Andrew Strominger: Black Holes, Quantum Gravity, and Theoretical Physics

00:00:00.000 | A black hole is a mirror.

00:00:01.640 | And the way it's a mirror is if light, a photon,

00:00:07.320 | bounces off your face towards the black hole,

00:00:11.660 | it goes straight to the black hole,

00:00:13.540 | just falls in, you never see it again.

00:00:15.740 | But if it just misses the black hole,

00:00:18.980 | it'll swing around the back and come back to you.

00:00:24.180 | And you see yourself from the photon

00:00:28.700 | that went around the back of the black hole.

00:00:31.320 | But not only can that happen,

00:00:33.700 | the black hole, the photon can swing around twice

00:00:37.680 | and come back.

00:00:40.000 | So you actually see an infinite number of copies of yourself.

00:00:44.480 | - The following is a conversation with Andrew Strominger,

00:00:49.720 | theoretical physicist at Harvard,

00:00:51.600 | whose research seeks to shed light

00:00:53.920 | on the unification of fundamental laws of nature,

00:00:56.760 | the origin of the universe, and the quantum structure

00:00:59.640 | of black holes and event horizons.

00:01:02.720 | This is the Lex Friedman Podcast.

00:01:04.680 | To support it, please check out our sponsors

00:01:06.760 | in the description.

00:01:08.040 | And now, dear friends, here's Andrew Strominger.

00:01:11.880 | You are part of the Harvard Black Hole Initiative,

00:01:16.740 | which has theoretical physicists, experimentalists,

00:01:19.800 | and even philosophers.

00:01:21.360 | So let me ask the big question.

00:01:23.280 | What is a black hole from a theoretical,

00:01:26.560 | from an experimental,

00:01:28.360 | maybe even from a philosophical perspective?

00:01:30.880 | - So a black hole is defined, theoretically,

00:01:35.880 | as a region of space-time

00:01:39.180 | from which light can never escape,

00:01:42.440 | therefore it's black.

00:01:44.960 | Now, that's just the starting point.

00:01:47.880 | Many weird things follow from that basic definition,

00:01:52.760 | but that is the basic definition.

00:01:56.640 | - What is light that can't escape from a black hole?

00:02:00.160 | - Well, light is the stuff that comes out of the sun,

00:02:03.920 | that stuff that goes into your eyes.

00:02:06.280 | Light is one of the stuff that disappears

00:02:09.760 | when the lights go off.

00:02:11.400 | This is stuff that appears when the lights come on.

00:02:14.240 | Of course, I could give you a mathematical definition,

00:02:18.240 | or a physical mathematical definition,

00:02:20.920 | but I think it's something that we all understand

00:02:26.040 | very intuitively what is light.

00:02:29.720 | Black holes, on the other hand,

00:02:31.560 | we don't understand intuitively.

00:02:33.280 | They're very weird.

00:02:35.040 | And one of the questions is, about black holes,

00:02:39.840 | which I think you were alluding to,

00:02:42.660 | is why doesn't light get out,

00:02:46.980 | or how is it that there can be a region of space-time

00:02:51.280 | from which light can't escape?

00:02:54.760 | It definitely happens.

00:02:57.040 | We've seen those regions.

00:02:58.980 | We have spectacular pictures,

00:03:01.040 | especially in the last several years of those regions.

00:03:05.280 | They're there.

00:03:08.080 | In fact, they're up in the sky,

00:03:10.260 | thousands or millions of them.

00:03:13.340 | We don't yet know how many.

00:03:14.820 | But the proper explanation

00:03:20.600 | of why light doesn't escape from a black hole

00:03:25.600 | is still a matter of some debate.

00:03:32.840 | And one explanation,

00:03:37.760 | which perhaps Einstein might have given,

00:03:43.060 | is that light carries energy.

00:03:48.280 | You know it carries energy because we have photocells

00:03:53.280 | and we can take the light from the sun and collect it,

00:03:56.640 | turn it into electricity.

00:03:58.180 | So there's energy in light.

00:04:01.040 | And anything that carries energy

00:04:03.480 | is subject to a gravitational pull.

00:04:06.620 | Gravity will pull at anything with energy.

00:04:09.280 | Now, it turns out that the gravitational pull

00:04:15.400 | exerted by an object is proportional to its mass.

00:04:20.400 | And so if you get enough mass in a small enough region,

00:04:27.000 | you can prevent light from escaping.

00:04:36.180 | And let me flesh that out a little more.

00:04:38.720 | If you're on the Earth

00:04:45.160 | and you're on a rocket ship leaving the surface of the Earth

00:04:50.160 | and if we ignore the friction from the air,

00:04:53.080 | if your rocket accelerates up to 11 kilometers per second,

00:04:59.880 | that's escape velocity.

00:05:02.040 | And if there were no friction,

00:05:04.240 | it could just continue forever to the next galaxy.

00:05:07.680 | On the moon, which has less mass,

00:05:11.560 | it's only seven kilometers per second.

00:05:15.440 | So, but going in the other direction,

00:05:18.400 | if you have enough mass in one place,

00:05:22.860 | the escape velocity can become the speed of light.

00:05:27.860 | If you shine light straight up away from the Earth,

00:05:32.560 | it doesn't have too much trouble.

00:05:34.000 | It's going way above the escape velocity.

00:05:36.320 | But if you have enough mass there,

00:05:41.420 | even light can't escape the escape velocity.

00:05:45.480 | And according to Einstein's theory of relativity,

00:05:50.480 | there is an absolute speed limit in the universe,

00:05:56.200 | the speed of light, and nothing makes any sense.

00:06:00.880 | Nothing could be self-consistent

00:06:03.120 | if there were objects that could exceed light speed.

00:06:08.200 | And so, in these very, very massive regions of space-time,

00:06:13.200 | even light cannot escape.

00:06:16.300 | - And the interesting thing is Einstein himself

00:06:18.100 | didn't think that these objects,

00:06:22.740 | we call the black holes, could exist.

00:06:25.020 | But let me actually linger on this.

00:06:26.180 | - Yeah, that's incredibly interesting.

00:06:27.820 | - There's a lot of interesting things here.

00:06:29.060 | First, the speed limit.

00:06:30.460 | How wild is it to you, if you put yourself in the mind

00:06:35.260 | in the time of Einstein before him,

00:06:37.900 | to come up with a speed limit, that there is a speed limit,

00:06:41.020 | and that speed limit is the speed of light?

00:06:44.160 | How difficult of an idea is that?

00:06:46.280 | You said from a mathematical physics perspective,

00:06:50.960 | everything just kind of falls into place,

00:06:52.520 | but he wasn't, perhaps, maybe initially had the luxury

00:06:57.520 | to think mathematically.

00:06:59.680 | He had to come up with it intuitively, yes?

00:07:02.280 | So how counterintuitive is this notion to you?

00:07:05.960 | Is it still crazy?

00:07:07.980 | - No, no.

00:07:08.900 | So it's a very funny thing in physics.

00:07:12.100 | The best discoveries seem completely obvious in retrospect.

00:07:17.100 | Even my own discoveries, which of course

00:07:21.820 | are far lesser than Einstein's,

00:07:24.420 | but many of my papers, many of my collaborators

00:07:27.620 | get all confused.

00:07:29.500 | We'll try to understand something.

00:07:30.780 | We say, we've got to solve this problem.

00:07:32.420 | We'll get all confused.

00:07:34.020 | Finally, we'll solve it.

00:07:35.020 | We'll get it all together, and then we'll,

00:07:40.020 | all of a sudden, everything will fall into place.

00:07:42.980 | We'll explain it, and then we'll look back

00:07:45.540 | at our discussions for the proceedings of months,

00:07:49.180 | and literally be unable to reconstruct how confused we were,

00:07:54.180 | and how we could ever have thought of it any other way.

00:08:00.020 | - That's so fascinating.

00:08:00.860 | - And so not only can I not fathom

00:08:04.780 | how confused Einstein was before he,

00:08:09.780 | when he started thinking about the issues,

00:08:14.100 | I can't even reconstruct my own confusion

00:08:17.940 | from two weeks ago.

00:08:19.740 | So the really beautiful ideas in physics

00:08:23.780 | have this very hard to get yourself back into the mindset.

00:08:28.500 | Of course, Einstein was confused about many, many things.

00:08:34.500 | It doesn't matter if you're a physicist.

00:08:38.140 | It's not how many things you got wrong.

00:08:40.220 | It's not the ratio of how many you got wrong

00:08:42.980 | to how many you got right.

00:08:44.620 | It's the number that you got right.

00:08:46.380 | So Einstein didn't believe black holes existed,

00:08:51.640 | even though he predicted them.

00:08:53.660 | And I went and I read that paper, which he wrote.

00:08:56.660 | Einstein wrote down his field equations in 1915,

00:09:01.980 | and Schwarzschild solved them

00:09:04.380 | and discovered the black hole solution

00:09:06.460 | three or four months later, in very early 1916.

00:09:10.980 | And 25 years later, Einstein wrote a paper.

00:09:17.020 | So with 25 years to think about what this solution means,

00:09:21.760 | wrote a paper in which he said that black holes didn't exist.

00:09:28.540 | And I'm like, well, if one of my students

00:09:33.540 | in my general relativity course wrote this,

00:09:36.160 | I wouldn't pass them.

00:09:38.780 | - You get a C minus, oh, you wouldn't pass them, okay.

00:09:40.900 | - I get a C minus, okay.

00:09:42.820 | Same thing with gravity waves.

00:09:44.260 | He didn't believe--

00:09:45.100 | - Oh, he didn't believe in gravitational waves either?

00:09:46.620 | - He went back and forth, but he wrote a paper in, I think,

00:09:49.620 | '34, saying that gravity waves didn't exist

00:09:52.620 | because people were very confused

00:09:55.300 | about what a coordinate transformation is.

00:09:57.540 | And in fact, this confusion

00:09:59.260 | about what a coordinate transformation is has persisted,

00:10:04.260 | and we actually think we're on the edge of solving it

00:10:10.060 | 100 years later.

00:10:15.880 | - Well, what--

00:10:17.660 | - 100 years later.

00:10:18.620 | - Well, what is coordinate transformation,

00:10:21.060 | as it was 100 years ago to today?

00:10:22.940 | Let's imagine I want to draw a map

00:10:27.260 | with pictures of all the states and the mountains,

00:10:30.700 | and then I wanna draw the weather forecast,

00:10:34.860 | what the temperatures are gonna be all over the country.

00:10:38.780 | And I do that using one set of weather stations,

00:10:44.860 | and I number the weather stations,

00:10:48.060 | and you have some other set of weather stations,

00:10:52.500 | and you do the same thing.

00:10:55.420 | So the coordinates are the locations of the weather stations.

00:10:58.780 | They're how we describe where the things are.

00:11:01.820 | At the end of the day, we should draw the same map.

00:11:06.820 | That is coordinate invariance.

00:11:10.200 | And if we're telling somebody,

00:11:12.300 | we're gonna tell somebody at a real physical operation,

00:11:16.220 | we want you to stay as dry as possible

00:11:18.940 | on your drive from here to California,

00:11:22.160 | we should give them exactly the same route.

00:11:24.500 | No matter which weather stations we use or how we,

00:11:28.780 | it's a very trivial,

00:11:31.180 | the labeling of points is an artifact

00:11:34.740 | and not in the real physics.

00:11:37.020 | So it turns out that that's almost true,

00:11:43.240 | but not quite.

00:11:46.740 | There's some subtleties to it.

00:11:49.440 | - The statement that you should always have the same,

00:11:51.820 | give at least the same kind of trajectory,

00:11:53.500 | the same kind of instructions,

00:11:56.100 | no matter the weather stations.

00:11:57.580 | - Yeah, there's some very delicate subtleties to that,

00:12:02.020 | which began to be noticed in the '50s.

00:12:06.780 | It's mostly true, but when you have a space-time with edges,

00:12:13.780 | it gets very tricky how you label the edges.

00:12:19.140 | - And space-time in terms of space or in terms of time,

00:12:21.300 | in terms of everything, just space-time?

00:12:22.620 | - Either one, space or time.

00:12:24.260 | That gets very tricky.

00:12:25.500 | And Einstein didn't have it right.

00:12:30.500 | In fact, he had an earlier version of general relativity

00:12:36.380 | in 1914, which he was very excited about,

00:12:42.160 | which was wrong,

00:12:46.360 | gave, it wasn't fully coordinate invariant,

00:12:50.580 | it was only partially coordinate invariant, it was wrong.

00:12:54.260 | It gave the wrong answer for bending light to the sun

00:12:58.260 | by a factor of two.

00:13:01.300 | There was an expedition sent out to measure it

00:13:04.980 | during World War I.

00:13:06.820 | They were captured before they could measure it.

00:13:10.620 | (both laughing)

00:13:12.100 | And that gave Einstein four more years to clean his act up,

00:13:17.100 | by which time he'd gotten it right.

00:13:20.580 | So it's a very tricky business,

00:13:22.820 | but once it's all laid out, it's clear.

00:13:25.820 | - Then why do you think Einstein

00:13:31.820 | didn't believe his own equations

00:13:35.420 | and didn't think that black holes are real?

00:13:37.220 | Why was that such a difficult idea for him?

00:13:39.860 | - Well, something very interesting happens

00:13:43.860 | in Schwarzschild's solution of the Einstein equation.

00:13:47.960 | (mouse clicking)

00:13:50.720 | I think his reasoning was ultimately wrong,

00:13:54.720 | but let me explain to you what it was.

00:13:59.720 | At the center of the black hole, behind the horizon,

00:14:06.780 | in a region that nobody can see and live to tell about it,

00:14:13.000 | as the center of the black hole, there's a singularity,

00:14:16.540 | and if you pass the horizon, you go into the singularity,

00:14:20.280 | you get crushed, and that's the end of everything.

00:14:23.420 | Now, the word singularity means that,

00:14:29.980 | it just means that Einstein's equations break down.

00:14:37.620 | They become infinite.

00:14:40.660 | You write them down, you put 'em on the computer.

00:14:43.240 | When the computer hits that singularity, it crashes.

00:14:47.380 | Everything becomes infinite, there's two.

00:14:49.580 | So the equations are just no good there.

00:14:52.760 | Now, that's actually not a bad thing.

00:14:59.440 | It's a really good thing, and let me explain why.

00:15:03.880 | So, it's an odd thing that Maxwell's equation

00:15:12.360 | and Newton's theory never exhibit this phenomena.

00:15:17.360 | You write them down, you can solve them exactly.

00:15:23.560 | They're really, Newton's theory of gravity,

00:15:25.340 | they're really very simple theories.

00:15:28.160 | You can solve them, well, you can't solve

00:15:30.120 | the three-body problem, but you can certainly

00:15:34.800 | solve a lot of things about them.

00:15:39.240 | Nevertheless, there was never any reason,

00:15:44.240 | even though Maxwell and Newton perhaps fell for this trap,

00:15:51.320 | there were never any reason to think

00:15:55.960 | that these equations were exact.

00:15:58.560 | And every, there's no equation, well,

00:16:07.800 | there's some equations that we've written down

00:16:09.960 | that we still think are exact.

00:16:12.340 | Some people still think are exact.

00:16:15.480 | My view is that there's no exact equation.

00:16:17.840 | - Everything is an approximation.

00:16:19.280 | - Everything is an approximation.

00:16:20.720 | - And you're trying to get as close as possible.

00:16:23.160 | - Yeah.

00:16:24.000 | - So you're saying objective truth

00:16:25.200 | doesn't exist in this world?

00:16:26.600 | The internet's gonna be very mad at you.

00:16:29.760 | - We could discuss that, but that's a different thing.

00:16:32.440 | We wouldn't say Newton's theory was wrong.

00:16:36.980 | It had very, very small corrections,

00:16:39.800 | incredibly small corrections.

00:16:42.380 | It's actually a puzzle why they're so small.

00:16:45.440 | So if you watch the precession of Mercury's perihelion,

00:16:48.400 | this was the first indication of something going wrong.

00:16:53.400 | According to Newton's theory,

00:16:55.080 | Mercury has an elliptical orbit.

00:16:57.960 | The long part of it moves around

00:17:02.440 | as other planets come by and perturb it and so on.

00:17:06.760 | And so this was measured by Le Verrier in 1859

00:17:11.140 | and he compared theory and experiment

00:17:15.140 | and he found out that the perihelion precess

00:17:19.140 | moves around the sun once every 233 centuries

00:17:24.140 | instead of every 231 centuries.

00:17:30.020 | Now this is the wonderful thing about science.

00:17:33.260 | Why was this guy?

00:17:36.760 | I mean, you don't get any idea how much work this is.

00:17:39.800 | But of course he made one of the greatest discoveries

00:17:43.640 | in the history of science without even knowing

00:17:47.680 | what good it was gonna be.

00:17:49.580 | So that's how small, that was the first sign

00:17:55.920 | that there was something wrong with Newton.

00:17:58.020 | Now, so the corrections to Newton's law

00:18:02.680 | are very, very small, but they're definitely there.

00:18:06.240 | The corrections to electromagnetism,

00:18:08.720 | they're mostly, the ones that we see

00:18:11.800 | are mostly coming from quantum effects.

00:18:14.700 | And-- - So the corrections

00:18:18.480 | for Maxwell's equations is when you get super tiny

00:18:23.480 | and then the corrections for Newton's laws of gravity

00:18:28.760 | is when you get super big.

00:18:31.760 | That's when you require corrections.

