physics, that's harder than it sounds.

So my favorite strategy moving forward is to stick with the general form of the laws of physics as we know them. not the specific laws, but the general form of those laws, and try to explain the arrow time within that. And within that, within our current framework for understanding laws of physics, the early universe has nowhere near as high of entropy as it could have had. Gary Miller says,

do physicists know for sure that infinity exists in the real world? No, physicists don't know anything for sure, literally nothing. No one knows anything for sure. You shouldn't care about knowing things for sure. You should care about what credence do we have on different ideas.

And we don't even know whether infinity exists in the real world with very high credence, right? If we're honest, we say we just don't know. That's one of these very, very deep questions where there's no experiment that we can currently put our fingers on that really gives us any information one way or the other. Bill Seltzer says, you've written that reality, all of it, is represented by a vector in Hilbert space. The world is a wave function in Hilbert space, and the world is a vector in Hilbert space. I think these are three different things that I'm purportedly said. I take these to be equivalent. Before Clinton, Herbert Feigel used to ask, what do you mean by is?

Is this the is of composition, as in a car is made of parts? The is of identity as in the morning star is the evening star, or the is of definition. A brother is a male sibling or predication. Ball is blue or some other is. So what kind of is do I mean when I make these statements? So when I say the world is a wave function in Hilbert space or the world is a vector in Hilbert space, I'm just being slightly sloppy. What I mean is the first formulation that you gave,

the world is represented by a vector in Hilbert space. A vector in Hilbert space is an abstract mathematical construction. As I've already said earlier in the AMA, the world is the world. The world is reality. So what I mean is representation. I forget whether this is one of your options. I think it's closest to identity, but not exactly.

I'm not saying that the world is identical to a vector in Hilper space, because vectors in Hilbert space are abstract, and the world is real and concrete, but all of the properties of the world map on isomorphically to the properties of a vector in Hilbert space. That is what I mean. Maxime Alexandrovich says,

if we are inside a black hole, could there still be branching happening, which would result in creating new two-dimensional layers on the event horizon as this would be according to the holographic principle? Well, there certainly could be branching inside a black hole. No problem with that whatsoever. You say, which would result in creating new two-dimensional layers on the event horizon?

I don't think that's how it would work. It would result in the... I mean, sorry, let me pause a little bit here, because I guess it depends on what we mean. You could define branches in different ways. This is why I'm hesitating. So here's an older question that is much easier to wrap your head around. If someone measures the spin of a particle 100 megaparsecs away, and they get spin up or spin down, are there now two copies of me right away? Like not 100 million years later, but right away, in some sense, did I branch all over the place?

And this is an interesting question for people who care about many worlds. And the answer is, you can say it either way. You know, you can read David Wallace's book, the Emergent Multiverse on the Many Worlds interpretation, and he talks about different schemes for how you could implement branching. So a similar thing, and the reason why is because branches are human contrivances, right? The reality, the quantum version of the Blas's demon knows the wave function of the universe. It's like a branches are like a coarse-grained emergent property, and so there's different ways to slice them, literally. Likewise for the black hole. So should I think of myself as branching when some quantum measurement happens inside a black hole?

You know, sadly, the answer is I could if I wanted to, but I don't have to. Okay. There's no observational difference to the exterior of the black hole. The black hole still seems to be a featureless thermal membrane as far as we're concerned. Pablo's Papa Georgi says, what shape is decoherence? Does the universe start out as a solid block of Hilbert space and decoherence hollows it out, giving zero amplitude to regions that are inconsistent and lowering its fractal dimension.

So no, you know, the universe is not a region of Hilbert space at all. It's one element of Hilbert space. There is a, there's some subtleties there because if you take two elements of Hilbert space that are just one as a numerical multiple of the other, then those count as the same physical state. You can scale the vector by any number. It's, you know, Hilber space is a vector space.

