Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas - AMA | March 2021

Episode Date: March 10, 2021

Welcome to the March 2021 Ask Me Anything episode of Mindscape! These are funded by Patreon supporters (who are also the ones asking the questions). With an expanding number of questions, it's become a bit impractical for me to try to rush through and answer them all. So instead, this time I have picked out certain questions to tackle, and grouped some together if they were related. I tried to pick questions on the basis of whether or not I had anything interesting to say in response, but that will of course be in the ear of the listener. Support Mindscape on Patreon.

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Ask Me Anything episode of the Mindscape Podcast. I'm your host, Sean Carroll. And as most of you know, this is something that is made possible by donations from patrons on Patreon. So once a month, we do an episode where Patreon, get to ask questions, and then I will try to answer them, and then a couple weeks later, we'll get the transcript made and release the results into the wild, into the public. So thanks to the patrons for supporting this, and the number of patrons and the number of questions has been growing, so we've finally reached a point where, rather than just trying

to answer every question, I will pick some subset of the questions to answer those. And so, therefore, this is the first time we've been doing it, and we'll see how it goes. let me know. Please do feel free to chime in your opinions as to how it's going. How do I choose which questions to answer you might ask? So it turns out that, well, you know, I'm still trying to answer a lot of questions. I don't want to just pick five questions out of 100 or 200 and answer those. I want to give as many people a chance to ask something as possible. But it's a weird, unpredictable set of criteria that I use, actually, and maybe not even completely explicable or articulatable. Some questions, for example, were perfectly good questions. Most questions are

really good in some context, but I just don't have anything interesting to say about them. So I skipped those. And some of those were sort of fun questions, like, what's your favorite dessert? Well, I don't have a very fun favorite dessert. I don't know, ice cream, apple pie, something like that. So it just didn't seem to be the kind of thing to, you know, give myself time to expostulate on when I have nothing to say. So oftentimes, if I don't pick your question, it's more a comment on me than a comment on your question, okay? And other questions are just also interesting, but in some realm that I either don't have anything to say about, or I've said too much about already, and I'm interested in saying other things. So, but I think it'll work out pretty well. Anyway, you'll let me

know, right? We still have plenty of questions to get through, so let's go. So before we officially start, let me have some words on time dilation and special relativity and those kinds of questions, because I actually called them out in the little call for questions. I mentioned that these are not my favorite kinds of questions, and yet some were asked, fewer than usual. But so let me, rather than answer them, explain why these are not my favorite questions. Because it's not like they're, again, bad questions or perfectly good questions, and it's certainly these questions are asked by people who are trying better to understand difficult physics, right?

So that's the kind of thing that I wanted to encourage. But it's a specific genre of question that I just get tired of answering myself because there is some kind of phase transition your brain goes through when you learn relativity from the point where these questions about time dilation and either in special relativity or in general relativity when you have black holes or whatever. They go from being just completely ineffably mysterious to like completely obvious. You get it, right? You like, it suddenly clicks and you understand it. And once you reach that point, then sort of articulating it over and over again, you just find yourself repeating yourself and it's less interesting. So let me try to sort of give my once and for all answer to how one should think about this kind of thing. You know, I noticed it again on Twitter recently when we just recently had the Lander, the Perseverance Lander landing on Mars.

And someone, you know, we know that Mars is seven light minutes away. roughly speaking, so it depends on the place it is in the orbit and so forth, but there's a delay from the signal getting from there to us. And there's a perfect opportunity to talk about special relativity, which I didn't really do, but I just, you know, made a joke about the fact that someone said, we know, we have, we know that the landing has either happened or it had not happened, but we don't yet know whether it's been successful. And I said on Twitter, you know, actually, I was that guy. I said, well, in some reference frame, it's happened. or not happened. In another reference frame, it hasn't happened yet, right? Because Mars is sufficiently

far away that you cannot blithely extend your local reference frame in a unique way. People moving at different speeds will get different answers to what's going on. And which is, you know, not the world's best observation or anything like that, but people objected to it, right? People are like, no, no, there is a fact of the matter about whether it's landed or not. You just don't know. And I think that represents just a misunderstanding of special relativity. I made a follow-up joke, which was better about the fact that, ah, the landing is now in our past light cone, because being in our past light cone is in fact a real physical thing. Happening at the same time is not a real physical thing. That's exactly what special relativity is about.

You cannot define simultaneity for events that are very, very far away in space. So what's going on there? What is the whole issue that is such a difficulty? And I think it comes back to the fact that our intuition, our view of the world that we grow up with is very much steeped in the Newtonian limit, if you want to think about it that way, of special relativity, right? Newtonian mechanics can be thought of as a limiting case of special relativity

that is perfectly valid as long as all of the objects that you're talking about are moving much more slowly than the speed of light, which you and I are, we are moving through our lives, even if we're in a very fast jet airplane, much slower than the speed of light, okay? So as a result, there are two different things, two different notions of the word time that are separate in special relativity, but are the same in Newtonian mechanics. So let's see where that comes from. You know, there's a notion of time that is our measured time on our clocks, okay? The time that clocks measure.

So you and I, we can have watches and we can measure time, and if we move through the universe, we can both equally measure time, right? and as long as we're moving slowly with respect to the speed of light, with respect to each other, then the following thing happens. You and I can get together and we can synchronize our watches, right? Assuming we're wearing accurate timekeeping devices, we can synchronize our watches so we know they measure the same time. And then we go off and we do our things, you know, we execute our bank heist or whatever it is, then we meet up afterward and we look at our watches, and if they read the same when we started

and their accurate timepieces, they still. measure the same. So, and that's going to be true for any number of people in the universe, along again, as you're in this Newtonian limit, you're moving slowly compared to the speed of light. So what this makes you do, what it makes Isaac Newton do and all of us and our intuitions, is to associate the passage of time, not with individual people or timekeeping devices, but with the universe, right? With something objective and real and out there, you think that we're just measuring the passage of time, right? That's what our... watches apparently are doing. So special relativity comes along and says, no, that is not what we're doing.

The objective, real physical thing is the time measured by observers along their path. But that time is not universal. It is not objective. It is, as I've often said, it is exactly like distance in space. You can have a path between two points in space, and you have a straight line path or you can have a curvy path, no one in their right minds ever thinks that all paths between two points in space must be of the same distance, right? The straight line path is the shortest distance one. There are also curved distance paths that will be longer distances. So special relativity says that the time between two events, as measured by observers carrying their watches, is like that. It's like distance. It will be different for different observers. You don't notice it usually, because you all move

slowly compared to the speed of light. But if you did, start moving fast compared to the speed of light, not fast compared to the speed of light, but at or near the speed of light, right? And you left the same event in space time where you synchronized your clocks and you go off on two different paths and you come back and you meet again at some other event,

the time that passes on your watch and the time that passes on someone else's watch will be different, okay? Because it's just like taking two different paths through space. you're measuring two different things. The spacetime interval, the temporal interval, are what physicists call the proper time on two very different trajectories.

Why should they be the same? Even if they begin at the same event and end at the same event, if they're different trajectories, they will have different proper times, okay? So what this means is that it separates the notion of the time that you feel and you experience the proper time,

and that's an objective, real, measurable thing from an entirely different idea, which is the coordinate that we put on space time to locate things. Okay, so we might put coordinates on the universe or even just on our local solar system or something like that that tell us where and when things are, a full four-dimensional coordinate system, okay? And we all know that coordinate systems are not physical, real objective things.

They are inventions of the human mind. one person can use one coordinate system, another person can use another coordinate system. So special relativity says that if you want to talk about the time it is right now here on Earth and try to extend that to a notion of time on Mars, you can do that by choosing a coordinate system, right? Typically, what we do implicitly is we imagine that we are moving slowly compared to the speed of light, And we, you know, imagine send out a whole army of little clocks that fill space that start their lives in our rest frame and move very, very slowly to get to their locations. And so we extend our rest frame throughout the universe. But that's highly non-unique.

If someone else have been moving at a different speed, or if you just pick some weird curvilinear coordinate system, that would also be just as good. Okay. So the very notion of when the thing on Mars landed, when perseverance landed from our point of view, that's just not a well-defined notion in special relativity. What you can say is it's in our past, if it's in our past light cone. If a signal from that event could get to us by moving at or more slowly than the speed of light, then it's in our past. If it's not, then all you can say is it's not. Technically the term would be it's space-like separated from us, but there's no deep reality to the idea of did it happen already yet or not.

That notion of simultaneity is just not well-defined. It's like saying, you know, it's really exactly like saying, okay, here I am sitting in Los Angeles. Where would I be if I were in New York City? I mean, that just makes no sense. There's no unique way of extending my location throughout space. space, right? I could put some coordinates, I could put X, Y coordinates, latitude, longitude coordinates, et cetera.

I could say, am I at the same latitude or whatever? But that's a completely arbitrary choice. So all of the questions in special relativity about time dilation, length contraction, all of these things, they're all instantly dissolved in your head if you really believe that how time should be measured is along individual trajectories. There's the extra little confusion that for space, the shortest distance between two points is a straight line. For time, the longest time between two events is a straight line. So if you zoom out of the speed of light or you hang out near a black hole, you will always have experienced less time when you reconvene to compare your clocks.

But otherwise, the math works out very, very similarly. So I just never like it when people talk about, well, time is flowing more slowly near a black hole, or time moves more slowly when you move near the speed of light. That's nonsense. Time moves at one second per second. When you're far away in relativity, you just shouldn't compare the rate at which time flows. You should only talk about the rate at which time is measured by individuals, and that rate is always one second per second. You can globally come back to where you left and meet up with the people that you left behind and then compare the total amount of elapsed time. But that's not a rate of anything flowing, okay?

And my belief is that once you truly internalize this way of thinking about time dilation and special relativity, you get it and you're free to ask questions on podcast AMAs that have nothing to do with time dilation anymore. And it's a wonderful moment in your life worth celebrating and remembering for the rest of the other. the time. All right. That's not a specific answer to any particular question, but those are my thoughts on time dilation. So I hope that those are useful. Now I can point here, uh, whenever anyone is tempted to ask such questions. So moving on to the actual questions, and I sort of scramble the order a little bit, but of course, uh, I will publish when I publish the transcripts, I'll publish, uh, the list of questions that were asked. So Duncan Palmer says, I'm grateful you shared your, you shared the podcast of your personal oral history and it's an oral history of my

personal history, and you are making a professional life change right now. If resources and money were not constraints, how would you devote your time and energy differently post-COVID than you are currently planning to do? So the reason I promoted this question to the very first slot here is because I wanted to say something that I should have said before, to, especially to those patrons out there who are supporting the podcast. For those of you who don't know the context here, I mentioned on Twitter and also on this AIP oral history interview that I did, that I'll be leaving Caltech next summer, so a year or year and a half from now, something like that. And, you know, they, it's, it's my choice. I have a contract that is renewed every five years and I'm choosing not to renew it. And so why am I doing that? Well, it's a complicated set of many different issues going into it. But, you know, basically what it comes down to is that it's, I am no longer, a very good fit for Caltech, and Caltech is not a very good fit for me and vice versa, okay? And this actually has, you know, there's a lot we could say about this, and it's nothing against

Caltech. Caltech is a tremendously wonderful place for the right person, for the right situation. It's just not my situation right now. My own research interests have drifted, as I hope that people's research interests do. You don't want to continue to do the same thing you did in graduate school for the rest of your career, although there are people who do that. And, you know, look, if it works for you, that's great. That's not my thing, I should say. That's not the way that I like to be. And in particular, you know, this goes back to a feature of modern fundamental physics, which is that it's a bit stuck right now, as I've mentioned in other places, in other contexts. Talked a little bit about this with Frank Wilczak. I've talked a little bit about this in the

biggest ideas videos, in the very last video, about science itself. You know, we have these incredibly successful models of the universe, the standard model of particle physics, general relativity, the standard cosmological model, et cetera, and all the experiments we do, roughly speaking, fit these models, okay? So we know these models are not the final answer. We need to do better. We haven't quantized gravity. We don't understand the black hole information problem or what the dark matter is or what happened at the Big Bang. There's a million questions. We don't understand. But our guidance to finding better answers is not coming from experimental surprises, as it usually does in physics. So we're reduced to a situation where, in order to get better answers, we kind of have to guess, right?

And so you guess how to extend beyond the standard model. There's a whole subfield of physics called Beyond the Standard Model, and that's particle physics in particular. And there's a whole other set of areas about quantizing gravity, unifying things, et cetera, et cetera. These are all guesses, the right model of inflation, the right model of dark matter. You guess, you hypothesize. And there's absolutely nothing wrong with that. That is the right thing to do. This is how science makes progress.

You make a guess, a hypothesis, tested against the data, etc. But I personally, and I played that game for a long time, but I personally have become convinced that I'm just not going to guess the right answer by this method. You know, when I was at the University of Chicago, I wrote a paper with some of my collaborators, on modifying gravity to explain the acceleration of the universe, right? Something that is now called F of R gravity, a function of R, F of R, R being the curvature scaler of space time. This is a way of modifying gravity, very simple, very, very straightforward, kind of obvious way of modifying gravity, makes the universe accelerate. The paper was helped inspire a lot of other people to do a lot of other work, lots of industry of sort of,

figuring out what are the cosmological implications of this, what are the predictions, et cetera, okay? So I could have just become the world's expert on F of R gravity or modified gravity and its cosmological implications, right? And that would have been a very standard, respectable cosmological career to have. I would have gotten tenure somewhere, no worries, you know, gotten citations, the whole bit. But, you know, I think that there's, it's just not very likely that F of R gravity is the correct theory that describes the universe. Number Number one, it's possible, so it was very worth doing and thinking about, but I wouldn't say it's probable. And, you know, furthermore, I'm just not constructed to, you know, come up with an

idea and then devote 20 years to investigating it. That's just not how I work. And again, nothing wrong with people who do work that way. But once I sort of figure out the idea, I want to move on to figure out other ideas. And it's just become harder and harder to figure out interesting ideas along those lines, which has caused me to shift my research direction. Like when I go to talks in the seminar series or the cloquia or whatever that are about someone's model for dark matter or dark energy or inflation or whatever, I just find it, you know, not that compelling because what are the chances that this is the real world, which is ultimately what I care about?

And as I say, in the biggest ideas video, under those kinds of circumstances, there's different things you can do, and one of the things you can do, which is what I have chosen to do, is sort of take a step backward and look at the underlying foundations of the field you're thinking about. So whether it's the sort of philosophy of quantum mechanics or cosmology or statistical mechanics or general questions about emergence and complexity and things like that, that's what I have decided is a much better place for me to spend my efforts. So, you know, like I said, when I go to particle physics seminars, I just, it's just not interesting to me personally these days in the way that it used to be, whereas when I go to

foundations of physics talks, you know, we have a, I go to several Zoom series, let's put it that way, about philosophy of physics and foundations of physics, or when I spend time at Santa Fe and talk to people about complexity and things like that, I have this, you know, enormous intellectual excitement, right, and that's what I want to have. That's what you should have as a working scientist. So I've shifted my research interests, and it's no longer. were a good fit. Caltech hired me to do this particular kind of research, and I'm not interested in doing that research anymore. I'm interested in doing other things. And furthermore, like my, other than just writing papers, my single most important job at Caltech was advising graduate

students, right, helping people get their PhDs, working with them, collaborating with them, launching them on their careers, you know, something that I care a lot about, take a lot of pride in and so forth. But it got to the point where, and I've had amazingly good graduate students at Caltech, and zero complaints about that. They've been fantastic. But I got to the point where I thought that I was letting them down and they were slowing me down through neither my fault or their fault, but there's a system that we're all embedded in, right? I mean, the people who were my graduate students entered physics, graduates to school at Caltech to get a PhD and become theoretical physicist doing particle physics, gravity, cosmology, that kind of thing. And I had

lost interest in doing those things to some extent. the kinds of things, the kind of research that I wanted to do, would not get these students' jobs, right? Would not launch them on a successful scientific career. My current interests are too idiosyncratic and weird to do that. And it wasn't what they wanted to do. Anyway, it's not why they came to grad school at Caltech because they were attracted by cosmology and particle physics and field theory. So I needed to sort of change what I wanted to do to help them out. And yet I wasn't very good at doing that, right?

Like, you can't really fake that kind of enthusiasm. So I don't think that I gave them the projects to work on that would absolutely maximize their chances of becoming successful physicists. So I thought that, you know, I wasn't a good fit either way. So anyway, long version of saying that I've decided that being a Caltech wasn't a good fit for me, and now we're coming to the punchline here.

So I don't need to be a Caltech anymore. and some large factor in not making me need to be at Caltech is you folks, the patrons that are helping out the Mindscape podcast. I can pay my salary through a combination of the podcast, writing books, giving talks, things like that. So I don't need to be at Caltech anymore. It was a good thing to be there when I got intellectual use out of it, but now I'm not doing that either, right?

So now I can do exactly the work I want to do because no one can tell me I don't need to worry about what the grant people think what my bosses think or anything like that I can do exactly the stuff I want to do and I spend, you know, a day a week doing the podcast and a day or two a week writing books or whatever or giving talks and the rest of the time I can do my work

and do my research. And it's a weird thing because, you know, exactly in the nature of podcasts and books and talks. that those things get a much bigger audience than the work I do, writing papers and things like that. But, you know, the papers that I want to write are the most important thing to me. And they do feed into, of course, the podcast and the books and so forth. So it's not, they're not completely independent. So now I can do things like, you know, this fall.