00:18:34.760 | - That's true, but I would phrase it as saying

00:18:36.840 | when it's super accurate.

00:18:38.920 | If you look at the Bohr atom, Maxwell electromagnetism

00:18:43.140 | is not a very good approximation to the force

00:18:47.400 | between the proton and the electron.

00:18:50.680 | The quantum mechanics, if you didn't have

00:18:54.760 | quantum mechanics, the electron would spiral

00:18:58.240 | into the proton and the atom would collapse.

00:19:02.600 | So that's a huge correction there.

00:19:04.040 | - Sure.

00:19:05.160 | - So every theory gets corrected as we learn more.

00:19:08.280 | Just be no reason to suppose that it should be otherwise.

00:19:14.640 | - How is this related to the singularity?

00:19:16.400 | Why the singularity-- - So when you hit

00:19:17.940 | the singularity, you know that you need some improvement

00:19:22.940 | to Einstein's theory of gravity.

00:19:29.000 | And that improvement, we understand what kind of things

00:19:32.520 | that improvement should involve.

00:19:35.400 | It should involve quantum mechanics,

00:19:37.320 | quantum effects become important there.

00:19:40.240 | It's a small thing.

00:19:42.160 | And we don't understand exactly what the theory is,

00:19:47.160 | but we know there's no reason to think,

00:19:50.680 | Einstein's theory was invented to describe

00:19:55.420 | weakly curved things, the solar system and so on.

00:19:59.920 | It's incredibly robust that we now see that it works

00:20:03.920 | very well near the horizons around black holes and so on.

00:20:08.920 | So it's a good thing that the theory drives itself

00:20:14.640 | that it predicts its own demise.

00:20:18.500 | Newton's gravity had its demise.

00:20:23.420 | There were regimes in which it wasn't valid.

00:20:26.760 | Maxwell's electromagnetism had its demise.

00:20:30.920 | There was regimes in which quantum effects

00:20:35.120 | greatly modified the equations.

00:20:39.740 | But general relativity all on its own

00:20:43.920 | found a system which originally was fine

00:20:51.760 | would perversely wander off into a configuration

00:20:56.760 | in which Einstein's equations no longer applied.

00:21:00.680 | - So to you, the edges of the theory are wonderful.

00:21:03.240 | The failures of the theory--

00:21:04.080 | - Edges are wonderful because that keeps us in business.

00:21:09.080 | - So one of the things you said, I think,

00:21:10.800 | in your TED Talk that the fact that quantum mechanics

00:21:15.800 | and relativity don't describe everything

00:21:20.600 | and then they clash is wonderful.

00:21:23.240 | I forget the adjective you used,

00:21:24.360 | but it was something like this.

00:21:25.920 | So why is that?

00:21:27.260 | Why is that interesting?

00:21:29.360 | Do you in that same way that there's contradictions

00:21:31.460 | that create discovery?

00:21:32.720 | - There's no question in my mind,

00:21:35.200 | of course many people would disagree with me,

00:21:37.560 | that now is the most wonderful time to be a physicist.

00:21:45.720 | So people look back at,

00:21:49.160 | it's a classical thing to say among physicists,

00:21:55.580 | I wish it were 1920.

00:21:58.100 | Quantum mechanics had been just understood.

00:22:02.660 | There was the periodic table.

00:22:05.920 | But in fact, that was such a rich thing

00:22:12.280 | that, well, so a lot of exciting stuff happened around 1920.

00:22:17.280 | It took a whole century to sort out

00:22:25.240 | the new insights that we got.

00:22:27.880 | - Especially adding some experimental stuff

00:22:29.680 | into the bunch, actually making observations

00:22:32.480 | and integrating all the data.

00:22:33.320 | - Adding the experimental thing.

00:22:34.160 | - All the computers also help with visualizations

00:22:36.240 | and all that kind of stuff.

00:22:37.080 | - Yeah, yeah, yeah.

00:22:37.900 | It was a whole sort of wonderful century.

00:22:41.160 | I mean, the seed of general relativity

00:22:45.200 | was the incompatibility of Maxwell's theory

00:22:50.200 | of the electromagnetic field

00:22:52.400 | with Newton's laws of gravity.

00:22:56.560 | They were incompatible because

00:22:59.720 | if you look at Maxwell's theory,

00:23:01.940 | there's a contradiction if anything goes faster

00:23:08.240 | than the speed of light.

00:23:10.440 | But Newton's theory of gravity,

00:23:14.080 | the gravitational field, the gravitational force,

00:23:19.080 | is instantaneously transmitted across the entire universe.

00:23:25.080 | So you could, if you had a friend in another galaxy

00:23:30.360 | with a very sensitive measuring device

00:23:36.200 | that could measure the gravitational field,

00:23:38.520 | they could just take this cup of coffee

00:23:40.640 | and move it up and down in Morse code

00:23:43.220 | and they could get the message instantaneously

00:23:46.600 | over another galaxy.

00:23:48.260 | That leads to all kinds of contradictions.

00:23:50.360 | It's not self-consistent.

00:23:52.440 | It was exactly in resolving those contradictions

00:23:56.480 | that Einstein came up with the general theory

00:23:59.160 | of relativity and it's fascinating

00:24:04.080 | how this contradiction, which seems like

00:24:07.420 | maybe it's kind of a technical thing,

00:24:09.340 | led to a whole new vision of the universe.

00:24:14.840 | Now, let's not get fooled because

00:24:17.880 | lots of contradictions are technical things.

00:24:22.520 | We haven't set up the,

00:24:23.840 | we run into other kinds of contradictions

00:24:27.480 | that are technical and they don't seem to,

00:24:30.360 | we understood something wrong, we made a mistake,

00:24:33.620 | we set up our equations in the wrong way,

00:24:35.500 | we didn't translate the formalisms.

00:24:37.680 | - As opposed to revealing some deep mystery

00:24:39.360 | that's yet to be uncovered.

00:24:40.440 | - Yeah, yeah, yeah.

00:24:41.280 | And so we're never very sure

00:24:43.920 | which are the really important ones.

00:24:47.320 | - But to you, the difference between quantum mechanics

00:24:49.360 | and general relativity, the tension,

00:24:52.440 | the contradiction there seems to hint

00:24:54.640 | at some deeper, deeper thing that's going

00:24:56.880 | to be discovered in the century.

00:24:58.240 | - Yes, because that one has been understood

00:25:01.600 | since the '50s.

00:25:03.340 | Pauli was the first person to notice it

00:25:08.340 | and Hawking in the early '70s gave it

00:25:11.820 | a really much more visceral form.

00:25:14.780 | And people have been hurling themselves at it,

00:25:21.100 | trying to reduce it to some technicality,

00:25:25.300 | but nobody has succeeded.

00:25:27.820 | And the efforts to understand it have led to

00:25:32.040 | all kinds of interesting relations

00:25:35.640 | between quantum systems and applications

00:25:40.440 | to other fields and so on.

00:25:42.520 | - Well, let's actually jump around.

00:25:44.280 | - Okay.

00:25:45.120 | - So we'll return to black holes,

00:25:45.940 | I have a million questions there,

00:25:46.800 | but let's go into this unification,

00:25:50.400 | the battle against the contradictions

00:25:53.200 | and the tensions between the theories of physics.

00:25:55.560 | What is quantum gravity?

00:25:58.060 | Maybe what is the standard model of physics?

00:26:00.760 | What is quantum mechanics?

00:26:01.960 | What is general relativity?

00:26:03.480 | What's quantum gravity?

00:26:04.740 | What are all the different unification efforts?

00:26:08.080 | - Okay, so--

00:26:11.440 | - Again, five questions.

00:26:12.660 | - Yeah.

00:26:14.040 | It's a theory that describes everything

00:26:18.920 | with astonishing accuracy.

00:26:22.400 | It's the most accurate theory

00:26:26.360 | in the history of human thought.

00:26:28.720 | Theory and experiment have been successfully compared

00:26:32.960 | to 16 decimal place.

00:26:36.120 | We have that stenciled on the door where I work.

00:26:41.120 | It's an amazing feat of the human mind.

00:26:46.480 | It describes the electromagnetic interaction,

00:26:53.240 | unifies the electromagnetic interaction

00:26:55.580 | with the so-called weak interaction,

00:26:58.040 | which you need some good tools

00:27:01.640 | to even view the weak interaction.

00:27:03.960 | And then there's the strong interaction,

00:27:06.060 | which binds the quarks into protons.

00:27:11.840 | And the forces between them are mediated

00:27:16.120 | by something called Yang-Mills theory,

00:27:19.480 | which is a beautiful mathematical generalization

00:27:24.720 | of electromagnetism in which the analogs

00:27:29.720 | of the photons themselves carry charge.

00:27:35.840 | And so this, the final piece of this,

00:27:41.040 | of the standard model, everything in the standard model

00:27:44.960 | has been observed, its properties have been measured.

00:27:48.920 | The final particle to be observed was the Higgs particle.

00:27:53.840 | It was observed over a decade ago.

00:27:56.400 | - The Higgs was observed a decade ago.

00:27:58.160 | - I think it is, yeah.

00:27:59.800 | - Wow, time flies.

00:28:01.120 | - But you better check me on that, yeah.

00:28:03.160 | - That's true, but so much fun has been happening.

00:28:06.480 | - So much fun has been happening.

00:28:08.560 | And so that's all pretty well understood.

00:28:13.560 | There's some things that might or might not,

00:28:20.200 | around the edges of that, you know,

00:28:22.280 | dark matter, neutrino masses, some sort of fine points

00:28:27.280 | or things we haven't quite measured perfectly and so on.

00:28:32.520 | But it's largely a very complete theory.

00:28:36.520 | And we don't expect anything very new conceptually

00:28:43.760 | in the completion of that.

00:28:49.680 | - Anything contradictory by new.

00:28:52.160 | - Anything contradictory, yeah.

00:28:54.200 | - I'll have some wild questions for you on that front.

00:28:58.000 | But yeah, anything that, yeah, 'cause there's no gaps.

00:29:01.120 | It's so accurate, so precise in its predictions,

00:29:03.160 | it's hard to imagine something completely new.

00:29:06.200 | - And it was all based on something called,

00:29:10.560 | let me not explain what it is,

00:29:11.880 | let me just throw out the buzzword,

00:29:14.120 | renormalizable quantum field theory.

00:29:16.720 | They all fall in the category

00:29:20.080 | of renormalizable quantum field theory.

00:29:22.960 | - I'm gonna throw that at a bar later to impress the girls.

00:29:27.400 | - Good luck.

00:29:29.520 | - Thank you.

00:29:30.360 | - They all fall under that rubric.

00:29:37.080 | Gravity will not put that suit on.

00:29:42.080 | So the force of gravity cannot be tamed

00:29:46.560 | by the same renormalizable quantum field theory

00:29:50.920 | to which all the other forces so eagerly submitted.

00:29:55.920 | - What is the effort of quantum gravity?

00:29:58.080 | What are the different efforts

00:29:59.640 | to have these two dance together effectively,

00:30:04.480 | to try to unify the standard model and general relativity,

00:30:09.480 | any kind of model of gravity?

00:30:13.240 | - Sort of the one fully consistent model that we have

00:30:18.240 | that reconciles, that sort of tames gravity

00:30:25.360 | and reconciles it with quantum mechanics

00:30:30.560 | is string theory and its cousins.

00:30:36.060 | And we don't know what, or if in any sense,

00:30:41.000 | string theory describes the world, the physical world,

00:30:45.040 | but we do know that it is a consistent reconciliation

00:30:50.040 | of quantum mechanics and general relativity,

00:30:54.960 | and moreover, one which is able to incorporate particles

00:30:59.960 | and forces like the ones we see around us.

00:31:08.360 | So it hasn't been ruled out as an actual

00:31:13.360 | sort of unified theory of nature,

00:31:16.160 | but there also isn't a, in my view,

00:31:22.520 | some people would disagree with me,

00:31:24.360 | but there isn't a reasonable possibility

00:31:30.200 | that we would be able to do an experiment

00:31:32.840 | in the foreseeable future,

00:31:34.720 | which would be sort of a yes or no to string theory.

00:31:39.720 | - Okay, so you've been there

00:31:41.400 | from the early days of string theory,

00:31:42.760 | you've seen its developments.

00:31:44.120 | What are some interesting developments?

00:31:46.120 | What do you see as also the future of string theory?

00:31:49.800 | And what is string theory?

00:31:51.120 | - Well, the basic idea which emerged in the early '70s

00:32:00.200 | was that if you take the notion of a particle

00:32:05.200 | and you literally replace it by a little loop of string,

00:32:15.200 | the strings are sort of softer than particles.

00:32:22.120 | - What do you mean by softer?

00:32:24.800 | - Well, you know, if you hit a particle,

00:32:29.000 | if there were a particle on this table, a big one,

00:32:31.040 | and you hit it, you might bruise yourself.

00:32:33.120 | But if there was a string on the table,

00:32:35.560 | you would probably just push it around.

00:32:38.320 | And the source of the infinities in quantum field theory

00:32:43.320 | is that when particles hit each other,

00:32:45.060 | it's a little bit of a jarring effect.

00:32:50.060 | And I've never described it this way before,

00:32:55.400 | but it's actually scientifically accurate.

00:32:57.680 | But if you throw strings at each other,

00:32:59.360 | it's a little more friendly.

00:33:01.000 | One thing I can't explain is how wonderfully precise

00:33:06.000 | all the mathematics is

00:33:08.760 | that goes into describing string theory.

00:33:10.600 | We don't just wave our hands and throw strings around.

00:33:13.560 | And there's some very compelling mathematical equations

00:33:18.560 | that describe it.

00:33:21.000 | Now, what was realized in the early '70s

00:33:25.120 | is that if you replace particles by strings,

00:33:28.680 | these infinities go away,

00:33:31.120 | and you get a consistent theory of gravity

00:33:36.120 | without the infinities.

00:33:39.300 | And that may sound a little trivial,

00:33:45.680 | but at that point, it had already been 15 years

00:33:50.800 | that people had been searching around

00:33:54.000 | for any kind of theory that could do this.

00:33:57.760 | And it was actually found kind of by accident.

00:34:02.760 | And there are a lot of accidental discoveries

00:34:07.240 | in this subject.

00:34:08.280 | Now, at the same time, it was believed then

00:34:12.520 | that string theory was an interesting sort of toy model

00:34:17.040 | for putting quantum mechanics

00:34:18.480 | and general relativity together on paper.

00:34:22.700 | But that it couldn't describe

00:34:27.440 | some of the very idiosyncratic phenomena

00:34:31.320 | that pertain to our own universe,

00:34:33.760 | in particular, the form of so-called parity violation.

00:34:37.660 | Our world is-- - Ooh, another term

00:34:40.360 | for the bar later tonight.

00:34:41.880 | - Yeah, yeah.

00:34:42.960 | - Parity violation.

00:34:44.360 | - So if you go to the bar and--

00:34:46.840 | - I already got the renormalizable quantum field theory.

00:34:48.520 | - And you look in the mirror across the bar,

00:34:51.440 | the universe that you see in the mirror is not identical.

00:34:55.480 | You would be able to tell if you show

00:34:59.840 | the lady in the bar a photograph

00:35:04.960 | that shows both the mirror and you.

00:35:07.080 | - There's a difference.

00:35:07.920 | - If she's smart enough, she'll be able to tell

00:35:11.160 | which one is the real world and which one is you.

00:35:14.480 | Now, she would have to do some very precise measurements.

00:35:17.400 | - Yes. (both laughing)

00:35:19.960 | - And if the photograph was too grainy,

00:35:21.680 | it might not be possible.

00:35:22.920 | But a principle is possible.

00:35:24.320 | - Why is this interesting?

00:35:25.920 | Does this mean that there is some not perfect determinism?

00:35:30.720 | Or what does that mean, there's some uncertainty?

00:35:32.800 | - No, it's a very interesting feature of the real world

00:35:36.520 | that it isn't parity invariant.

00:35:39.120 | In string theory, it was thought could not tolerate that.

00:35:42.920 | And then it was learned in the mid '80s

00:35:49.160 | that not only could it tolerate that,

00:35:51.740 | but if you did things in the right way,

00:35:54.580 | you could construct a world involving strings

00:35:59.580 | that reconciled quantum mechanics and general relativity,

00:36:04.440 | which looked more or less like the world that we live in.

00:36:09.440 | And now, that isn't to say

00:36:14.220 | that string theory predicted our world.

00:36:17.440 | It just meant that it was consistent,

00:36:19.520 | that the hypothesis that string theory describes our world

00:36:24.520 | can't be ruled out from the get-go.

00:36:30.640 | And it is also the only proposal for a complete theory

00:36:35.640 | that would describe our world.

00:36:41.200 | Still, nobody will believe it

00:36:46.840 | until there's some kind of direct experiment.

00:36:50.840 | And I don't even believe it myself.

00:36:53.080 | - Sure, which is a good place to be mentally

00:36:55.360 | as a physicist, right?

00:36:56.720 | I mean, Einstein didn't believe his own equations, right,

00:36:59.640 | with the black hole.

00:37:00.640 | Okay.

00:37:01.480 | - Well, then when he was wrong about that.

00:37:02.880 | (both laughing)

00:37:04.440 | He was wrong about that.

00:37:05.800 | - But you might be wrong too, right?

00:37:08.000 | So do you think string theory is dead

00:37:10.920 | if you were to bet all your money

00:37:12.080 | on the future of string theory?

00:37:15.000 | - I think it's a logical error

00:37:19.840 | to think that string theory is either right or wrong

00:37:24.840 | or dead or alive.

