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The following is a conversation with Andrew Strominger, theoretical physicist at Harvard, whose research seeks to shed light on the unification of fundamental laws of nature, the origin of the universe, and the quantum structure of black holes and event horizons.
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And now, a quick few second mention of each sponsor.
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Today is actually an example of a day where.
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It s been a pretty rough one.
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I didn t get much sleep the night before.
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I didn t get much sleep tonight.
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I had to do a bunch of really difficult work, especially programming work and then reviewing other people s code.
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I also had to record a podcast and also had to socialize, which is another kind of challenge altogether.
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And all of that combines to just this kind of mental state of maybe psychological, physical, all kinds of exhaustion.
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And it s kind of incredible how a short nap can fix so much of that.
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At least can fix the most important aspect, which is once you wake up, you see the world anew.
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That reinvigorated feeling of gratitude for being alive, for the beauty of the world, for being able to experience the beauty of the world.
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This show is also brought to you by Rocket Money, a personal finance app that finds and cancels your unwanted subscriptions, monitors your spending, and helps you lower your bills all in one place.
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It always cracks me up when people have something like a treadmill in their basement or in their apartment or wherever, and you can just tell that treadmill is not being used in months, if not years.
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And to me, for my life,
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I hate having that treadmill there because it s a kind of symbolic visceral reminder of promises to yourself that you have broken.
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Anyway, subscriptions are kind of like that except they also waste your money.
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It s like I plan to do this thing, I plan to consume this product or this service or to do this kind of reading or to do this kind of thing on the internet or whatever the subscription is and then you realize
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That s rocketmoney.com slash flex.
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I should also mention that we re currently hiring.
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beyond maybe educational, almost like exploring the human condition, whether that s podcasts or videos or all that kind of stuff.
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Anyway, it s an amazing team, and we re always growing it.
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That s the first step, the hardest step, and to do that fast is super cool.
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I ve used them for many years, with a big red sexy button that ll repress.
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And like Alice in Wonderland, escape into a world where nobody knows exactly where I am, but everybody is happy I m there.
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And...
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That s useful for Netflix, for watching content that s geographically restricted, but it s also useful in the way that the journey that Alice takes on Wonderland is useful because it allows you to do all kinds of stuff on the internet where your privacy is sensitive.
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I mean, everybody should be using a VPN, and maybe I m biased, but I don t think so.
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I think ExpressVPN is the best, and you should use it.
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It works on any device.
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It works on your iPhone, Android, it works on Windows and Mac, everything, everything, everywhere.
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And I should also mention, it s super fast on those operating systems too.
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This is the Lex Friedman Podcast.
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To support it, please check out our sponsors in the description.
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And now, dear friends, here s Andrew Strominger.
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You are part of the Harvard Black Hole Initiative, which has theoretical physicists, experimentalists, and even philosophers.
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So let me ask the big question.
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What is a black hole from a theoretical, from an experimental, maybe even from a philosophical perspective?
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So a black hole is defined, theoretically, as a region of space-time from which light can never escape, therefore it s black.
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Now, that s just the starting point.
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Many weird things follow from that basic definition, but that is the basic definition.
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What is light that can t escape from a black hole?
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Well, light is the stuff that comes out of the sun, the stuff that goes into your eyes.
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Light is one of the stuff that disappears when the lights go off, the stuff that appears when the lights come on.
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Of course, I could give you a mathematical definition, or a physical mathematical definition, but I think it s something that we all understand very intuitively, what is light.
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Black holes, on the other hand, we don t understand intuitively.
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They re very weird.
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And one of the questions is about black holes, which I think you were alluding to, is why doesn t light get out?
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Or how is it that there can be a region of space-time from which light can t escape?
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It definitely happens.
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We ve seen those regions.
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We have spectacular pictures, especially in the last several years of those regions.
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They re there.
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In fact, they re up in the sky, thousands or millions of them.
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We don t yet know how many.
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But the proper explanation of why light doesn t escape from a black hole is still a matter of some debate.
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And one explanation
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which perhaps Einstein might have given, is that light carries energy.
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You know it carries energy because we have photocells and we can take the light from the sun and collect it, turn it into electricity.
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So there s energy in light.
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And anything that carries energy is subject to a gravitational pull.
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Gravity will pull at anything with energy.
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Now it turns out that the gravitational pull exerted by an object is proportional to its mass.
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And so if you get enough mass in a small enough region,
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you can prevent light from escaping.
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And let me flesh that out a little more.
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If you re on the Earth and you re on a rocket ship leaving the surface of the Earth, and if we ignore the friction from the air,
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If your rocket accelerates up to 11 kilometers per second, that s escape velocity.
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And if there were no friction, it could just continue forever to the next galaxy.
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On the moon, which has less mass, it s only seven kilometers per second.
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But going in the other direction, if you have
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enough mass in one place, the escape velocity can become the speed of light.
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If you shine light straight up away from the Earth, it doesn t have too much trouble.
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It s going way above the escape velocity.
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But if you have enough mass there, even light can t escape the escape velocity, and according to
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Einstein s theory of relativity, there is an absolute speed limit in the universe, the speed of light, and nothing makes any sense.
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Nothing could be self-consistent if there were objects that could exceed light speed.
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And so in these very, very massive regions of space-time, even light cannot escape.
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And the interesting thing is Einstein himself didn t think that these objects, we call the black holes, could exist.
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But let me actually linger on this.
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Yeah, that s incredibly interesting.
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There s a lot of interesting things here.
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First of all, the speed limit.
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How wild is it to you, if you put yourself in the mind in the time of Einstein before him, to come up with a speed limit, that there is a speed limit, and that speed limit is the speed of light?
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How difficult of an idea is that?
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You said from a mathematical physics perspective, everything just kind of falls into place, but he wasn t,
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perhaps maybe initially had the luxury to think mathematically, he had to come up with it intuitively, yes?
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So how counterintuitive is this notion to you?
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Is it still crazy?
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What?
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No, no.
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It s a very funny thing in physics.
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The best discoveries seem completely obvious in retrospect.
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Even my own discoveries, which of course are far lesser than Einstein s, but many of my papers, many of my collaborators get all confused.
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We ll try to understand something.
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We say, we ve got to solve this problem.
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We ll get all confused.
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Finally, we ll solve it.
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We ll get it all together.
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And then we ll, all of a sudden, everything will fall into place.
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We ll explain it.
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And then we ll look back at our discussions for the proceedings of months.
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and literally be unable to reconstruct how confused we were and how we could ever have thought of it any other way.
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That s so fascinating.
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So not only can I not fathom how confused we were,
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Einstein was before he, when he started thinking about the issues.
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I can t even reconstruct my own confusion from two weeks ago.
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So the really beautiful ideas in physics have this very hard to get yourself back into the mindset.
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Of course, Einstein was confused about many, many things.
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It doesn t matter if you re a physicist.
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It s not how many things you got wrong.
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It s not the ratio of how many you got wrong to how many you got right.
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It s the number that you got right.
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So Einstein didn t believe black holes existed, even though he predicted them.
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And I went and I read that paper which he wrote.
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Einstein wrote down his field equations in 1915.
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and Schwarzschild solved them and discovered the black hole solution three or four months later in very early 1916.
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And 25 years later, Einstein wrote a paper.
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So with 25 years to think about what this solution means, wrote a paper in which he said that black holes didn t exist.
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And I m like, well,
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You know, if one of my students in my general relativity course wrote this, you know, I wouldn t pass them.
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You get a C-, oh, you went past him, okay.
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All right.
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Get a C-, okay.
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Same thing with gravity waves.
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He didn t believe.
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Oh, he didn t believe in gravitational waves either?
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He went back and forth, but he wrote a paper in, I think, 34, saying that gravity waves didn t exist because people were very confused about what a coordinate transformation is.
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And in fact, this confusion about what a coordinate transformation is has persisted, and we actually think
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we re on the edge of solving it 100 years later.
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100 years later.
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What is coordinate transformation, as it was 100 years ago to today?
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Let s imagine I want to draw a map with pictures of all the states and the mountains, and then I wanna draw the weather forecast, what the temperatures are gonna be.
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all over the country.
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And I do that using one set of weather stations, and I number the weather stations.
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And you have some other set of weather stations, and you do the same thing.
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So the coordinates are the locations of the weather stations.
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They re how we describe where the things are.
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At the end of the day, we should draw the same
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map, that is coordinate invariance.
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And if we re telling somebody, we re going to tell somebody at a real physical operation, we want you to stay as dry as possible on your drive from here to California, we should give them exactly the same route.
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No matter which weather stations we use or how we, it s a very trivial, the labeling of points is an artifact and not in the real physics.
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So it turns out that that s almost true.
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but not quite.
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There s some subtleties to it.
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The statement that you should always have the same, give at least the same kind of trajectory, the same kind of instructions, no matter the weather status.
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Yeah, yeah, there s some very delicate subtleties.
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to that, which began to be noticed in the 50s.
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It s mostly true, but when you have a space-time with edges, it gets very tricky how you label the edges.
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And space-time in terms of space or in terms of time, in terms of everything, just space-time?
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Either one, space or time.
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That gets very tricky.
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And Einstein didn t,
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didn t have it right.
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In fact, he had an earlier version of general relativity in 1914, which he was very excited about, which was wrong.
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It wasn t fully coordinate invariant.
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It was only partially coordinate invariant.
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It was wrong.
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It gave the wrong answer for bending light to the sun.
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by a factor of two.
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There was an expedition sent out to measure it during World War I. They were captured before they could measure it.
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And that gave Einstein four more years to clean his act up, by which time he d gotten it right.
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So it s a very tricky business, but once it s all laid out, it s clear.
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Then why do you think Einstein didn t believe his own equations and didn t think that black holes are real?
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Why was that such a difficult idea for him?
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Well, something very interesting happens in Schwarzschild s solution of the Einstein equation.
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I think his reasoning was ultimately wrong, but let me
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explained to you what it was.
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At the center of the black hole, behind the horizon, in a region that nobody can see and live to tell about it, at the center of the black hole, there s a singularity, and if you pass the horizon, you go into the singularity, you get crushed, and that s the end of everything.
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Now, the word singularity means that, it just means that Einstein s equations break down.
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They become infinite, you write them down, you put them on the computer, when the computer hits that singularity, it crashes.
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Everything becomes infinite, there s two.
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So the equations are just no good there.
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That s actually not a bad thing.
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It s a really good thing, and let me explain why.
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So it s an odd thing that Maxwell s theory and Newton s theory
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never exhibit this phenomenon.
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You write them down, you can solve them exactly.
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They re really Newton s theory of gravity.
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They re really very simple theories.
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You can solve them.
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Well, you can t solve the three-body problem, but you can certainly solve a lot of things about them.
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Nevertheless, there was never any reason
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even though Maxwell and Newton perhaps fell for this trap, there were never any reason to think that these equations were exact.
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And there s no equation, well, there s some equations that we ve written down that we still think are exact,
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Some people still think are exact.
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My view is that there s no exact equation.
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Everything is an approximation.
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Everything is an approximation.
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And you re trying to get as close as possible.
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So you re saying objective truth doesn t exist in this world?
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The internet s gonna be very mad at you.
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We could discuss that, but that s a different thing.
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We wouldn t say Newton s theory was wrong.
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it had very, very small corrections, incredibly small corrections.
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It s actually a puzzle why they re so small.
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So if you watch the precession of Mercury s perihelion, this was the first indication of something going wrong.
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According to Newton s theory, Mercury has an elliptical orbit.
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The long part of it moves around,
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as other planets come by and perturb it and so on.
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And so this was measured by Le Verrier in 1859 and he compared theory and experiment and he found out that the perihelion precess moves around the sun once every 233 centuries instead of every 231 centuries.
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Okay, now this is the wonderful thing about science.
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Why was this guy?
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I mean, do you have any idea how much work this is?
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But of course he made one of the greatest discoveries in the history of science without even knowing what good it was gonna be.
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So that s how small, that was the first sign that there was something wrong with Newton.
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Now, so the corrections to Newton s law are very, very small, but they re definitely there.
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The corrections to electromagnetism, they re mostly, the ones that we see are mostly coming from quantum effects.
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So the corrections for Maxwell s equations is when you get super tiny, and then the corrections for Newton s laws of gravity is when you get super big.
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That s when you require corrections.
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That s true, but I would phrase it as saying when it s super accurate.
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If you look at the Bohr atom,
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Maxwell electromagnetism is not a very good approximation to the force between the proton and the electron.
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The quantum mechanics, if you didn t have quantum mechanics, the electron would spiral into the proton and the atom would collapse.
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So that s a huge correction there.
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So every theory gets corrected as we learn more.