Visualize the two-dimensional vector space. If you multiply a vector by a number, it extends out in length, but goes in the same direction. So the higher-dimensional Hilbert space version of that is the same. You multiply by what matters is the direction in which the vector is pointing. But there's no block of Hilbert space, no region of Hilbert space that is being hollowed out. Now, having just a vector in a vector space doesn't seem like nearly enough information to be the world. And that is an ongoing research program that my friends and I are trying very hard to figure out.

How do you go from something as abstract and austere as a vector to the richness of the world? And the answer is going to depend on how you choose to represent that vector. Hilbert Space has a little bit of structure because of the Hamiltonian, which drives quantum states to evolve, gives it a bit of structure. but telling that story specifically is a hard and ongoing project that we're working on. Keith asks, do you know of any theories

that toy with the idea that dark matter is residual, heavily decayed gravitational interaction with wave function branches? I don't know of any theories like that, but you know,

there isn't any, there shouldn't be any residual gravitational interaction between wave function branches. The branches are really distinct. You know, you can numerically quantum how distinct they are. It might not be 100%, but it's 99.999,99, etc. And remember, there's more dark

matter in the universe than there is ordinary matter. But much more importantly, the dark matter acts just like a non-interacting, non-relativistic particle. You can make predictions on the basis of numerical simulations about how dark matter should cluster in galaxies and clusters, how it should affect the growth of structure and the cosmic microwave background, based on on the hypothesis that it's a particle that doesn't interact with other particles. It just moves slowly through space. And all those predictions come true. So you can bend over backwards to come up with a more elaborate theory of dark matter.

But it would be weird if a very elaborate theory of dark matter had all exactly of those properties. You know, once you have some kind of theory in which the dark matter interacts or is either self-interacting or interacting with ordinary matter, usually those theories are ruled out very, very quickly. Not always, but you have to work hard at it. Steve M. says, I ask your opinion on the quantum article about the velocity of tunneling particles faster than light. Nope, I have no idea what the article says, so I can't really give you any information there. But I can say, you know, as background, the whole idea of particles tunneling faster than light is only possible because

particles aren't particles in quantum mechanics. They're wave functions. And how do you define where a wave is? Well, you can try, but there's some ambiguity there. And that ambiguity gives you the wiggle room that you can use to say, oh, it move faster than light. But nothing is violating the rules of relativity, I promise you. Ben Nichols says, what are your thoughts on the axis of evil anomaly?

Do you think it is a statistical error bias or a real phenomenon? So the axis of evil is an idea. that there is some structure in the cosmic microwave background that picks out a preferred direction. Honestly, I'm not a super expert on it. So I have a small Bayesian prior that something like that is going to be real, just like I have small basium priors for any weird cosmological phenomena. So it's worth studying, worth getting more data on. But I don't, I haven't heard a lot of people talking about it lately. I think it's kind of faded away. I worked on similar ideas myself. I mean, maybe this is another

is it another way I could win the Nobel Prize? I did have a model with Lottie Ackerman and Mark Wise about preferred direction in space, but it turned out to make different predictions than the axis of evil. So that would be something different. Brad Malt says, if you were writing your Higgs boson book today instead of 2012, what would be the biggest difference? Or to put it another way, what is the most exciting

development to have come out of CERN that isn't included in your book? So the sad news, Brad, is, that the biggest difference is that I could say, you know, after we found the Higgs boson, we kept looking for other particles and we haven't found any. That's the big news. I forget, I wrote a blog post once where I made predictions before the LHC turned on about what it would see. And, you know, I had some hefty probability on finding nothing.

Not greater than 50%, though. It was less than 50%. I thought we would find something else besides the Higgs boson. But that seems to be what's happened. And, you know, that is a kind of clue that guides future exploration, but it's not the most helpful kind of clue, right? And, you know, okay, you found nothing. So what are you going to do? Like, believe me, if we found any other particles or even any other interactions between existing particles at the LHC, other than the minimal standard model Higgs boson, you would know.