00:37:26.200 | What it is is a stepping stone.

00:37:29.840 | And an analogy I like to draw is Yang-Mills theory,

00:37:36.440 | which I mentioned a few minutes ago

00:37:41.760 | in the context of standard model.

00:37:44.760 | Yang-Mills theory was discovered

00:37:47.120 | by Yang and Mills in the '50s.

00:37:49.120 | And they thought that the symmetry of Yang and Mills theory

00:37:55.000 | described the relationship between the proton

00:37:59.640 | and the neutron.

00:38:00.720 | That's why they invented it.

00:38:02.160 | That turned out to be completely wrong.

00:38:05.480 | It does, however, describe everything else

00:38:10.360 | in the standard model.

00:38:11.440 | (both laughing)

00:38:13.560 | And it had a kind of inevitability.

00:38:15.920 | They had some of the right pieces, but not the other ones.

00:38:19.280 | They didn't have it quite in the right context.

00:38:22.360 | And it had an inevitability to it,

00:38:24.920 | and it eventually sort of found its place.

00:38:27.280 | And it's also true of Einstein's

00:38:31.920 | theory of general relativity.

00:38:33.680 | He had the wrong version of it in 1914,

00:38:36.360 | and he was missing some pieces.

00:38:38.520 | And you wouldn't say that his early version

00:38:40.600 | was right or wrong.

00:38:41.640 | He'd understood the equivalence principle.

00:38:43.400 | He'd understood spacetime curvature.

00:38:45.840 | He just didn't have everything.

00:38:48.240 | I mean, technically, you would have to say it was wrong.

00:38:51.200 | And technically, you would have to say

00:38:52.480 | Yang and Mills were wrong.

00:38:54.480 | And I guess in that sense,

00:38:56.260 | I would believe just odds are.

00:39:00.880 | We always keep finding new wrinkles.

00:39:04.400 | Odds are we're gonna find new wrinkles in string theory,

00:39:07.020 | and technically what we call string theory now

00:39:09.700 | isn't quite right, but--

00:39:11.920 | - We're always going to be wrong,

00:39:13.400 | but hopefully a little bit less wrong every time.

00:39:16.080 | - Exactly.

00:39:17.320 | And I would bet the farm, as they say.

00:39:20.880 | - Do you have a farm?

00:39:21.920 | - I say that much more seriously,

00:39:24.720 | because not only do I have a farm,

00:39:26.280 | but we just renovated it.

00:39:27.620 | (laughing)

00:39:29.880 | - That's a very serious statement.

00:39:31.640 | - So before I renovated, betting at the farm,

00:39:33.960 | my wife and I spent five years renovating it.

00:39:36.200 | - You were much looser with that statement,

00:39:39.520 | but now it really means something.

00:39:41.480 | - Now it really means something.

00:39:43.080 | And I would bet the farm on the guess

00:39:47.080 | that 100 years from now, string theory will be viewed

00:39:57.580 | as a stepping stone towards a greater understanding

00:40:00.480 | of nature.

00:40:02.980 | And it would, I mean, another thing that I didn't mention

00:40:06.960 | about string theory is, of course,

00:40:08.520 | we knew that it solved the infinities problem,

00:40:13.520 | and then we later learned that it also solved

00:40:17.560 | Hawking's puzzle about what's inside of a black hole.

00:40:22.560 | And you put in one assumption, you get five things out,

00:40:28.400 | somehow you're doing something right.

00:40:31.200 | Probably not everything, but there's some good signposts.

00:40:35.700 | And there've been a lot of good signposts like that.

00:40:38.920 | - It is also a mathematical toolkit, and you've used it.

00:40:42.080 | You've used it with Kamran Vafa.

00:40:43.800 | Maybe we can sneak our way back from string theory

00:40:47.520 | into black holes.

00:40:48.600 | What was the idea that you and Kamran Vafa developed

00:40:52.320 | with the holographic principle and string theory?

00:40:55.160 | What were you able to discover through string theory

00:40:58.400 | about black holes?

00:41:00.320 | Or that connects us back to the reality of black holes?

00:41:05.000 | - Yeah, so that is a very interesting story.

00:41:08.980 | I was interested in black holes

00:41:11.980 | before I was interested in string theory.

00:41:14.540 | I was sort of a reluctance string theorist in the beginning.

00:41:19.540 | I thought I had to learn it

00:41:21.060 | 'cause people were talking about it.

00:41:22.760 | But once I studied it, I grew to love it.

00:41:26.120 | First, I did it in a sort of dutiful way.

00:41:28.420 | These people say they've claimed quantum gravity.

00:41:30.500 | I ought to read their papers at least.

00:41:33.180 | And then the more I read them,

00:41:35.160 | the more interested I got, and I began to see.

00:41:37.860 | They phrased it in a very clumsy way.

00:41:42.760 | The description of string theory was very clumsy.

00:41:46.220 | - Mathematically clumsy or just the interpretation?

00:41:47.860 | - Mathematically clumsy, yeah.

00:41:50.260 | It was all correct, but mathematically clumsy.

00:41:55.040 | But it often happens that in all kinds of branches

00:42:01.560 | of physics that people start working on it really hard

00:42:06.360 | and they sort of dream about it and live it and breathe it.

00:42:10.000 | And they begin to see inner relationships.

00:42:12.580 | And they see a beauty that is really there.

00:42:17.320 | They're not deceived.

00:42:18.820 | They're really seeing something that exists.

00:42:20.760 | But if you just kind of look at it,

00:42:24.160 | you can't grasp it all in the beginning.

00:42:30.540 | So our understanding of string theory in 1985

00:42:35.540 | was almost all about

00:42:42.380 | weakly coupled waves of strings colliding and so on.

00:42:49.980 | We didn't know how to describe a big thing

00:42:53.380 | like a black hole in string theory.

00:42:56.500 | Of course, we could show that strings in theory

00:43:00.020 | at some limit reproduced Einstein's theory

00:43:02.300 | of general relativity and corrected it.

00:43:05.240 | But we couldn't do any better with black holes

00:43:09.360 | than before my work with Kuhnman,

00:43:14.360 | we couldn't do any better than Einstein

00:43:16.740 | and Schwarzschild had done.

00:43:18.100 | Now, one of the puzzles,

00:43:22.660 | if you look at the Hawking's headstone

00:43:29.340 | and also Boltzmann's headstone and you put them together,

00:43:34.020 | you get a formula for,

00:43:37.580 | and they are really central equations

00:43:41.420 | in 20th century physics.

00:43:42.860 | I don't think there are many equations

00:43:46.520 | that made it to headstones.

00:43:48.020 | (both laughing)

00:43:51.100 | And they're really central equations.

00:43:54.380 | And you put them together and you get a formula

00:43:57.300 | for the number of gigabytes in a black hole.

00:44:00.320 | Now, in Schwarzschild's description,

00:44:04.480 | the black hole is literally a hole in space

00:44:08.900 | and there's no place to store the gigabytes.

00:44:11.640 | And it's not too hard to,

00:44:16.140 | and this really was Wheeler and Bekenstein,

00:44:19.340 | and Wheeler, Bekenstein, and Hawking

00:44:22.340 | to come to the conclusion that

00:44:27.020 | if there isn't a sense in which a black hole can store

00:44:32.020 | some large number of gigabytes,

00:44:35.580 | that quantum mechanics and gravity can't be consistent.

00:44:40.160 | - We gotta go there a little bit.

00:44:42.540 | So how is it possible,

00:44:45.220 | when we say gigabytes, so there's some information,

00:44:47.100 | so black holes can store information.

00:44:48.580 | How is this thing that sucks up all light

00:44:50.700 | and it's supposed to basically be super homogeneous

00:44:54.860 | and boring, how is that actually able to store information?

00:44:57.740 | Where does it store information?

00:44:59.260 | On the inside, on the surface?

00:45:01.140 | Where, where is, and what's information?

00:45:05.240 | I'm liking this ask five questions

00:45:08.160 | to see which one you actually answer.

00:45:10.220 | - Oh, okay.

00:45:11.060 | - I'm gonna ask you a question.

00:45:11.900 | - So if you say that, I should try to memorize them

00:45:13.420 | and answer each one in order just to answer them.

00:45:15.100 | - I don't know, I don't know what I'm doing.

00:45:16.740 | I'm desperately trying to figure it out as we go along here.

00:45:21.180 | So Einstein's black holes,

00:45:24.180 | Schwarzschild's black hole, they can't store information.

00:45:27.060 | The stuff goes in there and it just keeps flying

00:45:30.280 | and it goes to the singularity and it's gone.

00:45:32.580 | However, Einstein's theory is not exact.

00:45:37.980 | It has corrections.

00:45:39.160 | And string theory tells you what those corrections are.

00:45:44.400 | And so you should be able to find some way

00:45:50.860 | of some alternate way of describing the black hole

00:45:55.860 | that enables you to understand

00:46:01.100 | where the gigabytes are stored.

00:46:05.060 | So what Hawking and Bekenstein really did

00:46:08.020 | was they showed that physics is inconsistent

00:46:12.820 | unless a black hole can store a number of gigabytes

00:46:19.820 | proportional to its area divided by four times

00:46:24.820 | Newton's constant times Planck's constant.

00:46:28.900 | - And that's another wild idea.

00:46:30.300 | You said area, not volume.

00:46:32.380 | - Exactly.

00:46:33.900 | And that's the holographic principle.

00:46:35.540 | - The universe is so weird.

00:46:37.220 | - That's the holographic principle.

00:46:38.620 | - That's called the holographic principle,

00:46:40.180 | that it's the area.

00:46:42.740 | We're just jumping around.

00:46:44.180 | What is the holographic principle?

00:46:45.380 | What does that mean?

00:46:46.420 | Is there some kind of weird projection going on?

00:46:48.540 | What the heck?

00:46:50.260 | - Well, I was just before I came here

00:46:52.300 | writing an introduction to a paper

00:46:53.860 | and the first sentence was,

00:46:55.300 | the as yet imprecisely defined holographic principle.

00:47:01.180 | Blah, blah, blah, blah, blah.

00:47:05.580 | So nobody knows exactly what it is,

00:47:08.740 | but roughly speaking, it says just what we were alluding to

00:47:13.420 | that really all the information that is stored

00:47:18.780 | in some volume of space time can be stored

00:47:22.740 | on the boundary of that region.

00:47:25.020 | - So this is not just about black holes,

00:47:27.980 | it's about any area of space time.

00:47:29.740 | - Any area of space time.

00:47:30.660 | However, we've made sense of the holographic principle

00:47:33.980 | for black holes.

00:47:35.220 | We've made sense of the holographic principle

00:47:38.660 | for something which could be called anti-de Sitter space,

00:47:42.020 | which could be thought of as a giant,

00:47:47.460 | as a black hole turned into a whole universe.

00:47:49.860 | And we don't really understand how to talk

00:47:57.380 | about the holographic principle for either flat space,

00:48:02.500 | which we appear to live in,

00:48:04.760 | or asymptotically de Sitter space,

00:48:08.020 | which astronomers tell us we actually live in

00:48:11.100 | as the universe continues to expand.

00:48:14.860 | So it's one of the huge problems in physics

00:48:19.860 | is to apply or even formulate the holographic principle

00:48:26.340 | for more realistic, well, black holes are realistic,

00:48:32.900 | we see them, but yeah, in more general context.

00:48:37.900 | So a more general statement of the holographic principle.

00:48:41.500 | - What's the difference between flat space

00:48:43.100 | and asymptotic de Sitter space?

00:48:46.020 | So flat space is just an approximation

00:48:47.980 | of the world we live in.

00:48:49.500 | So de Sitter space, asymptotic,

00:48:52.780 | I wonder what that even means,

00:48:53.940 | meaning asymptotic over what?

00:48:57.860 | - Okay, so for thousands of years,

00:49:00.460 | until the last half of the 20th,

00:49:03.100 | well, sorry, until the 20th century,

00:49:04.940 | we thought space time was flat.

00:49:10.420 | - Can you elaborate on flat?

00:49:13.060 | What do we mean by flat?

00:49:14.660 | - Well, like the surface of this table is flat.

00:49:20.820 | Let me just give an intuitive explanation.

00:49:23.720 | Surface of the table is flat,

00:49:25.740 | but the surface of a basketball is curved.

00:49:29.420 | So the universe itself could be flat

00:49:35.420 | like the surface of a table,

00:49:37.320 | or it could be curved like a basketball,

00:49:39.640 | which actually has a positive curvature.

00:49:42.460 | And then there's another kind of curvature

00:49:44.260 | called the negative curvature.

00:49:46.580 | And curvature can be even weirder

00:49:48.820 | because that kind of curvature I've just described

00:49:53.380 | is the curvature of space,

00:49:55.640 | but Einstein taught us that we really live

00:49:57.740 | in a space-time continuum,

00:50:00.180 | so we can have curvature in a way

00:50:02.140 | that mixes up space and time.

00:50:04.200 | And that's kind of hard to visualize.

00:50:07.860 | - 'Cause you have to step, what, a couple of dimensions up,

00:50:10.600 | so it's hard to--

00:50:11.960 | - You have to step a couple,

00:50:13.880 | but even if you have flat space

00:50:18.880 | and it's expanding in time,

00:50:21.600 | we could imagine we're sitting here,

00:50:24.880 | this room, good approximation, it's flat,

00:50:27.100 | but imagine we suddenly start getting

00:50:28.680 | further and further apart.

00:50:30.180 | Then space is flat,

00:50:32.560 | but it's expanding,

00:50:35.520 | which means that space-time is curved.

00:50:38.400 | - Ultimately it's about space-time.

00:50:40.680 | Okay, so what's the de Sitter and anti-de Sitter space?

00:50:43.600 | - The three simplest space-times are flat space-time,

00:50:49.800 | which we call Minkowski space-time,

00:50:52.360 | and negatively curved space-time, anti-de Sitter space,

00:50:57.280 | and positively curved space-time, de Sitter space.

00:51:04.480 | Astronomers think that on large scales,

00:51:09.480 | even though for thousands of years we hadn't noticed it,

00:51:14.240 | beginning with Hubble,

00:51:16.240 | we started to notice that space-time was curved.

00:51:19.940 | Space is expanding in time,

00:51:22.840 | means that space-time is curved.

00:51:24.960 | And the nature of this curvature

00:51:29.320 | is affected by the matter in it,

00:51:31.960 | because matter itself causes the curvature of space-time.

00:51:36.960 | But as it expands, the matter gets more and more diluted.

00:51:42.520 | And one might ask, when it's all diluted away,

00:51:46.320 | is space-time still curved?

00:51:48.880 | And astronomers believe they've done

00:51:54.100 | precise enough measurements to determine this,

00:51:57.560 | and they believe that the answer is yes,

00:52:00.480 | the universe is now expanding.

00:52:02.580 | Eventually all the matter in it will be expanded away,

00:52:07.580 | but it will continue to expand because,

00:52:14.360 | well, they would call it the dark energy,

00:52:16.320 | Einstein would call it a cosmological constant.

00:52:19.200 | In any case, in the far future,

00:52:23.120 | matter will be expanded away,

00:52:24.720 | and we'll be left with empty de Sitter space.

00:52:27.200 | - Okay, so there's this cosmological,

00:52:28.760 | Einstein's cosmological constant

00:52:30.920 | that now hides this thing that we don't understand

00:52:34.240 | called dark energy.

00:52:35.120 | What's dark energy?

00:52:36.400 | What's your best guess at what this thing is?

00:52:39.040 | Why do we think it's there?

00:52:40.900 | It's because it comes from the astronomers.

00:52:43.200 | - Dark energy is synonymous

00:52:46.200 | with positive cosmological constant.

00:52:48.680 | And we think it's there

00:52:55.480 | because the astronomers have told us it's there,

00:52:59.280 | and they know what they're doing.

00:53:03.760 | - And we don't know what the heck it is.

00:53:04.600 | - It's a really, really hard measurement,

00:53:06.480 | but they really know what they're doing,

00:53:09.000 | and we have no friggin' idea why it's there.

00:53:13.420 | Another big mystery.

00:53:14.880 | Another reason it's fun to be a physicist.

00:53:18.320 | And if it is there, why should it be so small?

00:53:21.700 | Why should there be so little?

00:53:24.900 | Why should it have hid itself from us?

00:53:27.920 | Why shouldn't there be enough of it

00:53:29.960 | to substantially curve the space between us and the moon?

00:53:34.620 | Why did there have to be such a small amount

00:53:39.560 | that only the crazy best astronomers

00:53:41.640 | in the world could find it?

00:53:43.640 | - Well, can't the same thing be said

00:53:45.280 | about all the constants?

00:53:48.880 | All of the, can't that be said about gravity?

00:53:51.120 | Can't that be said about the speed of light?

00:53:53.480 | Like, why is the speed of light so slow?

00:53:56.060 | - So fast.

00:53:57.100 | - So slow.

00:53:57.940 | Relative to the size of the universe, can't it be faster?

00:54:01.900 | (laughing)

00:54:04.000 | Or no?

00:54:04.900 | - Well, the speed of light is a funny one

00:54:07.060 | because you could always choose units

00:54:09.980 | in which the speed of light is one.

00:54:13.060 | You know, we measure it in kilometers per second,

00:54:15.580 | and it's 186,000, or miles per second,

00:54:19.420 | it's 186,000 miles per second.

00:54:22.880 | But if we had used different units,

00:54:25.380 | then we could make it one.

