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Speaker 1

00:00

A black hole is a mirror. And the way it's a mirror is if light, a photon, bounces off your face towards the black hole, and it goes straight to the black hole, just falls in, you never see it again. But if it just misses the black hole, it'll swing around the back and come back to you. And you see yourself from the photon that went around the back of the black hole.

Speaker 1

00:31

But not only can that happen, the black hole, the photon can swing around twice and come back. So you actually see an infinite number of copies of yourself.

Speaker 2

00:46

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. This is the Lex Friedman Podcast. To support it, please check out our sponsors in the description.

Speaker 2

01:07

And now, dear friends, here's Andrew Strominger. You are part of the Harvard Black Hole Initiative, which has theoretical physicists, experimentalists, and even philosophers. So let me ask the big question. What is a black hole from a theoretical, from an experimental, maybe even from a philosophical perspective?

Speaker 1

01:30

So a black hole is defined, theoretically, as a region of space-time from which light can never escape, therefore it's black. Now, that's just the starting point. Many weird things follow from that basic definition, but that is the basic definition.

Speaker 2

01:56

What is light? I can't escape from a black hole.

Speaker 1

02:00

Well, light is the stuff that comes out of the sun, that stuff that goes into your eyes. Light is 1 of the stuff that disappears when the lights go off. This is stuff that appears when the lights come on.

Speaker 1

02:15

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. Black holes on the other hand, we don't understand intuitively, they're very weird. And 1 of the questions is about black holes, which I think you were alluding to, is why doesn't light get out or how is it that there can be a region of space-time from which light can't escape? It definitely happens.

Speaker 1

02:56

We've seen those regions. We have spectacular pictures, especially in the last several years of those regions, they're there. In fact, they're up in the sky, thousands or millions of them. We don't yet know how many.

Speaker 1

03:16

But the proper explanation of why light doesn't escape from a black hole is still a matter of some debate. And 1 explanation, which perhaps Einstein might have given, is that Light carries energy. You know it carries energy because we have photo cells and we can take the light from the sun and collect it, turn it into electricity. So there's energy in light.

Speaker 1

04:00

And anything that carries energy is subject to a gravitational pull. Gravity will pull at anything with energy. Now it turns out that the gravitational pull exerted by an object is proportional to its mass. And so if you get enough mass in a small enough region, you can prevent light from escaping.

Speaker 1

04:36

And let me flesh that out a little more. 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, if your rocket accelerates up to 11 kilometers per second, that's escape velocity. And if there were no friction, it could just continue forever to the next galaxy. On the moon, which has less mass, it's only 7 kilometers per second.

Speaker 1

05:16

But going in the other direction, if you have enough mass in 1 place, the escape velocity can become the speed of light. If you shine light straight up away from the Earth, it doesn't have too much trouble. It's going way above the escape velocity. But if you have enough mass there, even light can't escape the escape velocity.

Speaker 1

05:45

And according to Einstein's theory of relativity, there is an absolute speed limit in the universe, the speed of light, and nothing makes any sense, nothing could be self-consistent if there were objects that could exceed light speed. And so in these very, very massive regions of space-time, even light cannot escape.

Speaker 2

06:16

And the interesting thing is Einstein himself didn't think that these objects, we call the black holes, could exist. But let me actually linger on this.

Speaker 1

06:26

Yeah, that's incredibly interesting.

Speaker 2

06:27

There's a lot of interesting things here. First, the speed limit. 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, that in that speed limit is the speed of light?

Speaker 2

06:44

How difficult of an idea is that? You said from a mathematical physics perspective, everything just kind of falls into place, but he wasn't perhaps maybe initially had the luxury to think mathematically, he had to come up with it intuitively, yes? So like, how counterintuitive is this notion to you? Is it still crazy?

Speaker 1

07:07

No, no. So it's a very funny thing in physics. The best discoveries seem completely obvious in retrospect.

Speaker 1

07:19

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. We'll try to understand something, we say we've got to solve this problem, we'll get all confused. Finally, we'll solve it, we'll get it all together. And then we'll, all of a sudden, everything will fall into place, we'll explain it, and then we'll look back at our discussions for the proceedings of months and literally be unable to reconstruct how confused we were and how we could ever have thought of it any other way.