That would be front page news. Bendy Bruce says, should we build a bigger Hadron Collider and are there reasons why or why not? Well, you know, I think the basic idea here is extremely simple. If we don't find anything else at the LHC, which the jury is still out, finding new particles is extraordinarily helpful in pushing the frontiers of physics forward. So we can understand more and more things. And you can try to do that by a cleverness, right? You can try to do it in basement, tabletop experiments, or via astrophysical information or something like that.

But there's no guarantees you find anything, and those techniques are way less powerful than building a giant collider for finding new particles. By far, the best most productive way to find new particles or new physical phenomena is to build a giant collider. That's the one side. The other side is it costs money. That is literally the only reason not to build a bigger Hadron Collider is that it costs money. And that's not a trivial reason. It costs a lot of money, right? It's a non-trivial chunk of cash.

You can play the games where if you imagine every human being on earth contributing once per day or whatever, it's pennies or less that would require to build this thing, but you can play that game with lots of different costs. In any realistic accounting, it's expensive to build a bigger Hadron Collider. I would love to see it built. Is it worth the money? Is something that individual governments are going to have to decide?

Well, not individual governments. No individual government. has the capability to do it, except maybe the U.S., and we have no interest. So it's going to be an international collaboration, either in China or in Europe. Yet another sign that the United States has left behind the leadership of the world in an important way. Philip Maimon says, if someone discovers a new law of nature but dies before telling anyone, and no one ever learns it, no one else ever learns it, and no one's lives are changing anyway, was that still science?

Or does science require some communication or impact on the world? On the one hand, sure, of course it's science. You discovered something new about the world. You know, after all, eventually we'll all be dead, and science will have nevertheless happened in our lives while we were going on here. On the other hand, I can't really get that emotionally invested in questions like this. I mean, you're not asking about what happened, you're asking what label we should put on what happened. And I'm pretty pluralistic about that, but whatever labels on it you want.

If you don't want to call that science, knock yourself out. Chin says your take on the level of reality. I presume what you mean is how real is the following quote. In some way or other, space time itself seems to fall apart at a black hole, implying it space time is not the root level of reality, but immersion structure from something deeper. And this is from a recent quantum magazine article by George Musser

about the black hole information paradox. Yeah, I'm very sympathetic to exactly these words, exactly this philosophy. And by the way, this quantum magazine article is an explication of some of the stuff we talked about with Netta Englehart on the podcast recently. In fact, Netta's picture is in the article, so you should check that out. The article is good. The headline is way overblown. That's why I didn't retweet it or anything like that. It says basically we're close to solving the Black Hole Information Paradox. And if you remember

listening to Neta, she explicitly says, we do not know whether or not this is the right answer. So I'd be much more cautious than Quanta was in this case. Christoph Puransky says, Why do we insist that extra dimensions are small and curled up inside our 3D space? Why is our whole 3D universe not a slice of some higher dimensional space which we can't reach

the same way that Flatlanders can't leave Flatland? It's possible that we do live on a 3D universe embedded in a higher dimensional space. The traditional reason why that would have been hard to imagine being true is because even though you can confine things like the particles and forces of the standard model to a three-dimensional sub-manifold, which we call a brain, right, B-R-A-N-E from membrane, we could be confined to a three-brain in a higher-dimensional space.

But there's one thing that is hard to confine to a brain which is gravity. Gravity is the curvature of space-time. If you're in a bigger space-time, gravity leaks out into the bigger space-time. And if gravity leaks out into the bigger space-time, you don't have an inverse square law for gravity. The inverse square law comes about because space is three-dimensional. If there are extra hidden dimensions of space

that are big, then gravity can leak out into them. So the ways to get around that are either to make the extra dimensions small, so the gravity can't leak out in them, a traditional way, or to warp them. This is what Lisa Randall and Robin Sundrum became famous for, the idea of warped extra dimensions. They can literally be infinitely big,

but so highly curved that the ground, gravitational field cannot leak out into them. So that is absolutely a possibility on the table for how dimensions work. Martin Lesser says, a well-known biographer of Einstein, among others, has stated that to solve problems involving acceleration, one must use general relativity. The idea that special relativity cannot deal with acceleration seems to occur fairly often. What are your feelings about this, and have I missed something in understanding what these people are talking about?