00:54:28.020 | But you can make dimensionless ratios.

00:54:31.720 | So, you know, you could say,

00:54:34.340 | why is the time scale set by the expansion of the universe

00:54:38.720 | so large compared to the time scale of a human life,

00:54:42.900 | or so large compared to the time scale

00:54:46.140 | for a neutron to decay, you know?

00:54:48.220 | - Yeah, yeah, I mean, ultimately,

00:54:50.880 | the temporal reference frame here is a human life.

00:54:53.160 | - Maybe.

00:54:54.160 | - Isn't that the important thing for us,

00:54:55.840 | descendants of apes?

00:54:57.280 | Isn't that a really important aspect of physics?

00:55:00.400 | Like, because we kind of experience the world,

00:55:03.800 | we intuit the world through the eyes

00:55:06.160 | of these biological organisms.

00:55:10.240 | - Absolutely.

00:55:11.080 | - I guess mathematics helps you escape that for a time,

00:55:14.020 | but ultimately, isn't that how you wonder about the world?

00:55:18.240 | - Absolutely.

00:55:19.280 | - That like a human life time is only 100 years?

00:55:21.760 | 'Cause if you think of everything,

00:55:23.460 | if you're able to think in, I don't know,

00:55:26.400 | in billions of years,

00:55:27.860 | then maybe everything looks way different.

00:55:32.260 | Maybe universes are born and die,

00:55:36.040 | and maybe all of these physical phenomena

00:55:39.200 | become much more intuitive than we see

00:55:41.480 | at the grand scale of general relativity.

00:55:44.500 | - Well, that is one of the,

00:55:46.720 | a little off the track here,

00:55:47.800 | but that certainly is one of the nice things

00:55:49.600 | about being a physicist is you spend a lot of time

00:55:52.840 | thinking about insides of black holes

00:55:55.760 | and billions of years in the future,

00:55:58.440 | and it sort of gets you away from the day-to-day

00:56:03.080 | into another fantastic realm.

00:56:07.620 | But I was answering your question

00:56:12.160 | about how there could be information in a black hole.

00:56:15.920 | - Yes.

00:56:17.080 | So Einstein only gave us an approximate description,

00:56:22.080 | and we now have a theory that corrects it, string theory.

00:56:26.400 | And now sort of was the moment of truth.

00:56:31.760 | Well, when we first discovered string theory,

00:56:33.800 | we knew from the get-go that string theory

00:56:36.320 | would correct what Einstein said,

00:56:38.700 | just like Einstein corrected what Newton said.

00:56:41.140 | But we didn't understand it well enough

00:56:46.940 | to actually compute the correction,

00:56:49.360 | to compute how many gigabytes there were.

00:56:51.940 | And sometime in the early '90s,

00:56:57.340 | we began to understand the mathematics

00:57:02.240 | of string theory better and better,

00:57:04.880 | and it came to the point where it was clear

00:57:08.520 | that this was something we might be able to compute.

00:57:12.080 | And it was a kind of moment of truth for string theory

00:57:16.080 | because if it hadn't given the answer

00:57:20.880 | that Bekenstein and Hawking said it had to give

00:57:27.120 | for consistency, string theory itself

00:57:30.600 | would have been inconsistent

00:57:33.000 | and we wouldn't be doing this interview.

00:57:35.280 | - Wow. (laughs)

00:57:36.920 | That's a very dramatic statement, but yes.

00:57:39.320 | - That's not the most dramatic thing.

00:57:45.240 | - I mean, okay, that's very life and death.

00:57:47.920 | You mean like, because string theory

00:57:51.240 | was central to your work at that time.

00:57:53.320 | Is that what you mean?

00:57:54.560 | - Well, string theory would have been inconsistent.

00:57:57.000 | - Yeah, okay, so that'd be--

00:57:58.400 | - String theory would have been inconsistent.

00:58:00.000 | - But those inconsistencies can give birth

00:58:01.720 | to other theories, like you said.

00:58:03.200 | - The inconsistency, right, something else could have happened.

00:58:06.000 | It would have been a major change

00:58:10.560 | in the way we think about string theory,

00:58:12.880 | and it was a good thing that one supposition

00:58:17.280 | that the world is made of strings solves two problems,

00:58:21.240 | not one, it solves the infinity problem

00:58:24.240 | and it solved the Hawking's problem.

00:58:27.200 | And also the way that it did it was very beautiful.

00:58:32.200 | It gave an alternate description.

00:58:39.160 | So alternate description of things are very common.

00:58:44.160 | I mean, we could, to take a simple example,

00:58:50.120 | this bottle of water here is 90% full.

00:58:54.920 | I could say it's 90% full.

00:58:56.400 | I could also say it's 10% empty.

00:58:59.560 | Those are obviously the same statement.

00:59:01.640 | And it's trivial to see that they're the same,

00:59:05.880 | but there are many statements that can be made

00:59:08.840 | in mathematics and mathematical physics

00:59:12.000 | that are equivalent, but might take years

00:59:15.640 | to understand that they're equivalent,

00:59:18.040 | and might take the invention or discovery

00:59:20.560 | of whole new fields of mathematics

00:59:22.440 | to prove they're equivalent.

00:59:24.600 | And this was one of those.

00:59:26.840 | We found an alternate description

00:59:30.160 | of certain black holes in string theory,

00:59:34.680 | which we could prove was equivalent,

00:59:38.280 | and it was a description of the black hole

00:59:40.960 | as a hologram that can be thought of, a holographic plate,

00:59:45.920 | that could be thought of as sitting on the surface

00:59:50.440 | of the black hole, and the interior of the black hole itself

00:59:55.040 | sort of arises as a projection,

00:59:57.480 | or the near horizon region of the black hole

01:00:00.480 | arises as a projection of that holographic plate.

01:00:05.160 | So the two descriptions were the hologram,

01:00:08.640 | the three-dimensional image, and the holographic plate.

01:00:12.680 | And the hologram is what Einstein discovered,

01:00:16.880 | and the holographic plate is what we discovered.

01:00:20.200 | And this idea that you could describe things

01:00:26.300 | very, very concretely in string theory

01:00:29.120 | in these two different languages, of course, took off

01:00:32.840 | and was applied to many different contexts

01:00:37.840 | within string theory.

01:00:40.960 | - So you mentioned the infinity problem

01:00:43.080 | and the Hawking problem.

01:00:44.600 | Which Hawking problem?

01:00:45.880 | That the black hole destroys information,

01:00:48.760 | or which Hawking problem are we talking about?

01:00:51.800 | - Well, there's really two Hawking problems.

01:00:55.080 | They're very closely related.

01:00:56.640 | One is, how does the black hole store the information?

01:01:03.060 | And that is the one that we solved in some cases.

01:01:08.060 | So it's sort of like your smartphone,

01:01:17.620 | how does it store its 64 gigabytes?

01:01:21.200 | Well, you rip the cover off and you count the chips,

01:01:23.540 | and there's 64 of them, each with a gigabyte,

01:01:26.540 | and you know there's 64 gigabytes.

01:01:29.380 | But that does not solve the problem

01:01:31.900 | of how you get information in and out of your smartphone.

01:01:34.980 | You have to understand a lot more about the Wi-Fi

01:01:39.900 | and the internet and the cellular.

01:01:42.940 | - That's where Hawking radiation, this prediction,

01:01:46.900 | it starts to--

01:01:47.740 | - That's where Hawking radiation comes in.

01:01:50.060 | And that problem of how the information gets in and out,

01:01:54.160 | you couldn't have explained how information

01:01:58.500 | gets in and out of an iPhone without first explaining

01:02:02.380 | how it's stored in the first place.

01:02:04.980 | - So just to clarify, the storage is on the plate?

01:02:08.940 | - Is on the plate.

01:02:09.940 | - On the holographic plate,

01:02:10.980 | and then it projects somehow inside the--

01:02:14.140 | - The bulk, the space-time is the hologram.

01:02:18.100 | - The hologram, but man, I mean,

01:02:20.580 | do you have an intuitive, when you sit late at night

01:02:23.540 | and you stare at the stars,

01:02:25.580 | do you have an intuitive understanding

01:02:26.860 | of what a holographic plate is?

01:02:28.820 | Like that there's two dimension,

01:02:33.580 | no projections that store information?

01:02:35.720 | - How a black hole could store information

01:02:42.060 | on a holographic plate, I think we do understand

01:02:47.820 | in great mathematical detail and also intuitively,

01:02:52.340 | and it's very much like an ordinary hologram

01:02:56.100 | where you have a holographic plate

01:02:57.660 | and it contains all the information,

01:03:00.380 | you shine a light through it,

01:03:01.940 | you get an image which looks three-dimensional.

01:03:05.900 | - Yeah, but why should there be a holographic plate?

01:03:08.460 | - Why should there be?

01:03:12.300 | - Yeah, why?

01:03:13.500 | - That is the great thing about being

01:03:17.020 | a theoretical physicist is anybody can very quickly

01:03:22.340 | stump you with a going to the next level of whys.

01:03:26.460 | - Yeah, the whys is kind of, ooh, I can just keep asking.

01:03:29.180 | - Yeah, you could just keep asking

01:03:30.540 | and it won't take you very long to,

01:03:32.660 | so the trick in being a theoretical physicist

01:03:37.820 | is finding the questions that you can answer.

01:03:41.740 | - Sure.

01:03:43.060 | - So the questions that we think we might be able

01:03:45.620 | to answer now and we've partially answered

01:03:49.100 | is that there is a holographic explanation

01:03:54.100 | for certain kinds of things in string theory.

01:03:59.420 | - Sure.

01:04:00.300 | - We've answered that.

01:04:01.860 | Now we'd like to take what we've learned

01:04:04.100 | and that's what I've mostly been doing

01:04:06.140 | for the last 15, 20 years.

01:04:09.220 | I haven't really been working so much

01:04:11.340 | on string theory proper.

01:04:12.780 | I've been sort of taking the lessons

01:04:15.500 | that we learned in string theory

01:04:19.260 | and trying to apply them to the real world

01:04:22.260 | assuming only what we know for sure about the real world.

01:04:28.740 | - So on this topic, you co-authored a paper

01:04:32.500 | with Stephen Hawking called "Soft Hair on Black Holes"

01:04:36.300 | that makes the argument against Hawking's

01:04:37.860 | original prediction that black holes destroy information.

01:04:40.760 | Can you explain this paper?

01:04:42.820 | - Yes.

01:04:43.660 | - And the title?

01:04:44.500 | - Yeah, okay, so first of all,

01:04:47.600 | the hair on black holes is a word that was coined

01:04:54.180 | by the greatest phrase master in the history

01:04:59.500 | of physics, John Wheeler, invented the word black hole.

01:05:02.780 | And he also said that, he made the statement

01:05:08.580 | that black holes have no hair.

01:05:12.040 | That is, every black hole in the universe

01:05:15.360 | is described just by its mass and spin.

01:05:21.040 | They can also rotate as was later shown by Kerr.

01:05:26.000 | And this is very much unlike a star, right?

01:05:34.560 | Every star of the same mass is different

01:05:39.800 | in a multitude of different ways.

01:05:42.940 | Different chemical compositions,

01:05:46.240 | different motions of the individual molecules.

01:05:50.400 | Every star in the universe, even of the same mass,

01:05:55.040 | is different in many, many different ways.

01:05:58.400 | Black holes are all the same.

01:06:00.400 | And that means when you throw some,

01:06:03.160 | in Einstein's description of them,

01:06:05.020 | which we think must be corrected.

01:06:08.980 | And if you throw something into a black hole,

01:06:13.560 | it gets sucked in.

01:06:15.680 | And if you throw in a red book or a blue book,

01:06:20.680 | the black hole gets a little bigger,

01:06:24.240 | but there's no way within Einstein's theory

01:06:27.820 | of telling how they're different.

01:06:30.720 | And that was one of the assumptions

01:06:36.680 | that Hawking made in his 1974, '75 papers,

01:06:41.680 | in which he concluded that black holes destroy information.

01:06:48.780 | You can throw encyclopedias, thesis defenses,

01:06:52.600 | the Library of Congress.

01:06:54.480 | - Doesn't matter, it's going to behave

01:06:56.780 | exactly the same uniform way.

01:06:58.560 | - Yeah, so what Hawking and I showed,

01:07:03.120 | and also Malcolm Perry,

01:07:05.780 | is that one has to be very careful

01:07:10.780 | about what happens at the boundary of the black hole.

01:07:17.400 | And this gets back to something I mentioned earlier

01:07:21.640 | about when two things which are related

01:07:24.680 | by a coordinate transformation are and are not equivalent.

01:07:31.060 | And what we showed is that there are very subtle imprints

01:07:36.060 | when you throw something into a black hole,

01:07:41.260 | there are very subtle imprints left

01:07:43.620 | on the horizon of the black hole,

01:07:45.900 | which you can read off at least partially what went in.

01:07:50.740 | And so this invalidates Stephen's original argument

01:07:57.280 | that the information is destroyed.

01:08:02.280 | - And that's the soft hair, those imprints.

01:08:04.960 | - That's the soft hair, right.

01:08:06.840 | So, and soft is a word that is used in physics

01:08:10.400 | for things which have very low energy.

01:08:14.000 | And these things actually carry no energy.

01:08:17.160 | There are things in the universe which carry no energy.

01:08:20.600 | - You said, I think to Sean Carroll,

01:08:24.840 | by the way, everyone should go check out

01:08:26.320 | Sean Carroll's Mindscape podcast, it's incredible.

01:08:29.560 | And Sean Carroll's an incredible person.

01:08:31.320 | I think you said there, maybe in a paper, I have a quote.

01:08:34.200 | You said that a soft particle is a particle

01:08:36.760 | that has zero energy, just like you said now.

01:08:40.200 | And when the energy goes to zero,

01:08:41.840 | because the energy is proportionate to the wavelength,

01:08:44.600 | it's also spread over an infinitely large distance.

01:08:48.200 | If you like, it's spread over the whole universe.

01:08:51.320 | It somehow runs off to the boundary.

01:08:53.760 | What we learned from that is that

01:08:55.720 | if you add a zero energy particle to the vacuum,

01:08:58.120 | you get a new state.

01:08:59.480 | And so, there are infinitely many vacua,

01:09:03.320 | plural for vacuum, which can be thought of

01:09:07.320 | as being different from one another

01:09:10.080 | by the addition of soft photons or soft gravitons.

01:09:13.840 | Can you elaborate on this wild idea?

01:09:17.500 | If you like, it spreads over the whole universe.

01:09:21.920 | When the energy goes to zero,

01:09:23.120 | because the energy is proportionate to the wavelength,

01:09:25.440 | it also spreads over an infinitely large distance.

01:09:27.600 | If you like, it's spread over the whole universe.

01:09:30.280 | It's spread over the whole universe.

01:09:32.080 | Can you explain these soft gravitons and photons?

01:09:37.360 | - Yeah, so the soft gravitons and photons

01:09:40.520 | have been known about since the '60s,

01:09:46.840 | but exactly what we're supposed to do with them

01:09:51.280 | or how we're supposed to think about them,

01:09:53.680 | I think, has been well understood only recently.

01:10:00.080 | And in quantum mechanics, the energy of a particle

01:10:06.960 | is proportional to Planck's constant times its wavelength.

01:10:11.020 | So, when the energy goes to zero,

01:10:14.240 | the wavelength goes to infinity.

01:10:17.800 | Now, if something has zero energy

01:10:22.800 | and it's spread all over the universe,

01:10:26.240 | in what sense is it actually there?

01:10:28.600 | That's been the confusing thing,

01:10:32.840 | to make a precise statement

01:10:36.000 | about when something is and isn't there.

01:10:39.880 | Now, the simplest way of seeing,

01:10:43.040 | so people might have taken the point of view

01:10:47.960 | that if it has zero energy

01:10:51.440 | and it's spread all over the universe,

01:10:53.760 | it's not there, we can ignore it.

01:10:55.620 | But if you do this, you'll get into trouble.

01:11:03.240 | And one of the ways that you'll get into trouble

01:11:05.680 | is that even though it has zero energy,

01:11:08.600 | it doesn't have zero angular momentum.

01:11:10.880 | If it's a photon, it always has angular momentum one.

01:11:15.400 | If it's a graviton, it's angular momentum two.

01:11:19.480 | So, you can't say that the state of the system

01:11:26.600 | with the zero energy photon

01:11:29.840 | should be identified with the one

01:11:32.280 | without the zero energy photon,

01:11:35.160 | that we can just ignore them,

01:11:36.360 | because then you will conclude

01:11:38.520 | that angular momentum is not conserved.

01:11:40.840 | And if angular momentum is not conserved,

01:11:44.680 | things won't be consistent.

01:11:46.080 | And of course, you can have a lot of these things,

01:11:51.960 | and typically you do get a lot of them.

01:11:54.800 | And when you, you can actually do a calculation

01:11:57.820 | that shows that every time you scatter two particles,

01:12:02.460 | you create an infinite number of them.

01:12:04.680 | - Infinite number of the soft photons and gravitons?

01:12:06.680 | - Of the zero energy ones, yeah.

01:12:09.040 | - And so these are, and they're somehow everywhere.

01:12:11.920 | - They're everywhere.

01:12:12.760 | - But they also contain information,

01:12:14.360 | or they're able to store information?

01:12:16.200 | - And they're able to store information.

01:12:17.840 | They're able to store an arbitrary

01:12:20.120 | large amount of information.