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There d just be no reason to suppose that it should be otherwise.
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Well, how does this relate to the singularity?
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Why the singularity becomes possible?
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So when you hit the singularity, you know that you need some improvement to Einstein s theory of gravity.
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And that improvement, we understand what kind of things that improvement should involve.
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It should involve quantum mechanics, quantum effects become important there, it s a small thing.
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And we don t understand exactly what the theory is, but we know there s no reason to think Einstein s theory was invented to describe
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weakly curved things, the solar system and so on.
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It s incredibly robust that we now see that it works very well near the horizons around black holes and so on.
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So it s a good thing that the theory drives itself, that it predicts its own demise.
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Newton s gravity had its demise.
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There were regimes in which it wasn t valid.
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Maxwell s electromagnetism had its demise.
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There was regimes in which quantum effects greatly modified the equations.
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But general relativity all on its own found
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a system which originally was fine would perversely wander off into a configuration in which Einstein s equations no longer applied.
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So to you, the edges of the theory are wonderful, the failures of the theory.
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Edges are wonderful because that keeps us in business.
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So one of the things you said, I think, in your TED Talk, that the fact that quantum mechanics and relativity don t describe everything and then they clash is wonderful.
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I forget the adjective you used, but it was something like this.
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So why is that?
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Why is that interesting?
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Do you in that same way that there s contradictions that create discovery?
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There s no question in my mind, of course many people would disagree with me,
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that now is the most wonderful time to be a physicist.
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So people look back at, it s a classical thing to say among physicists, I wish it were 1920.
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Right.
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quantum mechanics had been just understood.
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There was the periodic table.
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But in fact, that was such a rich thing that, well, so a lot of exciting stuff happened around 1920.
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It took a whole century
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to sort out the new insights that we got.
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Especially adding some experimental stuff into the bunch, actually making observations and integrating all the data.
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Adding the experimental thing in.
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All the computers also help with visualizations and all that kind of stuff.
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Yeah, yeah, yeah.
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It was a whole sort of wonderful century.
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I mean, the seed of general relativity was the incompatibility of Maxwell s theory of the electromagnetic field
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with Newton s laws of gravity.
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They were incompatible because if you look at Maxwell s theory, there s a contradiction if anything goes faster than the speed of light.
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But Newton s theory of gravity, the,
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gravitational field, the gravitational force, is instantaneously transmitted across the entire universe.
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So you could, if you had a friend in another galaxy with a very sensitive
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measuring device that could measure the gravitational field, they could just take this cup of coffee and move it up and down in Morse code and they could get the message instantaneously over another galaxy.
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That leads to all kinds of contradictions.
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It s not self-consistent.
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It was exactly in resolving those contradictions that Einstein came up with the general theory of relativity.
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relativity, and it s fascinating how this contradiction, which seems like maybe it s a kind of technical thing, led to a whole new vision of the universe.
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Now, let s not get fooled because lots of contradictions are technical things.
1
SPEAKER_00
We haven t set up the
1
SPEAKER_00
We run into other kinds of contradictions that are technical and they don t seem to, we understood something wrong, we made a mistake, we set up our equations in the wrong way, we didn t translate the formalisms.
1
SPEAKER_01
As opposed to revealing some deep mystery that s yet to be uncovered.
1
SPEAKER_01
Yeah, yeah, yeah.
1
SPEAKER_00
And so we re never very sure which are the really important ones.
1
SPEAKER_01
But to you, the difference between quantum mechanics and general relativity, the tension, the contradiction there seems to hint at some deeper, deeper thing that s going to be discovered in the century.
1
SPEAKER_00
Yes, because that one has been understood since the 50s.
1
SPEAKER_00
Pauli was the first person to notice it.
1
SPEAKER_00
And Hawking in the early 70s gave it a really much more visceral form.
1
SPEAKER_00
And people have been hurling themselves at it, trying to reduce it to some technicality, but nobody has succeeded.
1
SPEAKER_00
And the efforts to understand it have led to all kinds of interesting relations between quantum systems and
1
SPEAKER_00
and applications to other fields and so on.
1
SPEAKER_01
Well, let s actually jump around.
1
SPEAKER_01
So we ll return to black holes.
1
SPEAKER_01
I have a million questions there, but let s go into this unification, the battle against the contradictions and the tensions between the theories of physics.
1
SPEAKER_01
What is quantum gravity?
1
SPEAKER_01
Maybe what is the standard model of physics?
1
SPEAKER_01
What is quantum mechanics?
1
SPEAKER_01
What is general relativity?
1
SPEAKER_01
What s quantum gravity?
1
SPEAKER_01
What are all the different unification efforts?
1
SPEAKER_00
Okay, so.
1
SPEAKER_01
Again, five questions.
1
SPEAKER_00
Yeah.
1
SPEAKER_00
It s a theory that describes everything with astonishing accuracy.
1
SPEAKER_00
It s the most,
1
SPEAKER_00
accurate theory in the history of human thought.
1
SPEAKER_00
Theory and experiment have been successfully compared to 16 decimal place.
1
SPEAKER_00
We have that stenciled on the door where I work.
1
SPEAKER_00
It s an amazing feat of the human mind.
1
SPEAKER_00
It describes the electromagnetic interaction.
1
SPEAKER_00
unifies the electromagnetic interaction with the so-called weak interaction, which you need some good tools to even view the weak interaction.
1
SPEAKER_00
And then there s the strong interaction, which binds the quarks into protons.
1
SPEAKER_00
And the forces between them are mediated by something called Yang-Mills theory, which is a beautiful mathematical generalization of electromagnetism in which the analogs of the photons themselves carry charge.
1
SPEAKER_00
And so this, the final piece of this
1
SPEAKER_00
of the standard model.
1
SPEAKER_00
Everything in the standard model has been observed, its properties have been measured.
1
SPEAKER_00
The final particle to be observed was the Higgs particle, observed like over a decade ago.
1
SPEAKER_01
Higgs is already a decade ago.
1
SPEAKER_01
I think it is, yeah.
1
SPEAKER_00
But you better check me on that, yeah.
1
SPEAKER_01
It s true, but so much fun has been happening.
1
SPEAKER_00
So much fun has been happening.
1
SPEAKER_00
And so that s all pretty well understood.
1
SPEAKER_00
There s some things that might or might not around the edges of that, you know, dark matter, neutrino masses, some sort of fine points or things we haven t quite measured perfectly and so on, but it s largely a very complete theory and we don t expect
1
SPEAKER_00
anything very new conceptually in the completion of that.
1
SPEAKER_01
Anything contradictory by Mnou.
1
SPEAKER_00
Anything contradictory, yeah.
1
SPEAKER_01
I ll have some wild questions for you on that front.
1
SPEAKER_01
But yeah, anything that, yeah, because there s no gaps.
1
SPEAKER_01
It s so accurate, so precise in its predictions, it s hard to imagine something completely different.
1
SPEAKER_00
Yeah, yeah, yeah.
1
SPEAKER_00
And it was all based on something called, let me not explain what it is, let me just throw out the buzzword.
1
SPEAKER_00
Renormalizable quantum field theory.
1
SPEAKER_00
They all fall in the category of renormalizable quantum field theory.
2
SPEAKER_00
I m gonna throw that at a bar later to impress the girls.
2
SPEAKER_01
Good luck.
2
SPEAKER_00
Thank you.
2
SPEAKER_01
They all fall under that rubric.
2
SPEAKER_01
Gravity will not put that suit on.
2
SPEAKER_01
So the force of gravity cannot be tamed by the same renormalizable quantum field theory to which all the other forces so eagerly submitted.
2
SPEAKER_00
What is the effort of quantum gravity?
2
SPEAKER_00
What are the different efforts to have these two dance together effectively, to try to unify the standard model and general relativity, any kind of model of gravity?
2
SPEAKER_01
sort of the one fully consistent model that we have.
2
SPEAKER_01
that reconciles, that sort of tames gravity and reconciles it with quantum mechanics is string theory and its cousins.
2
SPEAKER_01
And we don t know what or if in any sense string theory describes the world, the physical world, but we do know that it,
2
SPEAKER_01
is a consistent reconciliation of quantum mechanics and general relativity, and moreover one which is able to incorporate particles and forces like the ones we see around us.
2
SPEAKER_01
So it hasn t been ruled out as an actual sort of unified theory of nature.
2
SPEAKER_01
But there also isn t a, in my view, some people would disagree with me, but there isn t a reasonable
2
SPEAKER_01
possibility that we would be able to do an experiment in the foreseeable future, which would be sort of a yes or no to string theory.
2
SPEAKER_00
Okay, so you ve been there from the early days of string theory, you ve seen its developments.
2
SPEAKER_00
What are some interesting developments?
2
SPEAKER_00
What do you see as also the future of string theory?
2
SPEAKER_00
And what is string theory?
2
SPEAKER_01
Well, the basic idea which emerged in the early 70s was that if you take the notion of a particle and you literally replace it by a little loop of string, the strings are sort of softer than particles.
2
SPEAKER_00
What do you mean by softer?
2
SPEAKER_01
Well, if you hit a particle, if there were a particle on this table, a big one, and you hit it, you might bruise yourself.
2
SPEAKER_01
But if there was a string on the table, you would probably just push it around.
2
SPEAKER_01
And the source of the infinities in quantum field theory is that when particles hit each other, it s a little bit of a,
2
SPEAKER_01
a jarring effect.
2
SPEAKER_01
And I ve never described it this way before, but it s actually scientifically accurate.
2
SPEAKER_01
But if you throw strings at each other, it s a little more friendly.
2
SPEAKER_01
One thing I can t explain is how wonderfully precise, well, the mathematics is that goes into describing string theory.
2
SPEAKER_01
We don t just wave our hands and throw strings around and, you know, there s some very,
2
SPEAKER_01
compelling mathematical equations that describe it.
2
SPEAKER_01
Now, what was realized in the early 70s is that if you replace particles by strings, these infinities go away, and you get a consistent theory of gravity without the infinities.
2
SPEAKER_01
And,
2
SPEAKER_01
That may sound a little trivial, but at that point, it d already been 15 years that people had been searching around for any kind of theory that could do this.
2
SPEAKER_01
And it was actually found kind of by accident.
2
SPEAKER_01
And there are a lot of accidental discoveries in this subject.
2
SPEAKER_01
Now, at the same time, it was believed then that string theory was an interesting sort of toy model for putting quantum mechanics and general relativity together on paper, but that it couldn t describe some of the very idiosyncratic phenomena that pertain to our own universe, in particular, the form of so-called parity violation.
2
SPEAKER_00
Our world is- Ooh, another term for the bar later tonight.
2
SPEAKER_01
Yeah, yeah.
2
SPEAKER_00
Parody violation.
2
SPEAKER_00
So if you go to the bar and- I already got the renormalizable quantum field theory.
2
SPEAKER_01
And you look in the mirror across the bar.
2
SPEAKER_00
Yes.
2
SPEAKER_01
The universe that you see in the mirror is not identical.
2
SPEAKER_01
you would be able to tell if you show the lady in the bar a photograph that shows both the mirror and you.
2
SPEAKER_01
If she s smart enough, she ll be able to tell which one is the real world and which one is you.
2
SPEAKER_01
Now she would have to do some very precise measurements.
2
SPEAKER_01
And if the photograph was too grainy, it might not be possible.
2
SPEAKER_00
But in principle, it s possible.
2
SPEAKER_00
Why is this interesting?
2
SPEAKER_00
Does this mean that there is some not perfect determinism?
2
SPEAKER_00
Or what does that mean, there s some uncertainty?
2
SPEAKER_01
No, it s a very interesting feature of the real world, that it isn t parity of invariant.
2
SPEAKER_01
In string theory, it was thought could not tolerate that.
2
SPEAKER_01
And then it was learned in the mid-80s that not only could it tolerate that, but if you did things in the right way, you could construct a world involving strings that reconciled quantum mechanics and general relativity, which looked more or less like the world that we live in.
2
SPEAKER_01
And now, that isn t to say that string theory predicted our world.
2
SPEAKER_01
It just meant that it was consistent, that the hypothesis that string theory describes our world can t be ruled out from the get-go.
2
SPEAKER_01
And it is also the only hypothesis
2
SPEAKER_01
proposal for a complete theory that would describe our world.
2
SPEAKER_01
Still, nobody will believe it,
2
SPEAKER_01
until there s some kind of direct experiment.
2
SPEAKER_01
And I don t even believe it myself.
2
SPEAKER_00
Sure, which is a good place to be mentally as a physicist, right, always.
2
SPEAKER_00
I mean, Einstein didn t believe his own equations, right, with the black hole.
2
SPEAKER_00
Okay.
2
SPEAKER_01
Well, then when he was wrong about that.