So just to be clear, and I think Martin and I are on the same side here, there's zero problem with talking about acceleration in special relativity. The difference between general relativity and special relativity has to do with the fact that in special relativity, space time is a fixed background with zero curvature, which we call Minkowski space, and there's no such thing as gravity. In general relativity, space time is similar to special relativity, but it's dynamical. It can be warped and bent, and we experience that warping and bending as gravity. So you notice that nowhere in those words

did I use anything like acceleration, right? Of course, without gravity, I can still accelerate. Nothing wrong with being a rocket ship and accelerating whether or not there's any gravity around at all. The reason why people sometimes make the mistake, this mistake, is because there's something you can do in special relativity that you can't do in general relativity, precisely because space time is rigid and fixed,

And in a very special flat way, thus the name special relativity, there's a kind of coordinate system you can set up in space time that is adapted to the motion of unaccelerated observers, what we call inertial observers. So if you're moving in special relativity unaccelerated, you can define a reference frame and extend it uniquely throughout the universe. Okay? And you can define what you mean by simultaneity or distance or time

based on those reference frames. In general relativity, even if you're unaccelerated, you cannot extend your reference frame uniquely throughout the whole universe because space time is curved, and it would sort of get in the way in some way. So in some sense, the combination of being unaccelerated and in special relativity gives you a special ability to do something that you couldn't otherwise do.

But who cares? Who cares about extending your reference frame throughout the universe? just because I can extend my reference frame uniquely if I'm not accelerated doesn't mean I can't imagine being accelerated. I mean, my goodness, the twin paradox, one of the most popular things you talk about in special relativity requires that a twin accelerate off

in a rocket and then accelerate back. It'll be really weird to say you can't talk about the twin paradox in special relativity. All right, okay. So it's completely misapprehensible. even if you can try to invent some reason why they would have that misapprehension. George Robinson says, photon pairs can be created which are entangled. It is also the case that individual photon wave functions can pass right through each other without interference.

But an individual photon can interfere with itself, question mark. So I don't think you quite nailed having a question there, George, even though you have a question mark. But I think I get what you're at. However, let me back up a little bit here. You have to distinguish between the wave function of the photon and the classical electromagnetic field. They're related to each other conceptually, and if you sort of visualize them in your head, they might be similar, but they're different things. They're fundamentally different. So when you say words like individual photon wave functions can pass right through each other without interference,

I suspect that what you mean is the individual electromagnetic field pulses that you might associate with photons can travel through each other without interference. Of course, they don't travel through each other without interference. If you have an electromagnetic field that is vibrating up and down and you have two wave packets that are headed toward each other, then they absolutely will either constructively interfere or destructively interfere. with each other when they overlap. It is also true that an individual photon's wave function can interfere with itself, but that's a different kind of thing. So I hope that makes some sense.

I'm not quite sure if that's addressing your question, but I did my best. Nicholas Viberg says, I heard about ongoing activities to measure the impact of gravity on antimatter. Are there respectable theories in which antimatter behaves differently compared to normal matter, perhaps even with a repulsive force? How would that fit with general relativity where gravity is not a force, but a curvature in space time? So I think there are theories, but they are not respectable, is the right way to say it. I think there's basically zero good reason to think that matter and antimatter should behave any differently at all in a gravitational field.

You know, and we talked earlier about test particles, small particles, whether they're electrons or positrons, are roughly speaking test particles. They move on geodesics of spacetime. So they would behave exactly the same in the gravitational field of the Earth or the Sun, for example. You can try very hard to invent theories that predict something else. Then you can go experimentally test them. That's a good idea because if you find something, you'll win the Nobel Prize, much more likely than I will. But the reason why your overall chances are still small is because probably gravity doesn't behave like that.