01:12:22.320 | So, what we pointed out is,

01:12:25.840 | so what these things really do,

01:12:27.400 | one way of thinking of them is they rush off

01:12:29.440 | to the edges of the universe,

01:12:31.560 | spreading out all over the space.

01:12:33.360 | It's like saying they rush off

01:12:34.760 | to the energy edge of the universe.

01:12:37.520 | And that includes, if the interior of the black hole

01:12:40.880 | is not considered part of the universe,

01:12:42.800 | that includes the edge of the black hole.

01:12:44.900 | So we need to set up our description of physics

01:12:50.920 | so that all the things that are conserved

01:12:54.600 | are still conserved in the way that we're describing them.

01:12:59.560 | And that will not be true if we ignore these things.

01:13:01.920 | We have to keep careful track of these things.

01:13:04.240 | And people had been sloppy about that,

01:13:07.880 | and we learned how to be very precise and careful about it.

01:13:11.080 | - And once you're being precise,

01:13:13.120 | you can actually answer this kind of very problematic thing

01:13:18.040 | that Hawking suggested, that black holes destroy information.

01:13:21.120 | - Well, what we showed is that there's an error

01:13:25.760 | in the argument that all black holes are the same

01:13:30.040 | because they hadn't kept track of these,

01:13:32.600 | these very subtle things.

01:13:36.160 | And whether or not this is the key error

01:13:41.160 | in the argument remains to be seen,

01:13:45.440 | or whether this is a technical point.

01:13:47.560 | - Yes, but it is an error.

01:13:49.440 | - It is an error.

01:13:50.480 | - And Hawking obviously agreed with it.

01:13:53.880 | - Hawking agreed with it, and he was sure that this was the,

01:13:57.440 | he was sure that this was--

01:13:59.400 | - This was a critical error.

01:14:00.400 | - That this was the critical error,

01:14:01.760 | and that understanding this would,

01:14:05.280 | would get us the whole story, and that could well be.

01:14:09.520 | - What was it like working with Stephen Hawking

01:14:11.400 | on this particular problem?

01:14:13.480 | Because it's kind of a whole journey, right?

01:14:16.080 | - Well, you know, I love the guy.

01:14:19.120 | He's so passionate about physics.

01:14:24.120 | He just, yeah, his,

01:14:29.880 | his oneness with the problem,

01:14:33.680 | and I mean, it's--

01:14:36.440 | - So his mind is all occupied by the world that's--

01:14:39.760 | - Yeah, and let me tell you,

01:14:41.400 | there's a lot of other things,

01:14:43.040 | with his illness and with his celebrity,

01:14:45.280 | and a lot of other things.

01:14:48.000 | - A lot of distractions pulling at his,

01:14:51.480 | at his mind, he's still there.

01:14:54.120 | He's still right there. - That's right, that's right.

01:14:55.320 | I remember him turning down tea with Lady Gaga

01:14:59.080 | so we could spend another hour on our paper.

01:15:01.880 | (both laughing)

01:15:04.560 | - That, my friends, is dedication.

01:15:08.880 | What did you learn about physics?

01:15:10.520 | What did you learn about life

01:15:12.240 | from having worked with Stephen Hawking?

01:15:15.320 | - Well, he was one of my great teachers.

01:15:16.840 | Of course, he's older than me,

01:15:18.440 | and I was reading his textbooks in graduate school,

01:15:28.080 | and I learned a lot about relativity from him.

01:15:33.080 | I learned about passion for a problem.

01:15:37.680 | I learned about not caring what other people think.

01:15:42.680 | Physics is an interesting culture.

01:15:49.480 | Even if you make a great discovery like Hawking did,

01:15:55.680 | people don't believe everything you say.

01:15:58.760 | In fact, people love to disagree.

01:16:01.480 | It's a culture that cherishes disagreement,

01:16:08.440 | and so he kept ahead with what he believed in,

01:16:12.560 | and sometimes he was right, and sometimes he was wrong.

01:16:15.800 | - Do you feel pressure from the community?

01:16:17.280 | So, for example, with string theory,

01:16:19.720 | it was very popular for a time.

01:16:21.440 | There's a bit of criticism, or it's less popular now.

01:16:25.520 | Do you feel the forces of the community

01:16:27.600 | as it moves in and out of different fields,

01:16:29.920 | or do you try to stay, like how difficult is it

01:16:32.240 | to stay intellectually and mathematically independent

01:16:35.920 | from the community?

01:16:36.880 | - Personally, I'm lucky I'm well-equipped for that.

01:16:44.760 | When I started out in graduate school,

01:16:54.520 | the problem of quantum gravity

01:16:56.000 | was not considered interesting.

01:17:01.000 | - You still did it anyway.

01:17:02.480 | - I still did it anyway.

01:17:04.440 | I'm a little bit of a contrarian, I guess,

01:17:06.960 | and I think that has served me well.

01:17:09.920 | People are always sort of disagreeing with me,

01:17:18.160 | and they're usually right, but I'm right enough.

01:17:22.520 | - And like you said, the contradiction

01:17:24.520 | ultimately paves the path of discovery.

01:17:26.560 | Let me ask you, just on this tension,

01:17:29.360 | we've been dancing between physics and mathematics.

01:17:32.440 | What to you is an interesting line

01:17:35.220 | you can draw between the two?

01:17:36.960 | You have done some very complicated mathematics

01:17:39.400 | in your life to explore the laws of nature.

01:17:41.800 | What's the difference between physics and mathematics to you?

01:17:46.800 | - Well, I love math.

01:17:50.640 | I think my first love is physics,

01:17:53.240 | and the math that I've done, I've done to,

01:17:57.460 | because it was needed.

01:17:58.700 | - In service of physics.

01:18:00.960 | - In service of physics, but then, of course,

01:18:03.480 | in the heat of it, it has its own appeal.

01:18:08.480 | - In the heat of it, I like it, sure.

01:18:12.160 | - It has its own appeal, and I certainly enjoyed it.

01:18:15.760 | And ultimately, I would like to think,

01:18:19.060 | I wouldn't say I believe, but I would like to think

01:18:24.060 | that there's no difference between physics and mathematics,

01:18:27.760 | that all mathematics is realized in the physical world,

01:18:32.760 | and all physics has a firm mathematical basis,

01:18:38.360 | that they're really the same thing.

01:18:39.720 | I mean, why would there be math

01:18:42.320 | that had no physical manifestation?

01:18:47.680 | It seems a little odd, right?

01:18:49.340 | You have two kinds of math,

01:18:51.180 | some that are relevant to the real world.

01:18:54.260 | - Well, they don't have to be contradictory,

01:18:55.580 | but you can have, can't you not have mathematical objects

01:18:58.740 | that are not at all connected to the physical world?

01:19:01.480 | So, I mean, this is to the question

01:19:02.780 | of is math discovered or invented?

01:19:04.660 | So, to you, math is discovered,

01:19:08.580 | and there's a deep linkage between the two.

01:19:11.780 | - Yeah, yeah, yeah.

01:19:13.100 | - Do you find at all compelling these ideas,

01:19:16.020 | like something like Max Tegmark,

01:19:18.080 | where our universe is actually

01:19:19.880 | a fundamentally mathematical object,

01:19:22.440 | that math is, our universe is mathematical,

01:19:25.760 | fundamentally mathematical in nature?

01:19:27.760 | - My expertise is a physicist

01:19:33.160 | doesn't add anything to that.

01:19:35.820 | It's not really, you know, physics is,

01:19:40.840 | you know, I was once very interested in philosophy,

01:19:46.280 | and, you know, physics,

01:19:50.280 | physics, I like questions that can be answered,

01:19:54.680 | that it's not obvious what the answer is,

01:19:57.920 | and that you can find an answer to the question,

01:20:02.320 | and everybody will agree what the answer is,

01:20:06.080 | and that there's an algorithm for getting there.

01:20:09.040 | Not that these other questions aren't interesting,

01:20:14.520 | and they don't somehow have a way

01:20:16.160 | of presenting themselves,

01:20:18.160 | but to me, the interesting thing is to,

01:20:21.860 | is motion in what we know, is learning more,

01:20:27.480 | and understanding things

01:20:29.160 | that we didn't understand before,

01:20:31.400 | things that seemed totally confusing

01:20:34.080 | having them seem obvious, that's wonderful.

01:20:36.440 | So, I think that those questions are there.

01:20:40.320 | I mean, I would even go further, you know,

01:20:43.440 | the whole multiverse, I don't think there's too much

01:20:48.240 | concrete we're ever gonna be able to say about it.

01:20:52.520 | - This is fascinating, because you spend so much time

01:20:54.720 | in string theory, which is devoid from a connection

01:20:58.720 | to the physical world for a long time,

01:21:01.200 | like, not devoid, but it travels in a mathematical world

01:21:04.680 | that seems to be beautiful and consistent,

01:21:06.880 | and seems to indicate that it could be

01:21:11.400 | a good model of the laws of nature,

01:21:13.960 | but it's still traveling independently,

01:21:17.120 | 'cause it's very difficult to experimentally verify,

01:21:19.520 | but there's a promise laden in it,

01:21:21.520 | in the same way multiverse, or you can have a lot of kind of

01:21:25.280 | very far out there questions,

01:21:27.600 | where your gut and instinct and intuition says

01:21:30.000 | that maybe in 50, 100, 200 years,

01:21:32.080 | you'll be able to actually have

01:21:33.960 | strong experimental validation, right?

01:21:38.000 | - I think that with string theory,

01:21:40.960 | I don't think it's likely that we could measure it,

01:21:45.960 | but we could get lucky.

01:21:49.400 | In other words, just to take an example,

01:21:53.120 | about 10 or 20 years ago, it was thought that

01:21:56.880 | they had seen a string in the sky,

01:21:59.920 | and that it was seen by, you know,

01:22:04.920 | doubled stars that were gravitationally lensed

01:22:08.000 | around the gravitational field

01:22:10.320 | produced by some long string.

01:22:11.920 | There was a line of double lensed,

01:22:13.520 | now the signal went away, okay?

01:22:15.920 | But people were hoping that they'd seen a string,

01:22:19.640 | and it could be a fundamental string

01:22:21.280 | that somehow gotten stretched,

01:22:23.120 | and that would be some evidence for string theory.

01:22:26.580 | There was also BICEP2, which,

01:22:29.680 | it was, the experiment was wrong,

01:22:34.880 | but it could have happened.

01:22:37.920 | It could have happened that we got lucky,

01:22:39.800 | and this experiment was able to make direct measurements.

01:22:43.400 | Certainly would have been measurements of quantum gravity,

01:22:46.040 | if not string theory.

01:22:48.040 | So it's a logical, it's a very logical possibility

01:22:51.800 | that we could get experimental evidence from string.

01:22:54.960 | That is a very different thing than saying,

01:22:58.600 | do this experiment, here's a billion dollars,

01:23:02.080 | and after you do it,

01:23:02.940 | we'll know whether or not strings are real.

01:23:05.480 | But I think it's a crucial difference.

01:23:08.600 | It's measurable in principle,

01:23:11.280 | and we don't see how to get from here to there.

01:23:16.280 | If we see how to get from here to there,

01:23:21.840 | in my eyes, it's boring, right?

01:23:25.000 | So when I was a graduate student,

01:23:27.540 | they knew how to measure the Higgs boson.

01:23:32.040 | Took 40 years, but they didn't,

01:23:36.000 | not to say that stuff is boring.

01:23:38.480 | I don't want to say that stuff is boring.

01:23:40.400 | But I, you know, when Magellan set out,

01:23:44.600 | he didn't know he could get around the world.

01:23:47.440 | There was no map, you know?

01:23:49.600 | So I don't know how we're gonna

01:23:51.680 | connect in a concrete way

01:23:56.840 | all these ideas of string theory to the real world.

01:24:01.300 | And, you know, when I started out in graduate school,

01:24:04.360 | I said, what is the most interesting problem

01:24:09.200 | that there might be, the deepest, most interesting problem

01:24:12.360 | that there might be progress on in 60 years?

01:24:16.080 | And I think it could be, you know,

01:24:22.560 | that, you know, in another 30 years,

01:24:26.320 | that maybe we'll learn that we have understood

01:24:30.160 | how black holes store information, you know?

01:24:32.980 | That doesn't seem wild,

01:24:35.500 | that we're able to abstract what we learned

01:24:38.260 | from string theory and show that it's operative.

01:24:41.460 | And, you know, I mean, the Bose-Einstein condensate,

01:24:46.740 | they did, you know, when Bose and Einstein predicted it,

01:24:50.580 | when was that, the '30s maybe, early '30s?

01:24:56.440 | It took, there were 20 orders of magnitude

01:25:00.020 | that were needed in order to,

01:25:02.500 | an improvement in order to measure it,

01:25:05.320 | and they did, 50 years later.

01:25:08.540 | So, and you couldn't have guessed how that had happened,

01:25:11.900 | how they could have gotten that.

01:25:14.700 | And it could happen that we,

01:25:16.620 | I don't think we're gonna like see

01:25:19.220 | the heterotic string spectrum at an accelerator,

01:25:21.980 | but it could be that things come around

01:25:27.380 | and in an interesting way, and somehow it comes together.

01:25:32.380 | And the fact that we can't see to the end

01:25:35.620 | isn't a reason not to do it, you know?

01:25:39.740 | We're just, you know, what did they do

01:25:41.780 | when they were trying to find the Pacific, right?

01:25:44.780 | They just, they took every route.

01:25:46.500 | They just tried everything, and that's what we're doing.

01:25:49.420 | And we're taking, and I'm taking the one

01:25:52.340 | that my nose tells me is the best, you know?

01:25:56.060 | And other people are taking other ones, and that's good,

01:25:58.780 | because we need every person taking every route.

01:26:03.300 | And, you know, if somebody on another route

01:26:05.900 | finds something that looks really promising,

01:26:09.980 | you know, I'm gonna make a portage over the mountain

01:26:12.420 | and get on their stream, you know?

01:26:14.660 | So, the fact that you don't see the experiment now

01:26:19.660 | isn't, to me, a reason to give up on what I view

01:26:24.660 | as the most fundamental paradox in 20th century,

01:26:29.660 | 20th, in present physics, 20, 21st century physics.

01:26:33.340 | - Absolutely, you can see that it's possible.

01:26:35.580 | You just don't know the way.

01:26:37.100 | But that's what I mean,

01:26:38.140 | why some of the philosophical questions could be formulated

01:26:40.940 | in a way that's explorable scientifically.

01:26:43.540 | So, some of the stuff we've talked about,

01:26:46.500 | but, you know, for example,

01:26:49.100 | this topic that's become more okay to talk about,

01:26:52.460 | which is the topic of consciousness.

01:26:54.340 | You know, to me, as an artificial intelligence person,

01:26:57.460 | that's a very practically interesting topic.

01:26:59.740 | But there's also philosophers.

01:27:02.500 | Sean Carroll loves to argue against them.

01:27:05.740 | But there's some philosophers that are panpsychists.

01:27:07.780 | - I'm not against philosophers, it's just not as fun.

01:27:10.260 | I don't-- - It's not as fun, right.

01:27:11.820 | (both laughing)

01:27:13.260 | But they start a little flame of a fire going

01:27:18.260 | that some of those flames, I think,

01:27:21.380 | eventually become physics.

01:27:23.500 | So, eventually become something that we can really,

01:27:26.620 | like, having them around is really important

01:27:29.300 | because you'll discover something by modeling

01:27:32.420 | and exploring black holes that's really weird.

01:27:34.820 | And having these ideas around,

01:27:37.660 | like the ideas of panpsychists,

01:27:39.740 | that consciousness could be a fundamental force of nature.

01:27:43.020 | Just even having that crazy idea,

01:27:44.780 | swimming around in the background,

01:27:46.660 | could really spark something where

01:27:49.540 | that you were missing something completely.

01:27:51.700 | And it's just, that's where the philosophy done right,

01:27:56.300 | I think, is very useful.

01:27:58.620 | That's where even the, you know, these thought experiments,

01:28:01.020 | which is very fun in sort of the tech sci-fi world

01:28:04.420 | that we live in a simulation,

01:28:05.980 | that, you know, taking a perspective of the universe

01:28:11.180 | as a computer, as a computational system

01:28:14.620 | that processes information,

01:28:16.100 | which is a pretty intuitive notion,

01:28:18.040 | but you can just even reframing it that way for yourself,

01:28:21.260 | could really open up some different way of thinking.

01:28:25.140 | - Could be.

01:28:26.140 | - And then you have, I don't know if you're familiar

01:28:29.260 | with Stephen Wolfram's work

01:28:30.780 | of like cellular automata and complexity.

01:28:32.860 | - Yeah, I did a podcast with Stephen.

01:28:34.980 | - With Stephen, that's awesome.

01:28:36.260 | I mean, to me, forget physics, forget all that.

01:28:39.320 | Cellular automata make no sense.

01:28:43.500 | They're so beautiful.

01:28:44.340 | They're so, that from simple rules,

01:28:46.540 | you can create complexity.

01:28:48.480 | I just don't think, you know,

01:28:50.000 | he wrote a book, "A New Kind of Science,"

01:28:52.040 | basically hinting at, which a lot of people have hinted at,

01:28:57.680 | is like, we don't have a good way

01:28:59.280 | to talk about these objects.

01:29:01.000 | We can't figure out what is happening here.

01:29:03.000 | These simple, these trivial rules

01:29:04.920 | can create incredible complexity.

01:29:06.960 | - He's totally right about that, yeah.