I've been invited to spend a sabbatical at Harvard, talk to people there. Santa Fe Institute reached out and they want me to spend more time there and talking to those folks. And I can do all this because I'm 100% free from any obligation to any other place. Now, I say this with complete knowledge that it might very well be that if I come back to a year from now, I might say, you know, look, I got a job offer that was really good and I took it. So if I get, if there were an academic job that let me do the work I wanted to do in the way I want to do it, then that would be even better than being a freelancer because then I would, you know, it's all about, you know, the thing that you're missing by not having that. position is colleagues to talk to, right? So that's one of the reasons why I'm going to go around visiting places, spend more time at SFI and things like that, hire postdocs and do grant proposals at SFI so that I can keep, you know, talking to people who have the same interests as me.

But I do want to like send out a word of thanks to all of the people, um, supporting on Patreon because you're a big help in this project. And so it's not just the podcast that you're making possible by doing this, not just the AMAs. So, Duncan, to the answer to your question, what would I do differently? Nothing. What I'm doing now for, you know, maybe the first time of my life is nothing more or less than exactly what I want to do. And hopefully I can keep doing that. All right. Simo Visonin says, what was your journey into theoretical physics and how come you decided to become a physicist and not a mathematician? Yeah, I sometimes get asked this, and I don't think I ever give especially helpful answers because I was young and I just stumbled into it.

You know, it wasn't some choice. And I loved it and decided to do that. And here I am doing it. The difference between physicists and mathematician, again, I was never tempted to be a mathematician at all. I love math. I love certain things that one learns by studying mathematics. But doing math as a career was just never something I took as a serious possibility because I care about the real world. That's ultimately what's driving me is how does the world work? And mathematics sometimes applies to the real world, sometimes doesn't. The whole personality difference between physicists and mathematicians is that mathematicians are not content to stop when they're pretty sure something is right.

They need to prove it, right? That's not my personality. If I think that something is pretty much right, I want to move on to do something else. So, you know, on Twitter some friends of mine are discussing the question someone raised on Twitter the question, what do we know about the number pi to the power of pie to the power of pie to the power of pie?

So this is very more complicated than it sounds like you can't just plug it into your pocket calculator because the uncertainties when you raise one number to another power and then you do real number arithmetic rather than arbitrary arithmetic or finite power arithmetic I should say

the uncertainty's built up very, very quickly. And so there's an interesting math question about how well you can approximate such a number. And whof, I just have zero interest in this, like this little puzzle solving. Like, I'm just completely not into these weird math puzzles that people like to set for themselves. If it's not the world, if it's not a step to describing the actual world, it's not for me. Again, let a thousand flowers bloom. I'm very, very happy that there are people out there who do care about these things, but those people are not me.

So that was never a decision I was really faced with. Okay, we have three questions in a row that are on similar... I've sort of tried to group some similar questions together. So here's the first group. Mystery Horse says,

do you practice any kind of art or do you know of any other theoretical scientists who do? Tim Ryan says, if you could choose to excel at any art form, what would you choose? And Rocket Rat says,

do you have any hidden talents like juggling, making good wookie sounds, blowing bubble gum bubbles etc. Well, you know, so the short, to get to what people actually want to know, I'm not good at any other art form, no. I have no talents. But, you know, I wish I live in a world, and this is true for art as well as for science, I wish I lived in a world where we didn't try to confine the practice of art to experts. You know, science is a different thing because the practice

of science requires a certain amount of expertise. But in art, that's the difference between doing it and doing it well, right? Like, I don't think that people like me should be up there on stage singing. You don't want to hear me singing on stage. Trust me about that. But I think that people should, you know, play instruments more and they should paint more, especially when we have all these technologies to enable us to do it. You know, I think that the podcast did quite a while ago with Grimes was inspirational in this area. You know, she took a course in a neuroscience class, and they were told to, you know, do something about music, and she made an album and decided to realize that she could do it with the technology that was available. And that doesn't mean it's easy. That doesn't mean we can all be as good. But we should be able to democratize art a lot more. So as I think I have said before, one of my quarantine projects was learning bass guitar, something I've always wanted to learn to do. And look, there's no question. I have no talent for this at all. I cannot keep time.

I do not have a good ear. None of the things that would be good. I'm too old to really, you know, learn something from scratch. But it's fun. I like it. I do it. And, you know, I enjoy doing it. So that is my attitude towards art forms as scientists.

And I should say some other scientists are great at either art or music or whatever. I do also, I did go through a phase where I painted acrylics, abstracts. And again, they were not very good. But it was fun to do. And, you know, I think that should be an attitude that more people have. Okay, Jim Cecilian says, does the motion of the Earth around the sun,

the sun around the Milky Way, the Milky Way itself affect the cosmic microwave background? Does this make the CMB a preferred reference frame for the universe? So yes, it absolutely does. The CMB does have a rest frame as a sort of gas of particles. There is a frame in which the radiation coming from you

is on average, completely isotropic, same temperature in all directions. If you move, if you respect to that rest frame, then in one direction the photons get blue shifted and the opposite direction they get red shifted and this gives a dipole pattern of temperature variations on the CMB sky so it looks hotter in one direction, cooler in another direction And we absolutely observed that that was the first Anisotropy of the CB that was observed a long time ago But we know it's not intrinsic to the early universe it's something that is just due to our local motion with respect to the CMB

So yes, cosmologists know about this they take into account, they remove it from the maps that they show you usually, although you can find pictures of the cosmic microwave background dipole online if you want, just Google those words. Does it make the C&B preferred reference frame? You know, it's a reference frame, but whether or not it's preferred is a trickier question. What do you mean by preferred? You know, we always leave out some details when we try to translate the ideas of relativity

and anything else into ordinary English, but there's absolutely nothing wrong with having a preferred rest frame and special relativity, right? Here in my office, I have a preferred rest frame at rest with respect to tables and chairs that are around me. That does not violate the rules of relativity or physics.

What the real rules are is that there's no rest frame in the vacuum. There's no rest frame that is intrinsic to the laws of physics themselves. When you have stuff, whether it's a planet or the microwave background or whatever, then of course you can measure your speed with respect to that stuff and call that a rest frame when you're at rest with respect to that stuff. And the C&B lets you do that.

James Kittick says, when the super lotto got huge, I used the universe splitter app to generate enough bits of randomly chosen numbers and bought one ticket to each lottery. The me posting this comment didn't win, but do you believe one of me did? It's not that one of you did. Someone did. I do believe that someone did, but they're no longer you, right? This is the lesson of the many world's interpretation of quantum mechanics. All those other branches have different people that descended from the same past you, but now are no longer you. They're different people, just like if you had an identical twin. So, yep, I think that overall the collection of people who are descended from you lost money,

but there should be one out there who won money, unless the lottery was rigged, in which case I can't help you there. Okay. questions that I've grouped together. Groupen. Kirk Briggs says, why is temperature not symmetrical? Absolute zero is the minimum and there's no maximum. Could time be similar with a beginning and a no end? Michael Edelman says, at the cost of building particle accelerators, as the cost of building

particle accelerators continues to grow, the cost of doing some other kinds of science has decreased. Can you imagine that there might exist a new approach to high energy physics that could be done, if not on a tabletop, at least in a small level. And Anonymous says, we think there's plenty of particle physics we're missing because we can't get temperatures high enough. Do we have reasons to rule out there being major particle physics we're missing because we can't get to temperatures low enough? So I hope it's clear why these three are at least somewhat related. Maybe I didn't read them in the best possible order. Look, why is temperature not symmetrical? There's plenty of things that are not symmetrical, right? The set of all positive numbers is not symmetrical. And temperature is kind of like that.

Classically, temperature is just the average kinetic energy in a collection of molecules in thermal equilibrium. So kinetic energy is a non-negative number, right? If an object has zero velocity, it has zero kinetic energy, and the kinetic energy is 1⁄2MV squared. So v squared is a positive number. You're never going to get it to be below zero. So that's fine.

I mean, there's nothing really to be explained there. Could time be similar? Sure. Time can go to minus infinity to plus infinity, or time might go from zero to plus infinity, or time might go from zero to some big number that we don't know about. We don't know anything about what those possibilities,

which of those possibilities are really true. Is there something called low-temperature particle physics, just like there's high-temperature particle physics? You know, probably not, but we don't know for sure, right? We never know these things for sure. These are always our best guesses. But according to the philosophy of effective quantum field theories, Quantum field theories that work at one energy scale

typically work below that energy scale. It's just part of the structure of quantum field theory. But they might not work at energies above that energy scale. There really is a difference between going down in energy and going up in energy. And I talk about this, again, in the biggest ideas in the universe videos, we talk about why that is true when the renormalization video,

Ken Wilson's ideas about quantum field theory. So probably not, but you know, you never know. So I think that to get people interested in that, one would have to come up with a particular model in which new phenomena arose at low temperatures, low energies, long wavelengths. And that's hard to do. Maybe gravity has something to do with it.

Who knows? But it's not what you would typically expect. Which, by the way, is one of the reasons why it's probably not true that gravity is modified on cosmological scales, right? we have a very good theory of gravity, general relativity that works in the solar system, et cetera. It would be really weird for that theory to break down on galaxy or universe-sized scales,

but we don't know for sure, so we keep looking. As far as particle physics experiments are concerned, you know, just for the reasons I just gave, the best bet to find new phenomena experimentally in particle physics is to go to higher energies, smash particles together with more and more momentum. So you can be clever in particular models. Like there are particular models for dark matter

where if you shine light at a wall, sometimes the light will turn into dark matter and then turn back out. So some of the light will pass through the wall and you can actually use that to test ideas about axions and so forth. But those are very highly model dependent. And again, any one of them is unlikely to be the right answer.

Whereas if you just go to high energies and smash particles together, eventually you're guaranteed almost to find something new. So you can try to be more clever about doing particle physics at low energies, but it's just not as guaranteed a payoff as it is by going to higher energies. And furthermore, there are plenty of interesting phenomena that we can imagine that can only be found by going to high energy. It's not that we're just not thinking hard enough.

That's where they are and you've got to go there to see them. So there is a subversion of your question, which is, can we go to high energy? Can we smash particles together with enormous energies without building giant particle accelerators that are kilometers across and billions of dollars to build? You know, we would all love to be able to do that, and there are people who are thinking about doing it. There's a whole industry of what is called plasma wakefield accelerators, et cetera. So far they don't work. So far, it's not been very promising.

But that would change everything. If you could really do that, if you could build the equivalent of the large Hadron Collider in your basement, like Tony Stark somehow is able to do, then it would change the economics of high-energy particle physics enormously.

No one wants these experiments to cause $10 billion, right? We all want to be able to do them at our own universities, but we need a huge breakthrough in technology to make that happen. I'm not in a position to tell you whether or not that's a likely breakthrough or not.

Eric Klein says, if the mythical machines for going back in time exist and you travel back in time six months, would you appear in the point in space where you were in space currently,

but the earth is where it was six months ago, causing you to be floating in space on the other side of the sun? Well, there's a reason why these machines are mythical, right? Typically, you know,

the movie version of time travel is wrong in a million different ways. And you're pointing out one of the ways in which it's wrong. It takes you to typically the same location in Earth, right? Like if you travel back in time

in your basement, you end up in the future or the past in your basement. But we know that in fact what you're doing is you're traveling to a different location in space time. Why in the world should you still be in your basement? So it depends on what your mythical time machine

is. The most real world version would be a wormhole that literally connected to different regions of space time. So then the answer is you climb in your wormhole and you go to wherever the wormhole comes out. And you should be able to predict that ahead of time, I hope. So it's not like you're completely lost, but there's no reason why it has to be the same point in space with respect to the earth that it was when you left. Every day the headlines shift, but the uncertainty never seems to fade.

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Use as directed. James Kirkland says, I've long been fascinated by ElectraWeak false vacuum decay. I wondered if anyone has done the work to find out what the new stable symmetry breaking would look like. Yeah, no, absolutely.

So this idea is, we know in the standard model of particle physics, that there is a symmetry group, SU2 cross U1, that is broken by the Higgs field down to just a U1 amount of symmetry. And if all those letters and numbers don't mean anything, that's fine. What it means is that there's a certain amount of symmetry in what we call the Electro Week theory, and the Higgs field gets a non-zero value in empty space, which breaks that symmetry. And it's very much like, you know, imagine you have a sphere in space and you can rotate it, right? You can rotate it in all different directions, sort of three different directions. There's three different axes around which you can rotate it. But then you put a dot on the sphere and you say, I'm going to now, well, what do you say is, what is the symmetry group now that I have a dot on the sphere?

If I move the sphere that in a way that moves the dot, that's not a symmetry anymore, right? A symmetry is a transformation that keeps something unaffected. But there is still some symmetry. You can move around the axis defined by the dot, right? You can rotate the sphere in such a way so the dot doesn't move. So there's a little bit of unbroken symmetry in that case. And that's very analogous to what happens in the standard model of particle physics. The Higgs boson picks out a direction in field space, and some of it remains invariant,

and that gives rise to electromagnetism, as we know it. The rest remains broken, and that gives rise to the weak interactions, as we know them. And so it has been hypothesized that this Higgs field in empty space might be not completely stable, that the Higgs field could suddenly jump to a much bigger value at some point in time. That would be electro-week, false vacuum decay. And the question is, would the symmetry breaking change? And the answer in the models that I know about is no. So the amount of symmetry breaking would change, but which symmetry was broken would not change.

There would still be electromagnetism. there'd still be the W and Z bosons. So it would still be S-U-2-1 breaks down to just U-1. You would need a completely different field that is also charged under electromagnetism to get a non-zero expectation value in empty space to also break that U-1.

So that would be a different kind of thing. Peter Benham says, I was re-watching your Big Ideas videos on fields and was struck by how the infinite number of modes results in an infinite amount of energy, and how that sounds a lot like the same problem we had with the ultraviolet catastrophe. So yes and no, I think. Just because two things are infinite doesn't mean they're equal, right? You have to be a little bit careful. So there's quantum field theory and there's quantum gravity, okay? And these are slightly different things. In quantum field

theory, there's sort of things are infinite in at least three different ways. In quantum field theory, you can think of what's going on, there's a field pervading all of space, and it's very, very convenient to analyze what's happening to that field in terms of modes, as we call them, which are just sine waves, right, like plain waves stretching throughout all of space, with a fixed wavelength vibrating with some amplitude, okay? And so if you think in terms of those ways, just pick a certain mode, and there's three different ways that sort of things can go wrong. Not wrong, but, you know, you might get an infinite answer. One way is, if the wavelength of the modes you're thinking about is taken to be infinitely long, right? The energy in a mode goes down

when the wavelength goes up, especially if the field is massless, like the photon or the graviton or whatever. So there are what are called infrared divergences because very, very long wavelength modes of electromagnetism, for example, are easy to make. They cost no energy. And that's sort of a calculational problem when people learn to deal with that. There's another infinity that you could get when the modes become very, very short wavelength. This is an ultraviolet infinity, an ultraviolet divergence. And that's what happens in particle physics once you turn on interactions. The interactions naively can give you infinite answers.

Also, and maybe this is what you're referring to, Peter, when you calculate the vacuum energy, the energy in empty space. If you include the zero point energy, right, the quantum intrinsic energy in all of those modes, you also get an infinite answer to that because of the ultraviolet contributions. Okay, so that's the second infinity. There's infrared, ultraviolet. Then there's a third infinity

that for any fixed wavelength, you know, don't let the wavelength get really big or really small, just fix the wavelength, but let the amplitude of the wave become really, really big. Then any fixed wavelength mode can have an infinite amount of energy in it. And that is a difference

between quantum field theory and quantum gravity. In quantum field theory, that's true, but it's just true. You can get infinite of energy. You can get as much energy as you want. In gravity, if you try to put a lot of energy into a small region of space, the whole thing just collapses into a black hole. And this, even before you get onto anything fancy about holography or string theory or whatever,

this simple fact that you can't fit an arbitrary amount of energy into a region of space indicates that there is a deep fundamental difference between gravity and quantum field theory. And so you should not be surprised that just, trying to quantize general relativity, which is a field theory, doesn't work very well, okay? Or other approaches along those lines. Gravity is something very special, and it doesn't mean that it's not possible to fit it in to the bigger picture, but we're not going to get there by just building a more clever quantum field theory in some ways. So to your question, you know, there are all these infinities, but they're different infinities, okay? There's different kinds of

infinity to come up, and you need to just be on your toes when you refer to a problem with infinities in these contexts. Sam Bartah says, why do physicists give so much credence and real world significance to determinism given quantum mechanics, quantum randomness? I think it's a very misleading thing

to a general audience. Well, I don't know that physicists do do that. I'm not sure exactly what you're referring to. You know, some physicists, well, let's back up. You know, I wrote a whole blog post a while ago you can look up about determinism and when it appears and when it doesn't. Certainly in quantum mechanics,

the predictions we make are not deterministic. Let's put it that way. Whether the fundamental underlying structure is deterministic or not is less clear, but the empirical predictions we make for observable things are not deterministic. So, yeah, so in that sense, physics is not deterministic.

On the other hand, if you want to play baseball or play soccer or get a rocket to Mars, you don't need to worry about that quantum interminism. the relevant physical rules on the relevant scales are pretty darn deterministic, right? So it's, there is, to some extent, just a question of accuracy, right? You know, there's an extent to which physics is deterministic, and that's fine, and there's an extent to which it's not. Just, I mean, maybe what you have in mind is some question about free will.

Maybe not. I really don't know. But let me say this. free will has nothing whatsoever at all, even a little bit, to do with determinism. In fact, let's say it even stronger than that. When people are talking about free will and they start talking about determinism versus indeterminism, you instantly know not to take them seriously. This is not what the issue is.