2
SPEAKER_00
He was wrong about that.
2
SPEAKER_00
But you might be wrong too, right?
2
SPEAKER_00
So do you think string theory is dead, if you were to bet all your money on the future of string theory?
2
SPEAKER_01
I think it s a,
2
SPEAKER_01
a logical error to think that string theory is either right or wrong, or dead or alive.
2
SPEAKER_01
What it is, is a stepping stone.
2
SPEAKER_01
And an analogy I like to draw is Yang-Mills theory, which I mentioned a few minutes ago in the context of standard model.
2
SPEAKER_01
Yang-Mills theory was discovered by Yang and Mills in the 50s, and they thought that the symmetry of Yang and Mills theory described the relationship between the proton and the neutron.
2
SPEAKER_01
That s why they invented it.
2
SPEAKER_01
That turned out to be completely wrong.
2
SPEAKER_01
It does, however, describe everything else in the standard model.
2
SPEAKER_01
And it had a kind of inevitability.
2
SPEAKER_01
They had some of the right pieces, but not the other ones.
2
SPEAKER_01
They didn t have it quite in the right context.
2
SPEAKER_01
And it had an inevitability to it, and it eventually sort of found its place.
2
SPEAKER_01
And it s also true of Einstein s
2
SPEAKER_01
theory of general relativity.
2
SPEAKER_01
He had the wrong version of it in 1914, and he was missing some pieces.
2
SPEAKER_01
And you wouldn t say that his early version was right or wrong.
2
SPEAKER_01
He d understood the equivalence principle.
2
SPEAKER_01
He d understood space-time curvature.
2
SPEAKER_01
He just didn t have everything.
2
SPEAKER_01
I mean, technically, you would have to say it was wrong.
2
SPEAKER_01
And technically, you would have to say Yang and Mills were wrong.
2
SPEAKER_01
And I guess in that sense, I would believe just odds are
2
SPEAKER_01
We always keep finding new wrinkles.
2
SPEAKER_01
Odds are we re gonna find new wrinkles in string theory, and technically what we call string theory now isn t quite right, but... We re always going to be wrong, but hopefully a little bit less wrong every time.
2
SPEAKER_01
Exactly.
2
SPEAKER_01
And I would, you know, bet the farm, as they say.
2
SPEAKER_00
Do you have a farm?
2
SPEAKER_01
I say that much more seriously because not only do I have a farm, but we just renovated it.
2
SPEAKER_01
So before I renovated, betting at the farm, my wife and I spent five years renovating it before.
2
SPEAKER_00
You were much looser with that statement, but now it really means something.
2
SPEAKER_01
Now it really means something.
2
SPEAKER_01
And I would bet the farm on the,
2
SPEAKER_01
on the guess that 100 years from now, string theory will be viewed as a stepping stone towards a greater understanding of nature.
2
SPEAKER_01
And it would, I mean, another thing that I didn t mention about string theory is, of course, we knew that it solved the infinities problem, and then we later learned that it also solved Hoxhans  puzzle about what s inside of a black hole.
2
SPEAKER_01
And, you know,
2
SPEAKER_01
You put in one assumption, you get five things out, somehow you re doing something right.
2
SPEAKER_01
Probably not everything, but there s some good signposts.
2
SPEAKER_01
And there have been a lot of good signposts like that.
2
SPEAKER_00
It is also a mathematical toolkit, and you ve used it.
2
SPEAKER_00
You ve used it with Kamran Vafa.
2
SPEAKER_00
Maybe we can sneak our way back from string theory into black holes.
2
SPEAKER_00
What was the idea that you and Kamran Vafa developed with the holographic principle and string theory?
2
SPEAKER_00
What were you able to discover through string theory about black holes?
2
SPEAKER_00
Or...
2
SPEAKER_00
that connects us back to the reality of black holes.
2
SPEAKER_01
Yeah, so that is a very interesting story.
2
SPEAKER_01
I was interested in black holes before I was interested in string theory.
2
SPEAKER_01
I was sort of a reluctance string theorist in the beginning.
2
SPEAKER_01
I thought I had to learn it because people were talking about it, but once I studied it,
2
SPEAKER_01
I grew to love it.
2
SPEAKER_01
First, I did it in a sort of dutiful way.
2
SPEAKER_01
These people say they ve claimed quantum gravity, I ought to read their papers at least.
2
SPEAKER_01
And then the more I read them, the more interested I got, and I began to see, you know,
2
SPEAKER_01
They phrased it in a very clumsy way.
2
SPEAKER_01
The description of string theory was very clumsy.
2
SPEAKER_00
Mathematically clumsy or just the interpretation?
2
SPEAKER_01
Mathematically clumsy, yeah.
2
SPEAKER_01
It was all correct, but mathematically clumsy.
2
SPEAKER_01
But it often happens that in all kinds of branches of physics,
2
SPEAKER_01
people start working on it really hard, and they sort of dream about it and live it and breathe it, and they begin to see inner relationships, and they see a beauty that is really there.
2
SPEAKER_01
They re not deceived.
2
SPEAKER_01
They re really seeing something that exists, but if you just kind of look at it, you can t grasp it all in the beginning.
2
SPEAKER_01
And so,
2
SPEAKER_01
Our understanding of string theory in 1985 was almost all about, you know,
2
SPEAKER_01
weakly coupled waves of strings colliding and so on, we didn t know how to describe a big thing like a black hole in string theory.
2
SPEAKER_01
Of course, we could show that strings in theory in some limit reproduced Einstein s theory of general relativity and corrected it, but we couldn t do any better with black holes than Einstein s theory
2
SPEAKER_01
Before my work with Cormann, we couldn t do any better than Einstein and Schwarzschild had done.
2
SPEAKER_01
Now, one of the puzzles, if you look at the Hawking s headstone and also Boltzmann s headstone, and you put them together, you get a formula for their really central equations in 20th century physics.
2
SPEAKER_01
think there are many equations that made it to headstones.
2
SPEAKER_01
And they re really central equations and you put them together and you get a formula for the number of gigabytes in a black hole.
2
SPEAKER_01
Now in Schwarzschild s description, the black hole is literally a hole in space and there s no place to store the gigabytes.
2
SPEAKER_01
And it s not too hard to, and this really was Wheeler and Bekenstein, Wheeler, Bekenstein, and Hawking, to come to the conclusion that if there isn t a sense in which a black hole can store some large number of gigabytes,
2
SPEAKER_01
that quantum mechanics and gravity can t be consistent.
2
SPEAKER_00
We gotta go there a little bit.
2
SPEAKER_00
So how is it possible, when we say gigabyte, so there s some information.
2
SPEAKER_00
So black holes can store information.
2
SPEAKER_00
How is this thing that sucks up all light and it s supposed to basically be super homogeneous and boring, how is that actually able to store information?
2
SPEAKER_00
Where does it store information?
2
SPEAKER_00
On the inside, on the surface?
2
SPEAKER_00
Where, where is?
2
SPEAKER_00
And what s information?
2
SPEAKER_00
I m liking this ask five questions to see which one you actually answer.
2
SPEAKER_01
I m gonna ask you a bunch of them.
2
SPEAKER_01
I should try to memorize them and answer each one in order just to answer them.
2
SPEAKER_00
No, I don t know.
2
SPEAKER_00
I don t know what I m doing.
2
SPEAKER_00
I m desperately trying to figure it out as we go along here.
2
SPEAKER_01
Einstein s black hole, the Schwarzschild black hole, they can t store information.
2
SPEAKER_01
Stuff goes in there and it just keeps flying and it goes to the singularity and it s gone.
2
SPEAKER_01
However, Einstein s theory is not exact.
2
SPEAKER_01
It has corrections.
2
SPEAKER_01
And string theory tells you what those corrections are.
2
SPEAKER_01
And so you should be able to find some way of, some alternate way of describing the black hole that enables you
2
SPEAKER_01
to understand where the gigabytes are stored.
2
SPEAKER_01
So what Hawking and Bekenstein really did was they showed that physics is inconsistent unless a black hole can store a number of gigabytes proportional to its area divided by four times Newton s constant times Planck s constant.
2
SPEAKER_00
And that s another wild idea.
2
SPEAKER_00
You said area, not volume.
2
SPEAKER_00
Exactly.
2
SPEAKER_00
And that s the holographic principle.
2
SPEAKER_00
The universe is so weird.
2
SPEAKER_00
And that s the holographic principle.
2
SPEAKER_00
That s called the holographic principle, that it s the area.
2
SPEAKER_00
We re just jumping around.
2
SPEAKER_00
What is the holographic principle?
3
SPEAKER_01
What does that mean?
3
SPEAKER_01
Is there some kind of weird projection going on?
3
SPEAKER_01
What the heck?
3
SPEAKER_00
Well, I was just before I came here writing an introduction to a paper and the first sentence was, the as yet imprecisely defined holographic principle.
3
SPEAKER_00
Blah, blah, blah, blah, blah.
3
SPEAKER_00
So nobody knows exactly what it is.
3
SPEAKER_00
But roughly speaking, it says just what we were alluding to, that really all the information that is in some volume of spacetime can be stored on the boundary of that region.
3
SPEAKER_01
So this is not just about black holes, it s about any area of spacetime.
3
SPEAKER_00
Any area of spacetime.
3
SPEAKER_00
However, we ve made sense of the holographic principle for black holes.
3
SPEAKER_00
We ve made sense of the holographic principle for something which could be called anti-de Sitter space, which could be thought of as a giant, as a black hole turned into a whole universe.
3
SPEAKER_00
And we don t really understand how to talk about the holographic principle for either flat space, which we appear to live in, or
3
SPEAKER_00
asymptotically de Sitter space, which astronomers tell us we actually live in as the universe continues to expand.
3
SPEAKER_00
So it s one of the huge problems in physics is to apply or even formulate the holographic principle for more realistic
3
SPEAKER_00
Well, black holes are realistic, we see them, but yeah, in more general context, to give a more general statement of the holographic principle.
3
SPEAKER_01
What s the difference between flat space and asymptotic de Sitter space?
3
SPEAKER_01
So flat space is just an approximation of the world we live in.
3
SPEAKER_01
So de Sitter space, asymptotic, I wonder what that even means, meaning asymptotic over what?
3
SPEAKER_00
Okay, so for thousands of years, until the last half of the 20th, well, sorry, until the 20th century, we thought space-time was flat.
3
SPEAKER_01
Can you elaborate on flat?
3
SPEAKER_01
What do we mean by flat?
3
SPEAKER_00
Well, like the surface of this table is flat.
3
SPEAKER_00
Let me just give an intuitive explanation.
3
SPEAKER_00
Surface of the table is flat, but the surface of a basketball is curved.
3
SPEAKER_00
So the universe itself,
3
SPEAKER_00
could be flat, like the surface of a table, or it could be curved like a basketball, which actually has a positive curvature.
3
SPEAKER_00
And then there s another kind of curvature called the negative curvature.
3
SPEAKER_00
And curvature can be even weirder because that kind of curvature I ve just described is the curvature of space, but Einstein taught us that we really live in a space-time continuum, so we can have curvature in a way that mixes up space and time.
3
SPEAKER_00
And that s kind of hard to visualize.
3
SPEAKER_01
Because you have to step, what, a couple of dimensions up?
3
SPEAKER_00
So it s hard to... You have to step a couple.
3
SPEAKER_00
But even if you have flat space and it s expanding in time, we could imagine we re sitting here, this room, good approximation, it s flat, but imagine we suddenly start getting further and further apart.
3
SPEAKER_00
Then space is flat.
3
SPEAKER_00
but it s expanding, which means that spacetime is curved.
3
SPEAKER_01
Ultimately, it s about spacetime.
3
SPEAKER_01
Okay, so what s de Sitter and anti-de Sitter space?
3
SPEAKER_00
The three simplest spacetimes are flat spacetime, which we call Minkowski spacetime, and negatively curved spacetime, anti-de Sitter space, and positively curved spacetime, de Sitter space.
3
SPEAKER_00
And so astronomers think that on large scales, even though for thousands of years we hadn t noticed it, beginning with Hubble, we started to notice that space-time was curved.
3
SPEAKER_00
Space is expanding in time, means that space-time is curved.
3
SPEAKER_00
And the nature of this curvature is affected by the matter in it.
3
SPEAKER_00
because matter itself causes the curvature of spacetime.
3
SPEAKER_00
But as it expands, the matter gets more and more diluted.
3
SPEAKER_00
And one might ask, when it s all diluted away, is spacetime still curved?
3
SPEAKER_00
And astronomers believe they ve done precise enough measurements to determine this.
3
SPEAKER_00
and they believe that the answer is yes.