01:29:08.920 | - And physicists, I guess, don't have,

01:29:12.460 | don't know what to do with that.

01:29:14.360 | Don't know what to do with cellular automata,

01:29:16.440 | 'cause you can describe the simple rules

01:29:18.620 | that govern the system,

01:29:20.440 | but how complexity can emerge, like incredible complexity.

01:29:24.840 | - Yeah.

01:29:25.680 | Of course, Wolfram's version of that

01:29:28.140 | is that physicists will never be able to describe it.

01:29:31.220 | - Right, yeah, exactly.

01:29:32.900 | He tries to prove that it's impossible.

01:29:35.300 | What do you make of that?

01:29:36.380 | What do you make about the tension of being a physicist

01:29:40.500 | and potentially not being able to,

01:29:43.400 | it's like Freud or somebody

01:29:45.460 | that maybe, Sigmund Freud,

01:29:47.900 | maybe you'll never be able to actually describe

01:29:49.720 | the human psyche.

01:29:50.740 | Is that a possibility for you?

01:29:54.100 | That you will never be able to get to the core,

01:29:56.260 | fundamental description of the laws of nature?

01:29:58.760 | - Yeah, so I had this conversation with Weinberg.

01:30:05.040 | (laughing)

01:30:07.400 | - Yeah, how'd it go?

01:30:08.840 | - So Weinberg has this book called "Dreams of a Final Theory."

01:30:14.940 | - Yeah.

01:30:16.260 | - And I had this conversation with him.

01:30:18.140 | I said, "Why do you think

01:30:20.540 | "there's ever gonna be a final theory?

01:30:23.700 | "Why should there ever be a final theory?

01:30:25.420 | "I mean, what does that mean?

01:30:26.540 | "Do physics departments shut down?

01:30:28.260 | "We've solved everything?"

01:30:29.800 | And, you know,

01:30:32.960 | doesn't it seem that every time

01:30:35.980 | we answer some old questions,

01:30:37.660 | we'll just find new ones

01:30:40.100 | and that it will just keep going on forever and ever?

01:30:43.220 | He said, "Well, that's what they used to say

01:30:45.580 | "about the Nile.

01:30:46.580 | "They were never gonna find the end."

01:30:48.780 | (laughing)

01:30:51.140 | Then one day they found it.

01:30:53.100 | - Yeah.

01:30:53.940 | - So I don't know.

01:30:55.320 | String theory doesn't,

01:30:58.540 | string theory doesn't look like a candidate to me

01:31:02.940 | for a final theory.

01:31:05.260 | - As it stands now.

01:31:08.180 | It doesn't get to the bottom of the well

01:31:09.740 | and to the sides and to the whole thing.

01:31:11.340 | - Yeah, it seems to me that even if we kind of solved it

01:31:14.620 | and we did experiments,

01:31:17.180 | there still would be more questions,

01:31:18.780 | like why are there four dimensions instead of six?

01:31:21.540 | It doesn't seem to have anything in it

01:31:24.700 | that would explain that.

01:31:26.780 | You can always hope, you know,

01:31:28.980 | that there's something that we don't know

01:31:30.300 | about string theory that will explain it,

01:31:31.940 | but it still doesn't look like

01:31:35.100 | it's gonna answer every question.

01:31:37.720 | And why is there one time, not two?

01:31:42.080 | You know, why is there,

01:31:43.800 | you know, it doesn't seem like it's,

01:31:45.800 | I don't even know what it would mean

01:31:48.360 | to answer every question.

01:31:49.720 | - Well, to answer every question, obviously,

01:31:52.480 | so when you refer to the theory of everything,

01:31:55.520 | you'll be able to have a,

01:31:56.760 | if it exists, it would be a theory

01:32:01.000 | that allows you to predict precisely

01:32:03.640 | the behavior of objects in the universe

01:32:06.680 | and their movement, right?

01:32:09.040 | What about them, their movement?

01:32:11.760 | - Yeah.

01:32:12.600 | - Like precisely, no matter the object.

01:32:14.200 | - Right, that's true.

01:32:15.200 | So that would be a really interesting state of affairs

01:32:19.040 | if we could predict everything

01:32:22.080 | but not necessarily understand everything.

01:32:26.260 | So for example, let's just forget about gravity.

01:32:31.020 | I mean, we're not too far from that situation.

01:32:34.040 | If we forget about gravity,

01:32:36.080 | the standard model, in principle,

01:32:39.640 | given a big enough computer,

01:32:41.480 | predicts almost everything.

01:32:45.280 | But if you look at the standard model,

01:32:49.240 | it's kind of a laundry list

01:32:50.760 | with neutrino masses and all that stuff.

01:32:53.360 | There are hundreds of free parameters.

01:32:55.280 | Where do they come from?

01:32:56.700 | Is there an organizing principle?

01:33:00.440 | Is there some further unification?

01:33:02.560 | - Sure.

01:33:03.760 | - So being able to predict everything

01:33:07.840 | is not the only goal that physicists have.

01:33:10.520 | - So on the way to trying to predict,

01:33:13.400 | you're trying to understand.

01:33:14.360 | That's actually probably the goal is to understand.

01:33:16.320 | - Yeah.

01:33:17.760 | But right, we're more interested in understanding

01:33:20.800 | than actually doing the predictions

01:33:25.080 | but the predictions are more,

01:33:27.260 | focusing on how to make predictions

01:33:29.920 | is a good way to improve your understanding

01:33:33.560 | because you know you've understood it

01:33:35.120 | if you can do the predictions.

01:33:36.520 | - Yeah, one of the interesting things

01:33:39.000 | that might come to a head with is artificial intelligence.

01:33:43.220 | There's an increasing use of AI in physics.

01:33:47.800 | We might live in a world where AI

01:33:50.320 | would be able to predict perfectly what's happening.

01:33:53.520 | And so, as physicists, you'll have to come

01:33:56.800 | to the fact that you're actually

01:33:59.640 | not that interested in prediction.

01:34:01.400 | I mean, it's very useful but you're interested

01:34:04.640 | in really understanding the deep laws of nature

01:34:06.800 | versus a perfect predictor.

01:34:08.000 | - Yeah, yeah.

01:34:08.840 | - Like you wanna play chess.

01:34:09.680 | - But even within AI, AI people are trying

01:34:12.560 | to understand what it is that the AI bots have learned

01:34:16.880 | in order to produce whatever they produce.

01:34:21.880 | - For sure but you still don't understand deeply

01:34:24.120 | especially because they're getting,

01:34:25.820 | especially language models if you're paying attention,

01:34:30.160 | the systems that are able to generate text,

01:34:32.000 | they're able to have conversations,

01:34:33.200 | Chad GPT is the recent manifestation of that.

01:34:36.080 | They just seem to know everything.

01:34:38.740 | They're trained on the internet.

01:34:40.320 | They seem to be very, very good

01:34:43.080 | at something that looks like reasoning.

01:34:46.520 | They're able to generate, you can ask them questions,

01:34:49.120 | they can answer questions.

01:34:50.080 | It just feels like this thing is intelligent.

01:34:53.760 | And I could just see that being possible with physics.

01:34:56.520 | You ask any kind of physical question

01:34:58.240 | and it'll be able to, very precise

01:35:00.240 | about a particular star system or a particular black hole.

01:35:03.240 | It'll say, well, these are the numbers you see.

01:35:05.200 | It'll perfectly predict and then sure,

01:35:08.240 | you can understand how the neural network is,

01:35:12.440 | the architecture is structured.

01:35:13.840 | Actually, for most of them now, they're very simple.

01:35:16.240 | You can understand what data is trained on,

01:35:18.120 | huge amount of data.

01:35:19.200 | You're getting a huge amount of data

01:35:20.680 | from a very nice telescope or something.

01:35:23.200 | And then, but it seems to predict everything perfectly.

01:35:26.640 | How a banana falls when you throw it.

01:35:30.400 | Everything is perfectly predicted.

01:35:32.400 | You still don't have a deep understanding

01:35:33.980 | of what governs the whole thing.

01:35:35.580 | And maybe you can ask it a question.

01:35:38.720 | It'll be some kind of Hitchhiker's Guide

01:35:40.220 | to the Galaxy type answer.

01:35:41.700 | It's a funny world we live in.

01:35:46.440 | Of course, it's also possible that there's no such deep,

01:35:49.340 | simple governing laws of nature.

01:35:53.100 | Behind the whole thing.

01:35:54.540 | I mean, there's something in us humans--

01:35:57.060 | - It's possible.

01:35:57.900 | - That wants it there to be.

01:35:59.980 | - Yeah.

01:36:00.820 | - But doesn't have to be, right?

01:36:01.660 | - Right.

01:36:02.500 | - I do, again, you're betting, you already bet the farm.

01:36:06.340 | But if you were to have a second farm,

01:36:08.620 | do you think there is a theory of everything

01:36:10.260 | that we might get at?

01:36:11.100 | So, simple laws that govern the whole thing.

01:36:13.840 | - I don't, I don't, honestly, I don't know.

01:36:21.860 | But I'm pretty confident that if there is,

01:36:25.380 | we won't get to it in my lifetime.

01:36:27.560 | I don't think we're near it.

01:36:30.200 | - But doesn't it feel like there,

01:36:31.380 | like the fact that we have the laws we do,

01:36:33.940 | they're relatively simple already.

01:36:36.300 | That's kind of incredible.

01:36:37.580 | It's just, there seems to be,

01:36:39.420 | there seems to be simple laws that govern things, right?

01:36:42.760 | - By a theory of everything, you mean a theory of,

01:36:47.980 | of everything, an algorithm to predict everything.

01:36:52.420 | - But a simple algorithm.

01:36:54.500 | - A relatively simple algorithm to predict everything.

01:36:58.060 | So, for me, it would be a sad day

01:37:00.220 | if we arrived at that without answering

01:37:04.580 | some deeper questions.

01:37:06.040 | - Sure, of course, it definitely is.

01:37:08.820 | But the question, yes.

01:37:10.900 | But one of the questions before we arrive there,

01:37:14.580 | we can ask, does such a destination even exist?

01:37:17.820 | Because asking the question and the possible answers

01:37:23.060 | in the process of trying to answer that question

01:37:25.340 | is in itself super interesting.

01:37:27.320 | Is it even possible to get there,

01:37:31.020 | where there's an equals mc squared type of,

01:37:33.620 | there's a function, okay, you can have many parameters,

01:37:36.820 | but a finite number of parameter function

01:37:39.900 | that can predict a lot of things about our universe.

01:37:42.300 | - Well, okay, but just to sort of throw one thing in,

01:37:47.140 | in order to answer every question,

01:37:50.180 | we would need a theory of the origin of the universe.

01:37:53.340 | And that is a huge task, right?

01:37:59.980 | And the fact that the universe seems to have a beginning

01:38:05.060 | defies everything we know and love, right?

01:38:09.040 | Because one of the basic principles of physics

01:38:14.040 | is determinism, that the past follows from,

01:38:18.640 | the present follows from the past,

01:38:22.280 | the future follows from the present, and so on.

01:38:25.420 | But if you have the origin of the universe,

01:38:29.120 | if you have a Big Bang, that means before that,

01:38:31.000 | there was nothing.

01:38:32.560 | You can't have a theory in which

01:38:34.160 | something follows from nothing.

01:38:35.860 | So somehow--

01:38:38.520 | - Sounds like you don't like singularities.

01:38:40.620 | - Well--

01:38:43.220 | - I thought for somebody that works with black holes,

01:38:44.600 | you would get used to them by now.

01:38:45.800 | - No, no, I like this because it's so hard to understand.

01:38:50.440 | I like it 'cause it's hard to understand,

01:38:52.120 | but it's really challenging us.

01:38:54.600 | It's not a, I don't think we're close

01:38:56.400 | to solving that problem.

01:38:57.480 | - So even--

01:38:58.320 | - And string theory has basically had nothing,

01:39:02.440 | there's been almost nothing interesting said about that

01:39:06.400 | in the last many decades.

01:39:09.080 | - So string theory hasn't really looked at the Big Bang,

01:39:11.880 | it hasn't really tried to get to the origin.

01:39:14.160 | - Not successfully, not, there aren't compelling papers

01:39:18.760 | that lots of people have read that,

01:39:20.460 | people have taken it up and tried to go at it,

01:39:23.840 | but there aren't compelling.

01:39:28.360 | String theory doesn't seem to have a trick

01:39:31.800 | that helps us with that puzzle.

01:39:35.180 | - Do you think we'll be able to sneak up

01:39:36.440 | to the origin of the universe, like reverse engineer it,

01:39:40.320 | from experimental, from theoretical perspective?

01:39:43.320 | Okay, if we can, what would be the trajectory?

01:39:47.680 | - You've already gotten yourself in trouble,

01:39:50.200 | 'cause you used the word reverse engineer.

01:39:53.160 | So if you're gonna reverse engineer,

01:39:55.640 | that means you, forward engineering means

01:40:00.640 | that you take the present and determine the future,

01:40:03.200 | reverse engineering means that you take the present

01:40:06.100 | and determine the past.

01:40:07.340 | - Yeah, estimate the past, but yes, sure.

01:40:10.860 | - But if the past was nothing,

01:40:13.660 | how are you ever gonna reverse engineer to nothing?

01:40:16.620 | That's hard to do.

01:40:18.780 | - Run up against the nothing, right?

01:40:20.460 | Until, have mathematical models that break down nicely

01:40:23.740 | to where you can actually start to infer things.

01:40:26.140 | - Let's work on it.

01:40:32.220 | - No, but do you think that--

01:40:34.400 | - Maybe, but people have tried to do things like that.

01:40:38.240 | - Yeah, and have not succeeded.

01:40:40.200 | - It's not something that we're getting A pluses in.

01:40:44.120 | - Sure, let's pretend we live in a world

01:40:47.320 | where in 100 years we have an answer to that.

01:40:50.480 | What would that answer look like?

01:40:54.360 | What department is that from?

01:40:56.180 | What fields led us there?

01:41:01.480 | Not what fields, what set of ideas in theoretical physics?

01:41:04.380 | Is it experimental, is it theoretical?

01:41:07.680 | Like what can you imagine possibly

01:41:10.120 | could have possibly led us there?

01:41:11.440 | Is it through gravitational waves

01:41:12.700 | and some kind of observations there?

01:41:14.400 | Is it investigation of black holes?

01:41:16.520 | Is it simulation of universes?

01:41:18.840 | Is it maybe we start creating black holes somehow?

01:41:22.880 | I don't know.

01:41:23.720 | Maybe some kind of high energy physics type of experiments?

01:41:30.300 | - Well, I have some late night ideas about that

01:41:33.980 | that aren't really ready for prime time.

01:41:36.140 | - Okay, sure.

01:41:37.640 | But you have some ideas.

01:41:38.860 | - Yeah, yeah, and many people do.

01:41:42.800 | It could be that some of the advances

01:41:45.540 | in quantum information theory are important

01:41:49.140 | in that they kind of go beyond taking quantum systems

01:41:54.140 | and just replicating themselves

01:41:56.820 | but combining them with others.

01:41:59.380 | - Do you think, since you highlighted the issue

01:42:03.980 | with time and the origin of the universe,

01:42:06.060 | do you think time is fundamental or emergent?

01:42:08.940 | - I think ultimately it has to be emergent.

01:42:12.660 | - Yeah, what does it mean for time to be emergent?

01:42:16.320 | - Well, let's review what it means for space to be emergent.

01:42:22.940 | What it means for space to be emergent

01:42:27.100 | is that you have a holographic plate

01:42:32.100 | and you shine some light that's moving in space

01:42:37.980 | and it produces an image

01:42:41.900 | which contains an extra spatial dimension

01:42:45.100 | and time just goes along for the ride.

01:42:47.380 | So what we'd like to do,

01:42:53.560 | and indeed there is some rather concrete work

01:42:57.180 | in this direction, though again, I would say

01:42:59.200 | even within our stringing community,

01:43:02.040 | we're not getting A pluses on these efforts.

01:43:04.940 | But what we'd like to do is to see

01:43:09.660 | examples in which the extra space-time dimension is time.

01:43:17.720 | In other words, usually what we understand very well

01:43:23.560 | mathematically is how to take systems

01:43:26.800 | in some number of space-time dimensions

01:43:34.120 | and rewrite them as a plate in fewer space dimensions.

01:43:39.120 | What we'd like to do is to take systems

01:43:41.640 | with one time and some number of space dimensions

01:43:45.920 | and to rewrite them as a system

01:43:48.220 | that had only space dimensions in it,

01:43:50.640 | had no time evolution.

01:43:53.160 | And there's some fairly concrete ideas

01:43:56.040 | about how to do that, but they're not universally accepted

01:44:00.700 | even within the stringy community.

01:44:04.140 | - But isn't it wild to you?

01:44:07.840 | - Yes.

01:44:08.680 | - For it to be emergent?

01:44:10.000 | - Yes. - How do we intuit

01:44:11.900 | these kinds of ideas as human beings

01:44:13.760 | for whom space and time seems as fundamental

01:44:17.000 | as apples and oranges?

01:44:17.840 | - Well, they're both illusions.

01:44:19.680 | - Okay.

01:44:20.640 | - They're both illusions.

01:44:22.280 | Even time.

01:44:23.120 | - You co-authored a paper titled

01:44:26.760 | Photon Rings Around Warped Black Holes.

01:44:29.520 | First of all, whoever writes your paper titles,

01:44:32.460 | you like the soft hair and the term black hole

01:44:38.660 | and the big bang, you're very good

01:44:40.360 | at coming up with titles yourself.