If your question is whether or not there exists what we call libertarian free will, the ability to sort of be a law unto yourself in Emmanuel Kant's formulation, the ability to not be governed by the laws of physics. What matters is not whether the laws of physics are deterministic or not. What matters is just that there are laws. The fact that quantum mechanics is indeterministic has absolutely zero to do with free will. Because it's not your free will making the choice

about whether the spin is measured up or down, right? Laws can be random and stochastic or they can be deterministic. The question is whether or not you obey the laws. if your question is about libertarian free will. So I'm not sure why, in what context, the people you're thinking of are talking about determinism, but I would advocate being careful about that context. Jeff B. says, in the biggest idea series,

you started explaining why we see discrete particle phenomena despite the world being composed of wavy fields. Is it that the world is wavy but only interacts in small point-like locations, or is the particle-like behavior a more alive? elaborate emergent phenomenon from all the waving. I don't think it's either one of those, actually. You know, I did try to explain this both in the biggest ideas videos and in the book, something deeply hidden.

But, you know, it's a tricky thing. So I'll try again. Here we go. It's it. The simple glib answer is it's exactly like a vibrating string and the harmonics of a vibrating string, right? If you have a string and you tie it down or a rubber band or whatever and you tie it down on both ends, so there's boundary conditions where the string cannot move at the two ends,

and you pluck it, then there's a discrete set of vibrational modes that you can get. You can get the fundamental mode, and then you can get all the overtones, okay? You can get a wavelength of the whole thing between the two pegs, or half that, or a quarter of that, or whatever. Okay? That phenomenon is discreetness from continuity, right? the string, if it weren't tied down, there's nothing discreet about the string. You can go wherever it wants, or the rubber band or whatever. But it's because you tie it down and you

provide boundary conditions that in the presence of those boundary conditions, you get a discrete set of phenomena. That phenomenon is why particles arise in quantum mechanics and in quantum field theory. So in particular, in the original word quantum came to be because we were seeing discrete packets of energy released by atoms. So I'm not going to get the history

exactly right here, but you know that electrons can move to higher and lower energy levels in atoms and they emit or absorb photons while doing it and only certain photons are emitted or absorbed certain energies of those photons. And the reason why is because there's a discrete set of, orbitals that the electrons can be in. If the electron were not in the orbit of an atom,

it could have any energy at all. But once it falls into the potential field of the electromagnetic field of the nucleus of the atom, then it has a boundary condition, namely its wave function is near the nucleus and goes to zero far away, right? And that boundary condition is enough to say there's a discrete set of possibilities, exactly with the vibrating violin string or whatever. In quantum field theory, it's a little bit different, but the concepts are exactly analogous. So in quantum field theory, you do what we did before. You consider a certain wavelength of a mode of a quantum field, and that mode, you know, that wavelength of the field can vibrate up and down, right?

Its amplitude goes up and down, and you say, well, I don't see any boundary conditions there. It can vibrate up and down as much as it wants, right? But what you're thinking of, and this is why it's hard to understand, but what you're thinking of is the wave function of that field. Okay, so wave functions are not fields. Wave functions are assignments to every configuration of a physical system, a number, a complex number, the amplitude, and you would square that amplitude, find out what is the probability of getting an observational result, seeing the physical system in that configuration. So you say, I have a physical system, which is a wave, and it has an amplitude, and I have a wave function which says, what is the

probability, if I were to observe that mode of the field, to observe it with a certain amplitude or a different amplitude or a different amplitude, okay? And there is a boundary condition there. You want the amplitude, the wave function, for observing the field to have enormously big amplitude, enormously big vibrations to be zero, to go down to zero, to fade away. You do not want an arbitrarily large amount of energy in your quantum field. Any finite energy configuration will have the feature that the wave function assigns a very low probability to the field vibrating too much. That provides boundary conditions. That says that in the space of all possible vibrational amplitudes that the field can have,

the wave function goes to zero as the amplitude gets very, very big. And that once again gives you a discrete set of possible vibrational modes. And we interpret those vibrational modes as there's no particles there, there's one particle there, there's two particles there, etc. That's why we get discreetness in the sense of particles in quantum field theory because of the boundary conditions on the wave function of the quantum fields. So, you know, I've been thinking for years about how to best explain this. And that was it. And it's not very clear. I get that.

But I do, you know, having said that, go back to the videos or go back to the book and see if the description there makes any more sense. That's the general idea. Nate Wadoops says, what is it that makes general relativity and quantum mechanics incompatible? I've heard a few times that they are incompatible, but I love to know more about why. Well, I think that there are two whole big buckets of ideas that go into why general relativity and quantum mechanics seem to be incompatible. I won't say they are incompatible. but the straightforward ideas that we generally have don't quite work. We say that there are both technical issues and conceptual issues, okay?

So the technical issues are the following. If you, you know, make your life easy, just say, I'm going to only think about very, very tiny gravitational fields, right? Small perturbations, very weak field. So space time is almost flat by let it vibrate a little bit, okay? And there you can do quantum gravity perfectly well. that works pretty well, as long as you don't push it too hard. If you let those small violations of small vibrations in the gravitational field act as virtual particles, so you put them inside Feynman diagrams, then the rules say that you have to include contributions from all possible wavelengths of those vibrating gravitational fields all the way down to zero.

And maybe you want to cut it off at the plank length, but okay, that's not what the rule say. The rule say you go down to zero. And you get the infinities that we were talking about before. And this is a fancy way of saying that quantum gravity is not renormalizable. So what that means is, in practice, that there are corrections. You basically start out with a classical expectation, and then you quantize it. And what you hope is that the quantum corrections to your classical expectations are small and controllable. But in a non-renormalizable theory, they're not. They're big. uncontrollable. So you get infinities everywhere, and you don't know what to do, and you can cut them off, but you don't know where to cut them off or how to deal with that or anything

like that. So at the simple technical level of quantizing gravity and getting a sensible theory, it doesn't work. Okay, you need to think a little bit more cleverly about that, and obviously people have suggested schemes for thinking more cleverly. Conceptually, you know, gravity is different because it's a theory of space time in a way that other theories are not. All the other theories you have are theories on top of space time. Okay? So if you think about the Schrodinger equation, the Schrodinger equation says, I have some quantum state. It evolves with time in a way that is determined by what we call the Hamiltonian,

which is a fancy way of saying the energy of the quantum state. But you use the rules of general relativity, and you just naively plug it in. And unlike every other theory, general relativity is about time and space in a very direct way. and what pops out of the straightforward follow-your-nose kind of approach is that the wave function of the universe in quantum gravity doesn't evolve with time at all. It's just static. This is what is called the Wheeler-Dewitt equation after John Wheeler and Bryce DeWitt. So what do you do about that?

I mean, time seems to be happening all around us, and it seems that this most naive version of quantum gravity says the time doesn't exist. and this is literally what is called the problem of time in quantum gravity. So again, people have solutions to the problem of time, time as emergent or something like that, and that's great.

It's not an unsolvable problem, but it is a problem. And so people disagree about what the right solution is. So those two sets of things, both the technical problems that you just get infinite answers and the conceptual problems

that certain things that seemed clear and obvious for other quantum mechanical theories aren't quite so clear and obvious for general relativity. Alexander Kavanaugh says, have you ever had any PhD students who were violently opposed to the many worlds interpretation? So short answer, no.

I've never had any PhD students who were violently opposed to anything. I don't think. If the PhD students aren't that violent, usually, famous exceptions, of course. But, no, but I get the point. So let's interpret this as a broader question. What happens if,

an advisor to PhD students has certain ideological commitments, and the students themselves do not share them. You know, usually this just doesn't happen because why would you choose as your advisor, someone who you disagreed with in some very, very profound way, right? That doesn't really fit together. You generally choose someone as your advisor, who you're sympathetic with on the big questions.

But, yeah, students have minds of their own and opinions of their own. And I don't think I've ever had a PhD student who agreed with me about anything, about everything. Anything I said by, that was a Freudian slip. They agreed with me about many things. In fact, you know, it's funny. My very first PhD student gave a seminar while he was a graduate student,

gave a seminar at a different university. And, you know, he was answering some questions from the audience, from the seminar audience. And at some point, he mentioned that he was working with me. And then one of the audience member goes, oh, now I understand why you're saying all these things. So there's some, you know, you fit into a tradition. that is defined by the work that is being done by your advisor.

And also, by the way, you know, most of my research career, I was not thinking about the Everett or Many World's interpretation that much. I was just doing more or less standard cosmology, general relativity, field theory stuff, so the question just didn't arise. Okay, there's two questions that are different, but I'm going to group them together. Patrick Hall says, many of the contemporary debates in theoretical physics are very complicated to lay people such as myself. the only thing us lay people can do is trust the experts, but many of the experts disagree with each other.

How should we the lay people go about taking sides on issues within theoretical physics? The other question is from Alexander Cordova, who says, What is your process for obtaining reliable information about politics, social issues, etc? It sometimes feels like a full-time job's worth of work to just stay reasonably informed due to the sheer amount of information that we have access to nowadays and the increasingly polarized political climate we live in. I was wondering if you had any thought. about this or if you could discuss how you're able to stay informed while also having time for your actual job.

So here we have two different sides of a similar question, namely, how does one develop credences or beliefs or opinions in a world where one is not an expert and the experts disagree, right? So this is a question for non-physicists talking to people like me or for people like me thinking about politics or economics or whatever. So I think, you know, obviously, I do think that listening to experts is important. I think that experts generally know things. And if you don't know anything, if you're a true non-expert, and there is a consensus among experts that says one thing, then not 100%, but you should put a very high credence on that consensus simply because they're experts, right? I think that's a perfectly fair thing to do.

Again, not 100%. And if you have good reasons, if you're not completely on, you. If you have good reasons to doubt the experts, then do that. But if you don't, if you literally know nothing about a field, then your default should be to give most of your credence to what the consensus of experts is. But of course, both questions here are in the trickier situation where the experts do not have a consensus. Then what do you do?

Well, then you have to keep your words about you a little bit, and I think that there's two things to do. One is to listen to the actual reasoning, right? I mean, I think this is something that can be done, even if you're not an expert sometime. So people say two different things in some technical field, and you say, well, why? Why do you believe that? And sometimes, not always, but sometimes it is possible that even if you're not an expert, you can listen to their reasoning and go, oh yeah, that's the kind of reasoning that I'm sympathetic to, or, oh, no, that reasoning sounds like completely backwards to me,

even though I'm not an expert. So sometimes you're lucky about that. Like if you say, why do you not like the many world's interpretation of quantum mechanics. And someone says, well, it bugs me to think that there are other copies of me out there in the multiverse. Or even, or if they say something like, well, it's just not falsifiable because I can't see all those other universes. And maybe you have enough expertise to go, no, I don't, I don't really like those kinds of reasoning, so I'm not going to listen to you. Or to be fair, if someone said, I like the many worlds interpretation, because it's a very, very simple underlying ontology, even though it predicts lots of universes. And maybe you can personally say, nope, I don't find that kind of reasoning very persuasive myself.

So you can try to do that. The other thing, if you're not so lucky as to be able to sort of have a feeling about their process and evidence, even if you don't have a feeling about their conclusion, is listen to how they frame the controversy, right? So regardless of particular subject matter field, some people are just more trustworthy than others, right? And it's regardless of expertise also. I mean, there are people who are super duper experts, but not trustworthy, and vice versa. So when you talk to people and they're sort of giving you the sales pitch for believing something, listen to, you know, how much do they understand the alternatives to their point of view? Are they simply dismissing those alternatives?

Are they being fair? Are they straw manning the alternatives? Are they saying, well, here are the weaknesses in my view, but here's why it's nevertheless okay? Or are they hiding those weaknesses from you, right? There's a bunch of habits, a bunch of traits that you can pick out as trustworthy in different people, even if you are not in possession of the expert knowledge that they have. Neither one of these techniques is 100% reliable, and that's why life is hard.

Sorry, can't help you there. Sometimes we have to make leaps into beliefs of some sort or another. All right. Jorge says, what are the main obstacles or difficulties in quantizing gravity? I think I already did that one. I should have grouped that one. Sorry. Good.

I do have another group of questions here. Rasmus Case Nearbeck says, particles have antiparticles, but the force-carrying particles are their own antiparticle. I've always struggled to get a mental picture of this. Trilobite Tark says, looking back at your question. enjoyable and helpful summer series when you talk about fermions and bosons is it correct or too simplistic to say that since bosons can share locations that accumulations of bosons can account for forces and brian brunswick says there's a common illustration of the rarity of weak force interactions that neutrinos

would pass half pass through several light years of lead but what about the same question for gravitational waves how much interaction is there with matter so all of these questions all these three questions has something to do with the basic question of forces, which is sort of a macroscopic kind of human scale question, versus particles like bosons and fermions, right? And how do you get these macroscopic forces out of individual little particles? So to the middle question, yes,

the thing that you and I perceive as a classical force, electromagnetism, gravity, or whatever, in the language of particles, is just an accumulation of individual particles on top of each other. And bosons can accumulate, and therefore give rise to big classical forces, whereas fermions can not accumulate.

That's why we do not have fermion forces noticeable in the real world. But it's a fuzzy boundary. So sometimes particle physicists will talk about forces from the exchange of fermions. And what they mean is that two particles can interact

by exchanging fermions. So, for example, two photons can bounce off of each other, can scatter by exchanging electrons and positrons. It's a very weak thing. You need very high-energy photons to see it, but in some sense, that's kind of like a force. What it's not is a big macroscopic force

that we could notice in the real world. The bosonic nature of photons and gravitons is very important for creating those big macroscopic forces. To the first question, you have been told that particles have their own antiparticles, but you've just been lied to. some particles have their own antiparticles, and some don't.

The idea of antimatter is a convenient kind of classification in some cases, but it's not a fundamental idea. When you have particles that carry some conserved quantity like electric charge, then they will have antiparticles. A charged particle with a positive charge will always have a negatively charged antiparticle. But for particles like photons, they don't have their own antiparticles. Or sometimes you will hear people say, they are their own antiparticles, okay? And the same would be true for just a single, you could imagine a single fermion that didn't have its own antiparticle. You're allowed to imagine that possibility, although we just don't know of any in nature that are like that. So when you get into it, when you get into it a little bit more deeply, what matters is what the charges are, what the symmetries are, and what the interactions are between the different particles.

This particle and antiparticle classification doesn't always fit reality very well. Sorry about that. Sorry I've been misleading you for all these times. I do it myself. And about the passing through neutrinos or gravitons passing through light years of lead, gravitons, individual gravitons, interact much more weakly than neutrinos do. So if you're literally talking about a collection of gravitons, they would pass right through lead, no worries. The difference is, as we just talked about, gravitons can pile on top of each other.

They're bosons, unlike neutrinos, which are fermions. And so gravitons can give rise to big macroscopic classical force fields or classical excitations in the gravitational field. So classical gravitational waves can be thought of as a huge number of gravitons piling on top of each other. So in principle, a gravitational wave passing through matter would be distorted by it. It would lose energy because it would try to squeeze. the matter a little bit and pass energy onto it. And indeed, that process is at the heart of how

we can detect them. But you need a lot of gravitons pile on top of each other to make that a noticeable thing. If you're talking about individual gravitons versus individual neutrinos, individual gravitons will pass through matter much quickly, much, much more easily than even neutrinos will. Okay, Josh says, a field-like biology can have moral implications. For example, discovering which species feel pain? Do you think that anything we've learned in the field of physics has implications for morality? And there's a related question that I'll ask here. P. Walder says that this year's Darwin Day lecture, Oliver Scott Curry presented data from twin studies, showing that the moral qualities of kinship, mutualism, exchange, heroism, etc., are encoded in our genes. Data was also

presented for 60 different cultures showing that the same moral qualities were universal across the cultures. Assuming these data are replicable, should we now consider that ought can be derived from is. So in both cases, let me do the second question first. No. This is zero evidence that ought can be derived from is. You literally just mentioned a whole bunch of things that are ises. People behave heroically. People show deference. People feel kinship, right? And these are encode in our genes. That's an is. That's a fact about the world. Okay. There is no. There is no. nothing in our genes or our behavior that lets us then say, oh, and this is how we ought to behave, right? That's the question of morality. If you presuppose that kinship and heroism,

etc., are good, are things that we ought to do, then these genetic studies show us that these good behaviors that we ought to have are in our genes, right? And in fact, this is a well-known argument against moral realism because people say, look, there's something that evolves, right? There's some pressure to reproduce under the rules of natural selection to pass on our genes to future generations. And there is also something that is right and wrong. Why in the world should they match up? Why in the world should they agree? I mean, if you thought that there was some pre-existing objective morality, why should natural selection ever find that when natural selection just wants to make us survive.

So the inability to derive ought from is is simply a logical impossibility, right? Ought is a different property than is. It can't be derived from it. So there's no evidence that will ever change my mind. You cannot add two even numbers together and get an odd number, no matter how many mathematical experiments you end up doing.