Speaker 1

08:00

And So not only can I not fathom how confused Einstein was before he, when he started thinking about the issues? I can't even reconstruct my own confusion from 2 weeks ago. So the really beautiful ideas in physics have this, very hard to get yourself back into the mindset. Of course, Einstein was confused about many, many things.

Speaker 1

08:36

It doesn't matter if you're a physicist. It's not how many things you got wrong. It's not the ratio of how many you got wrong to how many you got right. It's the number that you got right.

Speaker 1

08:47

So Einstein didn't believe black holes existed, even though he predicted them. And I went and I read that paper, which he wrote, you know, Einstein wrote down his field equations in 1915, and Schwarzschild solved them and discovered the black hole solution 3 or 4 months later in very early 1916. And 25 years later, Einstein wrote a paper. So with 25 years to think about what this solution means, wrote a paper in which he said that black holes didn't exist.

Speaker 1

09:28

And I'm like, well, you know, if 1 of my students in my general relativity course wrote this, I wouldn't pass them.

Speaker 2

09:38

You get a C minus, oh you wouldn't pass them, okay.

Speaker 1

09:41

I get a C minus, okay. Same thing with gravity waves, he didn't believe.

Speaker 2

09:44

Oh, He didn't believe in gravitational waves either?

Speaker 1

09:46

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. And in fact, this confusion about what a coordinate transformation is has persisted and we actually think we're on the edge of solving it 100 years later. 100 years

Speaker 2

10:18

later. What is coordinate transformation, as it was 100 years ago to today?

Speaker 1

10:22

Let's imagine I want to draw a map with pictures of all the states and the mountains, and then I want to draw the weather forecast, what the temperatures are gonna be all over the country. And I do that using 1 set of weather stations, and I number the weather stations, and you have some other set of weather stations, and you do the same thing, so the coordinates are the locations of the weather stations. They're how we describe where the things are.

Speaker 1

11:01

At the end of the day, we should draw the same map. That is coordinate invariance. And if we're telling somebody, we're gonna 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. 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.

Speaker 1

11:38

So it turns out that that's almost true, but not quite. There's some subtleties to it.

Speaker 2

11:49

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 stations.

Speaker 1

11:57

Yeah, yeah, there's some very delicate subtleties to that, which began to be noticed in the 50s. It's mostly true but when you have a space-time with edges It gets very tricky how you label the edges.

Speaker 2

12:19

And space-time in terms of space or in terms of time, in terms of everything, just space-time?

Speaker 1

12:22

Either 1, space or time. That gets very tricky. And Einstein didn't have it right.

Speaker 1

12:31

And in fact, he had an earlier version of general relativity in 1914, which he was very excited about, which was wrong, gave, it wasn't fully coordinate invariant, it was only partially coordinate invariant, it was wrong. It gave the wrong answer for bending light to the sun by a factor of 2. There was an expedition sent out to measure it during World War I. They were captured before they could measure it.

Speaker 1

13:12

And that gave Einstein 4 more years to clean his act up, by which time he'd gotten it right. So it's a very tricky business, but once it's all laid out, it's clear.

Speaker 2

13:29

Then why do you think Einstein didn't believe his own equations and didn't think that black holes are real. Why was that such a difficult idea for him?

Speaker 1

13:39

Well, something very interesting happens in Schwarzschild's solution of the Einstein equation. I think his reasoning was ultimately wrong, but let me explain to you what it was. At the center of the black hole, behind the horizon, in a region that nobody can see and live to tell about it.

Speaker 1

14:14

As 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. Now, the word singularity means that, it just means that Einstein's equations break down. They become infinite, you write them down, you put them on the computer, when the computer hits that singularity, it crashes. Everything becomes infinite, there's 2.

Speaker 1

14:50

So the equations are just no good there. Now, that's actually not a bad thing, it's a really good thing, and let me explain why. So, it's an odd thing that Maxwell's theory and Newton's theory never exhibit this phenomenon. You write them down, you can solve them exactly.

Speaker 1

15:23

They're really Newton's theory of gravity. They're really very simple theories. You can solve them. Well, you can't solve the three-body problem, but you can certainly solve a lot of things about them.