That's why it's hard to win the Nobel Prize as a theorist, because either you can do sensible work within, the frameworks we already understand, and that's not the kind of thing Nobel Prizes are given for, or you can propose laws beyond what we understand, which is great, but they're probably wrong, because most proposals are going to be wrong. There's many more ways to be wrong than to be right. That's why physics is exciting, but also kind of hard. Steven Scully says, do you feel classical mechanics is adequately represented in academic pursuit of fundamental physics, and if so, why is no representation enough? Again, I'm not quite sure I parsed the sentence there.

But classical mechanics is absolutely part of academic physics. I mean, we teach it, obviously, but there are people who do research on it, usually under the name of non-linear dynamics, because linear dynamics is too simple. Like, you know, linear just means if I, the harder I push, the faster something moves, right? Non-linear dynamics means that the response that something has when I act on it obeys a complicated relationship. It's not a simple input-output proportionality. is something more elaborate than that.

And nonlinear dynamics is very important. Most real dynamics in the world is nonlinear. I think the point is the basic rules of classical mechanics were set down by Isaac Newton and they haven't changed. People like Hamilton and Lagrange and so forth came up with alternative formulations of the same rules, and that was good, that was fun. Those alternative formulations are useful for various purposes,

but the rules haven't changed. So it's not like particle physics. where you could imagine inventing a new particle and someday detecting it. There's not a lot of money to be made inventing new rules for classical mechanics. That game is kind of over. Gerard Driven says,

if spacetime is a lot of fields together and these same fields are here in us and around us and even torn apart in Geneva, then sometimes I think that the mass of the Earth is nothing else than rolled up space time. The question is, are there tiny black holes to be found in regions close to the smallest scale near the Planck scale?

We think so. So, you know, so this is a quantum gravity question. We don't understand quantum gravity. Whenever you're combining gravity and quantum mechanics, you should tread lightly because we don't know any answers for sure. So if I can translate your question into the more traditional language of particle physics,

you know, when two electrons interact with each other, in a Feynman diagram kind of description, we draw pictures of the two electrons passing photons back and forth to each other. We label these as virtual photons, and we say the reason why the electrons scatter off of each other is because of the exchange of virtual photons. So can you say the same thing about black holes? You know, since electrons interact with gravity and black holes are little bits of gravity, little bits of curved space time, is part of the interaction between two electrons due to the exchange of virtual black holes. holes. And the answer is yes. We believe the answer is yes. It's a little bit speculative, but not too much so. There's things that should happen just on the basis of the robust features of quantum field theory. The reason why no one cares too much is because gravity is

really, really, really, really weak. You know, if you've ever been an undergraduate physics major and had to do the Cavendish experiment, had to measure the gravitational force between two heavy objects, but in a lab, it's really, really hard because gravity is weak. Imagine measuring the gravitational force between two electrons. That's very, very, in fact, it's impossible, technologically speaking. And to imagine measuring the part of the gravitational force that is due to the exchange of virtual black holes is harder still. This goes, this harkens back a little bit to the earlier question about Wheeler's geometro dynamics, you know, is there space-time foam at the Planck scale? Does that contribute to particle physics in some way? These are the kind of words that are

fun to say, but don't quite add up to a sensible theory in the sense of our other theories of physics right now. So it's fun to think about, but until you turn it into a concrete mathematical proposal, it's really hard to know what to make of it. Yidna Ferdivik, sorry, I'm not sure I to pronounce the name, makes a suggestion about a future guest, Geraldine Heng, who wrote about England and the Jews. Again, thanks for the suggestion. Maybe the AMAs are not the best place to put the suggestions. You know, honestly, you can just go to Patreon and send me a message

if you will have a suggestion. I get a lot of suggestions. So please do not feel bad if your particular suggestion is not taken up on. But I do look into all of them. You know, I take them seriously. Oriah Biddle says, is creating a black hole in a lab on earth physically possible? and if so, why haven't we done it yet?