So likewise, the previous question, do you think that what we've learned in the field of physics has implications for morality, Here, it depends on what you mean by implications, because you begin the question, Josh begins his question with biology can have moral implications, for example, discovering which species feel pain. Discovering which species feel pain doesn't have implications for metaethics, metaethics being the job of justifying one's moral principles, right? Pain exists, I feel pain. if I discovered that earthworms felt pain

that would have zero impact on the way that I justify my moral principles. It might have an impact on how I behave if I already had a moral principle that said pain is bad and I should try to minimize it in the world, okay? So sure, physics can absolutely have implications for how we behave in the world

given what our moral principles are, physics helps us build bombs, okay? So I should not act in a certain way to build a bomb and drop it on people if I think that's morally wrong. But these discoveries in neither physics nor biology directly affect what I take to be the moral rules that I live my life by. Okay, Hilbert Spaceman says, probably not their given name. Suppose we have a two-state quantum system for which the born rule predicts equal probability to measure the system in either state. If an experimenter were to get the same measurement a million times in a row, would this be considered? an experimental violation of the Bourne Rule,

and if so, how does one experimentally test the Bourne Rule? You can't violate the Bourne Rule in quite that clear cut away, right? As I think you probably felt, as you were typing this question, you know, if you have a prediction via the Bourne Rule, that a certain system and experiment will give you 50-50 answers, spin up and spin down, and you get spin-up five times in a row, you go, huh, that's pretty rare. And if you get it 10 times in a row,

you're thinking, wow, that's really rare. And if you get it 100 times in a row, it's more rare, and a million times is more rare. So it never passes some threshold to go, oh, the born rule is wrong. But as a good Bayesian, what you should be doing is saying, well, there's a certain credence I give to the born rule being right. There's a certain credence I give to the born rule being wrong. And the more data I collect of all these spins being up when the born rule predicted that there would be 50-50, the more I deviate from the prediction of the born rule,

the lower my credence is that the born rule becomes, should be accepted, right? That the born rule is right. So it's exactly the same as with anything else. Look, if I say I had Newton's inverse square law of gravity here near the earth in the non-relativistic regime, places where it was all, you know, Newton's laws of gravity are very, very well tested, right? Could I imagine doing an experiment that seems incompatible with Newtonian gravity? Sure. Maybe I just make a mistake, right? Like, maybe my experimental setup isn't very good.

So you never just do an experiment and go, oh, I have ruled out my theory. You change your credence in your theory, and you change that gradually. This is what a good Bayesian does, and it applies just as well to the Bourne rule as to anything else. Greg Griffith says,

about your ruminations on the philosophy of mathematics a few months ago, what issues do you personally find most in need of thought in that field? So we will have podcast coming up about the philosophy of mathematics. Honestly, the most basic dumb questions are the ones that I am struggling with. What is real, right?

To what extent do we think of mathematical structures as real like a Platonist would do, or a mathematical realist more generally? My inclination is to say that there are senses in which mathematical things are real, but not the same sense and the tables and chairs are real. But honestly, you know, the real philosophers of mathematics, They thought through all this stuff, and this is a perfect example of a place where I am not an expert. So I'm trying to listen to them. And listening to them is hard because, again, they don't agree with each other, right?

As we said before, this is me. I'm trying to apply my criteria to the experts. So I'm reading the experts, and I'm, you know, trying to find the ones that, on the one hand, you know, overlap with my beliefs in other areas. On the other hand, seem trustworthy, you know, seem honest about what they're doing, and who give reasons that I find compelling. That's what I'm trying to do. Sam says, what are your thoughts on Cornell West's current situation at Harvard? So for those of you who don't know, Cornell West, previous podcast guest, is a professor at Harvard, but in a special position that is not technically a tenured position at Harvard. He used to be tenured at Harvard.

In fact, like a big university professor kind of job. Then he left because he was insulted by Larry Summers, who was the president at the time. He went to Princeton, and then he moved back. And I have zero knowledge of all the twists and turns in that saga, and I have zero knowledge of what Harvard is saying. But it's in the news recently because apparently he wants to be, he wants to have that tenured label attached to his current position, and Harvard is reluctant to do it. And so people have, you know, people have their opinions about this. So I don't have opinions about that specific situation. And as a general rule, it's not always true, but as a rough guide, I try not to comment on individual people's employment situations at institutions that I don't know much about.

Because you don't know. I mean, it's the kind of thing you can easily ride those kinds of controversies for your own political purposes. But in fact, there's usually about a billion factors going into these discussions that you're not aware of because you're not there, right? And not all the facts come to light, not all the parties involved, have any interests in sharing all the features that they're talking about. So you don't know, and maybe you should just be quiet about it. So I'm going to be quiet about the actual facts of Cornell West's current situation. we can think more broadly about the kind of situation, right, where we can abstract away from his personal situation. You know, what if you have someone like Cornell West, who is clearly very accomplished in their field, has also been extremely active in a more public sphere, right? So I think that it's fair to say that one of the reasons why there is some resistance to giving tenure to someone like Cornell West, these. days, even though there wouldn't have been 20 or 30 years ago, is because a lot of his work

is sort of more public facing than traditional and scholarly, right? You know, referee journals and things like that. So what should the role be of people like that? There's, of course, a whole huge extra set of considerations because Cornell West is a very, very famous African-American scholar, and there are black students at Harvard who says, like, he's our guy and you will really be treating us badly if you, you'd do not try your best to keep him, right? And I take all of those arguments very seriously. I think that's true. I think representation matters, role models matter, all that stuff. But again,

I'm not enough of an expert to opine intelligently on that. So when it comes to, you know, what do you do about people who, I mean, you can probably guess my answer here, right? But what do you do about people who have been doing most of their work in a more public-facing way than in the traditional academic channels? You know, I'm in favor of those people. guess what? You know, I try to do both myself, and not because I think it's the best for my career, but because that's what I enjoy doing. Like, I take real true pleasure in writing papers, technical papers for referee journals in the very standard academic way. But it's not the only thing I take pleasure in doing. I do things like what I'm doing at this very moment, which is much more

public-facing. So I'm a big believer in a diverse ecosystem when it comes to this stuff. I think that a good university should not devote all of its faculty positions purely 100% to people whose noses are to the grindstone in the academic treadmill, writing papers for referee journals, etc. That stuff is important, but it's not uniquely important. There are other things that are also important. I mean, say what you will about Cornell West. You cannot accuse him of not working hard.

I mean, the man puts in the hours, right? you know, even if it's kind of adorable. I don't even know if you know, but he doesn't use a laptop. So when we had him on the podcast, he uses an iPad. I don't know, he probably doesn't know how to type, but, you know, he must know how to type because he gets a lot of books written. But, yeah, he uses an iPad. So we had to, like, figure out what the right microphone and software was to do the podcast is. But he was willing to do it.

Like, he's all right, help me figure this out. Very prolific. He has his own podcast. He has done the academic. grind kind of work. I'm 100% in favor of people like Cornell West being tenured professors. Again, you wouldn't want every tenured professor to be like that, but you wouldn't want all the tenured professors to be stuck in their offices doing their work for the 12 other people in their field either. You need both. And so, you know, in terms of places like Harvard

devoting some tiny fraction of their resources to professors who are huge. hugely influential public figures, doing interdisciplinary work, being good role models, stuff like that? Yes, I think that's kind of important. Ashley Hyatt says, can you explain how hawking radiation causes a black hole to shrink? If only one particle from a pair created near the horizon is emitted as radiation, isn't the other one consumed by the black hole, thus increasing its mass? Yes, the other one is consumed by the black hole, but is a trick of general relativity.

So, you know, I don't know where you are in your education, Ashley, but if you ever get to read my graduate textbook on general relativity, space time and geometry, what you will learn is you can define the energy of a particle as seen by different observers. This is a feature of relativity, okay, so that different reference frames, if you want to put it that way, or different observers located different places, will define what you mean by the energy of a particle differently. And in particular, when you're close to a black hole in general relativity where space and time are being warped, that difference can be pretty big. So it turns out that if you are at the horizon, if somehow, and this is not actually possible, but if you could imagine seeing these two particles, a particle and an antiparticle being created, one goes out and one goes in, and as you're falling through the horizon, you would see both of them as ordinary positive mass particles. But from the point of view, someone who's very, very far away, they attribute a negative energy, a negative mass, effectively, to the particle that falls in. So, yes, the black hole, to the point of view an outside observer, the black hole is absorbing a particle, but it's absorbing a particle with a negative energy. So the total energy of the black hole goes down. And, of course, the energy that it's absorbing, the negative energy it's absorbing is exactly the same in magnitude as the positive energy of the particle that's emitted. So all the math works out.

Surprise, surprise. You're probably not surprised. All the math actually does work out. Okay. Two questions I'm going to group together. Sandro Stooky says, could you tell us a little bit about your podcasting setup?

What type of hardware and software do you use? You said at some point that you send mics to the guests. Would you mind telling us what type of mic those are? And Steve Louderbach says, you've mentioned you send a microphone to guests. So share it with us, the manufacturer, a name of that microphone. So, yeah, short version here,

if you go to the podcast homepage, preposterous universe.com slash podcast. One of the, you know, there's all sorts of goodies there. I encourage you to check it out. One of the things on the right-hand column is about Minescape. You can click there and you can find the complete description of my podcasting setup. I've typed that in, and there's also,

these are even links to buy it. So my microphone is what is called an electrovoice, R-E-320. It's a very nice voice microphone. You know, it's not going to be a good studio microphone for a professional musician, but for podcasters, it's just about perfect, I think. And for those, I don't know how up you are on microphone technology, but the big decision to make, if you're a potential podcaster,

and you're looking at microphones, is the wiring, believe it or not. I mean, there's other decisions to make, because there's like condensers versus dynamic microphones and, et cetera. But, okay, really the wiring matters because there are USB microphones. that you just plug right into your computer. No worries, right? But there's also XLR microphones. So the XLR cable is a different cable.

It's not USB, not at all USB. It's a more professional audio quality kind of cable, okay? And so the higher end microphones will tend to be XLR microphones, not USB microphones. Therefore, my microphone is an XLR microphone. I cannot plug it directly into my computer. And I knew this going in. It's not a surprise.

So what you do is you play. plug it into a recorder or a mixer. Okay, so the standard in the podcast land is something called a Zoom recorder. Nothing to do with Zoom, the online video thing, but there's only so many names you can have for your company. So there's a very nice company that makes small recording devices to which you can plug in XLR microphones. And so it's both like a recorder in of itself and also an interface for your mic.

And then you can attach the Zoom to your computer via U.S. I have a slightly higher level thing, which is called a MixPree 3 made by sound devices, and it's a little tiny mixer. Okay, so I can plug several microphones in, three, in fact, microphones, and they have their separate volume controls, gain controls, and I can also plug in the computer to the MixPree, and I can get an input from there. So usually when I'm actually recording a podcast, I have my microphone plugged into the Mix Pre three being recorded in the left channel of the stereo mix, and I have the guest coming into the computer, which is then plugged into the MixPree 3 going into the right-hand channel. But I don't actually use that guest audio for the podcast because I record using Zencaster, and Zencaster will record the guest locally themselves and then transfer the file to me. So I can download from my Mix Pre the

stereo version of what I've recorded with both my voice and their voice. And then I take from Zencaster, just their voice, and I go into this program called Audacity, which is, it's either free or it's, anyway, it's public. But it's a, you know, it's a, it's a, it's a for your computer and you do all the audio mixing from there, noise reduction, compression, all of those things. And so if you listen to the early podcast, I was not very good at that. I'm still not great at it, but I'm better than I was. at the beginning. So then I can line up the file that I get from Zencaster in time.

You can shift it right, right and left. So I line up the guest's audio from Zencaster with the guest's audio that I recorded. And then I can delete the guest's audio that I recorded so that the time sync is right, but the audio quality is better because I'm getting it directly from Zencaster. So the voice that you hear from the guest in my podcast did not come over the internet before. it was recorded. It recorded directly there at their house. So I'm not going to...

I mean, this stuff is too expensive. I'm not going to mail it all to them. What I generally mail to the guests is a Blue Yeti USB microphone. So you don't want the guest to do too much work. The Blue Yetis is a very, very standard microphone. It costs about $100. They went through a period

when we first entered quarantine. They became very expensive because everyone was starting a podcast. That's what else are you going to do. But a Blue Yeti is a wonderful microphone. It is the standard for very good research. It's plug-in-play, plug into your computer, and push the button, and there it goes, and it records quite well. Not as well as my RE320 or my MixPree 3. They're recording from a Blue Yeti directly into their computer.

So inevitably, my video, my audio sounds a little bit better, okay, but so be it. And then what was I going to say? Yeah, so then I think that if you're a starting out podcaster, you could just start. with the Blue Yeti yourself. Oh, I remember what I want to say. Thank you. Thank you once again to all the Patreon folks out there because you, you know, and of course also the ads that appear on the non-patri-on version of the podcast, but it's your

contributions that let me buy microphones and send them. Like the guests are always very, very happy that they get a microphone for free for appearing on the podcast because, you know, I don't ask them to send it back. I try to minimize work for my guests, right? So it's a gift. keep the microphone, use it for the next podcast you want to be on. So gradually, I am improving the sound quality of everyone's podcasts. I should give credit also to Sam Harris,

who actually gave me the idea for sending microphones to people. But you folks, the Patreon supporters, are paying for it in some very real sense. You're paying for that. You're paying for the transcripts of every episode. All this stuff costs money. And I do it because there are people who care enough about the podcast to give me money to do it. So I want to make the quality as high as I can for all of you folks. So thank you.

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What is your process with a typical guest before recording starts? Do you do a warm-up call? Do you go in cold? Do you send them an outline of points you'd like to hit? Don't do many of these things at all. No, I think that I try to get people, high-quality people as guests on the podcasts, they're usually very busy. I try to make their burden minimal.

I try to do very little to ask them to do things. other than the actual interview, I do let them suggest things to talk about if they have, you know, there's some subset of all the things that they've done in their life that they're very interested in talking about right now, then I'm very happy to take those suggestions. But there's no warm-up call. There's no outline or anything like that. You know, I am trying to get better at being the interviewer on the podcast. I probably over-prepared when I first started, you know, had a whole list of questions planned out. And I think that to make it more of an organic, conversation. These days, I try to go in with, you know, a couple, two, three, four main points I want to hit, maybe a little, a couple of things that I've read or heard in their stuff that is a sufficiently fun anecdote that I want to bring up. But otherwise, try to let the conversation be the conversation. I'm still not as good at that as I would like to be, but that is definitely the goal. And like I said, the guests are busy people and I want to make their burden as light as possible. In my view, the guests are being incredibly generous by offering their time to be on the podcast, so I don't want

them to do any work to do it. Peter Bamber says, if a proton is made of quarks, which themselves are disturbances in quantum fields, what does it mean to refer to the diameter of a proton? Well, it is a necessarily fuzzy concept, the diameter of a proton. You know, these fields have profiles, roughly speaking. That's a slightly overly classical notion, but maybe you know what I mean. and it's sort of like saying what is the width of a bell curve distribution right?

The bell curve Gaussian distribution never hits zero no matter how far you go away from it but you can talk about the width that half of its size right, half of its amplitude. So there's other ways

of talking about sizes of things that don't really have any well-defined boundary. That's what it is for the proton. You know, where do the fields fall off sufficiently far that you wouldn't say that you were in the proton

anymore? Something like that. that. Samuel Benjamin says, are you aware of any evidence that as humans, we try to fix the world around us into a highly organized lower entropy state because we like to make order from the chaos? I'm thinking specifically about efforts being put into highly coordinated and interconnected smart energy networks across the UK. These are super complex to design. No one really has an effective way of doing it. Wouldn't we be better off having more localized energy networks that are far less effort to design, i.e. higher entropy. I think that you're onto something here, but I'm not

sure if I would use exactly that terminology or necessarily that motivation. I don't think that human beings are trying to just be as organized as possible. What I think is that human beings, you know, when human beings design things, it is an example of intelligent design. It is different than things growing up organically. And I think that the real issue with things like that is not that they're too complicated or too simple or too global or too local, it's that when we design things,

there's a whole bunch of factors that go in, one of which is ease of being comprehended by us, right? We need to be able to look at it and fix it. And another thing that goes in is we can imagine a certain set of circumstances that would qualify as disasters, right, as bad as failure modes for the system we're looking at. And we're less imaginative in some sense than nature is.

So since nature does not design for specific purposes, we end up with organisms that are just more multi-purpose and, in a sense, much more flexible, much more able to adapt to unforeseen circumstances. But I don't think that it's necessarily the right thing to say, well, we need more localized energy networks. I think that it would be nice to have more robust energy networks and grids and things like that, but that might end up being higher entropy or low entropy.

I'm not quite sure what. I think that that is not necessarily, as far as I know, the right criterion to have in mind in a case like this. Victor Alejandro Weiner says, I grew up in the 60s in Argentina in a family of writers, painters, and actors. As a child and then as a young man I had the strong feeling that if you were a fan of the sciences, there was nothing for you in the arts and vice versa.

Later I saw this attitude ease, thanks in no small part to the Hofstadters, Sagan's, and so on. Does this fit with your personal experience at all? Did you ever feel that you needed to choose? No, I don't, actually, I don't really think so. I never, you know, look, I did not grow up in a family of either scientists or artists and writers.

I grew up, you know, in people who were working as secretaries and salespeople and steelworkers and so forth. So there was no science versus arts dialogue going on in the house that I was growing up in. And honestly, I never, I mean, I know what you're referring to. I get it. I have seen it later in life, but I never worried about that.

I never thought that, oh, I need to be either artsy or sciencey or something like that. That wasn't a bifurcation that I was aware of at the time in order to fret about. Steve Pilling says, according to PBS space time, gravity is due to the local time slowing effect due to the effect of mass on space time. This is mind-blowing. Can you discuss this and how it explains the gravity acceleration equivalence principle? I'm probably not going to exactly explain what you want me to explain. Let me say a few words about this.

For one thing, there is no local time slowing effect, as I said right at the beginning of the podcast. That is not the way to think about it. What there is, in general relativity, is a metric. on space time, which is a way of relating different curves in space time, which could either be literally a curve through space that you draw on the ground, or a curve through time, like the trajectory of a particle or something like that. And the metric tells you the length of these curves. If the curves extend mostly through space, it will tell you the spatial length. If the curves extend mostly through time, it will tell you the time elapsed on such a curve.

And, you know, this thing that you're calling, that probably PBS spacetime also called, the local time-slowing effect is exactly a comparison that you shouldn't be doing between a clock that is in deep into gravitational field and a clock that is far away. It's not a comparison you should make because time is measured locally and you should only compare elapsed time along trajectories that start and end at the same events. But I know what you mean. There is a metric on space time that has a form that depends on the distance, that you are away from some gravitating object.