Speaker 1

15:39

Nevertheless, there was never any reason, even though Maxwell and Newton perhaps fell for this trap, there were never any reason to think that these equations were exact. And every, there's no equation, Well, there's some equations that we've written down that we still think are exact. Some people still think are exact. My view is that there's no exact equation.

Speaker 2

16:17

Everything is an approximation.

Speaker 1

16:19

Everything is an approximation.

Speaker 2

16:20

And you're trying to get as close as possible. Yeah. So you're saying objective truth doesn't exist in this world?

Speaker 2

16:27

The internet's gonna be very mad

Speaker 1

16:29

at you. We could discuss that, but that's a different thing. We wouldn't say Newton's theory was wrong.

Speaker 1

16:36

It had very, very small corrections, incredibly small corrections. It's actually a puzzle why they're so small. So if you watch the precession of Mercury's perihelion, this was the first indication of something going wrong. According to Newton's theory, Mercury has an elliptical orbit.

Speaker 1

16:58

The long part of it moves around as other planets come by and perturb it and so on. And so this was measured by Le Verrier in 1859 and he compared theory and experiment and he found out that the perihelion process moves around the sun once every 233 centuries instead of every 231 centuries. Now this is the wonderful thing about science. Why was this guy?

Speaker 1

17:36

I mean, you don't get any idea how much work this is. But of course he made 1 of the greatest discoveries in the history of science without even knowing what good it was gonna be. So that's how small, that was the first sign that there was something wrong with Newton. Now, so the corrections to Newton's law are very, very small, but they're definitely there.

Speaker 1

18:06

The corrections to electromagnetism, they're mostly, the ones that we see are mostly coming from quantum effects.

Speaker 2

18:16

And- 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. That's when you require corrections.

Speaker 1

18:34

That's true, but I would phrase it as saying when it's super accurate. If you look at the Bohr atom, Maxwell electromagnetism is not a very good approximation to the force between the proton and the electron. The quantum mechanics, if you didn't have quantum mechanics, the electron would spiral into the proton and the atom would collapse.

Speaker 1

19:01

It's quantum, so that's a huge correction there. So every theory gets corrected as we learn more. Just be no reason to suppose that it should be otherwise.

Speaker 2

19:14

How does this relate to the singularity? Why the singularity is uncomfortable?

Speaker 1

19:17

So when you hit the singularity, you know that you need some improvement to Einstein's theory of gravity. And that improvement, we understand what kind of things that improvement should involve. It should involve quantum mechanics, quantum effects become important there, it's a small thing.

Speaker 1

19:44

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 weakly curved things, the solar system and so on. It's incredibly robust that we now see that it works very well near the horizons around black holes and so on. So it's a good thing that the theory drives itself that it predicts its own demise. Newton's gravity had its demise.

Speaker 1

20:23

There were regimes in which it wasn't valid. Maxwell's electromagnetism had its demise. There was regimes in which quantum effects greatly modified the equations. But general relativity all on its own found a system which originally was fine would perversely wander off into a configuration in which Einstein's equations no longer applied.

Speaker 2

21:00

So to you, the edges of the theory are wonderful. The failures of

Speaker 1

21:03

the theory. Edges are wonderful because that keeps us in business.

Speaker 2

21:08

So 1 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. I forget the adjective you used, but it was something like this. So why is that?

Speaker 2

21:28

Why is that interesting? Do you in that same way that there's contradictions that create discovery?

Speaker 1

21:32

There's no question in my mind, of course many people would disagree with me, that now is the most wonderful time to be a physicist. So people look back at, it's a classical thing to say among physicists, I wish it were 1920. Quantum mechanics had been just understood.

Speaker 1

22:03

There was the periodic table. But in fact, that was such a rich thing that, Well, so that what lot of exciting stuff happened around 1920. It took a whole century to sort out the new insights that we got.

Speaker 2

22:27

Especially adding some experimental stuff into the bunch, actually making observations and

Speaker 1

22:32

integrating all the data.

Speaker 2

22:33

All the computers also help with visualizations and all that kind of stuff.

Speaker 1

22:37

Yeah, yeah, yeah. It was a whole sort of wonderful century. I mean, the seed of general relativity was the incompatibility of Maxwell's theory of the electromagnetic field with Newton's laws of gravity.