3
SPEAKER_00
The universe is now expanding.
3
SPEAKER_00
Eventually all the matter in it will be expanded away, but it will continue to expand because, well, they would call it the dark energy.
3
SPEAKER_00
Einstein would call it a cosmological constant.
3
SPEAKER_00
In any case, in the far future, matter will be expanded away and we ll be left with empty decider space.
3
SPEAKER_01
Okay, so there s this cosmological, Einstein s cosmological constant that now hides this thing that we don t understand called dark energy.
3
SPEAKER_01
What s dark energy?
3
SPEAKER_01
What s your best guess at what this thing is?
3
SPEAKER_01
Why do we think it s there?
3
SPEAKER_01
It s because it comes from the astronomers.
3
SPEAKER_00
Dark energy is synonymous with positive cosmological constant.
3
SPEAKER_00
And,
3
SPEAKER_00
We think it s there because the astronomers have told us it s there.
3
SPEAKER_00
And they know what they re doing.
3
SPEAKER_00
It s a really, really hard measurement, but they really know what they re doing.
3
SPEAKER_00
And we have no friggin  idea why it s there.
3
SPEAKER_00
Another big mystery.
3
SPEAKER_00
Another reason it s fun to be a physicist.
3
SPEAKER_00
And if it is there, why should it be so small?
3
SPEAKER_00
Why should there be so little?
3
SPEAKER_00
Why should it have hid itself from us?
3
SPEAKER_00
Why shouldn t there be enough of it to substantially curve the space between us and the moon?
3
SPEAKER_00
Why did there have to be such a small amount that only the crazy best astronomers in the world could find it?
3
SPEAKER_01
Well, can t the same thing be said about all the constants?
3
SPEAKER_01
Can t that be said about gravity?
3
SPEAKER_01
Can t that be said about the speed of light?
3
SPEAKER_01
Like, why is the speed of light so slow?
3
SPEAKER_00
So fast.
3
SPEAKER_01
So slow.
3
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Relative to the size of the universe, can t it be faster?
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Or no?
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Well, the speed of light is a funny one because you could always choose units
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in which the speed of light is one.
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We measure it in kilometers per second, and it s 186,000, or miles per second, it s 186,000 miles per second.
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But if we had used different units, then we could make it one.
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But you can make dimensionless ratios.
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So you could say why is the time scale set by the expansion of the universe so large compared to the time scale of a human life or so large compared to the time scale for a neutron to decay?
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I mean, ultimately, the temporal reference frame here is a human life.
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Isn t that the important thing for us descendants of apes?
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Isn t that a really important aspect of physics?
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Because we kind of experience the world, we intuit the world through the eyes of these biological organisms.
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I guess mathematics helps you escape that for a time, but ultimately isn t that how you wonder about the world?
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SPEAKER_00
Absolutely.
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That like a human life time is only 100 years?
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Because if you think of everything, if you re able to think in, I don t know, in billions of years, then maybe everything looks way different.
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SPEAKER_01
Maybe universes are born and die and maybe all of these physical phenomena become much more intuitive than we see at the grand scale of general relativity.
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SPEAKER_00
Well, that is one of the,
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SPEAKER_00
a little off the track here, but that certainly is one of the nice things about being a physicist is you spend a lot of time thinking about insides of black holes and billions of years in the future, and it sort of gets you away from the day-to-day into another fantastic realm.
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SPEAKER_00
But I was answering your question about how there could be information in a black hole.
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So Einstein only gave us an approximate description, and we now have a theory that corrects it, string theory.
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And now sort of was the moment of truth.
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SPEAKER_00
Well, when we first discovered string theory, we knew from the get-go that string theory would correct what Einstein said, just like Einstein corrected what Newton said.
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But we didn t understand it well enough
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to actually compute the correction, to compute how many gigabytes there were.
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And sometime in the early 90s, we began to understand the mathematics of string theory better and better.
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And it came to the point where it was clear that this was something we might be able to compute.
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And it was a kind of moment of truth for string theory because if it hadn t given the answer that Bekenstein and Hawking said it had to give for consistency, string theory itself would have been inconsistent and we wouldn t be doing this interview.
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That s a very dramatic statement, but yes.
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SPEAKER_00
That s not the most dramatic thing.
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I mean, okay, that s very life and death.
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SPEAKER_01
You mean like, because string theory was central to your work at that time?
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Is that what you mean?
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SPEAKER_00
Well, string theory would have been inconsistent.
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SPEAKER_01
Yeah, okay.
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String theory would have been inconsistent.
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But those inconsistencies can give birth to other theories, like you said.
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The inconsistency, right, something else could have happened.
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It would have been a major change in the way we think about string theory.
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And it was a good thing that one supposition that the world is made of strings solves two problems, not one.
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It solves the infinity problem and it solved the Hawking s problem.
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And also the way that it did it,
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was very beautiful.
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It gave an alternate description.
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So, alternate description of things are, you know,
3
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are very common.
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I mean, we could, to take a simple example, this bottle of water here is 90% full.
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I could say it s 90% full.
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I could also say it s 10% empty.
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Those are obviously the same statement.
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And it s trivial to see that they re the same, but there are many statements
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that can be made in mathematics and mathematical physics that are equivalent but might take years to understand that they re equivalent and might take the invention or discovery of whole new fields of mathematics to prove they re equivalent.
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And this was one of those.
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We found an alternate description
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of certain black holes in string theory, which we could prove was equivalent, and it was a description of the black hole as a hologram that can be thought of a holographic plate.
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that could be thought of as sitting on the surface of the black hole, and the interior of the black hole itself sort of arises as a projection, or the near horizon region of the black hole arises as a projection.
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of that holographic plate.
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So, the two descriptions were the hologram, the three-dimensional image, and the holographic plate.
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And the hologram is what Einstein discovered, and the holographic plate is what we discovered.
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And this idea that you could describe things very, very concretely in string theory in these two different languages, of course, took off and was applied to many different contexts within string theory.
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SPEAKER_01
So you mentioned the infinity problem and the Hawking problem.
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SPEAKER_01
Which Hawking problem?
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That the black hole destroys information or which Hawking problem are we talking about?
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SPEAKER_00
Well, there s really two Hawking problems.
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They re very closely related.
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One is, how does the black hole store the information?
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And the other one is,
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That is the one that we solved in some cases.
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So it s sort of like, you know, your smartphone, how does it store its 64 gigabytes?
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SPEAKER_00
Well, you rip the cover off and you count the chips and there s 64 of them, each with a gigabyte, and you know there s 64 gigabytes.
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SPEAKER_00
But that does not solve the problem of how you get information in and out of your smartphone.
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SPEAKER_00
You have to understand a lot more about the Wi-Fi and the internet and the cellular.
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SPEAKER_01
And so- And that s where Hawking radiation, this prediction, it starts to- That s where Hawking radiation comes in.
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SPEAKER_00
And that problem of how the information gets in and out, you can t, you couldn t have explained how information gets in and out of an iPhone without first explaining
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how it s stored in the first place.
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So just to clarify, the storage is on the plate?
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Is on the plate.
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On the holographic plate, and then it projects somehow inside the... The bulk, the space-time is the hologram.
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The hologram.
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Man, I mean, do you have an intuitive, when you sit late at night and you stare at the stars, do you have an intuitive understanding of what a holographic plate is?
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SPEAKER_01
like that there s two-dimension projections that store information?
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SPEAKER_00
How a black hole could store information on a holographic plate, I think we do understand in great mathematical detail and also intuitively, and it s very much like an ordinary
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SPEAKER_00
hologram where you have a holographic plate and it contains all the information, you shine a light through it, and you get an image which looks three-dimensional.
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SPEAKER_01
Yeah, but why should there be a holographic plate?
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SPEAKER_00
Why should there be?
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SPEAKER_01
Yeah, why?
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SPEAKER_00
That is the great thing about being a theoretical physicist is
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SPEAKER_00
anybody can very quickly stump you with a going to the next level of wise.
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SPEAKER_01
Yeah, I can just keep asking.
3
SPEAKER_00
Yeah, you could just keep asking and it won t take you very long to
3
SPEAKER_00
So the trick in being a theoretical physics is finding the questions that you can answer.
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SPEAKER_00
So the questions that we think we might be able to answer now, and we ve partially answered, is that there is a holographic explanation for certain kinds of things in string theory.
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We ve answered that.
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Now we d like to take what we ve learned, and that s what I ve mostly been doing for the last...
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15, 20 years, I haven t really been working so much on string theory proper.
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I ve been sort of taking the lessons that we learned in string theory and trying to apply them to the real world.
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using only, assuming only what we know for sure about the real world.
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SPEAKER_01
So on this topic, you co-authored a paper with Stephen Hawking called Soft Hair on Black Holes.
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That makes the argument against Hawking s original prediction that black holes destroy information.
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SPEAKER_01
Can you explain this paper?
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And the title?
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Okay, so first of all, the hair on black holes
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is a word that was coined by the greatest phrase master in the history of physics, John Wheeler, invented the word black hole.
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And he also said that, he made the statement that black holes have no hair.
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That is, every black hole in the universe is described just by its mass and spin.
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They can also rotate, as was later shown by Kerr.
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And this is very much unlike a star, right?
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Every star of the same,
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mass is different in a multitude of different ways.
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Different chemical compositions, different motions of the individual molecules.
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Every star in the universe, even of the same mass, is different in many, many different ways.
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Black holes are all the same.
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And that means when you throw some, in Einstein s description of them,
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which we think must be corrected.
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And if you throw something into a black hole, it gets sucked in.
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And if you throw in a red book or a blue book, the black hole gets a little bigger, but there s no way within Einstein s theory of telling how they re different.
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And that was one of the assumptions that Hawking made in his 1974, 75 papers in which he concluded that black holes destroy information.
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SPEAKER_00
You can throw encyclopedias, thesis defenses, the Library of Congress.
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SPEAKER_01
It doesn t matter, it s going to behave exactly the same uniform way.
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Yeah.
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So, what Hawking and I showed, and also Malcolm Perry, is that one has to be very careful about what happens at the boundary of the black hole.
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And this gets back to something I mentioned earlier about when two things which are related by a coordinate transformation are and are not equivalent.
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And what we showed is that there are very subtle imprints when you throw something into a black hole, there are very subtle imprints left on the horizon of the black hole, which you can read off at least partially what went in.
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And so this invalidates Stephen s original argument that the information is destroyed.
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And that s the soft hair.
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SPEAKER_00
That s the soft hair, right.
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SPEAKER_00
And soft is a word that is used in physics for things which have very low energy.
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SPEAKER_00
And these things actually carry no energy.
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There are things in the universe which carry no energy.
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You said, I think to Sean Carroll, by the way, everyone should go check out Sean Carroll s Mindscape podcast, it s incredible.
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SPEAKER_01
And Sean Carroll s an incredible person.
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SPEAKER_01
I think you said there, maybe in a paper, I have a quote, you said that a soft particle is a particle
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that has zero energy, just like you said now.
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And when the energy goes to zero, because the energy is proportionate to the wavelength, it s also spread over an infinitely large distance.
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If you like, it s spread over the whole universe.
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It somehow runs off to the boundary.
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What we learned from that is that if you add a zero energy particle to the vacuum, you get a new state.
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And so there are infinitely many vacua.
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plural for vacuum, which can be thought of as being different from one another by the addition of soft photons or soft gravitons.
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Can you elaborate on this wild idea?
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SPEAKER_01
If you like, it spreads over the whole universe.
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SPEAKER_01
When the energy goes to zero, because the energy is proportionate to the wavelength, it also spreads over an infinitely large distance, if you like.
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SPEAKER_01
it s spread over the whole universe.
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SPEAKER_01
Can you explain these soft gravitons and photons?
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SPEAKER_00
Yeah, so the soft gravitons and photons have been known about since the 60s, but exactly what we re supposed to do with them or how we re supposed to think about them I think has been
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well understood only recently.
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SPEAKER_00
And in quantum mechanics, the energy of a particle is proportional to Planck s constant times its wavelength.
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So when the energy goes to zero, the wavelength goes to infinity.
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SPEAKER_00
Now, if something has zero energy and it s spread all over the universe, in what sense is it actually there?
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SPEAKER_00
That s been the confusing thing, to make a precise statement about when something is and isn t there.
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Now, the simplest way of seeing, so people might have taken the point of view
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that if it has zero energy and is spread all over the universe, it s not there, we can ignore it.
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SPEAKER_00
But if you do this, you ll get into trouble.
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SPEAKER_00
And one of the ways that you ll get into trouble is that even though it has zero energy, it doesn t have zero angular momentum.
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SPEAKER_00
If it s a photon, it always has angular momentum one.