And furthermore, the way that the actual metric of space time works in general relativity, there's sort of two factors that come in. There's a factor that multiplies the time coordinate, and there's a factor that multiplies the spatial coordinates. And here's what they're getting at. Here's the actual thing that they're trying to say. You might think that gravity, if you ever think about all those pictures of general relativity,

with like a rubber sheet with a bowling ball or whatever, right, bending space around it, you might think that the gravitational field that we feel is mostly due to this stretching of space, not the stretching of time.

Like, time isn't being bent in these rubber sheet examples, right? But that's not true. The reality is that if you think about your trajectory through space time, you're traveling much slower than the speed of light, right? So in some very real quantifiable sense,

you are traveling more through time than through space. You're not moving that fast through space, and you're moving one second per second through time. So it turns out that it's the warping of the time component in the metric that is actually more responsible for gravity in the Newtonian limit, the inverse square law of gravity. The fact that apples fall from trees, the fact that you do not float away from the earth,

this is mostly because of the warping of that time coordinate, in that time-like direction. Now, I would be reluctant to allow you to sort of extend this in any simple-minded way to how you experience time or acceleration or the equivalence principle or any of those other things. It is just a true fact about the Newtonian limit of general relativity, and the fact that you're moving slowly compared to the speed of light, you feel the warping of time more than the warping of space. That's really all it is.

It's not even that important. I wouldn't make too big a deal of it. John Lounsbury says, as quantum computers move from theoretical to practical applications, can you envision any ways in which their computations could actually help solve other quantum mysteries? Yeah, sure, it's definitely possible to imagine.

Indeed, that was the big motivation that Richard Feynman had when he was one of the first people to discuss quantum computers. And basically, if you want to simulate a quantum mechanical system, what better place to do it than in another quantum mechanical system? You don't have to do it.

We can simulate the wave function in the Schrodinger equation on classical computers, but quantum computers are sort of tailor-made for that kind of thing. So whether it's things like chemistry or, you know, down at the single molecule level, how do different molecules come together

to do chemical reactions, or quantum field theory. My Caltech colleague, John Preskell, and his friends have done a lot of work on simulating quantum field theory with quantum computers. I'm not an expert on any of those things, but if you want to know

whether quantum computers will be used to solve quantum problems, the answer is yes. You say quantum mysteries, the mysteries in quantum mechanics are more like the foundational questions, right? The measurement problem, the reality problem. I don't think the quantum computers

are going to say anything about those things, but they will help us do practical calculations. Daniela Cortesi says, what do you think about Penrose's argument that consciousness is not a computation. Well, I do not agree with it. I mean, I get it. I get where it's coming from.

You know, there is this argument from Gertel's theorem, essentially, or even, I guess, from Turing, that there's some things that computations can't do, right? There's some things that you can't reach axiomatically on the basis of some axioms and then do computations on the axioms to prove theorems.

There are conclusions, you can't reach in that axiomatic way, according to what we know about logic and mathematics. Whereas it seems that human beings can reach conclusions. So, you know, Girdle's theorem says, if you have a sufficiently powerful formal system, then there are statements in that formal system that cannot be proven if the system is consistent. So either the system is inconsistent and you can end up proving it or the system is consistent and you can't prove it, in which case maybe it's true, but you can't prove it. And you get into sort of philosophy of math questions very, very quickly. Whereas Penrose and others would say, look,

I can see that statement, I can just look at it, and I know it's true, even though the formal system and therefore a computer can't prove it. Well, the question is, and Scott Aronson, who previously appeared in the podcast, is the one who basically taught me this, you know, how do you know that that formal system is consistent, right? We think it's consistent, but do you know, like, how do you prove that? Have you proven it? And, you, a computer, you could just say, well, you know, let's assume it's consistent and see what we get out of that, right? I don't, I think that the human brain is physical, right?

I think that we are sloppy thinkers, our human brains, and we can often attribute certainty to some of our thoughts that are, in fact, don't deserve to have certainty attributed to them, and therefore it can lead us to believe that we've, quote, unquote, reached conclusions that a rigorous algorithmic computer wouldn't. but I think that's just giving us too much credit, honestly. Horst Wurst says, What is your opinion on black heterodox public intellectuals like John McWhorter or Coleman Hughes? Do you share their concerns regarding critical race theory or identity politics?

No, I don't generally share their concerns. Also, I should say I don't have great familiarity with either one of them. I know who they are. I was on Coleman Hughes's podcast, but I don't know their work in any. detail. I am a big believer in the existence of heterodox public intellectuals. In a very similar vein to what I just said about people like Cornell West, I do think that there's, you know, there's usefulness in pluralism and variety in intellectual life. We talked about this on the podcast with Musa Al-Gabri. You know, it's just good to have a whole bunch of different ideas

out there. So I'm all in favor of people like John McWhorter and Coleman Hughes existing. Having said that, let me say two other things. One is, you know, there's an obvious bias at work here that people can fall into if you're not black, if you're a white person, and you kind of are rubbed the wrong way by black people going around saying, you know, we've been oppressed, we've been discriminated against. There's a legacy of slavery. There still exists biases in discrimination. A lot of white people are defensive about that. And so when they can find a black person who says, oh, no, no, no, it's more black people's fault than white people's fault, which is it, I know a dramatic oversimplification, but for this thought experiment, there's a huge bias that you can fall into in saying, ah, you're a good black person, you're saying what I want to hear, let me talk to you and elevate your opinions. This is not a criticism of the opinions themselves.

This is a criticism of who is listening to the opinions and who is giving them voice, et cetera. You know, and again, not to say that these opinions are the wrong ones, but you should be very, very wary about your own vices, right? Your own biases, your own ability to want to hear certain things and to agree when the people you're talking to say the things you want to hear. I think that it's more important to be confronting and to be facing the opinions you don't want to hear, right? So I think that it's important that we have heterodox black public intellectuals, but also it's super important, arguably more important, that we take seriously what the more orthodox black public intellectuals are saying that we don't want to hear if we, we in the sense of white people who might take some of what they say as criticism, right? that's what we have to face up to. The other thing is, you know, I have a complicated relationship with the word heterodox, you know, because I do believe in pluralism, and I do believe that there's a lot of questions to which we don't know the right answer. Some amount of heterodoxy, I think, is important. Heterodoxy being not being orthodoxy, right? You know, going against the tide. But I believe that because I believe in pluralism, not because I believe that heterodoxy by itself is a virtue. You know, there's a very thing. thin line between saying, you know, I'm heterodox. I don't fall into the establishment, man,

and just being a crackpot, right? Or just valorizing heterodoxy. Like, you know, the heterodox academy is a whole thing. And I would never want to belong to something called the heterodox academy, because I don't value being heterodox. I value being correct. And I worry that there's a whole, again, a temptation, a bias, whatever you want to call it, to valorize being anti-establishment for the sake of being anti-establishment, not for the sake of being correct. Sometimes you've got to be anti-establishment because you think it's correct. That I'm all in favor of. But as soon as you sort of start taking pleasure in being naughty and contrarian, just for the sake of being naughty and contrarian, that's equally bad of a vice or a bias or a mistake as it would be to just always go

along with the orthodoxy, right? That's why intellectual life is hard, because there's a balance you have to be able to strike between listening to experts, knowing what the orthodoxy is, trying to understand what they're saying, and also be willing to be heterodox and anti-establishment when you think it's the right thing to do. Okay? That's a really, really difficult balance to get right. I struggle with it. Everyone struggles with it. So I'm all in favor of the existence of black heterodox public intellectuals, along with the orthodox ones. And we all, white, black, yellow, purple, whatever, have the difficult task ahead of us of listening to all of them and listening to them for the value in what they're saying, not because they happen to be saying what we do or do

not want to hear. Johnny says, Frank Wilczak spoke recently with Sam Harris and said something to the effect of, it is enough to make the calculations in quantum physics and the interpretation of what is happening is basically semantics. Is this actually the prominent view in the field? It's certainly a prominent view. I mean, a lot of people think that. I mean, maybe probably, you might be right. I'm not you, but the question might be, the answer might be yes, that it's the most prominent view in the field. It always depends on how you define the field, right? It's obviously not the prominent view among people who specialize in foundations of quantum mechanics. But then there's obviously also a huge selection effect. Like, if you thought that was the right view, you would not

specialize in the foundations of quantum mechanics. It's like most philosophers are atheists, but most philosophers of religion are not, right? Okay, that's that least surprising thing in the world. If you're an atheist, probably not going to specialize in philosophy of religion. If you think that the interpretation is basically semantics, you're not going to specialize in the foundations of quantum mechanics. I disagree, obviously. I have thought about this carefully, I think,

and other people also have thought about it carefully, and I completely wildly dramatically disagree that the interpretation is just semantics. There are physical questions. Is the wave function of representation of reality, or is it just a calculational tool that we use to make predictions? That's a question.

I can't even imagine thinking that that question is somehow basically semantics. It's a question about the fundamental nature of reality. Are there hidden variables? Are there objective wave function collapses? Are there branches into other universes? These are all direct, physical, real questions that are certainly not semantics.

And, you know, I know perfectly well that Frank and others have these points of view. And so I had him on the podcast, and we didn't talk about that. And, you know, I don't want to talk about that stuff with Frank Wilczek because Frank Wilczek has made enormously big contributions in a bunch of really creative ways to all sorts of interesting questions in fundamental physics. I want to talk about that with him. I do not consider—I know this wasn't your question, Johnny, but let me just say it anyway.

I mean, again, part of why I'm picking certain questions to answer here is because they spark in me other things I want to say. So, you know, I don't bring people on the podcast to debate them. I certainly do not bring people on the podcast to ride my hobby horses and, you know, confront them with my opinions about things about which we might disagree. When I bring people on the podcast, I want them to look good. I want the people who are listening to the podcast to say, oh, yes, I see why this person is an interesting person to listen to. Whether or not I agree with them. I'm very happy to bring people I disagree on the podcast, but I want to disagree with them in a way that is potentially profitable, potentially constructive, that I can learn something from them, even if I disagree with them. And mostly, they're there on the podcast to say their spiel, not to listen to me.

You know, the total number of words I've said on the podcast over 130-some episodes is way bigger than any of the guests. So I don't need to, you know, spend the whole time talking to Frank Wilczek, writing my hobby horses about the foundations of quantum mechanics. I would much rather learn from him about particle physics and his views of fundamental physics today, just in case you were wondering, why did not give him a hard time about the foundations of quantum theory. Okay, so I have another group of three questions. Yes. Anders says, in your interview with Brian Green, he mentioned that some people suspect that string theory might not allow for a positive cosmological constant, after all.

Let's say that that's definitely true. Would you say, what is more probable? A, string theory is wrong, or B, the cause of the universe's acceleration is not a cosmological constant. So that's one question. Daniel Westwater says, I was wondering why dark energy is known to be the cause of the expansion of the universe.

universe, increasing the speed of galaxies moving apart, but seems like it's only an empty space. Why don't we see dark energy and effects in galaxies themselves? And the third question is, you often mention, sorry, from David DeCloat, you often mention that the cosmological constant is the best candidate for what the dark energy is. I can see how a constant in a formula can describe what dark energy does, but how does that explain anything? How can it be dark energy? So yeah, let's go, I should have ordered these better once again.

Let's do the last one first here. The cosmontal constant, that's a label, right? Okay. So, yes, it is true that I have said, and I continue to believe that the cosmontal constant is the best candidate for what dark energy is.

But the phrase, cosmological constant, is a label for what is alternatively known as the vacuum energy. It's the energy inherent in empty space itself. It's a thing. It's not just a number in a formula. It's a real thing. It's the energy of empty space.

in every cubic centimeter of space, if there's no stuff there, no particles, no radiation, or whatever, you can still ask how much energy it has. That's a fact about empty space itself, and that fact is the vacuum energy, aka the cosmological constant. That's how it can be dark energy.

It's just an inherent property of space time itself. For the effect on galaxies themselves question from Daniel, well, there is an effect. So if you have the cosmological constant in particular, but also if you have other forms of dark energy, in principle, that affects the motions of the planets around the sun. It affects the motion of the stars in the galaxy. But in practice, when you calculate the numbers,

the effects are really, really, really, really small. Why? Because the amount of dark energy is really, really, really small. The difference between it and other things is that it accumulates in effect because it's everywhere. It's literally in every cubic centimeter, okay? So to see the effects of dark energy, you need to let that effect build up,

which means that it's easiest to see when you look at galaxies very, very far away, so you're really seeing the whole expansion rate of the universe. Here in the solar system, in principle, the procession of Mercury is affected by the value of the cosmological constant, but the precision to which we can measure that is nowhere, anywhere close

to what it would need to be to detect the effect of the cosmological constant. Finally, in reverse order here, for Anders's question, So it is possible, although people don't really have an agreement on this. This is another place on which experts disagree is string theory compatible with a positive cosmological constant. I think it would be a very good question. You know, I'm only two cheers for string theory, not three cheers kind of person.

I will continue to say until I no longer believe it, that string theory is the most promising approach that we have on the market now to a quantum theory of gravity. but it doesn't mean it's right. None of the approaches are home runs quite at this point in time. So string theory could easily be wrong. If the cause of the universe's acceleration is not a cosmontrial constant, that also leads to other problems. It leads to problems with fifth forces and constants of nature changing and other new fine-tunings and so forth.

So neither one of these options would be plausible. or sorry, not plausible. Neither one of these options would be, you know, easily embraced by someone like me. But that's okay, you know, sometimes you're forced to take action. So I think I can't quite answer the question definitively if the cosmontal constant were positive and, sorry, if string theory were incompatible with the cosmotial constant,

would I first give up on string theory or that the acceleration of the universe is a cosmotral constant? I think I would decrease my credence in both of those by a certain amount and wait for more evidence to come in to decide between them. Angela Howard says, if the wave function represents reality, and we're living in a particular branch, why do we see a wave pattern in the unmeasured double slit experiment? Are we seeing part of the wave function of the universe?

Well, in my way of saying it, so I need to confess, different fans of the Everett interpretation will answer this question differently. You know, famously, I think David Deutsch, if I get his view correctly, talks about the two different slits that the electron can go through as being two different branches of the wave function in the universe. I do not think that way at all. In my way of thinking, branches happen when quantum mechanical systems decoher. That is to say, when they become entangled with their environment. So in my way of talking, your question is easily

answered. The electron going through two different slits is still one branch of the wave. function, okay? It's doing two different things because its wave function is spread out, but its wave function is not entangled with anything else. That's why you can still see the interference pattern. It's not two different branches. And honestly, I'm not sure how you can consistently hold the other point of view. I mean, if you think about a spin instead of an electron, rather than instead of the position of the electron, think of its spin. So we know that if you measure the electron spin in a plus z axis. Okay, then you will always get either spin up or spin down.

Those are the only two choices. Likewise, if you measure it along a plus x axis or a plus y axis, you will always get either spin up or spin down along those individual axes. So let's say you have an electron that you've just measured, you know it is spin up along the x-axis, okay? So you know what the spin state is of the electron. up along the x-axis.

But if you can express that in the z-axis basis, and an electron that is pointing along the x-axis is exactly the same as an electron that is half pointing up along the z-axis and half pointing down along the z-axis. So with respect to the x-axis, it's a single thing. It's just pointing in the plus direction. With respect to the z-axis, it's a combination of two things. one spin up and one spin down.

So is that one branch or two? I think the answer is, it's one branch in either case, because as long as it's not entangled with the rest of the world, it's still one branch. Anonymous says, could the expansion of space ever move the Earth and the Sun so far apart that we all freeze, or does the local gravitational traction of the Earth and the Sun keep us safe? It's the latter.

I probably should have grouped this with the previous question, but the Earth and the Sun are not moving apart because of the expansion of space. the existence of, in fact, so here's something you can think about. If you had a universe that was completely uniform, right, the matter particles, and let's say that they're super duper cold, dark matter particles. So they're literally zero velocity with respect to each other, just a lattice of particles sitting there in a universe that it's expanding,

perfectly smoothly distributed. So you know exactly what the solution is, an exact solution to Einstein's equation with an expanding universe. What you can do is you can take some sphere, Okay, imagine some sphere that you invent with particles inside and particles outside, and you can take all the particles inside and collapse them to the middle, okay, so that you now have the same number of particles you had before, but instead of being uniformly spread, they're all collapsed in, let's say, a black hole or a planet or something or a star,

or a star, inside this sphere, and there's a sharp boundary where you're vacuum inside, except for the planet or star or black hole or whatever, and outside there's this uniform gas of particles. So it turns out that this is another situation that in general relativity, you can solve exactly. There's actually not that many solutions in general relativity that are exact. There's a lot of approximate solutions. But this is what is called a vacuole model where you take a uniform collection of dust, but you take some sphere and collapse all the particles to the middle.

So we know exactly what the metric of space time is inside that hole, right? you have a hole in space with all the matter in the middle rather than being uniformly spread, and we know exactly how much expansion of the universe there is inside that hole, and the answer is zero. There is no expansion of space whatsoever inside that hole, even though the whole rest of the universe is expanding. Okay? So as long as the expansion of the universe is spherically symmetric around us, it's exactly analogous to a question of, what if you have, you know, a spherical shell of metal that you put an electric charge on, right? So from outside, it looks like a ball that is electrically charged and there's a charge.

But what's inside? What is the electric field inside? And the answer is zero. As long as it's spherically symmetric, there is literally zero field inside. Likewise for the solar system, as long as the universe around us is expanding spirically symmetrically around us, there is literally no effect on what goes on here in the solar system. Okay, here are three questions that I'm grouping together, and it's a little bit complicated, so bear with me. They're long questions.