Well, it might be hard to get permits from the International Regulatory Commission's, but it's possible, sure, to make a black hole. If you mean metaphysically possible, is it conceivable? Sure. Is it technologically practical? Is there any way that we know how to do it? No, you need an enormous amount of matter and energy

in a very small region of space to make a black hole. We don't know how to do that. Now, there was an idea that was very, popular and is sort of faded away a little bit that the large Hadron Collider or another big particle accelerator could make black holes. And that was based on the idea of extra dimensions of space, because if you have large extra dimensions of space, then gravity can leak out into them a little bit. And what that means is that gravity is stronger than you think, because the reason why we perceive gravity is weak in our world, according to these models, is because it's

largely leaking out into the extra dimensions, and we only feel a little remnant of it in some sense. And what that means is that if gravity is much stronger than you think, it's much easier to make black holes than you think. So this was the idea that maybe you could actually make black holes of the Large Hadron Collider. But as we said, it didn't happen. Nothing happened other than the Higgs boson.

So there's been no evidence so far for black holes at the LHC. Probably those models were just on the wrong track. And if they are, if they're not right, then there's no feasible way that we can think up to make black holes here on Earth. Abdul Uppsal says, I've seen William Lane Craig and others argue that causality can be atemporal. One example they often cite is taken from Kant, where they invite you to imagine a heavy ball resting on a cushion from eternity past

and causing an indentation of the cushion. Prima Fashi, this makes no sense to me, as it seems to me to be a snapshot of a dynamical process in time, where exactly is my reasoning going wrong? I agree with you. That is a bad example, the heavy ball causing an indentation on the cushion. But there might be better examples. So it depends on what you mean by causality.

Okay. I mean, causality is an ancient concept. Aristotle talked about it 2,500 years ago. And Aristotle didn't know about modern physics. He didn't know about Lepas's demon or quantum field theory or any of those things. So it should be the least surprising thing in the world to imagine that our modern view of causality, needs to be a little bit different than our conventional,

intuitive, natural language view of causality. So there's a view of causality, which is fundamentally temporal, right? Causes precede effects, we hear. But there's other notions of causality. So, for example, you can ask, why is electric charge conserved? And someone could say,

well, because of gauge invariance, because of a symmetry of the underlying equations of electromagnetism. So in some perfectly reasonable sense, someone can say electric charge is conserved because of gauge invariance in the underlying equations. That would be a good example of an atemporal cause. It's not that gauge invariance happens and then electric charge is conserved. They both happen simultaneously. But more important, you have to recognize that this notion of the word cause is just one of the various notions.

So you have to be very careful to specify what you mean when you say X causes Y. Sam Hartzog says, could you elaborate a little on the concept of scale invariance and what if any relation it has to inflationary cosmology? Yeah, sure. You know, I talked about scale invariance again in one of the biggest ideas videos. I had a video on criticality and complexity. And criticality in this sense is very closely related to the concept of scale invariance. It means something is happening on all scale.

Right. So rather than, so the example that is often used is the height of, let's say, you know, adult American women, okay? Not all adult American women are the same height, but there is a common height. There's sort of a traditional height. There's an average height that is well defined. You know, women are not scale invariant, nor are men or are any other organisms. So it's, it's very unlikely to find one foot tall adult American women. And it's very unlikely to find 20-foot-tall adult American women. Somewhere in between is where they lie. Whereas scale invariant phenomena, you can find something going on at every possible scale. And there are examples of that in nature. The application to cosmology is that when we look at large-scale structure in the universe, either directly in galaxies or in the microwave background, the evidence seems to be compatible with the idea that at early times,

the initial primordial perturbations in density that grew into large-scale structure were scale-free. We're basically the same amplitude at every length scale. So the variations on large scales that stretched over what are now megaparsecs and the variations on small scales that stretched over what are now meters or kilometers were of the same rough size. That seems to be an experimentally, observational, empirically true fact about the universe. inflation can explain that. Inflationary cosmology, when thought of as a theory of the origin of density perturbations,

generically robustly predicts approximately scale-free perturbations because the rate of inflation is approximately constant. So inflation is continually creating new fluctuations, and they get stretched by the expansion of the universe. So inflation creates fluctuations at all length scales of approximately, but not exactly the same size. And that is size in the sense of amplitude, not a linear size. And that's more or less what we see.