Speaker 1

22:56

They were incompatible because if you look at Maxwell's theory, There's a contradiction if anything goes faster than the speed of light. But Newton's theory of gravity, the gravitational field, the gravitational force, is instantaneously transmitted across the entire universe. So you could, if you had a friend in another galaxy with a very sensitive 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. That leads to all kinds of contradictions.

Speaker 1

23:50

It's not self-consistent. It was exactly in resolving those contradictions that Einstein came up with the general theory of relativity, and it's fascinating how this contradiction, which seems like maybe it's kind of technical thing, led to a whole new vision of the universe. Now, let's not get fooled because lots of contradictions are technical things. We haven't set up the, 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.

Speaker 2

24:37

As opposed to revealing some deep mystery that's yet to be uncovered.

Speaker 1

24:40

Yeah, yeah, yeah. And so we're never very sure which are the really important ones.

Speaker 2

24:47

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.

Speaker 1

24:58

Yes, because That 1 has been understood since the 50s. Polly was the first person to notice it and Hawking in the early 70s gave it a really much more visceral form. And people have been hurling themselves at it, trying to reduce it to some technicality, but nobody has succeeded.

Speaker 1

25:27

And the efforts to understand it have led to all kinds of interesting relations between quantum systems and applications to other fields and so on.

Speaker 2

25:42

Well, let's actually jump around. So we'll return to black holes. 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.

Speaker 2

25:55

What is quantum gravity? Maybe what is the standard model of physics? What is quantum mechanics? What is general relativity?

Speaker 2

26:03

What's quantum gravity? What are all the different unification efforts? Okay, so. Again, 5 questions.

Speaker 1

26:12

Yeah. It's a theory that describes everything with astonishing accuracy. It's the most accurate theory in the history of human thought. Theory and experiment have been successfully compared to 16 decimal place.

Speaker 1

26:36

We have that stenciled on the door where I work. It's an amazing feat of the human mind. It describes the electromagnetic interaction, unifies the electromagnetic interaction with the so-called weak interaction, which you need some good tools to even view the weak interaction. And then there's the strong interaction, which binds the quarks into protons.

Speaker 1

27:11

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. And so this, the final piece of this, of the standard model, everything in the standard model has been observed, its properties have been measured. The final particle to be observed was the Higgs particle, observed like over a decade ago. Higgs is already a decade ago.

Speaker 1

27:58

I think it is, yeah.

Speaker 2

27:59

Wow, time flies.

Speaker 1

28:01

But you better check me on that, yeah.

Speaker 2

28:04

That's true, but so much fun has been happening.

Speaker 1

28:06

So much fun has been happening. And so that's all pretty well understood. 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 anything very new conceptually in the completion of that.

Speaker 2

28:49

Anything contradictory by Mnou. Because can't you?

Speaker 1

28:52

Anything contradictory, yeah.

Speaker 2

28:54

I'll have some wild questions for you on that front. But yeah, anything that, yeah. Because there's no gaps.

Speaker 2

29:01

It's so accurate, so precise in its predictions, it's hard to imagine.

Speaker 1

29:04

Something completely different. Yeah, yeah, yeah. And it was all based on something called, let me not explain what it is, let me just throw out the buzzword, renormalizable quantum field theory.

Speaker 1

29:16

They all fall in the category of renormalizable quantum field theory.

Speaker 2

29:22

I'm gonna throw that at a bar later to impress the girls.

Speaker 1

29:28

Good luck.

Speaker 2

29:29

Thank you. All right, so.

Speaker 1

29:34

They all fall under that rubric. Gravity will not put that suit on. So the force of gravity cannot be tamed by the same renormalizable quantum field theory to which all the other forces so eagerly submitted.

Speaker 2

29:55

What is the effort of quantum gravity? What are the different efforts to have these 2 dance together effectively, to try to unify the standard model and general relativity, any kind of model of gravity?

Speaker 1

30:13

Sort of the 1 fully consistent model that we have that reconciles, that sort of tames gravity and reconciles it with quantum mechanics, is string theory and its cousins. 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 is a consistent reconciliation of quantum mechanics and general relativity, and moreover, 1 which is able to incorporate particles and forces like the ones we see around us. So it hasn't been ruled out as an actual sort of unified theory of nature, but there also isn't a, in my view, some people would disagree with me, but there isn't a reasonable 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.