James says, I haven't quite wrapped my mind around the need for a past hypothesis to explain the arrow of time. If I zoom out and look at the model of the universe as a whole, it seems that simply having a boundary condition at the Big Bang, along with the second law, is sufficient to result in a model of the universe where entropy increases when moving away from that boundary condition. And the entropy will be low, by definition, at that boundary. Am I thinking about this wrong? Matt, Matt, Matt, Matt, three mats in a row is the person's name. I have a question about your thoughts on time direction and Landauer's principle. Landauer's principle, by the way, parenthetically, is what attributes a growth of entropy when you erase some bit of information in a thermal bath.

So the question continues. Supposing that the erasure of a memory increases the entropy of the relevant system, what does this mean when we choose to describe things in the opposite direction of time? Surely we say that towards the past memories are erased, but that there is not an accompanying increase in entropy. Should we say that Landauer's principle only applies to the future? And finally, Nick Shorten says, I've heard physicists talk about the time reversibility of the laws of physics and the linking of the arrow of time with entropy.

In many worlds, where the Schrodinger equation is presumably time reversible, the splitting seems to only go one way. Is this related to entropy or an unrelated and separate arrow of time? Good. So all these three questions relate to, the idea that the second law of thermodynamics that says that entropy increases with time is explained by this combination of two things.

One thing is the definition of entropy, which says that high entropy states are associated with macroscopic configurations that have many, many microscopic configurations associated with them. The entropy is the number of, the logarithm, I should say, of the number of microscopic configurations that look macroscopically the same.

That's thing number one. And thing number two is there's a boundary condition. So if you start the system in a low entropy state in that case, there are just more ways to be high entropy than low entropy. So if you let it go, almost all, or at least the vast majority of trajectories will increase in entropy over time. So that second condition, that there is a boundary condition with low entropy, is called the past hypothesis. And so James is saying, so James, I'll confess, I don't quite exactly understand your question. What you've described is exactly what people say, that there is a boundary condition at one end of time, and that's the boundary condition

says that entropy is low there, and we define that direction of time to be the past, and that's why we call it the past hypothesis. So that is exactly what we do, and you're saying you don't understand why, but you just described exactly what is the correct thing to do. So I think you're thinking about it right. I'm not quite sure why you think you're disagreeing with the usual way of stating it. There's no pre-existing definition of past and future. There is the direction of lower entropy and the direction of higher entropy,

and we label the direction of lower entropy to be the past, right? And to next question, yes, this is exactly the same thing going on with the Schrodinger equation and branching in many worlds. The fact that there were fewer branches in the past and the number of branches increases toward the future is exactly because of a low entropy boundary condition in the past. Now, that's not to be too glib about it. There's still issues to make that exactly rigorously defined, et cetera.

What do you mean by low entropy, et cetera? Do you have a pre-existing division of the world into environment and system and stuff like that? But basically, it's the same kind of thing. It's a cosmological boundary condition that gives us the apparent asymmetry, time asymmetry, of branching from an underlying equation, which is perfectly symmetric.

Then the Landauer's principle question, I think that you're cheating a little bit. I actually haven't thought about this one very deeply, so I might be getting the wrong impression here. I think you're cheating when you say, surely we say that toward the past, memories are erased. I think it's a difference between being erased

and not yet having been created, right? I mean, it's true. As we go into the past, there are fewer memories because there's not as much past to remember in some sense. But if you actually take, so consider a memory. And the memory, for those of you who don't listen to this kind of thing all the time, a memory in this case is just some physical artifact,

some feature of the present state of the world, to which we can attach a correlation with some event in the past. Okay, so it's not just necessarily memory in your brain, any record, any photograph, any document that says something about the past. One of my favorite examples, is a footprint on the beach. If you come across a footprint on a beach, you will be quite right in saying,

I bet there used to be a foot that landed there. I bet that this is a record of the passage of someone who left a footprint. You might not be right. There's a possibility that the random motion of the ocean and the sand and so forth left a footprint there. But given the evolution of entropy in the universe

and so forth, most such footprints are going to be associated with a past event. namely the walking down the beach of some person with a foot that landed there and left the footprint, okay? So when, if you were to erase that, so if you take a bucket of water and you pour it over the footprint and you've now erased it, right, you've increased the entropy of the universe. You've shaken things up. It's an irreversible process.

If someone comes across the same patch of beach where the footprint used to be, they don't know whether there was a footprint there that you erased or, where there was never a footprint there at all, right? So that's the definition of an irreversible process. You don't know how to go backwards. Whereas if you take the footprint and evolve backward in time, follow where it came from, right? So unwind the clock and see the person walking backwards

and their foot exactly lands on where the footprint was and they raise the foot and the beach goes back to being perfectly flat. In some sense, that's removing the memory. It's removing the footprint. But it's not erasing the memory in the same. way that you erased it by pouring a bucket of water over it, right? And the reason why the two processes can be so different is because the reversed time one is lowering entropy all the way, right? Is the universe is being traced toward the direction of lower entropy in the Big Bang?

So I think, if I understand the question correctly, I think the answer is that your notion of erasing a memory is not quite the same in those two cases. Roy Rodenstein says, in your February ask me anything, in 51 minutes, you cleanly dispel free will, then in 56 minutes you offer advice to take initiative. Can you share how you think about this? Sure. So I did not cleanly dispel free will. If anything, I cleanly dispelled libertarian free will, right? Libertarian free will is exactly the idea that we talked about earlier, that somehow you are a law unto yourself, you are not beholden to the laws of physics, okay? I don't believe in that. Very few people do believe in that, some do, actually probably the majority of people in the world, very few, you know,

philosophers and neuroscientists and physicists believe in that. Let's put it that way. But I'm a compatibilist. So I think that it is still useful to talk about free will. For example, I give advice to people to take initiative. So what I would say is that the laws of physics are the laws of physics. My willpower does not overcome them. My willpower is an emergent creation of the laws of physics. So there's no liberty. free will in that sense. And yet, when I describe human beings, when I talk about human beings, when I interact with human beings, including myself, it is overwhelmingly useful to use that description of human beings as decision-making agents. As people who can think about reasons for

doing things, and on the basis of those reasons, they can make decisions, okay? That's what a compatibilist is. A compatibilist is someone who recognizes that the laws of physics or the laws of physics. I cannot overcome them. And yet there is an absolutely clear sense of volition and decision-making and free will that we have at the emergent level of being human beings. And honestly, in my less generous moments, I think that literally everyone is a compatibleist, but some of us are willing to admit it and some of them, some of us are not. I have never met a free will skeptic, someone who doesn't believe in free will, who also refuses to use the language of people making choices, who always refuses to try to convince people of things, right?

I mean, if you don't have the ability to be convinced, because everything is predetermined, why would you ever convince people of anything? Why would you ever think that anything is right or wrong? You can't believe in morality if you don't believe in free will. You can't believe that I can make a right decision because you don't believe I'm making decisions, okay? So I think that a lot of people are compatible as they just don't like to use that language. That's my theory.

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that somewhere out there there's a thickest branch? Could there be any significance to this idea of the trunk of the many worlds tree? Sure, there should be a thickest branch, but I don't think that there's any significance to it being, I wouldn't call it the trunk, right? I mean, a trunk is not just the thickest part of the tree. There's something kind of central about it. in some very real sense, whereas there's no center to the set of all branches of the wave function of the universe. So, but yeah, different branches will have different weights. If there's a finite number of them, one of them will be the thickest. There you go.

Will Robinson says, in the laws of physics, in the laws underlying the physics of everyday life are completely understood, you seem to focus on what we can observe in the everyday world, but in some podcast statements, you also extend your conclusions to cover what we could possibly build tomorrow. No Star Trek force field. etc. You seem very confident that a future understanding of dark matter will not lead to a similar technological change in our everyday lives. Is this right? And if so, what is that certainty rooted in? It's not certainty. It's never certainty. This is science. Okay. So you should never be certain, but we have different credences for different things we might believe in. I did just recently

write a paper about this, where I tried to make the case at a more technical level than I have in blog posts and things like that. So you're welcome to look up that paper called the Quantum Field Theory on which everyday life supervenes or the everyday world supervenes, something like that. And the point is that there are very good reasons to believe that dark matter

or Higgs bosons or anything like that, you know, grand unification particles are not going to lead to technological changes, namely because we can't interact with them very much.

You know, the Higgs boson we made. We did interact with it, right? We discovered it, large Hadron Collider. And then it decays away in 10 to the minus 21 seconds. It is completely impractical for any technological purpose that you might want to put it to. It goes away. First off, you need a $10 billion particle accelerator to make one, and then it disappears in a zepto-second. Not very useful for technological changes. Likewise, dark matter. Dark matter seems to be stable, right? Otherwise, it wouldn't have survived for the whole age of the universe. But you can't make it. It interacts too weakly with us. If you had a dark matter particle in front of you, you couldn't hold it in your hand. It would pass right through your hand.

We know that's true because if it wouldn't pass through your hand, if it would interact with you strongly enough, that not only could you hold in your hand, but you could trap it using anything that we have at our disposal, we would have done that a long time ago. People have been trying to do exactly that and failing. So dark matter just goes right through you. And the simpler reasoning goes for anything else you might want to imagine. So there are plenty of possible new particles, new forces, et cetera. but we will not have the ability to manipulate them in interesting technological ways. And in fact, that point is long past.

You know, as I mentioned in this paper, the last time we made a discovery in fundamental physics, not higher-level physics, not emergent-level materials or anything like that, but fundamental physics, you know, particle physics, quantum field theory, gravity, stuff like that. The last time we made a new discovery that actually had technological implications

was basically like the 1950s. We discovered pyons and things like that. And those are like just, they're not stable, but they're stable enough that you can imagine putting them to some use. Since then, for the past more than half of a century, technological change has all been putting the stuff that we know about,

the fundamental ingredients we already discovered long ago, to work in different ways in more complicated configurations. right? And I think that's what technological change is going to continue to be. And there's a lot of ways to do that. So we have a lot of room for new technological change, but it will not be based in new fundamental physics discoveries. Again, not a certainty. I could be wrong. It would be great if I were wrong, but that's the argument. Saraj Raj Rajan says, as a medical professional during my European stint, I had appreciated the large population medical data sets we had access to due to the centralized medical data collection systems that they have, there. They also have a more interconnected network of labs, etc. Are there any interesting practices or traditions you've noticed in non-American physics or cosmology universities that you wish you could implement in the U.S.? So I included this among the questions to say out loud, because

I think it's an interesting question. Sadly, I have nothing interesting to add to it. I'm just offering it up to other listeners, if they have anything that they want to think about spurred by this question. You know, and partly, this is because I have not spent an extremely large amount of time in foreign universities. I've visited them for a few days or a couple weeks at a time. Even a couple weeks might be a slight exaggeration. I visited plenty of foreign universities, but not really hung out with enough time to get deeply immersed in the culture of them. And also partly because my particular kind of theoretical physics is, you know, pencil and paper. There's not a lot of things you can do in different ways.

You know, theoretical physics of this form I do it is basically the same, whether you're in Los Angeles or Paris or Beijing or Seoul or wherever, okay? So there's not a lot of differences. If anything, the most noticeable thing is that, believe it or not, a lot of foreign universities have way worse bureaucracy than the United States does. To get reimbursed for travel expenses, if you go to a conference in France, is just such. an enormous pain that it's almost not worth it sometimes. They love their paperwork there, man, like no one's business.

And so in that sense, I'm happier for the American system. But, you know, I'm sure there are different things. But the things I notice are like, you know, yeah, they have really good coffee and wine at the conferences in Paris. So that makes up for the paperwork. Anonymous says the game Minecraft has seed numbers used to generate their worlds. If two players generate a massive world with the same seed number, they would both start and walk hundreds of miles in a direction

and get the same random block in front of them without communicating. Is this analogous to quantum entanglement? So no, it is not analogous to quantum entanglement. And this is a crucial thing. This is why I think this is a good question to answer. Entanglement is not correlation. Okay? I mean, it is correlation.

It's different than classical correlation. This is not exactly your question, but there's a related famous example given by John Bell of Bertelman's socks. right? Berlman always used to wear two different colored socks. He would never wear two socks that were the same color. So whenever you saw him and if you saw that on one foot he was wearing a green sock, you would instantly know without actually seeing it, the other sock was not green. Maybe it's purple or red or whatever, but you know it's not green. And so Bell, John Bell, is saying, isn't that like entanglement? But he knows better. He's using an example. He's saying, no, entanglement is not like that. It's not just like that. And the reason why is because there is some fact of the matter about the color of Burtleman's left sock and right sock. There is some fact of the matter about what your seed number is, and therefore what block will randomly, quote unquote, randomly drop down in front of you miles, hundreds of miles away, okay? There's a fact of the matter.

So there's a correlation. You don't know that these two things are in certain states, but they are in certain states, okay? The number has been generated. The sock has been put on the foot. Whereas entanglement is a different kind of thing, because if I have two spins, and let's say they are oppositely aligned, so that an entangled state of two spins, so that I know that if one is spin up, the other is spin down, or if the first is spin down, the other is spin up. Okay? So two things to note. Number one, neither one of them has a definite state of its spin. It's not spin up or spin down. It's in a superposition of both. That's different than the classical analogy. But if I observe one spin, then the other one I will know, okay? But more importantly, I could choose to measure

along a different direction. Remember, we just talked about how you can relate spins in the Z direction versus spins in the X direction or Y direction, et cetera. So for this kind of entangled state of two spins held by Alice and Bob, I could choose to measure something different. I could choose to measure the spin along the X axis rather than along the Z axis. And it would still be true that the spins would be anti-aligned. So if I measure my spin, or Alice measures her spin along the x-axis, whatever answer she gets, I now know that Bob, very, very far away, will get the opposite answer. So it wasn't a pre-existing correlation. It wasn't until I chose to measure that, that I could say what Bob was going to see. So it's not just the existence of two copies of the same thing

from a common cause, even from an accidental cause. It's a kind of relationship that is intrinsically quantum mechanical, and you can't really analogize very well classically. UC Polvi says, I was wondering, does it feel different now that Biden and Harris are in charge? At least looking from afar, it seems that the first month has been full of smaller and bigger changes in the right direction. Yeah, I think it absolutely feels different. I think that's actually the good way of putting it, you know? I mean, the people who are in charge of different government organizations are now experts in their fields rather than close relatives and lawyers of the president. Decisions are being made. I'm not going to agree with all the decisions, but they're

being made by people, you know, who are pretty competent in their fields. And also, you know, this feeling question, a lot of it comes down to what in both physics and in poker, which is another, you know, regime arena that I always used to reach for for my metaphors, we would say has to do the lowering of the temperature. Okay? You know, in physics, if you have a box of gas, a very low temperature, everything, the molecules are not moving very much.

They're just sitting there. If you raise the temperature, heat it up, put your cup of coffee in the microwave. Everything moves faster, right? Everything's jumping around and going crazy. Poker players use this as a metaphor. If you're at a table of poker players and they're all very tight, right?

So they're all very reluctant to bet. Like, if someone does make a bet, everyone else folds. It's very kind of predictable. That's a low temperature situation. And if you just bring in a couple of new players who suddenly like always are raising and betting and going crazy, in order to adapt to, you know, play well against that different kind of style, you have to increase your frequency of raising and betting and stuff like that.

So the whole temperature of the table goes up. And the switch from Trump Pence to Biden Harris has lowered our political temperature. That doesn't mean it's lowered the polarization or the commitment of Republicans to oppose the Democratic agenda or anything like that. But the discourse is just a fundamentally different kind of style that is going on. You know, we're talking about the actual technical problems of getting vaccine doses into the right hands as quickly as possible. We are not talking about, well, you know, the governor of the government, that state was mean to me, therefore I'm not going to give them any aid or something like that. You know, the state of Texas did not vote for Joe Biden.

Nevertheless, no one ever even imagined that he would fail to give them aid when they recently had to go through a lot of troubles with the temperature was very low. It was very, very bad weather, and the electrical grid in Texas failed, and a lot of people were in trouble. And, yeah, the federal government helped out because that's what the federal government is supposed to do. it's not an issue now in the way that it would have been a short while ago. Tieri-Leroux-Packett says, Why are you uncertain about Hilbert Space's size?

So Hilbert Space is the space of all possible wave functions of a physical system. So if you have a spin, that can either be spin up or spin down, that's a two-dimensional Hilbert space. But very quickly, if you have something like the position of a particle, just in good old non-relativistic quantum mechanics, Well, how many different positions could you observe if you measured the position of a particle? An infinite number, right? Any real number you could get as an answer.

And therefore, the Hilbert space, the space of all wave functions for a particle moving in one dimension in non-relevistic quantum mechanics, is infinite dimensional. The number of dimensions of Hilbert space is just the number of possible observational outcomes that are truly distinct from one another. That don't, as they say, that commute with each other, a complete set of commuting observables. So what about the real world, right? I mean, if a single particle has an infinite dimensional Hilbert space, surely the world, which has lots of particles, has an infinite dimensional Hilbert space. How could it be finite? Well, that idea of a single particle described by non-reliabistic quantum mechanics is not the real world, right? It's a model, and it's a model that imagines that we could divide space arbitrarily into tiny regions, which maybe we can't do in the real world because the real world.

because the real world has gravity. So again, I'll refer to a paper that I wrote, but I wrote a paper with Ning Bao and Ashmeet Singh, I think, saying that the Hilbert space of quantum gravity is locally finite dimensional. And the reason why is because something we mentioned way earlier in the podcast, talking about black holes,

when you try to put a lot of energy into a quantum field in a world with gravity, you don't create whatever you want. You create a black hole. There's too much energy in a region. And we know that black holes have an entropy, Stephen Hawking told us that, and we know that entropy is finite. And there's a relationship, I won't go into details, but there's a relationship between the entropy of a black hole and the dimensionality of its Hilbert space. So it's a finite number.