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SPEAKER_00
If it s a graviton, it s angular momentum two.
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So you can t say that the state of the system with the zero energy photon should be identified with the one without the zero energy photon, that we can just ignore them, because then you will conclude that angular momentum is not conserved.
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SPEAKER_00
And if angular momentum is not conserved, things won t be consistent.
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SPEAKER_00
And of course, you can have a lot of these things, and typically you do get a lot of them.
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SPEAKER_00
And when you, you can actually do a calculation that shows that every time you scatter two particles, you create an infinite number of them.
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SPEAKER_01
infinite number of the soft photons and gases.
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SPEAKER_01
Of the zero energy ones, yeah.
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SPEAKER_01
And so these are, and they re somehow everywhere.
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SPEAKER_01
They re everywhere.
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SPEAKER_01
But they also contain information, or they re able to store information.
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SPEAKER_00
And they re able to store information.
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SPEAKER_00
They re able to store an arbitrary large amount of information.
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SPEAKER_00
So what we pointed out is, so what these things really do, one way of thinking of them is they rush off to the edges of the universe, spreading out all over the space.
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It s like saying they rush off to the edge of the universe.
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And that includes, if the interior of the black hole is not considered part of the universe, that includes the edge of the black hole.
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SPEAKER_00
So we need to set up our description of physics so that all the things that are conserved are still conserved in the way that we re describing them, and that will not be true if we ignore these things.
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We have to keep careful track of these things.
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and people had been sloppy about that, and we learned how to be very precise and careful about it.
4
SPEAKER_01
And once you re being precise, you can actually answer this kind of very problematic thing that Hawking suggested, that black holes destroy information.
4
SPEAKER_00
Well, what we showed is that there s an error in the argument, that all black holes are the same, because they hadn t kept track of these, these very subtle things.
4
SPEAKER_00
And whether or not this is the key error in the argument remains to be seen, or whether this is a technical point.
4
SPEAKER_00
Yes, but it is an error.
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SPEAKER_00
It is an error.
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SPEAKER_00
And Hawking obviously agreed with it.
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SPEAKER_00
Hawking agreed with it, and he was sure that this was the, he was sure that this was, This was a critical error.
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SPEAKER_00
That this was the critical error and that understanding this would,
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SPEAKER_00
would get us the whole story, and that could well be.
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SPEAKER_01
What was it like working with Stephen Hawking on this particular problem, because it s kind of a whole journey, right?
4
SPEAKER_00
Well, you know, I love the guy.
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SPEAKER_00
He s so passionate about physics.
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SPEAKER_00
He just, yeah, his oneness with
4
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the problem and I mean it s... So his mind is all occupied by the world that s... Yeah, and let me tell you there s a lot of other things with his illness and with his celebrity and a lot of other things.
4
SPEAKER_01
A lot of distractions pulling at his mind, he s still there.
4
SPEAKER_01
He s still right there.
4
SPEAKER_01
That s right, that s right.
4
SPEAKER_00
I remember him churning down tea with Lady Gaga so we could spend another hour on our paper.
4
SPEAKER_01
That, my friends, is dedication.
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SPEAKER_01
What did you learn about physics?
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SPEAKER_01
What did you learn about life from having worked with Stephen Hawking?
4
SPEAKER_00
Well, he was one of my great teachers.
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SPEAKER_00
Of course, he s older than me, and I was reading his textbooks in graduate school.
4
SPEAKER_00
And, you know, I learned a lot about relativity from him.
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SPEAKER_00
I learned about passion for a problem.
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SPEAKER_00
I learned about
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SPEAKER_00
not caring what other people think.
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SPEAKER_00
You know, physics is an interesting culture.
4
SPEAKER_00
Even if you make a great discovery, like Hawking did, people don t believe everything you say.
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SPEAKER_00
In fact, people love to disagree.
4
SPEAKER_00
It s a culture that cherishes disagreement.
4
SPEAKER_00
And so, you know, he kept ahead with what he believed in and sometimes he was right and sometimes he was wrong.
4
SPEAKER_01
Do you feel pressure from the community?
4
SPEAKER_01
So for example, with string theory, it was very popular for a time, there s a bit of criticism, or it s less popular now.
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SPEAKER_01
Do you feel the forces of the community as it moves in and out of different fields, or do you try to stay, like how difficult is it to stay intellectually and mathematically independent from the community?
4
SPEAKER_00
Personally, I m lucky.
4
SPEAKER_00
I m well-equipped for that.
4
SPEAKER_00
When I started out in graduate school, the problem of quantum gravity was not considered interesting.
4
SPEAKER_01
You still did it anyway.
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SPEAKER_00
I still did it anyway.
4
SPEAKER_00
I m a little bit of a contrarian, I guess, and I think that has served me well.
4
SPEAKER_00
And
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people are always sort of disagreeing with me.
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SPEAKER_00
They re usually right, but I m right enough.
4
SPEAKER_01
And like you said, the contradiction ultimately paves the path of discovery.
4
SPEAKER_01
Yeah.
4
SPEAKER_01
Let me ask you just on this tension, we ve been dancing between physics and mathematics.
4
SPEAKER_01
What to you is an interesting line you can draw between the two?
4
SPEAKER_01
You have done some very complicated mathematics in your life to explore the laws of nature.
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SPEAKER_01
What s the difference between physics and mathematics to you?
4
SPEAKER_00
Well,
4
SPEAKER_00
I love math, I think my first love is physics, and the math that I ve done, I ve done to, because it was needed.
4
SPEAKER_00
In service of physics.
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SPEAKER_00
In service of physics, but then of course, in the heat of it, it has its own appeal.
4
SPEAKER_01
In the heat of it, I like it, sure.
4
SPEAKER_00
It has its own appeal, and I certainly enjoyed it,
4
SPEAKER_00
Ultimately, I would like to think, I wouldn t say I believe, but I would like to think that there s no difference between physics and mathematics, that all mathematics is realized in the physical world and all physics has a firm mathematical basis, that they re really the same thing.
4
SPEAKER_00
I mean, why would there be math that had no physical
4
SPEAKER_00
manifestation seems a little odd, right?
4
SPEAKER_00
You have two kinds of math, some that are relevant to the real world.
4
SPEAKER_01
Well, they don t have to be contradictory, but you can have, can t you not have mathematical objects that are not at all connected to the physical world?
4
SPEAKER_01
So I mean, this is to the question of is math discovered or invented?
4
SPEAKER_01
So to you, math is discovered, and there s a deep linkage between the two.
4
SPEAKER_01
Yeah, yeah, yeah.
4
SPEAKER_01
Do you find at all compelling these ideas, like something like Max Tegmark, where our universe is actually a fundamentally mathematical object, that math is, our universe is mathematical, fundamentally mathematical in nature?
4
SPEAKER_00
My expertise as a physicist,
4
SPEAKER_00
doesn t add anything to that.
4
SPEAKER_00
It s not really, you know, physics is, you know, I was once very interested in philosophy and, you know, physics, I like questions that can be answered, that it s not obvious what the answer is and that you can find an answer to the question.
4
SPEAKER_00
and everybody will agree what the answer is, and that there s an algorithm for getting there.
4
SPEAKER_00
Not that these other questions aren t interesting, and they don t somehow have a way of presenting themselves, but to me, the interesting thing is to,
4
SPEAKER_00
is motion in what we know, is learning more, and understanding things that we didn t understand before.
4
SPEAKER_00
Things that seemed totally confusing, having them seem obvious, that s wonderful.
4
SPEAKER_00
So I think those questions are there.
4
SPEAKER_00
I mean, I would even go further, you know, the whole multiverse, I don t think there s too much concrete we re ever gonna be able to say about it.
4
SPEAKER_01
This is fascinating because you spend so much time in string theory, which is devoid from a connection to the physical world for a long time.
4
SPEAKER_01
Not devoid, but it travels in a mathematical world that seems to be beautiful and consistent and seems to indicate that it could be.
4
SPEAKER_01
a good model of the laws of nature, but it s still traveling independently, because it s very difficult to experimentally verify.
4
SPEAKER_01
But there s a promise laden in it, in the same way multiverse, or you can have a lot of kind of very far out there questions, where your gut instinctively
5
SPEAKER_00
I think the intuition says that maybe in 50, 100, 200 years, you ll be able to actually have strong experimental validation, right?
5
SPEAKER_01
I think that with string theory, I don t think it s likely that we could measure it, but we could get lucky.
5
SPEAKER_01
In other words, just to take an example, about 10 or 20 years ago, it was thought that they had seen a string in the sky, and that it was seen by doubled stars that were gravitationally lensed around the gravitational field produced by some long string.
5
SPEAKER_01
There was a line of double lens.
5
SPEAKER_01
Now, the signal went away, okay?
5
SPEAKER_01
people were hoping that they d seen a string, and it could be a fundamental string that had somehow gotten stretched, and that would be some evidence for string theory.
5
SPEAKER_01
There was also BICEP2, which the experiment was wrong, but it could have happened.
5
SPEAKER_01
It could have happened that we got lucky, and this experiment was able to make direct measurements.
5
SPEAKER_01
Certainly would have been measurements of quantum gravity, if not string theory.
5
SPEAKER_01
So it s a very logical possibility that we could get experimental evidence from string.
5
SPEAKER_01
That is a very different thing than saying, do this experiment, here s a billion dollars, and after you do it, we ll know whether or not strings are real.
5
SPEAKER_01
But I think it s a crucial difference.
5
SPEAKER_01
It s measurable in principle, and we don t see how to get from here to there.
5
SPEAKER_01
If we see how to get from here to there, in my eyes, it s boring, right?
5
SPEAKER_01
So when I was a graduate student, they knew how to measure the Higgs boson.
5
SPEAKER_01
Took 40 years, but they did it.
5
SPEAKER_01
Not to say that stuff is boring.
5
SPEAKER_01
I don t want to say that stuff is boring.
5
SPEAKER_01
But when Magellan set out, he didn t know he could get around the world.
5
SPEAKER_01
There was no map.
5
SPEAKER_01
So I don t know how we re gonna connect in a concrete way all these ideas of string theory to the real world.
5
SPEAKER_01
You know, when I started out in graduate school, I said, what is the most interesting problem that there might be, the deepest, most interesting problem that there might be progress on in 60 years?
5
SPEAKER_01
And I think it could be, you know, that, you know, in another 30 years that maybe we ll learn that we have understood how black holes store information.
5
SPEAKER_01
You know, that doesn t seem wild, that we re able to abstract what we learned from string theory and show that it s operative.
5
SPEAKER_01
And, you know, I mean, the Bose-Einstein condensate, they did, you know, when Bose and Einstein predicted it, when was that, the 30s maybe, early 30s, it took, there were 20 orders of magnitude that were needed
5
SPEAKER_01
in order to improvement, in order to measure it.
5
SPEAKER_01
And they did 50 years later.
5
SPEAKER_01
So, and then you couldn t have guessed how that had happened, how they could have gotten that.
5
SPEAKER_01
And it could happen that we, I don t think we re gonna like see the heterotic string spectrum at an accelerator, but it could be that
5
SPEAKER_01
things come around in an interesting way, and somehow it comes together.
5
SPEAKER_01
And the fact that we can t see to the end,
5
SPEAKER_01
isn t a reason not to do it.
5
SPEAKER_01
What did they do when they were trying to find the Pacific, right?
5
SPEAKER_01
They took every route.
5
SPEAKER_01
They just tried everything.
5
SPEAKER_01
And that s what we re doing.
5
SPEAKER_01
And I m taking the one that my nose tells me is the best.
5
SPEAKER_01
And other people are taking other ones, and that s good, because we need every person taking every route.
5
SPEAKER_01
And if somebody on another route
5
SPEAKER_01
find something that looks really promising, I m going to make a portage over the mountain and get on their stream. 
5
SPEAKER_01
So the fact that you don t see the experiment now isn t, to me, a reason to give up on what I view as the most fundamental
5
SPEAKER_01
paradox in 20th century, in present physics, 20th, 21st century physics.
5
SPEAKER_00
Absolutely, you can see that it s possible, you just don t know the way.
5
SPEAKER_00
But that s what I mean why some of the philosophical questions could be formulated in a way that s explorable scientifically.
5
SPEAKER_00
So some of the stuff we ve talked about, but for example,
5
SPEAKER_00
this topic that s become more okay to talk about, which is the topic of consciousness.
5
SPEAKER_00
To me as an artificial intelligence person, that s a very practically interesting topic, but there s also philosophers.
5
SPEAKER_00
Sean Carroll loves to argue against them, but there s some philosophers that are panpsychics.
5
SPEAKER_00
I m not against philosophers, it s just not as fun.
5
SPEAKER_00
It s not as fun, right.