Speaker 2

31:40

Okay, so you've been there from the early days of string theory. You've seen its developments. What are some interesting developments?

Speaker 2

31:46

What do you see as also the future of string theory? And what is string theory?

Speaker 1

31:53

Well, the basic idea which emerged in the early 70s was that if you, 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.

Speaker 2

32:22

What do you mean by softer?

Speaker 1

32:24

Well, you know, if you hit a particle, if there were a particle on this table, a big 1, and you hit it, you might bruise yourself. But if there was a string on the table, you would probably just push it around. And the source of the infinities in quantum field theory is that when particles hit each other, it's a little bit of a jarring effect.

Speaker 1

32:50

And I've never described it this way before, but it's actually scientifically accurate. But if you throw strings at each other, it's a little more friendly. 1 thing I can't explain is how wonderfully precise all the mathematics is that goes into describing string theory. We don't just wave our hands and throw strings around.

Speaker 1

33:14

And there's some very compelling mathematical equations that describe it. 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. And that may sound a little trivial, but at that point, it had already been 15 years that people had been searching around for any kind of theory that could do this. And it was actually found kind of by accident.

Speaker 1

34:03

And there are a lot of accidental discoveries in this subject. 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. Our world

Speaker 2

34:39

is- Oh, another term for the bar later tonight. Yeah, yeah. Parity violation.

Speaker 1

34:44

So if you go to the bar and-

Speaker 2

34:46

I already got the renormalizable quantum field theory.

Speaker 1

34:48

And you look in the mirror across the bar. The universe that you see in the mirror is not identical. You would be able to tell if you show your, the lady in the bar, a photograph that shows both the mirror and you.

Speaker 2

35:07

If

Speaker 1

35:07

she's smart enough, she'll be able to tell which 1 is the real world and which 1 is you. Now she would have to do some very precise measurements. And if the photograph was too grainy, it might not be possible, but in principle it's possible.

Speaker 2

35:24

Why is this interesting? Why is it, does this mean that there is some, not perfect determinism, or what does that mean? There's some uncertainty?

Speaker 1

35:32

No, it's a very interesting feature of the real world that it isn't parity of invariant. In string theory it was thought could not tolerate that. 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.

Speaker 1

36:10

And now, that isn't to say that string theory predicted our world. 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. And it is also the only proposal for a complete theory that would describe our world. Still, nobody will believe it until there's some kind of direct experiment.

Speaker 1

36:50

And I don't even believe it myself.

Speaker 2

36:52

Sure, which is a good place to be mentally as a physicist, right? Always, I mean Einstein didn't believe his own equations, right, with the black hole. Okay.

Speaker 1

37:01

Well, that when he was wrong about that. I don't have it. But he was wrong about that.

Speaker 2

37:05

But you might be wrong too, right? So do you think string theory is dead if you were to bet all your money on the future of string theory?

Speaker 1

37:15

I think it's a logical error to think that string theory is either right or wrong or dead or alive. What it is is a stepping stone. And an analogy I like to draw is Yang-Mills theory, which I mentioned a few minutes ago in the context of standard model.

Speaker 1

37:45

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. That's why they invented it. That turned out to be completely wrong. It does, however, describe everything else in the standard model.

Speaker 1

38:13

And it had a kind of inevitability. They had some of the right pieces, but not the other ones. They didn't have it quite in the right context. And it had an inevitability to it, and it eventually sort of found its place.

Speaker 1

38:28

And it's also true of Einstein's theory of general relativity. You know, he had the wrong version of it in 1914, and he was missing some pieces. And you wouldn't say that his early version was right or wrong. He'd understood the equivalence principle.

Speaker 1

38:43

He'd understood space-time curvature. He just didn't have everything. I mean, technically you would have to say it was wrong. And technically you would have to say Yang and Mills were wrong.

Speaker 1

38:54

And I guess in that sense, I would believe just odds are, we always keep finding new wrinkles. Odds are we're gonna find new wrinkles in string theory, and technically what we call string theory now isn't quite right, but...