So we think that in a world with gravity, in any region of space, there's only a finite number of things that can happen. There's only a finite number of quantum mechanical states that are truly distinct from each other. That would lead us to believe that Hilbert space is locally finite dimension. And by the word locally, I mean in a single region of space. But then, how many regions of space are there, right? We have our observable universe, which is a region of space. Maybe it has a finite dimensional Hilbert space, but maybe there's an infinite number of regions of space outside.

So that's why we don't know. We're a little bit unsure about whether this argument that quantum gravity makes Hilbert space locally finite dimensional is even true. I think it's right, but we don't know for sure because quantum gravity is tricky. And we don't know the total dimensionality of Hilbert space, because we don't know how many local regions of space there are. That's okay. Lots of

things we don't know about the universe. Andre Dino says, as an advocate of many worlds, you probably agree with the philosophical perspective labeled as wave function realism by David Albert. If so, then what do you make of the criticisms against that position, considering the waveform

function of the universe as the fundamental ontological entity which describes our world? So I need to be a little bit technical and nitpicky here. I am not a fan of wave function realism, but you have to understand what wave function realism, as David Albert posited it, is supposed to mean. So David, of course, one of the world's leading philosophers of physics, former mindscape guest, and what he says is the following. And I'm not even sure if he believes it. I mean, this is something which we do, as thinkers. You know, we put forward ideas as possibly true, and then we investigate them, right? We don't just say,

well, because I thought of it, it must be true. So what David says is, look, if I have, let's say I have a particle in a box, a three-dimensional box. And in classical mechanics, I would say, okay, the particle lives in the three-dimensional box. And if I have a quantum theory of that particle, then its wave function can be thought of as a function of space, as a function of the three-dimensional space in which the box lives,

because the wave function assigns an amplitude, which I would square to get the probability to every point in that box, every point in that three-dimensional space. So far, so good. But David says, look, what if you have two particles in the box? Okay? Then because of entanglement,

there are not two different wave functions for the two-particles. There's one wave function for the two-particle system. And the two-particle system, even though each individual particle lives in a three-dimensional box, the two-particle system

lives in a six-dimensional configuration space. three dimensions telling you where one particle is, three dimensions telling you where the other particle is. Let's assume they're not identical particles, so an electron and a proton, for example. And so David says, in that case, the quantum mechanical wave function is a function of the six-dimensional configuration space.

And if I have n particles in a three-dimensional box, the wave function is a function of a three-n-dimensional configuration space. So he wants us to imagine the idea that that, kind of wave function, not just an abstract quantum state, but a function on configuration space, three-end dimensional configuration space, is what is real, is the fundamental ontology of the world, okay? So in some sense, the three-end dimensional configuration space for this view is more fundamental than the three-dimensional space that we call space, okay? So it's a slightly cheeky point of view because we all think that space is more fundamental than configuration space in some sense.

So I admire that cheekiness. I think that's a good thing to do. But look, I don't buy that ontology, as it were, in a straightforward way. And for a number of different reasons, the most basic is the world is not made of particles. That's not the world. Sorry, right? I mean, the world is maybe made of quantum fields, but like I said, gravity is not a quantum field theory itself. So the world's made of something. We don't know what it is, but it's definitely not a bunch of non-relativistic particles. So that straightforward literal wave function of n particles ontology certainly is a non-starter, in my view, describing fundamental reality. But also, that wave function is, in my mind, you know, a representation of a quantum state. It's a way of

describing and giving the information contained in a quantum state. But it's a highly non-unique way of giving that information. For example, I'm not sure, Andre, how much into quantum mechanics you are, but there is the position basis. There's also the momentum basis, right? So I can equally well describe that n-particle state in the momentum basis, in which case the wave functions of function of momentum space, not of configuration space.

So I certainly can't imagine taking configuration space as the fundamental arena in which wave functions live. What I can do is be even more abstract than David was proposing to be, and say what really exists is just a vector in Hilbert space. And you can represent such vectors in Hilbert space as functions of configuration space. You can represent them just as well as functions of momentum space, or literally an infinite number of other choices. It's just like when we were talking in the very first special relativity talk

when I said choosing coordinate systems is not an objective physical thing. It's a choice made by human beings. Likewise, choosing representations of quantum states is not an objective physical thing. So the thing that is real, in my mind, is the vector in Hilbert space. Now, most philosophers of physics

would say that's crazy talk. and they would say it because there's not enough structure in Hilbert space to recover the reality of the world. And David Wallace, who's another leading philosopher of physics, has said this out loud. I believe the phrase he used was it would be naive in the extreme to believe that. And I know that off the top of my head because I'm writing a paper right now. I'm supposed to finish it this week. And yet here I am doing AMAs, but I love doing the AMA, so it's okay. Anyway, I'm writing a paper that gives a defense and Apologia for this.

view that really is just the wave function as a vector in Hilbert space that should be considered real, not configuration space or momentum space. So I'm quoting David Wallace as saying that my view is naive in the extreme. But I think that's just because people don't appreciate there is just enough structure in Hilbert space to do everything you want, and that's what you want. You want to be able to reconstruct the complicated world around us from the minimal amount of structure. I think that's what we can do. Okay, Nathaniel Zabel says, I've always sort of romanticized academia and studying physics. I gave it a go and realized that it may not be for me, and now I'm most of the way through an electrical engineering undergrad degree. So my question is,

how, if at all, do you see engineers involved in real physics? Well, they're all over the place. You know, I mean, it depends on what you mean by real physics. But most of physics, not what I do, but most of physics, involves experiments in a very intimate way, right? Building apparatuses, designing detectors that are the world's most precise and robust. There's an enormous amount of engineering that goes into this. You know, you don't think that large Hadron Collider was built by physicists, do you? I mean, there's a lot of physicists involved, but there's a lot of engineers, technical people, you know, contractors, etc. Someone had to pour concrete, et cetera. So, yeah, if you're interested in being involved in the broader project

of physics from an electrical engineering degree point of view, that's 100% possible. I'm not the world's expert about how to do it, but it's absolutely possible. Carlos Nunez says, if you could have a superpower, what would it be and why? Yeah, I think, let me see. I think the answer is obvious, which is teleportation. And, you know, look, there's slight ill-definedness in the question because what counts as a superpower? Like, is omniscience a superpower? Will I get to do that?

You know, another obvious choice would be flying or, you know, telepathy is a good one, reading other people's minds. Certainly reading other people's minds would give you great advantages going through life. I also think it would be really depressing, right? I think that minds are messy places that people don't have complete control over, and I don't necessarily want to know what other people are thinking about everything. So telepathy might be more than I want access to. Flying seems cool, but if you believe that you have some limitation on your speed, then teleportation is way cooler.

So I'm interpreting the question as, you know, going through the lists of super powers that superheroes tend to have. So I'm excluding omniscience or something like that. Teleportation would be great. Do you think I can walk out the door and suddenly have lunch in Paris or, you know, breakfast in Tokyo? That would be pretty awesome. Don't have to fly. Don't have to wait in line with TSA. Yeah, that would be a good superpower in my view. Stephen Klein says the sun is hot. The 93 or so million miles of space between Earth and the Sun is cold near absolute zero. How is it that the heat of the sun can travel that distance through cold without heating space up and then get hot again when it hits Earth?

Well, the reason why I include this question is because it's a good one exactly because we are usually sloppy about using words like hot and cold. If you're really, really, really strict, and you shouldn't be, but if you were really, really strict, you should only attribute a temperature to a system that is at or near thermal equilibrium, okay, that a system that has been given a chance to equilibrate and come to a constant temperature everywhere, then we have a well-defined notion of what it means to be at a temperature. If you are near the surface of the sun, well, then you're pretty close, okay?

Then everywhere around you is the same temperature, everywhere around you is the same density of matter, etc. You're pretty, if you're not outside, but right inside the sun, that's a pretty good approximation. But if you're in the space in between the earth and the sun, you're nowhere near. equilibrium. Exactly because in most directions you look, it's really cold. The sky is dark. But in one direction you look toward the sun, it's really hot and bright. Okay. That is a paradigmatic example of not being in thermal equilibrium. So it's not that space is cold in between the earth and the sun. It's that space doesn't have a temperature, strictly speaking. It's not a system that is in thermal equilibrium. There's the other part of the answer, which is that space is not a thing. You can't heat up

space. Space is the thing through which other things travel at this level of description. So another reason why you should not attribute a temperature to it. But if that's a deflationary answer, then sorry, but that's the right way I think to think about these things. Costel Rotari says, why isn't there an Italian translation of the big picture? Will it happen and can I do it myself? So I answer this question because maybe it helps up. I get this kind of question, usually by email, a bunch of times. You know, can I translate your book? As far as I remember,

there is no Italian translation of the big picture and no current plans for one. I believe they are currently in the process of translating something deeply hidden, but the big picture I don't think I have an Italian contract for.

So the point is, you know, look, the publishing business is a business, okay? People get paid. People earn their living doing it. You need to have contracts and dollar amounts or lira amounts

or euro amounts or whatever. So a lot of people from different countries write in and say, there is no translation in my language of your book. Can I translate it? And the answer is, no, you can't. What you can do, I'm not sure if this ever works, but in principle, you could interest a publishing house in getting a translation done.

And maybe you can even volunteer your service or let yourself be hired for money as the translator, okay? But the actual translation and publishing of a foreign language edition of a book has to be done through a publishing house with a contract, etc. So if you have a favorite Italian publisher, especially one that you know does translations of English language science books, then by all means pester them to offering a contract to my agent, to do a translation. Usually, you know, for those of you who are out there,

interested in writing books and things like that in the economics of the business, foreign language editions of books are not a good way to get rich. Like if you write a book in English, most of the people buying your book will buy the English version, not a foreign language version. So, you know, you can make a few hundred, a few thousand dollars here and there, depending on the version, but I'm not saying all of this because I want to get rich off the Italian edition of the big picture. That is not going to happen. But there are legal things to keep in mind, and there are Italians who work for publishing houses who need to earn a living. So that's the way to get it done. Ben Turner says, you mentioned

before you had previously considered going to law school. If you'd been a lawyer in another life, what kind of lawyer would you have been? You know, honestly, I would have been a law professor. I don't like, I don't know because I haven't tried it, but the romance here is not me being in court and being Perry Mason or whatever. It's thinking about the idea of the attraction of law school was that law is sort of an intersection of logical thinking and reasoning with down-to-earth human concerns, right? It's not the world, you know, finding out the truth about the world outside, but there is legal theory, constitutional theory, things like that, and that's what attracts me. So I think that doing that as a law professor would have been a better fit to me than to be like

a corporate lawyer or even a constitutional lawyer or something like that. Chris Fotash says you talk about anti-desitter space often, but where do we find that in our universe? Isn't space time a D-sitter space? So D-sitter space was, is a solution to Einstein's equations for general relativity that you can get if you start with an empty universe that has a non-zero vacuum energy, that has a non-zero cosmotical constant, a positive value of cosmotial constant. So De Sitter, Billum DeSitter, way back in the days, I don't know, 1917 or something like that,

very soon after general relativity came along, solved the equations for general relativity with a positive cosmotical constant. We call it DeCitter space. We don't live in DeCidder space. Our space time is not DeCidder space because there is stuff in our universe other than the vacuum, right? There are stars and planets and dark matter. The better thing to say is that as time goes on, that stuff is, you know,

emptying out, and we are approaching, we're asymptoting toward decider space, and maybe in the future we're going to look more and more like decider space. Anti-de-sitter space is an analogous thing, but what would happen if you get a negative cosmological constant? So there's no worries about a negative number for the vacuum energy. It's just as physically allowed. So you can imagine cosmologies with the negative vacuum energy, solve the equations. We call it anti-de-sitter space. There's no Mr. anti-decitter, but I don't know who did anti-desider space first. The reason why anti-de-sitter space is so often mentioned is because it is a wonderful toy model for quantum gravity. And it's wonderful because of the discovery in the mid-1990s from Juan Maldesana that quantum gravity with anti-de-sitter

boundary conditions is related to and maybe is exactly the same as a quantum field theory without gravity living on the boundary. That's the ADS-CFT correspondence. ADS being anti-decider. CFT meaning conformal field theory, a particular kind of quantum field theory. So the idea, so you might say, well, okay, that's good. If you have this correspondence

between a theory of gravity and a theory without gravity, then we can use our knowledge of non-gravitational quantum field theory to learn something about gravity. Yes, that is the motivation behind ADS-CFT correspondence being so popular in the physics literature.

But you might say, but who cares if it's not our world, right? And the answer would be, well, we know so little about quantum gravity. Like, we think that information is preserved when black holes evaporate, but we don't know how. Maybe studying a problem like that in the context of the ADS-CFT correspondence teaches us something fundamental about quantum gravity that will continue to be true, or maybe less so but equally good will inspire some true thing in the real world, in the decider space world. And I think this is a reason why, I mean, let's put it this way. This is the generous way of spinning

why so many people care about ADS CFT, because they're trying to learn robust features of quantum gravity in a well-controlled example that hopefully we can then port over to more realistic situations. There is a less generous construction, which would say it's just full employment. Like, once you have ADS-C-F-T, there's a million questions you can ask and answer, a million equations, you can write down and solve them, and whether or not it's related to the real world, that's what physicists like to do, so they do it, okay? The truth is probably somewhere in between those two things. There is definitely some, you know, treadmill kind of work. You're on the hamster wheel of solving equations in the ADS-C-F-T correspondence,

and you lose sight of the bigger picture, but there's absolutely also work that has come out of ADS-C-F-T that potentially has interesting ramifications for the real world. So somewhere in between probably is the truth. Humberto Nani says, is it possible that the universe looks flat because it has not had enough time or space to express its curvature,

or is that possibility ruled out by observations? No, it's absolutely possible. you know, we sometimes say that when we talk about the curvature of space in cosmology, there are three choices. There's positively curved, negatively curved, and zero, flat, right? Well, you know, that's not an false statement, but it's also not the most information conveying statement. A better way to say it is that there is, for every possible geometry of space, and these are not arbitrary geometries, but homogeneous and isotropic geometries. There's only those three choices. You can attach a real number to any such geometry

called the radius of curvature. The radius of curvature of a sphere is literally the radius, right? How big is the sphere? Spheres are positively curved. So if you take a sphere and make it bigger and bigger and bigger radius of curvature, in the limit where it becomes an infinitely big radius of curvature, it's flat. You get the plane. You can check this mathematically. And then you can sort of come back the other side. You can say, well, what if I have a negative radius of curvature? That's what you would attach to a hyperbolic geometry, a negatively curved surface. So even though there are three classes of geometries of universe, positively curved, negatively curved or flat,

there's really a continuum of actual geometries from negatively infinite curvature to positively infinite curvature. So what we're measuring when we measure the curvature of the universe is a real number, not a discrete choice of three different possibilities. So the measurement that we do of the curvature of the universe has error bars. And right now, the curvature of the universe, as we've measured, is compatible with zero curvature, compatible with being flat. But it's also compatible with a very large radius of curvature that is either positive or negative.

So as those error bars get better, or as our observations get stronger, we might discover that the universe is a little bit negatively curved or positively curved. That is absolutely possible. Christopher Matthews says, as someone who hasn't solved an equation since high school, I don't have a great sense for what is actually involved when you talk about solving the sort of equations a working physicist would solve.

Is that something you could describe or would I need a stronger background to grasp it? It's hard to describe just because there's so many different equations, right? I mean, there's a lot of different techniques for solving equations that physicists use. Sometimes pretty simple, like, you know, oftentimes, you know, I already mentioned earlier that if you have a bunch of particles in a box, you can describe them in position space or in momentum space.

Well, there's a technique. There's a mathematical technique for transforming from one to the other called Fourier transforms. And so many systems that are very complicated in position space are easy to solve in momentum space. You do that kind of integral transform. Other times you have a simple differential equation like the freedom in equation for the expansion of the universe, and even though it's very simple, if you have some complicated stuff in the universe, you're just going to solve it numerically.

You're just going to put it on a computer and solve it. In yet other cases, you have something like Feynman diagrams for a scattering process, and Feynman has given you a little recipe for attaching to each diagram an integral or a set of integrals to do. And a set of tricks, like literally there's something that you can look up in quantum field theory books

called Feynman's trick for doing integrals. And it's a trick that is specific. for the kind of integrals that appear in Feynman diagram calculations. Other areas of physics will have matrices that you need to diagonalize or find the eigenvalues of. You know, there's just a whole bunch

of different techniques. So I'm not giving you a very helpful answer, but it's a big heterogeneous mess. That's all I can really say for it. Sorry. Okay, here's a group of questions. I'm going to group together. Robert Casson says,

did your conversation with Russ Schaefer Landau change your degree of belief in moral constructivism? Stefan Berniger says, did your podcast with Robert Sapolsky yield any new insights about how we humans should deal with moral challenges? And Andre says, in your recent podcast with Robert Sapolsky, you didn't push back

on his views against free will. What is your response to his arguments against compatibilism? So the grouping is, did my mind change when I did these podcasts? And nope, my minds didn't really change. Like my amount of information increased, I learned something about it. But for the Rush Schaefer Landau podcast, for example, you know, he is really quite explicitly, you know, he's very honest and enamel about it.