01:44:41.920 | Anyway, you co-authored a paper titled

01:44:43.800 | Photon Rings Around Warped Black Holes.

01:44:46.160 | In it you write, quote, "Recent work has identified

01:44:48.640 | "a number of emergent symmetries

01:44:50.000 | "related to the intricate self-similar structure

01:44:52.520 | "of the photon ring."

01:44:53.720 | So what are photon rings?

01:44:55.360 | What are some interesting characteristics of a photon ring?

01:44:58.320 | - So that was a paper with Dan Kopitz

01:45:02.240 | and Alex Lipsaska that just came out.

01:45:05.160 | And this paper is kind of a wonderful example

01:45:10.920 | of what happens when you start to talk to people

01:45:17.400 | who are way out of your comfort zone

01:45:21.920 | of no different stuff and look at the world a different way.

01:45:26.920 | And some two or three years ago,

01:45:31.400 | I'm part of this, the Black Hole Initiative,

01:45:35.440 | I'm also part of this Event Horizon Telescope collaboration

01:45:39.320 | that took the famous, though I had nothing to do

01:45:43.320 | with the experiment, but that took the famous picture

01:45:48.120 | of the donut of M87, and through conversations with them,

01:45:53.120 | which started out in an effort to understand

01:46:00.920 | the image that they'd seen.

01:46:04.600 | So it's a great thing for somebody like me,

01:46:07.760 | a theoretical physicist, lost, seemingly lost

01:46:12.680 | in string land to be presented with an actual picture

01:46:17.680 | of a black hole. - And asked what?

01:46:22.320 | - To be asked, what can we learn from this?

01:46:25.320 | So with some help from Michael Johnson, Alex Lipsaska,

01:46:30.320 | and a bunch of other people, Event Horizon collaboration,

01:46:39.760 | we came up with a fantastic, beautiful answer

01:46:43.560 | using Einstein's theory that is both shaping the future of,

01:46:48.560 | now it is shaping the future of improved

01:46:57.680 | black hole photographs, what do you wanna concentrate on

01:47:02.040 | in the photograph?

01:47:03.160 | You know, you just point it at the sky and click?

01:47:05.680 | No, you don't do that.

01:47:07.320 | You optimize for various features.

01:47:10.240 | And it's both shaping that, and in the process

01:47:15.240 | of talking to them and thinking about how light behaves

01:47:22.320 | around a black hole, black holes just have so many

01:47:27.320 | magic tricks and they do so many weird things.

01:47:31.200 | And the photon ring is among the weirdest of them.

01:47:35.120 | We understood this photon ring, and in the process of this,

01:47:39.920 | we said, hey, this photon ring has gotta be telling us

01:47:44.920 | something about the puzzle of where the holographic plate is

01:47:51.920 | outside of a ordinary astrophysical black hole.

01:47:59.880 | And we nailed it for the stringy black holes,

01:48:03.180 | but they have a somewhat different character.

01:48:06.960 | - What's a stringy black hole?

01:48:08.640 | The black holes that describe a string theory?

01:48:10.480 | - The black holes that are contained in string theory

01:48:13.280 | and they have different structure in them.

01:48:16.640 | - Well, but actually, can we step back?

01:48:17.760 | So what was the light in the image taken in 2019?

01:48:21.080 | No, not taken in 2019, presented in 2019.

01:48:24.760 | - So here's the puzzle.

01:48:26.000 | What they really saw, so the black holes tend to gather

01:48:33.260 | stuff that swirls around it.

01:48:36.540 | And they don't know what that stuff is made of,

01:48:41.780 | they don't know what its temperature is,

01:48:44.220 | they don't know what kind of magnetic fields

01:48:45.980 | there are around there.

01:48:48.360 | So the form of the image has a lot of unknowns in it

01:48:53.360 | that it's dependent on many other things

01:48:56.580 | other than the geometry of the black hole.

01:49:00.180 | So most of what you're learning is about the stuff.

01:49:05.180 | Now the stuff, the swirling stuff,

01:49:10.600 | the hot swirling stuff is interesting as hell,

01:49:15.100 | but it's not as interesting as the black hole,

01:49:18.020 | which are the most, in my view,

01:49:21.080 | the most interesting things in the universe.

01:49:23.640 | So you don't wanna just learn about the stuff,

01:49:27.000 | you wanna learn about the black hole

01:49:29.840 | that it's swirling around.

01:49:31.120 | - So one of the, at the very first step,

01:49:34.800 | at the very primitive level,

01:49:36.720 | this is just a big leap for human civilization

01:49:39.240 | to be able to see a black hole,

01:49:43.300 | and the way you can see it

01:49:44.480 | is because there's stuff around it.

01:49:46.680 | But you don't get to learn much about the black hole,

01:49:50.440 | but you get to learn more about the stuff

01:49:51.960 | just from the image.

01:49:54.000 | - Yeah, but you're not gonna learn about the details

01:49:56.920 | before you've even seen it.

01:49:58.320 | - 'Cause there's too many parameters,

01:49:59.560 | there's too many variables that govern the stuff.

01:50:01.480 | - Yeah, so then we found a very wonderful way

01:50:06.480 | to learn about the black hole,

01:50:09.120 | and here's how it works.

01:50:10.580 | A black hole is a mirror,

01:50:13.700 | and the way it's a mirror is if light,

01:50:18.040 | a photon, bounces off your face

01:50:21.040 | towards the black hole,

01:50:23.720 | and it goes straight to the black hole,

01:50:25.600 | just falls in, you never see it again.

01:50:28.880 | But if it just misses the black hole,

01:50:31.040 | it'll swing around the back and come back to you,

01:50:36.240 | and you see yourself from the photon

01:50:40.760 | that went around the back of the black hole.

01:50:43.400 | But not only can that happen,

01:50:45.780 | the black hole, the photon can swing around twice

01:50:49.760 | and come back, so you actually see

01:50:53.000 | an infinite number of copies of yourself.

01:50:56.780 | - Like with a little bit of a delay.

01:50:58.960 | - With a little bit of a delay, right.

01:51:01.000 | - This is awesome.

01:51:03.560 | - Yeah, and in fact--

01:51:06.920 | - I mean, we're not used to an object

01:51:08.360 | that bends light like that, right?

01:51:10.040 | - Yeah, yeah.

01:51:10.880 | - So you're gonna get some trippy effects.

01:51:13.000 | - And in fact, one of my students

01:51:15.560 | has made a really awesome computer animation of this,

01:51:18.960 | which I'm gonna show at a public lecture

01:51:22.040 | in a couple weeks where the audience

01:51:23.640 | will see infinitely many copies of themselves.

01:51:26.920 | That's all swirling around the black hole.

01:51:29.020 | So a black hole is like a hall of mirrors,

01:51:36.320 | like an apartment store where you go

01:51:37.880 | and there's the three mirrors

01:51:39.780 | and you see infinitely many copies of yourself.

01:51:42.180 | Think of the black hole as the mirror.

01:51:45.280 | And you go in there with your clothes.

01:51:49.980 | If you wanna know about your clothes,

01:51:51.100 | you just look at the direct image.

01:51:52.540 | You're not learning anything

01:51:53.700 | about the configuration of mirrors.

01:51:56.140 | But the relation of the image you see in front of you

01:52:02.020 | to the one you see at the side

01:52:07.660 | and the next one and so on depends only on the mirrors.

01:52:11.740 | It doesn't matter what clothes you're wearing.

01:52:14.800 | So you can go there 1,000 times wearing different clothes,

01:52:18.900 | but each time there will be the same relation

01:52:21.940 | between the subsequent images.

01:52:23.780 | And that is how we're gonna learn about the black holes.

01:52:29.740 | We're gonna take the stuff that is swirling around

01:52:34.660 | and we're gonna tease out the subsequent images

01:52:39.180 | and look at their relation.

01:52:41.380 | And there's some very beautiful,

01:52:43.180 | really beautiful mathematics,

01:52:45.820 | which we were surprised to realize

01:52:48.380 | with the volumes and volumes of papers

01:52:52.380 | on black holes and their properties,

01:52:55.220 | this particular, 'cause it was a physical question

01:52:58.820 | that had never been asked in exactly this way.

01:53:01.580 | - So basically you're looking at the--

01:53:03.380 | - The relation between the subsequent images.

01:53:06.580 | - But those are ultimately formed by photons

01:53:08.380 | that are swirling around.

01:53:09.860 | - Photons that are orbiting.

01:53:11.140 | So the photon ring are the photons that orbit around.

01:53:15.140 | - And beyond, so like orbit and lose orbit.

01:53:17.620 | Like are they, like so, wow.

01:53:22.300 | And that starts to give you,

01:53:23.380 | what can you possibly figure out mathematically

01:53:25.540 | about the black hole?

01:53:26.380 | Can you, the geometry of it?

01:53:27.860 | Does the spin of it? - The geometry, the spin.

01:53:30.740 | And you can verify things behaving.

01:53:35.740 | We have never seen a region of space-time

01:53:40.980 | with such high curvature.

01:53:43.660 | I mean, the region around a black hole is crazy.

01:53:46.920 | It's not like in this room.

01:53:48.820 | The curvature is everything.

01:53:51.020 | - You spend probably enough time with the math

01:53:52.700 | and the photons.

01:53:54.300 | Can you put yourself in that space?

01:53:57.460 | So we're like having a conversation

01:53:59.420 | in pretty peaceful, comfortable, flat space.

01:54:02.060 | Are you able to put yourself in a place

01:54:04.420 | around a black hole?

01:54:05.780 | - Yeah, I'm able to imagine that kind of thing, yeah.

01:54:08.780 | So for example, and actually there's a wonderful movie,

01:54:11.920 | Interstellar.

01:54:15.180 | And in that movie, you know, Kip Thorne,

01:54:20.180 | of course is a great theoretical physicist,

01:54:24.000 | experimental, who later won the Nobel Prize for LIGO.

01:54:29.000 | And that movie is very accurate scientifically.

01:54:34.560 | And there's some funny statements in there

01:54:37.520 | that of the, you know, 100 million people

01:54:43.840 | who saw that movie, there can't be more than 10 or 20

01:54:47.500 | understood about why Matthew McConaughey

01:54:51.140 | is ejecting the trash in a certain direction in order to.

01:54:55.300 | But you know, for example, if I were a spinning black hole

01:54:59.900 | right here, if I was spinning fast enough,

01:55:02.520 | you wouldn't be able to stay still there.

01:55:05.200 | You'd have to be orbiting around like that, you know?

01:55:08.820 | You'd have to have your microphone on a rotating--

01:55:11.380 | - Yeah, but I wonder what the experience is

01:55:13.500 | with the actual experience,

01:55:14.480 | 'cause I mean, space itself is curved.

01:55:16.500 | - Well, if space gets very curved, you get crushed.

01:55:19.100 | You know, body gets ripped apart,

01:55:23.420 | 'cause the forces are different on different parts of the--

01:55:25.460 | - Sure, okay, so that would be--

01:55:27.460 | - But if it can be less curved,

01:55:29.020 | so that the curvature is very noticeable,

01:55:31.380 | but you're not ripped apart.

01:55:32.860 | - The fact that this was just nonchalantly stated

01:55:39.420 | is just beautiful.

01:55:41.620 | Like two biological systems discussing

01:55:43.940 | which level of curvature is required

01:55:46.780 | to rip apart said biological system.

01:55:49.940 | Very well.

01:55:50.780 | So you propose in the paper that a photon ring

01:55:53.820 | of a warped black hole is indeed part

01:55:55.940 | of the black hole hologram.

01:55:57.480 | A photon ring of a warped black hole

01:56:01.860 | is indeed part of the black hole hologram.

01:56:04.800 | So what can you intuit about the hologram?

01:56:11.580 | And the holographic plate from looking at the photon rings?

01:56:16.580 | - Well, this paper is exploring a new idea.

01:56:24.740 | It's not making a new discovery, so to speak.

01:56:30.060 | It's exploring an idea and the ins and outs of it,

01:56:40.300 | and what might work and what might not.

01:56:44.540 | And this photon ring, somehow everybody always thought

01:56:48.580 | that the holographic plate sat at the horizon

01:56:51.540 | of the black hole.

01:56:52.640 | And that the quantum system that describes the black hole

01:56:57.540 | is inside the horizon.

01:56:59.060 | And in fact, we think it's plausible,

01:57:08.500 | and we give some evidence in some soluble examples,

01:57:13.500 | in this case, in an example in one lower dimension

01:57:16.740 | where we can handle the equations better,

01:57:19.280 | that the quantum system that describes the black hole

01:57:23.660 | should correspond to a region of space-time

01:57:28.300 | which includes the photon ring.

01:57:30.860 | So it's bigger.

01:57:32.700 | - So that would be the holographic plate.

01:57:36.980 | - That would be the holographic plate.

01:57:38.380 | - All of that.

01:57:39.740 | - I mean, we didn't prove this.

01:57:41.720 | We put it out there.

01:57:47.500 | It hadn't really been considered previously.

01:57:50.700 | We put it out there, and it does seem more plausible

01:57:55.460 | than the idea that it sits literally at the horizon.

01:57:58.860 | And it is a big outstanding problem

01:58:03.100 | of how you have a holographic reconstruction

01:58:07.260 | of black holes like M87.

01:58:10.280 | - Do you think there could be further experimental data

01:58:15.660 | that helps explore some of these ideas

01:58:17.740 | that you have for photon rings and holographic plates

01:58:21.660 | through imaging and through high and high resolution images

01:58:24.660 | and also just more and more data?

01:58:25.820 | - I wish so, but I don't think so.

01:58:28.180 | But what I think already has happened

01:58:32.420 | and will continue to happen is that the,

01:58:36.100 | there are many different ways that theorists

01:58:42.420 | and observers can interact.

01:58:45.720 | The gold standard is the theorist makes a prediction,

01:58:51.900 | the observer measures it and confirms it,

01:58:55.500 | or the observer makes a discovery

01:59:00.340 | and the theorist explains it.

01:59:02.700 | But there's a lot less than that,

01:59:05.860 | which is really kind of the bread and butter of,

01:59:10.300 | those are dramatic moments when that happens, right?

01:59:13.040 | Those are once in a lifetime moments when that happens.

01:59:16.300 | But the bread and butter is more when,

01:59:18.780 | and it has already happened,

01:59:21.520 | they came to us and said,

01:59:22.900 | "What is the interesting theoretical things

01:59:25.380 | "we can understand in this swirl around the black hole?"

01:59:30.740 | And we give an answer and then that in turn jogged us

01:59:35.740 | to think about the holographic principle

01:59:42.060 | in the context of M87 a little bit differently.

01:59:46.300 | And so it's a useful, and in the same vein,

01:59:48.900 | it's useful to talk to the philosophers

01:59:51.520 | and it's useful to talk to the mathematicians

01:59:55.280 | and a lot of, you gotta,

02:00:00.060 | we just gotta, we don't know where we're going,

02:00:02.460 | we just gotta do everything.

02:00:05.860 | - Let me ask you another sort of

02:00:08.020 | philosophical type question, but not really actually.

02:00:10.720 | It seems that thought experiments are used,

02:00:14.260 | so it's not just mathematics that makes progress

02:00:16.160 | in theoretical physics, but thought experiments do.

02:00:18.340 | They did for Einstein as well.

02:00:20.640 | They did for a lot of great physicists throughout history.

02:00:24.300 | Over the years, how's your ability

02:00:26.300 | to generate thought experiments?

02:00:27.540 | Or just your intuition about some of these weird things

02:00:30.780 | like quantum mechanics or strength theory

02:00:35.780 | or quantum gravity or yeah, even general relativity,

02:00:40.040 | how's your intuition improved over the years?

02:00:42.060 | Have you been able to make progress?

02:00:43.980 | - The hard part in physics is most problems are

02:00:54.420 | either doable, most problems that a theoretical calculation

02:00:58.860 | that a theoretical physicist would do,

02:01:01.820 | there's no end of problems whose answer is uninteresting.

02:01:06.720 | Can be solved, but the answer is uninteresting.

02:01:11.500 | There's also no end of problems that are very interesting,

02:01:17.700 | some of which you've asked me,

02:01:21.660 | but we don't have a clue how to solve them.

02:01:24.820 | And when first presented with a problem,

02:01:29.460 | almost every problem is one or the other.

02:01:31.560 | It's the jackpot when you find one

02:01:35.360 | that isn't one or the other.

02:01:36.760 | And--

02:01:38.860 | - It seems like there's a gray area between the two, right?

02:01:41.460 | That's where you should be looking.

02:01:44.000 | - Well, I wouldn't describe it as a gray area.

02:01:45.900 | I would describe it as a knife edge.

02:01:47.760 | - So it's a very small area.

02:01:51.180 | - There isn't like a huge area with a sign.

02:01:55.140 | Here lie problems that are doable

02:01:57.420 | and people wanna know the answer.

02:01:59.060 | - And that's in some deep sense,

02:02:00.740 | that's where timing is everything with physics,

02:02:02.680 | with science, with discovery.

02:02:04.180 | - With timing.

02:02:05.100 | I mean, I think earlier in my career,

02:02:07.420 | I erred more on the side of problems that were not solvable.

02:02:12.420 | - The ambition of youth.

02:02:17.900 | - Yeah.

02:02:19.980 | - What made you fall in love with physics at first?

02:02:24.060 | If we can go back to the early days.

02:02:27.220 | You said black holes were there in the beginning.