So to that extent, inflation is a very nice way of explaining the scale-free behavior that we see in cosmological perturbations. John Eastman says, are atoms shrinking rather than space expanding? Why is this the question? Why is this the big question today? I don't really know. See earlier comments. You can think of it that way, but I wouldn't recommend it. see my answers get shorter every time I answer the same question.

Mark Moore says, is it reasonable to ask what the pulse width of a photon is? No, not really. There's really no such well-defined thing, you know. It's tricky once you get into it because photons are intrinsically quantum mechanical objects, right? So you should first, you know, get familiar with the classical theory of the electromagnetic field. And so you can say, okay, if you ever take an electromagnetic, if you've taken a class in it, you realize that the first thing that they tell you about when you study waves or radiation are what are called plane waves. That is to say like a sine wave or a cosine wave that literally stretches forever. It has no edges, right? It just goes on throughout the whole universe. We all know that's unrealistic, but it's very easy to solve the equations in that case. And there's no size of it. It has a wavelength. It oscillates up and down, but it stretches throughout the universe. And then you learn a little bit

bit about Fourier transforms and other mathematical techniques, and you learn that you can take those kinds of plane waves with different frequencies, different wavelengths. Every individual plane wave stretches forever throughout the universe, but a specific combination of plane waves can describe a localized wave packet. We already mentioned wave packets when we're talking about the speed of quantum tunneling. So you can get a classical electromagnetic field that is sort of more or less zero outside a certain region and inside a certain region, it's oscillating and that little pulse oscillation moves at the speed of light. But that's all classical talk, right? We haven't talked yet about quantum mechanics. I'm not going to go into the whole thing, but each individual plane wave

is what we first, we play the same game over again in quantum mechanics that I just mentioned in classical mechanics. Every plane wave is the thing that can quantum mechanically describe zero particles, zero photons, one photon, two photons, et cetera. And then you can superimpose, you can combine the one photon state corresponding to many different wavelengths to make a one photon state that is a localized wave packet. But you can do that with any width you want, as long as it's compatible with, you know, being at least as long as the wavelength itself of the electromagnetic waves. So James O'Sullivan says, Veritasium just made a video. about how it's impossible to measure the one-way speed of light, which blew my mind.

Do you have any ideas about what this might mean in terms of new theories of gravity? So I did not see the video, so I can't comment on that, and I'm not exactly sure what the argument is that he has in mind. I'm sure it's respectable, and I'm pretty sure it has zero meaning for new theories of gravity. You know, measuring the speed of light is a question whether or not, you know, the extent of which you can do it and what it means to do it is formulated purely within the context of special relativity, where there's no gravity at all. So whatever your difficulties are in measuring the speed of light, it has zero impact on gravity, which is another kind of thing.

Patrick Hall says, what is your opinion on the reality of ordinary macroscopic objects? Are they real, or are they simply illusions of our object discriminatory faculties, or neither of those? Yeah, I think they're real. I'm sitting on a chair. I think it's real. I do not think a chair I'm sitting on is an illusion

of my discriminatory faculties. You know, you can talk yourself into thinking that the chair is an illusion, but why would you do that? What do you gain by doing that? You know, I think that it's a weird definition of illusion that you would have to believe in to think that the chair I'm sitting on is an illusion. It's not fundamental. Chairs are nowhere to be found in the standard model of particle physics. It's a macroscopic emergent feature of the world, but it is part of what Daniel Dennett