He's basing his moral realism on our intuitions. We have an intuition about what is right and what is wrong, and he thinks that deserves, you know, being taken seriously as a moral realist. And I just don't. I'm like, wow, our intuitions, which clearly were shaped by evolution and all sorts of weird things in the structure of our brain. and so forth, I can't imagine attaching moral heft to what those intuitions are. So no, I understand better where he's coming from, but my mind did not change. With Robert Sapolsky, yes, you know, he knows much better than I do. What are the different actual characteristics of the world that go into making us make certain

choices and do certain behaviors? and he concludes from that that there's no free will because he can see why we're making all these different choices. From a philosophical point of view, I don't think this has much to do with compatibilism. The philosophical case is about what I said before is the best language that we have for talking about human beings and describing their behaviors,

one in which they are agents making choices. And even if I know that there is something called the amygdala and there are hormones and there are genetic behaviors and there are things that happen in your childhood or in the womb and etc. All of these things. I can know all of these things, but as long as a person is not sufficiently brain damaged or something like that, that my best way of thinking about them is as a person I could give reasons to and they could respond to those reasons and make choices based on those reasons, then that's what I mean. by free will. So I don't think that Sapolsky has

really thought about the philosophical argument one way or the other, which is fine. That's not his job. That's not what he is doing. So it does not change my personal view on free will. And again, as I said before in the podcast, I'm not there to ride my hobby horses. You know, if I have Robert Sapolsky on the podcast, I want to hear his insight about how the brain works. I don't want to have a long, complicated semantic argument over how we should think about free will. Okay. And how the question about how whether or not that podcast change, how we humans should deal with moral challenges, again, you know, no, not really. And I think this is why I don't like the free will discussion that we have in the modern world. Because there are really important questions about practical things we should do. Punishing people who behave badly, treating people who have mental disorders or addictions or things like that. These are really important questions. These are really. You're really. You're really. You're real questions, okay, that I don't have any simple answers to. To me, these questions are

largely orthogonal to the free will questions. You know, these are all questions about which I'm very much a consequentialist. What is the thing we can do to make things better to the extent that we agree on what should be better? You know, if someone is addicted to heroin, it'll be better if they were not. If someone murdered someone, it would be better if they didn't murder people, okay? So we agree on what the goals are, and we have a very instrument. instrumentalist question. How do we get those goals? And that seems to have nothing to do with how we think about free will in my mind. It has a lot to do with the biology, sure. So that's the discussion I think should be going on, not should we think about free will or not, but given what we know about biology and psychology and pharmacology and the legal system and all these things, how should we make all those systems the best to get the outcomes we want? And if no one ever used words like free will, will in that discussion, I think the discussion would be way better than it is. Okay, Gary Miller says, why are you skeptical of Avi Loeb's view that umamuma, sorry, uh, uh, uh-uh-mua, umuamua, man, I was really

good at pronouncing that when I talked to Avi, but I've lost the ability. Why are you skeptical that it's artificially made? As a lay person, I think it's a fascinating idea, but rely on Tari Lee on what the experts think. What do you think on Amuamua being artificial versus natural? Well, partially it's a question of priors, right? You know, I think that most objects flying through the solar system are much more likely to be natural than artificial. So I would need really good evidence to change my mind before I sort of switched over. You know, just for the simple reason that, you know, I presume in my way of thinking about life in the galaxy, that most of the stuff in the galaxy is natural, not artificial. And, you know, the, the, the, the,

specific scenario being put forward is a little bit weird, right? I mean, on the one hand, it's supposed to be a solar sail. You're supposed to say, well, it's more likely that it would pass close by the sun because it's meant to be a solar sail and use the sun. But why exactly? Like, is it, you know, where is it aiming? Why didn't they just aim it where are they supposed to aim it rather than like send it on a trajectory past the sun? Why is the solar sale the right thing to be. Why didn't it give off some signal that makes it perfectly clear that it's artificial and so forth? Like the bigger context here is not very convincing to me. On the other hand, I'm very much in agreement that we should take it seriously. That's why I had Avi on the podcast. You know, again, I don't, I invite people on the podcast who I disagree with, but not people who I don't think are worth listening to. So it's very much worth taking that seriously. And, you know, I do think that Avi, like, more than Sapolsky or Schaefer Landau, Avi did shift my credences a little bit toward taking the possibility of detecting extraterrestrial

life a little bit more seriously than I usually do. He didn't change my credences on whether artificial life exists, but there is some, you know, payoff to finding it if it's out there, especially defining it before they find us, so I think that's a good lesson to learn. Joseph Tangretti says, if two massive objects exist in spacetime and are X distance apart, how does space time know the distance

and impute the proper amount of gravity between the two objects? Well, the answer is because space time is not empty. Space time contains fields at every point. In particular, in this case, it contains the metric tensor field, which we previously referenced. The metric tensor is just a fancy way of saying,

space time has a geometry. The metric tensor is what keeps track of what that geometry is. And so there is something that stretches in between, these two objects, and that thing, that metric obeys an equation, Einstein's equation, of general relativity. That's how it knows. That's why there's not any question of spooky action at a distance in general relativity. Of course, Isaac Newton did worry about exactly this, because he didn't know about the metric tensor.

He didn't know about fields more generally, so he didn't know how objects did this, but we've learned a lot since Newton's time. Okay. Amon Nilapa says, I'm a naturalist such as yourself, however, having been brought up in India, I've always been cognizant of multiple claims that there is a deep, non-naturalistic, perhaps metaphysical truth on offer to someone who's prepared to dedicate themselves to a rigorous and long-term mental and physical discipline. The catch is that one has to dedicate one's life to such a pursuit. And another catch seems to be that the said truth is claimed to be only, expressable by a metaphors, and eludes a direct and precise description. However, from all accounts,

the nature of the truth that one discovers is not just deeply gratifying and beautiful, but also largely consistent, as can be evidenced from the similarity in the writings of those who are believed to have done a particularly good job of walking these disciplines. What, in your opinion, should be one's attitude towards claims such as these? So this is a really good question. You know, I don't, I hope that no one listening is dismissing this question. So the claim is that there are truths of some sort, which are left vague, the sort of truth that you're getting here is left vague, that can only be reached with a certain kind of investment, right? And that once you do reach that truth, you can't even convey it to those who have not made that investment. So is this worth

taking seriously, right? Like if it's all just hoagum, then you would waste a lot of time if you put in that investment. So I don't, you know, so I'm a little skeptical, honestly, but I'm willing to be open-minded about something like this. You know, so let's think about the in-principle question before we think about specific examples. In principle, is it possible? There are kinds of truths, realizations, revelations, if you want to call them that, that can only be reached by meditation or practice or something like that. And that even when you attain them, you can't share them with anyone. So I think that's possible. I can imagine something like that is correct. I don't think that the kinds of truths being obtained that way would be physics truths or math truths. I think those kinds of truths are essentially always expressible in non-metaphorical ways. But you might be able to reach truths about yourself, you know, about how you should think about yourself as a person in the world, how you should live your life, what you should value, those kinds of things. I certainly would be. much more. So what I just said is I think it's possible in principle. Yes, it's conceivable. On the other hand, I still am

skeptical, in part because I think it would be much more convincing if you could give some flavor of those truths even to those of us who've not gone through the discipline, right? You know, being a scientist, doing science, one of the lessons you learn is, as I think it was Feynman who said, you know, you have to be careful not to fool yourself because you're the easiest person to fool. And this kind of thing where, oh, I have some special knowledge, but I can't tell you what it is, man, that just seems ripe for fooling yourself, right?

Like, you have a vested interest because you've put in a lot of work, put a lot of your own time in attaining this kind of truth. And so you're certainly predisposed to think that it's there, and then you're predisposed to think that you've done it, right? So it's hard to be skeptical. It's hard to stop yourself from fooling yourself.

And finally, if it were true that these kinds of truths were obtained, then I would expect there to be, even if the people who gained that truth were not able to convey it non-metaphorically, I would expect there to be some kind of tangible difference in that person, a wisdom or knowledge or something like that. And I don't see it. I think that people who spend a lot of time meditating or monks or priests or mystics or so forth in my experience, which is admittedly limited in these regimes, but they're the same kind of people that I see otherwise. They're not any better doing physics. I know that. They may be more peaceful, but I don't always see that even, to be very honest. I went to a Catholic school. It's not quite the same, but, you know, there's something. similar there. So, yeah, you know, so I would tend to be skeptical, but I don't in any sense claim to know for sure or to even be unpersuadable about those things.

Greg says, what are your thoughts about the risk of setting off an AI apocalypse, i.e. the possibility of creating hyper-intelligence, self-replicating and self-improving beings that would ultimately result in a net detriment to humanity. So I have conflicting thoughts about this. You're asking me my thoughts. On one side, I absolutely think there is a worry here worth worrying about, right? And putting aside the buzzword language of AI and hyperintelligence, we're creating systems we don't understand. We're creating complex systems artificially constructed that it is hard for us to be specific about what they do, how they will respond under different circumstances. And that's just dangerous, right? We're, you know, technological progress has brought us to the point where human beings can create enormously powerful things.

It would make me happier if we really, really, really understood those things we're creating and we don't always. Okay, that's one side of it. The other side, the much more skeptical side is, I think that people who I've heard talk about this, to my mind, are just being way, naive about how they anthropomorphize AI systems. They use words like hyper-intelligent. They use words like values and morals that they attribute to these systems.

And there might be a sense in which these words have some relation to what's going on. And this is not, I'm not trying to say that artificially constructed systems can't be intelligent or even conscious or whatever. I'm just saying that to the extent that we create systems that have something like

intelligence or consciousness or whatever, or values or morals, I suspect that those systems will be wildly different than human intelligence and values and morals. And I think that I'm just very unconvinced by the discussions I've heard about, you know, how to make them value the same thing we value? Like, how in the world do you think that we can tell them, to value. Like, if they're really hyper-intelligent to the point where they're like us, they're going to decide what to value. They're not going to listen to us. We can't bake it in. If we bake in instructions, then they're not really as intelligent and self-aware and

self-controlling to the extent that we want them to be. So I think that the real problem is not this simple-minded thing that they're so smarter, so much smarter than we are, that we become sort of lesser beings. The real problem is that these systems are going to be so different than what we think of as an intelligent agent, that they'll be totally unpredictable. And that's a worry. I agree that that's a worry. I hope that we are worrying about it to the right extent. Katen says, in your book from Eternity Here, you wrote to this day, scientists haven't determined yet to anyone's satisfaction, whether the universe will continue to evolve forever or whether it will eventually settle into a placid state of equilibrium.

But given an infinite period of time for something to happen, why wouldn't any equilibrium eventually break, perhaps as a result of quantum fluctuations? So, yeah, there's, the short answer is because there are competing infinities here. If you have a system, which is sort of a finite state system,

there's only finite number of things that can happen, or even a bounded state system. So there's sort of a real number of things that can happen, but they happen in a circle or in some bounded region of space or of configuration space. And that thing lasts for an infinitely long time. And the dynamics are sort of irreversible. Okay.

I'm sorry. The dynamics are reversible. So let me back up. I'm making too many assumptions here to even make the conclusion clear. Think of all the different possible dynamics of a particle moving on a disk. Okay. So you have a disk. So you have literally some little.

circular thing, and it's interior. And you say, I have a particle, so a point, a little mathematical idealized point moving on the disk. And there are different kinds of dynamics, I can imagine. One would be a kind of random dynamics where the particle wanders around in a random walk, and it will come back to where it started, et cetera.

But there are other kinds of dynamics where the particleist goes in a circle forever, or a figure eight forever, or whatever. And there's a third kind of dynamics where it can just sort of spiral into the middle and just stay in the middle forever. So in all of these cases, the particle evolves for an infinitely long time,

but in some cases, it just fills the space of possibilities over and over again, and in other cases, it just goes to a single point and stops. So in this space of all possible dynamics, you can have anything you want in that set of circumstances.

If you put on the extra restriction that the dynamics are like the reversible dynamics of the real world, of the Schrodinger equation or whatever, then the system will not simply wind down to a point, because that's not reversible. Once you're at the point, you don't know where you came from. However, maybe the world, the universe,

doesn't have a bounded space of possibilities, right? If you have both infinite time but also an infinite space of possibilities, then any given region of the universe can wander into an equilibrium and never leave. that is absolutely mathematically possible. So there's no, to get to your question, there's no guarantee that just because there's an infinite amount of time, you will return to any particular place. Like if you are walking on the real numbers

and you're walking in the increasing direction and you go, you know, one, two, three, four, five, you can keep going forever. You're never going to get back to one if you just keep walking in the same direction. And the universe might be like that. Gordon Bamber says, my assumption is that dark matter interacts only gravitationally with normal matter.

There must be occasions when a black hole accretes a large amount of dark matter. If dark matter does not emit electromagnetic radiation, then could the resulting energetic accretion disk be detected at all? So the assumptions are not quite mutually compatible, Gordon. It's fine to assume the dark matter interacts only gravitationally with normal matter. That's a possibility. The question is, does the dark matter interact only gravitationally? with itself. And maybe even only gravitationally is an exaggeration, maybe just doesn't interact

very strongly with itself or with ordinary matter. Okay? In that case, so if dark matter mostly interacts gravitationally, sure, dark matter can be absorbed by black holes, but not a lot of it, because the reason why ordinary matter gets absorbed by black holes is because ordinary matter undergoes dissipation. When atoms of ordinary matter, matter bump into each other, they can emit photons and lose energy. That's why you make an accretion disk, and the accretion disk is shining, because there's a dissipation-filled system where the matter bumps into other matter, loses energy, increases the entropy of the universe, and settles into a disk. Okay? None of that happens for dark matter. Dark matter typically would just

zoom right by the black hole. It would have to hit it right on to be absorbed by it. Whereas ordinary matter, it can come near the black hole, and if it were all by itself in the universe, it would go right by. But it's not. It bumps into some other ordinary matter, loses energy and spirals into the black hole. So mostly black holes are going to be eating up ordinary matter, not dark matter. So there is no accretion disk of dark matter, even if there is dark matter being absorbed by the black hole. And if there were, we couldn't detect it because it would be subdominant to the ordinary matter in a typical situation. Okay, Jim Burnside says, If dark energy is causing the galaxies to travel apart at an accelerating rate at very high velocities,

then I don't understand how the Milky Way can be on a collision course with Andromeda. Well, I should have put this up with the previous questions about dark energy in the solar system. Dark energy is not pushing the Milky Way away from Andromeda. That idea that the universe is overall expanding is only true on the larger scales. It's an approximation that gets better and better, as you consider galaxies that are further and further away. The Milky Way and Andromeda are near to each other. They are gravitationally bound to each other.

It's just like the Earth and the Sun are gravitationally bound to each other. So the Milky Way and Andromeda have broken away from the larger expansion of the universe, and dark energy does not change that fact. Last question is from ACAC, ACAC. I was re-listening to your Minescape episode with Leonard Suskind. In the end, he talks about his father and wanting to teach science to him and others above a scientific American level. Was this an inspiration for your biggest ideas in the universe series and the textbook you are or were working on? Not quite in exactly that way.

You know, I had less lofty inspirations, or at least different inspirations. You know, the big picture question here, as it were, is that I've always had an interest in explaining and talking about and discussing physics with a broad audience, not just my fellow physicists. I think it's part of the fun, honestly, in doing it. You know, I've said it many times before. The kind of physics that I do doesn't cure cancer or lead to technological investigations, improvements. All it does is help us understand the universe better.

So to understand the universe better and then to not tell anyone that you've understood it better, seems crazy to me. So I really like that part of the enterprise, including making videos. and so forth. The biggest ideas, videos in particular, honestly, I think I said this somewhere, but maybe didn't get enough credit, but my friend Lauren Gunderson,

who's a playwright, she's actually the most published playwright in the United States working today. And when the pandemic hit, and when we started to be locked down, she started doing playwriting lessons online, you know, as a way to keep people connected and talking to each other and doing things and learning, improving themselves.

And so that fact that she did that was the immediate inspiration for me for doing the biggest ideas in the universe series. I'm like, well, that's something I can do. I can make little videos teaching people about physics. And, you know, the Q&A videos were part of that inspiration, that it was not going to be just me talking. It would be some interaction of some form, right? And now the question that I honestly, even though it was only, last year, I don't even remember how I chose exactly the level, right? So the thing that makes

those videos interesting in my mind is that their higher level, in a form of, in the sense of being technical and mathematical, than most popular physics discussions, while being lower level than most textbooky discussions, right? So, and that's a niche that is, that is unfilled to a very large extent. And so I'm very happy that the videos ended up being in that niche. And I would like to turn them into a book someday that is also in that niche, because I think that is not filled as much as it could be. But I really don't remember how conscious that choice was or it was just like I started talking and that's where they went. I would have to think back. And, you know, it all moved very quickly because, honestly, I had to rush out and get the equipment to start the videos before everything

locked down. And then once I started them, I had put myself on this weekly schedule. I didn't have a lot of time to sit back and reflect. So they happened. You know, they were, I didn't do a lot of research for them. It was just mostly stuff that I thought that I knew already that I could share with a broad audience. And as everyone knows who saw them, my video quality fluctuated with time, mostly improved, but fluctuated a little bit. And it was great fun. But it wasn't something that I put a lot of pre-planning into. I'm not a really great pre-planter kind of guy. Like, once I'd gone out and purchased the green screen and everything, I wanted to start

making videos. I didn't want to, what I should have done was like watch 20 hours of other YouTube videos about how to make good videos, right? That's what I should have done. I didn't do that. I just plunged into it and the result is up there and we'll be there on YouTube forever, or at least as long as there is YouTube. Forever is an exaggeration.

But I hope people enjoyed it. I hope people enjoy the podcast. I hope people enjoy the AMA. I hope you like how it went. I still did probably many, probably a little bit too many questions. I went too long in this AMA. But I think it works picking some questions

that I want to answer because I'm more enthusiastic about the answering. I think maybe that happened. Maybe I'll try to be even a little bit more persnickety about which questions to answer next time. so that I can just take the questions that are inspiring me to give interesting answers and focus on those. But I do solicit feedback. Let me know how you think it went.

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