Speaker 2

39:11

We're always going to be wrong, but hopefully a little bit less wrong every time.

Speaker 1

39:16

Exactly. And I would bet the farm, as they say. Do you

Speaker 2

39:20

have a farm?

Speaker 1

39:22

I say that much more seriously because not only do I have a farm, but we just renovated it. So before I renovated, betting at the farm, my wife and I spent 5 years renovating it before I.

Speaker 2

39:37

You were much looser with that statement, but now it really means something.

Speaker 1

39:41

Now it really means something. And I would bet the farm on the, on the guess that 100 years from now, string theory will be viewed as a stepping stone towards a greater understanding of nature. 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 Hawking's puzzle about what's inside of a black hole.

Speaker 1

40:23

And you put in 1 assumption, you get 5 things out, somehow you're doing something right. Probably not everything, but there's some good signposts. And there have been a lot of good signposts like that.

Speaker 2

40:38

It is also a mathematical toolkit, and you've used it, you've used it with Kamran Vafa. Maybe we can sneak our way back from string theory into black holes. What was the idea that you and Kamran Vafa developed with the holographic principle and string theory?

Speaker 2

40:54

What were you able to discover through string theory about black holes? Or that connects us back to the reality of black holes.

Speaker 1

41:04

Yeah, so that is a very interesting story. I was interested in black holes before I was interested in string theory. I was sort of a reluctance string theorist in the beginning.

Speaker 1

41:19

I thought I had to learn it because people were talking about it, but once I studied it, I grew to love it. First, I did it in a sort of dutiful way. These people say they've claimed quantum gravity. I ought to read their papers at least.

Speaker 1

41:33

And then the more I read them, the more interested I got and I began to see, they phrased it in a very clumsy way. The description of string theory was very clumsy.

Speaker 2

41:46

Mathematically clumsy or just

Speaker 1

41:47

the interpretation? Mathematically clumsy, yeah. It was all correct, but mathematically clumsy, but it often happens that in all kinds of branches of physics that 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.

Speaker 1

42:17

They're not deceived. 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. So our understanding of string theory in 1985 was almost all about, you know, 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. 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 before my work with Kuhnman, we couldn't do any better than Einstein and Schwarzschild had done.

Speaker 1

43:20

Now, 1 of the puzzles, you know, if you look at the Hawking's headstone and also Boltzmann's headstone and you put them together, you get a formula for, and they are really central equations in 20th century physics. I don't think there are many equations that made it to headstones. 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. Now in Schwarzschild's description, the black hole is literally a hole in space and there's no place to store the gigabytes.

Speaker 1

44:13

And it's not too hard to, and this really was Wheeler and Bekenstein, and 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 that quantum mechanics and gravity can't be consistent.

Speaker 2

44:41

We gotta go there a little bit. So how is it possible, when we say gigabytes, so there's some information, So black holes can store information. 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?

Speaker 2

44:57

Where does it store information? On the inside, on the surface? Where, where is, and what's information? I'm liking this ask 5 questions to see which 1 you actually answer.

Speaker 1

45:10

Oh, okay. So if you say that, I should try to memorize them and answer each 1 in order just to answer them.

Speaker 2

45:14

I don't know, I don't know what I'm doing. I'm desperately trying to figure it out as we go along here.

Speaker 1

45:21

So Einstein's black hole, the Schwarzschild black hole, they can't store information. The stuff goes in there and it just keeps flying and it goes to the singularity and it's gone. However, Einstein's theory is not exact.

Speaker 1

45:37

It has corrections. And string theory tells you what those corrections are. And so you should be able to find some way of some alternate way of describing the black hole that enables you to understand where the gigabytes are stored. 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 4 times Newton's constant times Planck's constant.

Speaker 2

46:28

And that's another wild idea. You said area, not volume.

Speaker 1

46:32

Exactly. And that's the holographic principle.

Speaker 2

46:35

The universe is so weird.

Speaker 1

46:36

That's the holographic principle.

Speaker 2

46:38

That's called the holographic principle, that it's the area. We're just jumping around. What is the holographic principle?

Speaker 2

46:45

What does that mean? Is there some kind of weird projection going on? What the heck?