5
SPEAKER_00
They start a little flame of a fire going that some of those flames, I think, eventually become physics.
5
SPEAKER_00
So eventually become something that we can really, like having them around is really important because you ll discover something by modeling and exploring black holes that s really weird and having these ideas around.
5
SPEAKER_00
like the ideas of panpsychists that consciousness could be a fundamental force of nature.
5
SPEAKER_00
Just even having that crazy idea, swimming around in the background, could really spark something where you were missing something completely.
5
SPEAKER_00
And it s just, that s where the philosophy done right, I think, is very useful.
5
SPEAKER_00
That s where even the, you know, these thought experiments, which is very fun in sort of the tech sci-fi world that we live in a simulation, that, you know,
5
SPEAKER_00
you know, taking a perspective of the universe as a computer, as a computational system that processes information, which is a pretty intuitive notion, but you can, just even reframing it that way for yourself could really open up some different way of thinking.
5
SPEAKER_00
And then you have, I don t know if you re familiar with Stephen Wolfram s work of like cellular atomic complexity.
5
SPEAKER_01
Yeah, I did a podcast with Stephen.
5
SPEAKER_00
With Stephen, that s awesome.
5
SPEAKER_00
I mean, to me, forget physics, forget all that, cellular automata.
5
SPEAKER_00
make no sense, they re so beautiful, they re so, that from simple rules you can create complexity.
5
SPEAKER_00
I just don t think, you know, he wrote a book, A New Kind of Science, basically hinting at, which a lot of people have hinted at, is like, we don t have a good way to talk about these objects.
5
SPEAKER_00
We can t figure out what is happening here.
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SPEAKER_00
These simple, these trivial rules can create incredible complexity.
5
SPEAKER_01
He s totally right about that.
5
SPEAKER_00
And physicists, I guess, don t know what to do with that, don t know what to do with cellular automata, because you can describe the simple rules that govern the system, but how complexity can emerge, like incredible complexity.
5
SPEAKER_01
Of course, Wolfram s version of that is that physicists will never be able to describe.
5
SPEAKER_00
Right, yeah, exactly, he tries to prove that it s impossible.
5
SPEAKER_00
What do you make of that?
5
SPEAKER_00
What do you make about the tension of being a physicist and potentially not being able to, it s like Freud or somebody that maybe, Sigmund Freud, that maybe you ll never be able to actually describe the human psyche.
5
SPEAKER_00
Is that a possibility for you?
5
SPEAKER_00
That you will never be able to get to the core fundamental description of the laws of nature?
5
SPEAKER_01
Yeah, so I had this conversation with Weinberg.
5
SPEAKER_00
Yeah, how d it go?
5
SPEAKER_01
So Weinberg has this book called Dreams of a Final Theory.
5
SPEAKER_01
And I had this conversation with him, I said,
5
SPEAKER_01
why do you think there s ever gonna be a final theory?
5
SPEAKER_01
Why should there ever be a final theory?
5
SPEAKER_01
I mean, what does that mean?
5
SPEAKER_01
Do physics departments shut down?
5
SPEAKER_01
We ve solved everything.
5
SPEAKER_01
Doesn t it seem that every time we answer some old questions, we ll just find new ones and that it will just keep going on forever and ever?
5
SPEAKER_01
He said, well, that s what they used to say about the Nile.
5
SPEAKER_01
They were never gonna find the end.
5
SPEAKER_01
Then one day they found it.
5
SPEAKER_01
So I don t know.
5
SPEAKER_01
String theory doesn t, string theory doesn t look like a candidate to me for a final theory.
5
SPEAKER_00
As it stands now.
5
SPEAKER_00
It doesn t get to the bottom of the well, to the sides and to the whole thing.
5
SPEAKER_01
Yeah, it seems to me that even if we kind of solved it and we ve,
5
SPEAKER_01
did experiments, there still would be more questions, like why are there four dimensions instead of six?
5
SPEAKER_01
It doesn t seem to have anything in it that would explain that.
5
SPEAKER_01
You can always hope that there s something that we don t know about string theory that will explain it, but it still doesn t look like it s gonna answer every question.
5
SPEAKER_01
And why is there one time, not two?
5
SPEAKER_01
Why is there, you know, it doesn t seem like
5
SPEAKER_01
I don t even know what it would mean to answer every question.
5
SPEAKER_00
Lex.
5
SPEAKER_00
Well, to answer every question, obviously, so when you refer to the theory of everything, you ll be able to have a, if it exists, it would be a theory that allows you to predict precisely the behavior of objects in the universe and their movement, right?
5
SPEAKER_00
What about them, their movement?
5
SPEAKER_01
David.
5
SPEAKER_01
Yeah.
5
SPEAKER_00
like precisely no matter the object.
5
SPEAKER_01
Right, that s true.
5
SPEAKER_01
So that would be a really interesting state of affairs if we could predict everything but not necessarily understand everything.
5
SPEAKER_01
So for example,
5
SPEAKER_01
Let s just forget about gravity.
5
SPEAKER_01
I mean, we re not too far from that situation.
5
SPEAKER_01
If we forget about gravity, the standard model, in principle, given a big enough computer, predicts almost everything.
5
SPEAKER_01
But if you look at the standard model, it s kind of a laundry list with neutrino masses and all that stuff.
5
SPEAKER_01
There are hundreds of free parameters.
5
SPEAKER_01
Where do they come from?
5
SPEAKER_01
Is there an organizing principle?
5
SPEAKER_01
Is there some further unification?
5
SPEAKER_01
So being able to predict everything is not the only goal that physicists have.
5
SPEAKER_00
So on the way to trying to predict, you re trying to understand.
5
SPEAKER_00
That s actually probably the goal, is to understand.
5
SPEAKER_01
Yeah.
5
SPEAKER_01
Right, we re more interested in understanding than actually doing the predictions, but the predictions are more,
5
SPEAKER_01
Focusing on how to make predictions is a good way to improve your understanding because you know you ve understood it if you can do the predictions.
5
SPEAKER_00
Yeah, one of the interesting things that might come to a head with is artificial intelligence.
5
SPEAKER_00
There s an increasing use of AI in physics.
5
SPEAKER_00
We might live in a world where AI would be able to predict perfectly what s happening.
5
SPEAKER_00
And so, as physicists, you ll have to come to the fact that you re actually not that interested in prediction.
5
SPEAKER_00
I mean, it s very useful, but you re interested in really understanding the deep laws of nature versus a perfect predictor.
5
SPEAKER_00
Like you wanna play chess.
5
SPEAKER_01
But even within AI, AI people are trying to understand what it is that the AI bots have learned in order to...
5
SPEAKER_01
produce whatever they produce.
5
SPEAKER_00
For sure, but you still don t understand deeply, especially because they re getting, especially language models, if you re paying attention, the systems that are able to generate text, they re able to have conversations, chat GPTs, the recent manifestation of that.
5
SPEAKER_00
they just seem to know everything.
5
SPEAKER_00
They re trained on the internet.
5
SPEAKER_00
They seem to be very, very good at something that looks like reasoning.
5
SPEAKER_00
They re able to generate, you can ask them questions, they can answer questions.
5
SPEAKER_00
It just feels like this thing is intelligent.
5
SPEAKER_00
And I could just see that being possible with physics.
5
SPEAKER_00
You ask any kind of physical question, and it ll be able to, very precise about a particular star system or a particular black hole.
5
SPEAKER_00
It ll say, well, these are the numbers you see.
5
SPEAKER_00
It ll perfectly predict.
5
SPEAKER_00
And then, sure, you can understand how the neural network is, the architecture is structured.
5
SPEAKER_00
Actually, for most of them now, they re very simple.
5
SPEAKER_00
You can understand what data is trained on, huge amount of data.
5
SPEAKER_00
You re getting a huge amount of data from a very nice telescope or something.
5
SPEAKER_00
But it seems to predict everything perfectly.
5
SPEAKER_00
How a banana falls when you throw it, everything is perfectly predicted.
5
SPEAKER_00
You still don t have a deep understanding of what governs the whole thing.
5
SPEAKER_00
And maybe you can ask it a question, it ll be some kind of Hitchhiker s Guide to the Galaxy type answer.
5
SPEAKER_00
It s a funny world we live in.
5
SPEAKER_00
Of course, it s also possible that there s no such deep, simple, governing laws of nature.
5
SPEAKER_00
behind the whole thing.
5
SPEAKER_00
I mean, there s something in us humans that wants it there to be, but doesn t have to be, right?
5
SPEAKER_00
Again, you already bet the farm, but if you were to have a second farm, do you think there is a theory of everything that we might get at?
5
SPEAKER_00
So, simple laws that govern the whole thing.
5
SPEAKER_01
Honestly, I don t know, but I m pretty confident that if there is, we won t get to it in my lifetime.
5
SPEAKER_01
I don t think we re near it.
5
SPEAKER_00
But doesn t it feel like the fact that we have the laws we do that are relatively simple already, that s kind of incredible.
5
SPEAKER_00
It s just there seems to be simple laws that govern things, right?
5
SPEAKER_01
By theory of everything, you mean a theory of
5
SPEAKER_01
of everything, an algorithm to predict everything.
5
SPEAKER_00
But a simple algorithm.
5
SPEAKER_01
A relatively simple algorithm to predict everything.
5
SPEAKER_01
So for me, it would be a sad day if we arrived at that without answering some deeper questions.
5
SPEAKER_00
Sure, of course, it definitely is, but the question, yes.
5
SPEAKER_00
But one of the questions before we arrive there we can ask, does such a destination even exist?
5
SPEAKER_00
Because asking the question and the possible answers and the process of trying to answer that question is in itself super interesting.
5
SPEAKER_00
Is it even possible to get there where there s an equals MC squared type of, there s a function, okay, you can have many parameters,
5
SPEAKER_00
a finite number of parameter function that can predict a lot of things about our universe.
5
SPEAKER_01
Well, okay, but just to sort of throw one thing in, in order to answer every question, we would need a theory of the origin of the universe.
5
SPEAKER_01
Right.
5
SPEAKER_01
And that is a huge task, right?
5
SPEAKER_01
And the fact that the universe seems to have a beginning
5
SPEAKER_01
defies everything we know and love, right?
5
SPEAKER_01
Because one of the basic principles of physics is determinism, that the past follows from, the present follows from the past, the future follows from the present, and so on.
5
SPEAKER_01
But if you have the origin of the universe, if you have a Big Bang, that means before that, there was nothing.
5
SPEAKER_01
And you can t have a theory in which something follows from nothing.
5
SPEAKER_01
So somehow.
5
SPEAKER_00
Sounds like you don t like singularities.
5
SPEAKER_01
Well.
5
SPEAKER_00
I thought for somebody that works with black holes you would get used to them by now.
5
SPEAKER_00
No, no, I like this because it s so hard to understand.
5
SPEAKER_01
I like it.
6
SPEAKER_00
because it s hard to understand, but it s really challenging us.
6
SPEAKER_00
I don t think we re close to solving that problem.
6
SPEAKER_00
And string theory has basically had nothing, there s been almost nothing interesting said about that in the last many decades.
6
SPEAKER_01
So string theory hasn t really looked at the Big Bang.
6
SPEAKER_01
It hasn t really tried to get to the origin.
6
SPEAKER_00
Not successfully.
6
SPEAKER_00
There aren t compelling papers that lots of people have read
6
SPEAKER_00
People have taken it up and tried to go at it, but there aren t compelling.
6
SPEAKER_00
String theory doesn t seem to have a trick that
6
SPEAKER_00
that helps us with that puzzle.
6
SPEAKER_01
Do you think we ll be able to sneak up to the origin of the universe, like reverse engineer it, from experimental, from theoretical perspective?
6
SPEAKER_01
Like, okay, if we can, what would be the trajectory?
6
SPEAKER_00
You ve already gotten yourself in trouble, you see, because you used the word reverse engineer.
6
SPEAKER_00
So if you re gonna reverse engineer, that means, you know, you,
6
SPEAKER_00
Forward engineering means that you take the present and determine the future.
6
SPEAKER_00
Reverse engineering means that you take the present and determine the past.
6
SPEAKER_01
But.
6
SPEAKER_01
Estimate the past, but yes, sure.
6
SPEAKER_00
But if the past was nothing, how are you ever gonna reverse engineer to nothing?
6
SPEAKER_01
That s hard to do.
6
SPEAKER_01
Run up against the nothing, right?
6
SPEAKER_01
Until have mathematical models that break down nicely to where you can actually start to infer things.
6
SPEAKER_00
Let s work on it.
6
SPEAKER_00
Maybe, but people have tried to do things like that, but it s not something that we re getting A pluses in.
6
SPEAKER_01
Sure.