calls real patterns. Go back to the podcast I did with Dan. The idea, the concept of a chair, is very useful when you discuss the world, right? You know, if you think about the world as a giant vector in Hilbert space or even just classically as a collection of a whole bunch of particles, there's an infinite, literally infinite number of ways you can imagine carving up that world into parts and then describing how the parts interact with each other. Almost all of those ways are going to be useless. But some ways of carving the world up into parts are really, really useful. When someone points over to one side of the room and says,

could you bring that chair over, you know what that means, right? Information has successfully been conveyed to you. That's not what an illusion is like. An illusion is like, you know, you see an oasis in the desert. But it's really just vibrations from the heat that are fooling you into thinking that there's water there. That illusion has no causal efficacy. You can say, oh, I'm thirsty, I will walk over to the oasis, but there's nothing there. You're not going to get any water, right?

When you have concepts that really do have useful causal efficacy in the world, I think that we should just call those real. Okay, last question. We've reached the end. Robert Grinice says, can you explain the page curve, aimed after a dawn page, and what it tells us about the information escaping a black hole? So this is also something, a concept that's mentioned. I think that Neta Englehart mentioned it, and it's certainly mentioned in the Quanta article that we referred to that is very recent.

The page curve is just this. When you make a black hole and you let it evaporate, like Stephen Hawking says you might imagine doing, you want to know, is it possible that the information gets out, that the information that went into the black hole is contained in the radiation that is leaving it, right? And if it is, what would that allow you to say about the radiation?

Because Hawking's calculation, what Hawking did in the 1970s gives you no way for the information to get out. It makes a prediction based on some assumptions, and we think those assumptions can't be right. But under Hawking's assumptions, the radiation that comes out is exactly the same for every black hole, regardless what kind of information went into making it. So Don Page asked, well, what if the information somehow does get out? And what if, which many of us think, but it's not exactly completely proven, you know, what if the entropy that we're talking about here is the entanglement entropy, the quantum mechanical entanglement entropy, okay? So as soon as you make the black hole, information starts coming out, or sorry, photons start coming out, right?

Radiation starts coming out. And even if that, well, it's not even if, if the information is going to get out, then those photons have to be entangled with what's going on inside the black hole. Otherwise, they can't carry out information. Okay, the quantum information is largely contained in the entanglement. So what that means is that the photons coming out have entropy. Entropy is one way quantum mechanical entropy is called sometimes entanglement entropy. It's a way of quantifying the degree of entanglement.

So as more and more radiation comes out, the entropy of that whole set of radiation goes up. It increases with time until you hit a point where, roughly speaking, the number of degrees of freedom inside the black hole becomes equal to or less than the number of degrees of freedom in the radiation that's already escaped. And in that case, it is the black hole that is the smaller subsystem and the radiation that is the bigger subsystem. and there's sort of less room for there to be entanglement because the black hole is shrinking and shrinking and shrinking. There's less stuff for the radiation to be entangled with. So the page curve is just the curve, the plot, the graphical representation, of the entanglement entropy of the radiation escaping a black hole.

And it's the simplest curve in the world. It goes up and then it hits a peak and then it goes down. That's the page curve. And the point of the reason why it's relevant is because it's a nice little quantitative thing that it doesn't follow from an elaborate calculation. It follows from the robust basic features that you want information to get out.

And it's one of the things that is a target for you if you're inventing a new theory of quantum gravity or a new way of thinking about quantum gravity. You want to be able to predict the page curve. So when Page predicted it, he didn't have a theory that predicted it. He just said this had better be the case

if information gets out. So a sort of standard test that you want to put your idea through, if you have an idea about how the information gets out, is it better reproduce the page curve? I'm not sure if that made sense that explanation, but I think what I said is true. It's also explaining the Quanta article, so you can check it out there. And I think that is the end of the questions.

At least, I mean, this is a long thing. Probably more questions have appeared as I'm doing it, but I'm going to call it victorious for the day. Happy election week, everybody. Happy quarantine. Happy whatever we're going through in the world. Hang in there. We'll all make it. It'll all be okay. I promise. Okay. Bye-bye.