Speaker 1

46:50

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. Blah, blah, blah, blah, blah. So nobody knows exactly what it is, but roughly speaking, it says just what we were alluding to that really all the information that is in some volume of space-time can be stored on the boundary of that region.

Speaker 2

47:26

So this is not just about black holes, it's about any area of space-time?

Speaker 1

47:29

Any area of space-time. However, we've made sense of the holographic principle for black holes. 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.

Speaker 1

47:52

And we don't really understand how to talk about the holographic principle for either flat space, which we appear to live in, or asymptotically de Sitter space, which astronomers tell us we actually live in as the universe continues to expand. So it's 1 of the huge problems in physics is to apply or even formulate the holographic principle for more realistic, well, black holes are realistic, we see them. But yeah, in more general context. So a more general statement of the holographic principle.

Speaker 2

48:41

What's the difference between flat space and asymptotic de Sitter space? So flat space is just an approximation of the world we live in. So like, de Sitter space, I wonder what that even means, meaning like, asymptotic over what?

Speaker 1

48:57

Okay, so for thousands of years, you know, until the last half of the 20th, well, sorry, until the 20th century, we thought space-time was flat.

Speaker 2

49:10

Can you elaborate on flat?

Speaker 1

49:12

What do we mean by flat? Well, like the surface of this table is flat. Let me just give an intuitive explanation.

Speaker 1

49:23

Surface of the table is flat, but the surface of a basketball is curved. So the universe itself could be flat, like the surface of a table, or it could be curved like a basketball, which actually has a positive curvature. And then there's another kind of curvature called the negative curvature. 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.

Speaker 1

50:05

And that's kind of hard to visualize.

Speaker 2

50:07

Because you have to step, what, a couple of dimensions up? So it's hard to...

Speaker 1

50:11

You have to step a couple, but even a, if you have flat space and it's expanding in time, you know, we could imagine we're sitting here, this room, good approximation, it's flat, but imagine we suddenly start getting further and further apart. Then space is flat, but it's expanding, which means that space-time is curved.

Speaker 2

50:39

Ultimately, it's about space-time. Okay, so what's the de Sitter and anti-de Sitter space?

Speaker 1

50:45

The 3 simplest space-times are flat space-time, which call Minkowski space time, and negatively curved space time, anti-de Sitter space, and positively curved space time, de Sitter space. 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. Space is expanding in time, means that space-time is curved.

Speaker 1

51:26

And the nature of this curvature is affected by the matter in it, because matter itself causes the curvature of space-time. But as it expands, the matter gets more and more diluted. And 1 might ask, when it's all diluted away, is space-time still curved? And astronomers believe they've done precise enough measurements to determine this, and they believe that the answer is yes, The universe is now expanding.

Speaker 1

52:02

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. Einstein would call it a cosmological constant. In any case, in the far future, matter will be expanded away and we'll be left with empty de Sitter space.

Speaker 2

52:26

Okay, so there's this cosmological, Einstein's cosmological constant that now hides this thing that we don't understand called dark energy. What's dark energy? What's your best guess at what this thing is?

Speaker 2

52:38

Why do we think it's there? It's because it comes from the astronomers.

Speaker 1

52:44

Dark energy is synonymous with positive cosmological constant. And we think it's there because astronomers have told us it's there and they know what they're doing. And It's a really, really hard measurement, but they really know what they're doing.

Speaker 1

53:10

And we have no frigging idea why it's there. Another big mystery. Another reason it's fun to be a physicist. And if it is there, why should it be so small?

Speaker 1

53:23

Why should there be so little? Why should it have hid itself from us? Why shouldn't there be enough of it to substantially curve the space between us and the moon. Why did there have to be such a small amount that only the crazy best astronomers in the world could find it?

Speaker 2

53:43

Well, can't the same thing be said about all the constants? All of the, can't that be said about gravity, can't that be said about the speed of light? Like why is the speed of light so slow?

Speaker 1

53:55

So fast.

Speaker 2

53:56

So slow. Relative to the size of the universe, can't it be faster? Or no?

Speaker 1

54:04

Well, the speed of light is a funny 1 because you could always choose units in which the speed of light is 1. You know, we measure it in kilometers per second and it's 186, 000 or miles per second, it's 186, 000 miles per second. But if we had used different units, then we could make it 1.