6
SPEAKER_01
Let s pretend we live in a world where in 100 years we have an answer to that.
6
SPEAKER_01
What would that answer look like?
6
SPEAKER_01
What department is that from?
6
SPEAKER_01
What fields led us there?
6
SPEAKER_01
Not what fields, what set of ideas in theoretical physics?
6
SPEAKER_01
Is it experimental, is it theoretical?
6
SPEAKER_01
Like what can you imagine possibly could have possibly lead us there?
6
SPEAKER_01
Is it through gravitational waves and some kind of observations there?
6
SPEAKER_01
Is it investigation of black holes?
6
SPEAKER_01
Is it simulation of universes?
6
SPEAKER_01
Is it maybe we start creating black holes somehow?
6
SPEAKER_01
I don t know.
6
SPEAKER_01
Maybe some kind of high energy physics type of experiments?
6
SPEAKER_00
Well, I have some late night ideas about that that aren t really ready for prime time.
6
SPEAKER_01
Okay, sure, but you have some ideas.
6
SPEAKER_00
Yeah, yeah, but, and many people do.
6
SPEAKER_00
It could be that some of the advances in quantum information theory are
6
SPEAKER_00
important in that they kind of go beyond taking quantum systems and just replicating themselves, but combining them with others.
6
SPEAKER_01
Do you think, since you highlighted the issue with time and the origin of the universe, do you think time is fundamental or emergent?
6
SPEAKER_00
I think ultimately it has to be emergent.
6
SPEAKER_01
Yeah, what does it mean for time to be emergent?
6
SPEAKER_00
Well, let s review what it means for space to be emergent.
6
SPEAKER_00
What it means for space to be emergent is that you have a holographic plate and you shine some light that s moving in space and it produces an image which contains an extra spatial dimension and time just goes along for the ride.
6
SPEAKER_00
So what we d like to do, and indeed, there is some rather concrete work in this direction, though again, I would say, even within our stringing community, we re not getting A pluses on these efforts.
6
SPEAKER_00
But what we d like to do is to see examples in which the extra space-time dimension is time.
6
SPEAKER_00
In other words,
6
SPEAKER_00
Usually what we understand very well mathematically is how to take systems in some number of space-time dimensions and rewrite them as a plate in fewer space dimensions.
6
SPEAKER_00
What we d like to do is to take systems with
6
SPEAKER_00
at one time in some number of space dimensions, and to rewrite them as a system that had only space dimensions in it, had no time evolution.
6
SPEAKER_00
And there s some fairly concrete ideas about how to do that, but they re not universally accepted even within the stringy community.
6
SPEAKER_01
But isn t it wild to you?
6
SPEAKER_01
To be emergent?
6
SPEAKER_01
How do we intuit these kinds of ideas as human beings for whom space and time seems as fundamental as apples and oranges?
6
SPEAKER_00
They re both illusions.
6
SPEAKER_00
They re both illusions, even time.
6
SPEAKER_01
You co-authored a paper titled Photon Rings Around Warped Black Holes.
6
SPEAKER_01
First of all, whoever writes your paper titles, you like the soft hair and the term black hole and the big bang, you re very good at coming up with titles yourself.
6
SPEAKER_01
Anyway, you co-authored a paper titled Photon Rings Around Warped Black Holes.
6
SPEAKER_01
In it you write, quote, recent work has identified a number of emergent symmetries related to the intricate self-similar structure of the photon ring.
6
SPEAKER_01
So what are photon rings?
6
SPEAKER_01
What are some interesting characteristics of a photon ring?
6
SPEAKER_00
So that was a paper with Dan Kopitz and Alex Lipsaska that just came out.
6
SPEAKER_00
And this paper is kind of a wonderful example
6
SPEAKER_00
of what happens when you start to talk to people who are way out of your comfort zone of no different stuff and look at the world a different way.
6
SPEAKER_00
And some two or three years ago, I m part of this group
6
SPEAKER_00
the Black Hole Initiative, and I m also part of this Event Horizon Telescope collaboration that took the famous, though I had nothing to do with the experiment, but that took the famous picture of the donut of M87, and through conversations with them,
6
SPEAKER_00
which started out in an effort to understand the image that they d seen.
6
SPEAKER_00
So it s a great thing for somebody like me, a theoretical physicist, lost, seemingly lost in string land, to be presented with an actual picture of a black hole, and to be asked, what can we learn from this?
6
SPEAKER_00
So, you know, with some help from, you know, Michael Johnson, Alex Lapsaska, and a bunch of other people at the Venturizing Collaboration, we came up with a fantastic, beautiful answer using Einstein s theory.
6
SPEAKER_00
that is both shaping the future of, now it is shaping the future of improved black hole photographs.
6
SPEAKER_00
What do you wanna concentrate on in the photograph?
6
SPEAKER_00
You just point it at the sky and click?
6
SPEAKER_00
No, you don t do that.
6
SPEAKER_00
You optimize for various features.
6
SPEAKER_00
And it s both shaping that and in the process of talking to them and thinking about how light behaves around a black hole, black holes just have so many magic tricks and they do so many weird things.
6
SPEAKER_00
And the photon ring is among the weirdest of them.
6
SPEAKER_00
We understood this photon ring, and in the process of this, we said, hey, this photon ring has got to be telling us something about the puzzle of where the holographic plate is.
6
SPEAKER_00
outside of a ordinary astrophysical black hole.
6
SPEAKER_00
I mean, we nailed it for the stringy black holes, but they have a somewhat different character.
6
SPEAKER_01
What s a stringy black hole?
6
SPEAKER_01
The black holes that describe us?
6
SPEAKER_00
The black holes that are contained in string theory and they have different structure in them.
6
SPEAKER_00
Sure.
6
SPEAKER_01
But actually, can we step back?
6
SPEAKER_01
So what was the light in the image taken in 2019?
6
SPEAKER_01
Not taken in 2019, presented in 2019.
6
SPEAKER_00
So here s the puzzle.
6
SPEAKER_00
What they really saw, so the black holes tend to gather stuff that swirls around it.
6
SPEAKER_00
And they don t know what that stuff is made of, they don t know what its temperature is, they don t know what kind of magnetic fields there are around there.
6
SPEAKER_00
So the form of the image has a lot of unknowns in it that it s dependent on many other things other than the geometry of the black hole.
6
SPEAKER_00
So most of what you re learning is about the stuff.
6
SPEAKER_00
Now the stuff, the swirling stuff, the hot swirling stuff is interesting as hell, but it s not as interesting as the black hole, which are the most, in my view, the most interesting things in the universe.
6
SPEAKER_00
So you don t wanna just learn about the stuff.
6
SPEAKER_00
You wanna learn about the black hole that is swirling around.
6
SPEAKER_01
at the very first step, at the very primitive level, this is just a big leap for human civilization to be able to see a black hole.
6
SPEAKER_01
And the way you can see it is because there s stuff around it.
6
SPEAKER_01
But you don t get to learn much about the black hole.
6
SPEAKER_01
You get to learn more about the stuff just from the image.
6
SPEAKER_00
Yeah, but you re not gonna learn about the details before you ve even seen it.
6
SPEAKER_01
Because there s too many parameters, there s too many variables that govern the stuff.
6
SPEAKER_00
Yeah, so then we found a very wonderful way to learn about the black hole.
6
SPEAKER_00
And here s how it works.
6
SPEAKER_00
A black hole is a mirror.
6
SPEAKER_00
And the way it s a mirror is if light, a photon, bounces off your face towards the black hole, it goes straight to the black hole, just falls in, you never see it again.
6
SPEAKER_00
But if it just misses the black hole, it ll swing around the back and come back to you.
6
SPEAKER_00
And you see yourself from the photon that went around the back of the black hole.
6
SPEAKER_00
But not only can that happen, the black hole, the photon can swing around twice and come back.
6
SPEAKER_00
So you actually see an infinite number of copies of yourself.
6
SPEAKER_01
Like with a little bit of a delay.
6
SPEAKER_00
with a little bit of a delay, right?
6
SPEAKER_01
This is awesome.
6
SPEAKER_00
Yeah, and in fact... I mean, we re not used to an object that bends light like that, right?
6
SPEAKER_00
Yeah, yeah.
6
SPEAKER_01
So you re gonna get some trippy effects.
6
SPEAKER_00
Yeah, and in fact, one of my students has made a really awesome computer animation of this, which I m gonna show at a public lecture in a couple weeks, where the audience will see infinitely many copies of this.
6
SPEAKER_00
That s all swirling around the black hole.
6
SPEAKER_00
So a black hole is like a hall of mirrors, like in a department store where you go and there s the three mirrors and you see infinitely many copies of yourself.
6
SPEAKER_00
Think of the black hole as the mirror.
6
SPEAKER_00
And you go in there with your clothes.
6
SPEAKER_00
If you wanna know about your clothes, you just look at the direct image.
6
SPEAKER_00
You re not learning anything about the configuration of mirrors.
6
SPEAKER_00
But the relation of the image you see in front of you to the one you see at the side and the next one and so on depends only on the mirrors.
6
SPEAKER_00
It doesn t matter what clothes you re wearing.
6
SPEAKER_00
So you can go there 1,000 times wearing different clothes, but each time there will be the same relation between the subsequent images.
6
SPEAKER_00
And that is how
6
SPEAKER_00
we re gonna learn about the black holes.
6
SPEAKER_00
We re gonna take the stuff that is swirling around and we re gonna tease out the subsequent images and look at their relation.
6
SPEAKER_00
And there s some very beautiful, really beautiful mathematics which we were surprised to realize with the volumes and volumes of papers on black holes and their properties, this particular
6
SPEAKER_00
because it was a physical question that had never been asked in exactly this way.
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The relationship between the subsequent images.
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Photons that are orbiting, so the photon ring
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are the photons that orbit around.
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And beyond, so like orbit and lose orbit.
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Wow, and that starts to give you, what can you possibly figure out mathematically about the black hole?
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The geometry of it, the spin of it?
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The geometry, the spin, and you can verify things behaving.
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We have never seen a region of space-time
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with such high curvature.
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I mean, the region around a black hole is crazy.
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It s not like in this room.
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The curvature is everything, you know?
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You spent probably enough time with the math and the photons.
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Can you put yourself in that space?
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So we re like having a conversation in pretty peaceful, comfortable, flat space.
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Are you able to put yourself in a place around a black hole?
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Yeah, I m able to imagine that kind of thing, yeah.
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So for example, and actually, there s a wonderful movie, Interstellar.
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And in that movie, Kip Thorne, of course, is a great theoretical physicist, experimental, who later won the Nobel Prize for LIGO.
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And that movie is very accurate, scientifically.
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And there s some funny statements in there that of the 100 million people who saw that movie, there can t be more than 10 or 20 understood about why Matthew McConaughey is ejecting the trash in a certain direction in order to... For example, if I were a spinning black hole right here, if I was spinning fast enough, you wouldn t be able to stay still there.
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you d have to be orbiting around like that, you know?
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You d have to have your microphone on a rotating basis.
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But I wonder what the experience is, what the actual experience, because I mean, space itself is curved.
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Well, if space gets very curved, you get crushed.
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Your body gets ripped apart, because the forces are different on different parts of your body.
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Sure, okay, so that would be.
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But it can be less curved, so that the curvature is very noticeable, but you re not ripped apart.
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The fact that this was just nonchalantly stated is just beautiful.
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Like two biological systems discussing which level of curvature is required to rip apart said biological system.
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Very well.
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So you propose in the paper that a photon ring of a warped black hole is indeed part of the black hole hologram.
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A photon ring of a warped black hole is indeed part of the black hole hologram.
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So what can you intuit about the hologram?
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and the holographic plate from looking at the photon rings?
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Well, this paper is exploring a new idea.
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It s not making a new discovery, so to speak.
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It s exploring an idea and the ins and outs of it and what might work.
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and what might not.
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And this photon ring, somehow everybody always thought that the holographic plate sat at the horizon of the black hole.
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And that the quantum system that describes the black hole is inside the horizon.
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And in fact, we think it s plausible
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and we give some evidence in some soluble examples, in this case in an example in one lower dimension where we can handle the equations better, that the quantum system that describes the black hole should correspond to a region of space-time which includes the photon ring.
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So it s bigger.
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So that would be the holographic plate.
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That would be the holographic plate.
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All of that.
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I mean, we didn t prove this.
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We put it out there.
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It hadn t really been considered previously.
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We put it out there, and it does seem more plausible than the idea that it sits literally at the horizon.
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And it is a big, outstanding problem of how you have a holographic reconstruction of black holes like M87.
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Do you think there could be further experimental data that helps explore some of these ideas that you have for photon rings and holographic plates through imaging and through high and high resolution images and also just more and more data?
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I wish so, but I don t think so.
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But what I think already has happened and will continue to happen is that there are many different ways
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theorists and observers can interact.
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The gold standard is the theorist makes a prediction, the observer measures it and confirms it, or the observer makes a discovery and the theorist explains it.
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But there s a lot less than that, which is really kind of the bread and butter of, those are dramatic moments when that happens, right?
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Those are once-in-a-lifetime moments when that happens.
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But the bread and butter is more when, and it has already happened,
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they came to us and said, what is the interesting theoretical things we can understand in this swirl around the black hole?
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And we gave an answer, and then that, in turn, jogged us to
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think about the holographic principle in the context of M87 a little bit differently, and so it s a useful, and in the same vein, it s useful to talk to the philosophers, and it s useful to talk to the mathematicians, and a lot of, you gotta, we just gotta, we don t know where we re going, we just gotta do everything.
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Let me ask you another sort of philosophical type of question, but not really, actually.
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It seems that thought experiments are used, so it s not just mathematics that makes progress in theoretical physics, but thought experiments do.
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They did for Einstein as well.
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They did for a lot of great physicists throughout history.
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Over the years, how s your ability to generate thought experiments, or just your intuition about some of these weird things like quantum mechanics, or strength theory?
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or quantum gravity, or yeah, even general relativity.
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How s your intuition improved over the years?
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Have you been able to make progress?
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The hard part in physics is most problems are either doable, most problems, the theoretical calculation that a theoretical physicist would do, there s no end of problems.
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whose answer is uninteresting can be solved, but the answer is uninteresting.
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There s also no end of problems that are very interesting, some of which you ve asked me, but we don t have a clue how to solve them.
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And when first presented with a problem, almost every problem is one or the other.
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It s the jackpot when you find one that isn t one or the other.
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And... It seems like there s a gray area between the two, right?
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That s where you should be looking.
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Well, I wouldn t describe it as a gray area.
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I would describe it as a knife edge.
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So it s a very small...
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Small area.
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There isn t like a huge area with a sign.
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Here lie problems that are doable and people want to know the answer.
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And in some deep sense, that s where timing is everything with physics, with science, with discovery.
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With timing.
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I mean, I think earlier in my career, I erred more on the side of problems that were not solvable.
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The ambition of youth.
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What made you fall in love with physics at first?
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If we can go back to the early days.
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You said black holes were there in the beginning, but what made you, do you remember what really made you fall in love?
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you know, I wanted to reach nirvana, and I sort of realized that wasn t gonna happen.
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And then after that, I wanted to know the meaning of life, and I realized I wasn t, probably wasn t gonna figure that out, and then I wanted to, like, understand, you know,
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justice and socialism and world things, and couldn t figure those out either.
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The simplest- Smaller and smaller problems until you arrived at Blackwell s.
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Smaller and smaller problems.
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I mean, most of this I m talking about adolescence, but it was the biggest problem
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that I thought that there was a prospect of, but not 100%, you know?
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And I was definitely ready to spend my money.
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my life in the wilderness, knocking my head against the wall, but I haven t had to.
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I haven t solved them, but I ve said enough interesting things that you re interviewing me.
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So I m not in the wilderness, but yeah, so.
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Do you remember the early days?
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Do you feel nostalgic when you think back to the ideas, the circumstances that led you down the path?
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towards black holes, towards theoretical physics, towards the tools of physics, towards this really fascinating world of theoretical physics?
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Well, I wouldn t add nostalgia to it because it s not like a,
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you know, a summer in Italy or something, it s like there s results that are there that people are, and that s what s so gratifying.
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I mean, of course one s name disappears from these things, unless you re Einstein or Newton or something.
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People are not gonna remember my name in 50 years.
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almost basically every name would be forgotten in hundreds of years, yeah?
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Are you able to, by the way, love the idea, the exploration of ideas themselves without the names, the recognition, the name?
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Yeah, yeah, that s what I m saying.
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So I have not, I hope someday, but I have not,
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You know, there are some experiments now to verify some of my predictions about, you know, properties of gravity and so on, but I have not liked
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You know, most of what I ve done is in the, you know, it could happen still.
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It s still a logical possibility that everything having to do with string theory and the, I mean, as we mentioned, I m betting the farm that it s not, but it is indeed a logical possibility that people will say, can you believe Lex Fridman interviewed Elon Musk and Kenya West
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And he interviewed Strominger, who was working on this theory that just completely went into the toilet.
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I m gonna get, with a wife I don t have, I m gonna make a public statement, she ll be on stage, I ll say, I m really sorry, I made this giant mistake.
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of platforming this wild-eyed physicist that believed for decades in the power of theoretical physics, yes.
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No, like you said.
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So that could happen, it could happen, it could happen.
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And of course, if that couldn t happen, it wouldn t be real exploration, right?
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Absolutely.
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So, but I, you know, I do take a lot of satisfaction that some of the things I discovered are at the minimum mathematical truths and they re still, so you don t have that sort of nostalgic feeling of it being something that was gone and I m still making discoveries now that I m as excited about.
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We ll see if they,
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hold the test of time that, stand the test of time that these other ones did, but that I m as excited about as I was about those when I made them.
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I am easily excitable, as my friends will tell you.
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And I have been very excited about things which turned out to be completely wrong, you know?
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The excitement is a precondition for breakthroughs.
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But you re also somebody, just like you said, you don t have a cynical view of the modern state of physics.
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So there s a lot of people that glorify the early days of string theory and all the discussions that we made.
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But you re saying this to you might be one of, if not the most exciting times to be a theoretical physicist.
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When the alien civilizations 500 years from now that visit Earth will look back, they ll think the 21st century, some of the biggest discoveries ever were made in the 21st century.
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Then we have to go on to something new, you know?
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No, there s deep, there s going to be deep, the fun is over.
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Oh, man.
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But there is an end to the Nile, right?
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Is there?
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Who told you?
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Some Weinberg guy.
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Let me ask you another trippy out there question.
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So again, perhaps unanswerable from a physics perspective, but do you wonder about alien civilizations?
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Do you wonder about other intelligent beings out there making up their own math and physics trying to figure out the world?
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Do you think they re out there?
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It is hard to understand why there would, given that there s so many planets, and of course, there s Drake s formula, and we don t exactly know what the, but I mean, I think Fermi s paradox is a real paradox, and I think there probably are, and I think it s very exciting that,
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you know, we might, you know, find some, it s a logical possibility that we could learn about it.
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I mean, to me, it s super interesting to think about aliens from a perspective of physics, because so any intelligent civilization is going to be contending with the ideas, trying to understand the world around it.
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So I think that the universe is filled with alien civilizations.
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So they all have their physicists, right?
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They re all trying to understand the world around them.
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And it s just interesting to me to imagine all these different perspectives, all these different Einsteins.
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Like trying to make sense of like- Though they might be more different than we think.
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They might be different in a way that we haven t even thought of.
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Like smarter or different?
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just different, something that we re not even able to describe now.
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We just haven t thought of it, you know?
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Yeah, this is a really frustrating thing when we think, from me as an AI person, you start to think about what is intelligence, what is consciousness, and you start to sometimes, again, evening thoughts is, you know,
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how little we understand, how narrow our thinking is about these concepts.
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That it could be intelligence, could be, something could be intelligent and be very different.
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Intelligent in a very different way that we won t be able to detect because we re not keeping an open mind, open enough mind,
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And that s kind of sad because to me there s also just a strong possibility that aliens or something like alien intelligence or some fascinating, beautiful, physical phenomena are all around us.
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and we re too dumb to see it, for now.
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Or too close-minded to see it.
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There s something we re just deeply missing.
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Whether it s like fundamental limitations of our cognitive abilities, or just because our tools are too primitive right now.
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Or like the way we, it s like you said, like the idea seemed trivial once you figured it all out, looking back.
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Yeah.
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That kind of makes me sad, because there could be so much beauty in the world we re not seeing.
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Because we re too dumb.
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There surely is.
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And that s, I guess, the process of science and physics is to keep exploring, to keep exploring, to find the thing that will, in a century, seem obvious.
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Well, it s something we know for sure.
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I mean, the brain we don t really understand, and that s gotta be some,
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fabulously beautiful story.
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I m hoping some of that story will be written through the process of trying to build a brain, so the process of engineering intelligence, not just the neuroscience perspective of just looking at the brain, but trying to create it.
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But yeah, that story hasn t been written almost at all.
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We re just at the early days of figuring that one out.
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But see, like you said, that math is discovered.
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Aliens should at least have the same math as us, right?
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I think so.
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Maybe different symbols?
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They might have discovered it differently, and they might have had a different idea of what a proof is.
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Sure, yeah.
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We re very like black and white with the proof thing.
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Maybe they re looser.
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Right, well, so you can know something is true.
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First of all, you never know something is true with 100% uncertainty.
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I mean, you might have had a blackout just to be, it s never 100%, right?
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You might have had a momentary lapse of consciousness as the key step in the proof and nobody read it and whatever.
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Okay, so you never know for sure.
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But you can have a preponderance of evidence which makes it...
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And preponderance of evidence is not accepted very much in mathematics.
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And that was sort of how the famous Ramanujan work.
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He had formulas which he guessed at, and then he gathered a preponderance of evidence that he was sure they were true.
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So there might be, or something completely different.
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They might function in a very different way.
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Let me ask you kind of a heavy question for a physicist, but one on nuclear weapons.
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Just in general, what do you think about nuclear weapons where, like philosophical level, where brilliant physicists and brilliant engineering leads to a thing that can destroy human civilization?
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Sort of like some of the ideas that you re working on
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have power when engineered into machines, into systems?
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Is there some aspect of you that worries about that?
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Well, first of all, I don t know what the brilliant had to do with it, because, of course, Oppenheimer and all that, okay, they did it really fast, but if you didn t have Oppenheimer, I mean, what will have happened anyway?
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It had a reality of its own.
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The possibility of making a nuclear, it didn t depend on the fact that the physicists who built it were brilliant.
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Maybe that sped it up by a year or two years, but by now we d have nuclear weapons.
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It s something that
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So the ideas have momentum and that they re unstoppable.
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Right, the possibility of making nuclear weapons was discovered, right?
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It was there before, it s not like somebody made it, right?
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Without Picasso, there would never have been a Guernica, but without Oppenheimer, there would surely have still been an atom bomb.
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But timing matters, right?
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Timing s very important.
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There s a guy with a mustache.
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Of course, of course, of course.
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Of course, the timing mattered there.
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But I, yeah, okay, I mean, you could try to make a case for stopping,
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No, no, no, no, it s the case of carrying the burden of the responsibility of the power of ideas when manifested into systems.
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So it s not a game.
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It s not just a game of fun mathematics.
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It s the same with artificial intelligence.
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You have this, a lot of people in AI,
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A lot of people in the AI community, it s a fascinating, fun puzzle, how to make systems more and more intelligent, how to, you have a bunch of benchmarks, you try to make them perform better and better and better, and all of a sudden, you have a system that s able to outsmart people.
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it s now able to be used in geopolitics.
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It s able to create super-intelligent bots that are able to, at scale, control the belief of a population of people.
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And now you can have world wars.
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You can have a lot of really risky instabilities that are created.
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And so, there is some responsibility.
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This is not sort of, it s a beauty and a terror to these ideas, you know?
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At that moment, it was certainly a question for Oppenheimer and everybody who participated in that.
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What is the responsible way to serve society when you re sort of accidentally in this position of being at the forefront of a development that has a huge impact on society?
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I don t see my work
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a likelihood of having a huge impact on the development of society itself.
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But if I were you, working on AI, I think that there is a possibility there.
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And that it is, as a responsible scientist, that it s really not a good thing to say, I m just the scientist here and I m figuring out what s possible.
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Because you re in a role where you
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you have more of a podium to influence things than other people, and it s your responsibility as a citizen
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of the planet, or let me phrase it a little less shooty, you have an opportunity as a citizen of the planet to make the world a better place, which it would be sad to bypass.
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Yeah, it s a nice world without going.
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It d be nice to keep it going for a little bit longer.
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Andrew, I m really honored that you sat down with me.
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Thank you for your work.
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Thank you for your time.
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Well, it was a really great conversation.
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I really enjoyed it.
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You really covered a lot.
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I can t believe you re able to discuss at this level on so many different topics.
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So it s a pleasure.
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It was super fun, thank you.
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Thanks for listening to this conversation with Andrew Stromager.
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To support this podcast, please check out our sponsors in the description.
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And now, let me leave you with some words from Werner Heisenberg.
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Not only is the universe stranger than we think, it is stranger than we can think.
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Thank you for listening, and hope to see you next time.
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you