02:02:30.500 | But what made you, do you remember

02:02:32.020 | what really made you fall in love?

02:02:33.940 | - You know, I wanted to reach nirvana

02:02:36.500 | and I sort of realized that wasn't gonna happen.

02:02:38.580 | And then after that, I wanted to know the meaning of life

02:02:41.780 | and I realized I probably wasn't gonna figure that out.

02:02:45.720 | And then I wanted to understand justice

02:02:50.720 | and socialism and world things

02:02:57.100 | and couldn't figure those out either.

02:02:59.980 | (laughing)

02:03:02.220 | - Smaller and smaller problems

02:03:05.020 | until you arrived at black holes.

02:03:06.580 | - And then, I mean, most of this stuff

02:03:08.580 | I'm talking about adolescence,

02:03:10.060 | but it was the biggest problem

02:03:14.580 | that I thought that there was a prospect of,

02:03:18.940 | but not 100%, you know?

02:03:20.980 | And I was definitely ready to spend my life

02:03:25.340 | in the wilderness knocking my head against the wall,

02:03:31.300 | but I haven't had to.

02:03:34.460 | I haven't solved them,

02:03:36.380 | but I've said enough interesting things

02:03:39.300 | that you're interviewing me.

02:03:42.660 | So I'm not in the wilderness,

02:03:44.220 | but yeah, so.

02:03:46.140 | - Do you remember the early days?

02:03:48.020 | Do you feel nostalgic when you think back

02:03:51.020 | to the ideas, the circumstances that led down,

02:03:54.100 | that led you down the path towards black holes,

02:03:58.180 | towards theoretical physics, towards the tools of physics,

02:04:00.740 | towards this really fascinating world

02:04:02.420 | of theoretical physics?

02:04:03.580 | - Well, I wouldn't add nostalgia to it

02:04:09.300 | because

02:04:10.260 | (silence)

02:04:15.260 | it's not like a,

02:04:17.580 | you know, a summer in Italy or something.

02:04:23.380 | It's like there's results that are there,

02:04:28.380 | that people are, and that's what's so gratifying.

02:04:36.500 | I mean, of course one's name disappears from these things,

02:04:41.500 | unless you're Einstein or Newton or something.

02:04:46.540 | People are not gonna remember my name in 50 years.

02:04:51.740 | - Well, most, basically every name will be forgotten

02:04:54.380 | in hundreds of years, yeah.

02:04:56.060 | - Yeah.

02:04:56.900 | - Are you able to, by the way,

02:04:58.580 | love the idea, the exploration of ideas themselves

02:05:03.140 | without the names, the recognition, the fame?

02:05:05.100 | - Yeah, yeah, that's what I'm saying.

02:05:06.460 | I have not, I hope someday, but I have not,

02:05:10.260 | there are some experiments now to verify

02:05:15.180 | some of my predictions about properties of gravity

02:05:20.180 | and so on, but I have not.

02:05:22.260 | Most of what I've done is in the,

02:05:28.900 | it could happen still.

02:05:31.380 | It's still a logical possibility that everything

02:05:33.720 | having to do with string theory and,

02:05:36.180 | I mean, as we mentioned, I'm betting the farm

02:05:40.580 | that it's not, but it is indeed a logical possibility

02:05:44.700 | that people will say, "Can you believe

02:05:48.860 | "Lex Fridman interviewed Elon Musk in Kenya West,

02:05:53.060 | "and then he interviewed Strominger,

02:05:55.700 | "who was working on this theory

02:05:59.200 | "that just completely went into the,

02:06:02.080 | "completely went into the toilet?"

02:06:04.020 | - I'm gonna make, I'm gonna get,

02:06:06.420 | with a wife I don't have, I'm gonna make

02:06:07.740 | a public statement, she'll be on stage,

02:06:09.420 | I'll say, "I'm really sorry I made this giant mistake

02:06:12.020 | "of platforming this wild-eyed physicist

02:06:17.440 | "that believed for decades in the power

02:06:19.520 | "of theoretical physics."

02:06:20.440 | Yes, no, like you said.

02:06:22.700 | - So that could happen, it could happen, it could happen.

02:06:25.580 | And of course, if that couldn't happen,

02:06:28.660 | it wouldn't be real exploration, right?

02:06:30.780 | - Absolutely.

02:06:32.100 | - And so, but I, you know, I do take a lot of satisfaction

02:06:37.100 | that some of the things I discovered

02:06:41.260 | are at the minimum mathematical truths,

02:06:45.020 | and they're still, so you don't have that sort of

02:06:49.080 | nostalgic feeling of it being something that was gone,

02:06:54.080 | and I'm still making discoveries now

02:06:58.980 | that I'm as excited about.

02:07:01.460 | We'll see if they hold the test of time,

02:07:05.020 | that stand the test of time that these other ones did,

02:07:09.700 | but that I'm as excited about as I was about those

02:07:13.900 | when I made them.

02:07:16.140 | I am easily excitable, as my friends will tell you.

02:07:18.860 | - Well, one interesting thing about you is--

02:07:20.620 | - And I have been very excited about things

02:07:22.180 | which turned out to be completely wrong, you know?

02:07:24.180 | - Well, that's, the excitement is a precondition

02:07:26.540 | for breakthroughs, but you're also somebody,

02:07:30.860 | like you said, you don't have a cynical view

02:07:33.700 | of the modern state of physics.

02:07:36.060 | - No.

02:07:36.940 | - So there's a lot of people that glorify

02:07:38.500 | the early days of string theory,

02:07:39.820 | and all the discoveries that were made in the '20s.

02:07:42.180 | - Yeah, people are always, yeah, yeah, yeah.

02:07:43.580 | - But you're saying this to you might be one of,

02:07:48.580 | if not the most exciting times to be a theoretical physicist.

02:07:51.780 | Like when the alien civilizations, 500 years from now,

02:07:55.740 | that visit Earth will look back,

02:07:57.820 | they'll think the 21st century,

02:07:59.780 | some of the biggest discoveries ever

02:08:01.180 | were made in the 21st century.

02:08:02.020 | - Yeah, I mean, when they have a measurement

02:08:04.140 | of string theory, the fun's over.

02:08:05.880 | Then we have to go on to something new, you know?

02:08:10.380 | - No, there's deep, there's going to be deep,

02:08:12.820 | the fun is over.

02:08:14.260 | Oh, man.

02:08:15.860 | But there is an end to the Nile, right?

02:08:17.420 | I mean, there's--

02:08:18.380 | - Is there?

02:08:19.220 | Who told you?

02:08:21.340 | - Some Weinberg guy.

02:08:24.840 | (laughing)

02:08:26.960 | - Let me ask you another trippy out there question now.

02:08:29.080 | So again, perhaps unanswerable from a physics perspective,

02:08:33.960 | but do you wonder about alien civilizations?

02:08:38.760 | Do you wonder about other intelligent beings out there

02:08:42.440 | making up their own math and physics,

02:08:44.040 | trying to figure out the world?

02:08:45.560 | Do you think they're out there?

02:08:47.120 | - It is hard to understand

02:08:53.160 | why there would, given that there's so many planets,

02:08:56.360 | and of course there's Drake's formula,

02:08:58.960 | and we don't exactly know what the,

02:09:01.720 | but I mean, I think Fermi's paradox,

02:09:04.880 | that, you know, is a real paradox,

02:09:07.560 | and I think there probably are,

02:09:12.560 | and I think it's very exciting that,

02:09:18.680 | you know, we might, you know, find some,

02:09:23.040 | it's a logical possibility that we could learn about it.

02:09:26.600 | - I mean, to me, it's super interesting to think about

02:09:28.480 | aliens from a perspective of physics,

02:09:30.840 | because, so any intelligent civilization

02:09:33.280 | is going to be contending with the ideas,

02:09:36.360 | or just trying to understand the world around it.

02:09:39.280 | So I think that the universe is filled

02:09:43.600 | with alien civilizations.

02:09:45.180 | So they all have their physicists, right?

02:09:47.500 | They all have their,

02:09:48.680 | they're all trying to understand the world around them.

02:09:50.760 | And it's just interesting to me

02:09:52.160 | to imagine all these different perspectives,

02:09:54.240 | all these different Einsteins.

02:09:55.760 | - Yeah.

02:09:56.600 | - Like trying to make sense of like--

02:09:57.440 | - Though they might be more different than we think.

02:10:00.600 | - What, why--

02:10:01.440 | - They might be different in a way

02:10:02.260 | that we haven't even thought of.

02:10:04.800 | - Like smarter or different?

02:10:06.960 | - Just different, something that we don't even,

02:10:09.240 | we're not even able to describe now.

02:10:11.720 | We just haven't thought of it, you know?

02:10:13.720 | - Yeah, yeah, this is a really frustrating thing

02:10:16.440 | when we think, from me as an AI person,

02:10:19.280 | you start to think about what is intelligence,

02:10:20.820 | what is consciousness, and you start to sometimes,

02:10:25.000 | again, like evening thoughts is

02:10:28.740 | how little we understand,

02:10:31.320 | how narrow our thinking is about these concepts.

02:10:34.700 | - Yeah, yeah.

02:10:35.540 | - That it could be intelligence,

02:10:38.840 | could be, something could be intelligent

02:10:42.200 | and be very different, intelligent in a very different way.

02:10:46.240 | It won't be able to detect

02:10:47.800 | because we're not keeping an open mind,

02:10:50.960 | open enough mind, and that's kind of sad

02:10:54.040 | because to me there's also just a strong possibility

02:10:56.840 | that aliens or something like alien intelligence

02:10:59.840 | or some fascinating, beautiful physical phenomena

02:11:04.840 | are all around us, and we're too dumb to see it for now,

02:11:10.320 | or too closed-minded to see it.

02:11:14.860 | There's something we're just deeply missing,

02:11:16.620 | whether it's fundamental limitations

02:11:18.680 | of our cognitive abilities

02:11:20.240 | or just because our tools are too primitive right now.

02:11:23.520 | Or like the way we, it's like you said,

02:11:26.440 | the idea seemed trivial once you figured it all out,

02:11:30.840 | looking back.

02:11:32.080 | - Yeah.

02:11:32.960 | - But that kind of makes me sad

02:11:36.640 | because there could be so much beauty in the world

02:11:38.320 | we're not seeing 'cause we're too dumb.

02:11:41.960 | - There surely is.

02:11:43.400 | And that's, I guess, the process of science and physics

02:11:45.540 | is to keep exploring, to keep exploring,

02:11:48.480 | to find the thing that will in a century seem obvious.

02:11:53.040 | - Well, it's something we know for sure.

02:11:55.980 | I mean, the brain we don't really understand,

02:11:59.720 | and that's gotta be some fabulously beautiful story.

02:12:04.720 | - I'm hoping some of that story will be written

02:12:09.180 | through the process of trying to build a brain,

02:12:10.920 | so the process of engineering intelligence,

02:12:14.060 | not just the neuroscience perspective

02:12:15.680 | of just looking at the brain, but trying to create it.

02:12:18.640 | But yeah, that story hasn't been written almost at all,

02:12:25.060 | or just the early days of figuring that one out.

02:12:27.240 | But see, like you said that math is discovered,

02:12:29.220 | so aliens should at least have the same math as us, right?

02:12:34.220 | - I think so.

02:12:36.880 | - Maybe different symbols?

02:12:38.520 | - They might have discovered different,

02:12:41.240 | they might have discovered it differently,

02:12:43.600 | and they might have had a different idea

02:12:47.600 | of what a proof is.

02:12:49.280 | - Sure, yeah.

02:12:54.040 | We're very like black and white with the proof thing.

02:12:58.000 | Maybe they're looser?

02:12:59.080 | - Right, well, so you can know something is true.

02:13:04.460 | First of all, you never know something is true

02:13:08.120 | with 100% uncertainty.

02:13:09.800 | I mean, you might have had a blackout,

02:13:14.200 | just to be, it's never 100%, right?

02:13:17.440 | You might have had a momentary lapse of consciousness

02:13:20.240 | as the key step in the proof,

02:13:21.760 | and nobody read it, and whatever.

02:13:24.400 | Okay, so you never know for sure.

02:13:26.040 | But you can be, you have a preponderance of evidence,

02:13:32.280 | which makes it, and preponderance of evidence

02:13:38.080 | is not accepted very much in mathematics.

02:13:42.240 | And that was sort of how the famous Ramanujan work.

02:13:47.240 | He had formulas which he guessed at,

02:13:51.920 | and then he gathered a preponderance of evidence

02:13:54.960 | that he was sure they were true.

02:13:58.000 | So there might be, or something completely different.

02:14:00.480 | They might function in a very different way.

02:14:03.300 | - Let me ask you kind of a heavy question for a physicist,

02:14:07.840 | but one on nuclear weapons.

02:14:10.920 | Just in general, what do you think about nuclear weapons

02:14:13.400 | where, like philosophical level,

02:14:16.080 | where brilliant physicists and brilliant engineering

02:14:20.100 | leads to things that can destroy human civilization?

02:14:23.080 | Sort of like some of the ideas that you're working on

02:14:27.120 | have power when engineered into machines, into systems.

02:14:34.240 | Is there some aspect of you that worries about that?

02:14:36.440 | - I don't know what the brilliant had to do with it,

02:14:38.520 | because of course, Oppenheimer and all that,

02:14:41.920 | okay, they did it really fast,

02:14:43.320 | but if you didn't have Oppenheimer,

02:14:45.860 | I mean, it would all have happened anyway.

02:14:49.020 | It had a reality of its own.

02:14:53.440 | The possibility of making a nuclear,

02:14:55.640 | it didn't depend on the fact that the physicists

02:14:57.800 | who built it were brilliant.

02:14:58.960 | Maybe that sped it up by a year or two years,

02:15:03.160 | but by now we'd have nuclear weapons.

02:15:05.480 | It's something that--

02:15:06.800 | - So the ideas have momentum, that they're unstoppable.

02:15:11.040 | - Right, the possibility of making nuclear weapons

02:15:15.120 | was discovered, right?

02:15:17.340 | It was there before.

02:15:19.240 | It's not like somebody made it, right?

02:15:22.820 | Without Picasso, there would never have been a Guernica,

02:15:31.020 | but without Oppenheimer, there would surely

02:15:33.920 | have still been an atom bomb.

02:15:36.920 | - But timing matters, right?

02:15:38.400 | Timing's very important.

02:15:39.560 | There's a guy with a mustache--

02:15:41.040 | - Of course, of course, of course.

02:15:43.740 | The timing mattered there.

02:15:45.940 | But I, yeah, okay, I mean,

02:15:53.000 | you could try to make a case for stopping--

02:15:58.000 | - No, no, no, no, it's the case of carrying the burden

02:16:01.100 | of the responsibility of the power of ideas

02:16:04.180 | when manifested into systems.

02:16:06.880 | So it's not a game.

02:16:09.220 | It's not just a game of fun mathematics.

02:16:11.660 | Just same with artificial intelligence.

02:16:13.160 | You have this, a lot of people in AI,

02:16:15.720 | in a lot of people in the AI community,

02:16:20.300 | it's a fascinating, fun puzzle,

02:16:22.360 | how to make systems more and more intelligent,

02:16:24.140 | how to, you have a bunch of benchmarks,

02:16:26.240 | you try to make them perform better and better and better,

02:16:27.980 | and all of a sudden, you have a system

02:16:30.200 | that's able to outsmart people.

02:16:33.180 | It's now able to be used in geopolitics.

02:16:35.920 | It's able to create super intelligent bots

02:16:38.740 | that are able to, at scale,

02:16:40.780 | control the belief of a population of people.

02:16:43.180 | And now you can have world wars.

02:16:44.860 | You can have a lot of really risky instabilities--

02:16:47.780 | - They're incredible, they really are incredible.

02:16:49.540 | - And so, there's some responsibility.

02:16:53.100 | This is not sort of, it's a beauty and a terror to these ideas

02:16:58.100 | and a terror to these ideas, you know?

02:17:00.980 | - Yeah.

02:17:03.240 | At that moment, it was certainly a question for Oppenheimer

02:17:08.240 | and everybody who participated in that.

02:17:10.560 | What is the responsible way to serve society

02:17:15.560 | when you're sort of accidentally in this position

02:17:20.800 | of being at the forefront of a development

02:17:25.640 | that has a huge impact on society?

02:17:28.500 | I don't see my work a likelihood of having a huge impact

02:17:33.500 | on the development of society itself,

02:17:36.780 | but if I were you--

02:17:38.400 | - Oh, I love this.

02:17:40.620 | - Working on AI, I think that there is a possibility there,

02:17:45.420 | and that it is, as a responsible scientist,

02:17:49.100 | that it's really not a good thing to say,

02:17:52.540 | I'm just the scientist here

02:17:54.020 | and I'm figuring out what's possible,

02:17:56.660 | because you're in a role where you have more of a podium

02:18:01.660 | to influence things than other people,

02:18:09.340 | and it's your responsibility as a citizen of the planet,

02:18:14.340 | or let me phrase it a little less shooty,

02:18:22.460 | it's, you have an opportunity,

02:18:25.560 | - Yeah.

02:18:26.680 | - as a citizen of the planet to make the world

02:18:30.240 | a better place, which it would be sad to bypass.

02:18:34.400 | - Yeah, it's a nice world we got going.

02:18:38.160 | It'd be nice to keep it going for a little bit longer.

02:18:40.500 | Andrew, I'm really honored that you sat down with me.

02:18:43.480 | This is, thank you for your work, thank you for your time.

02:18:45.560 | - Well, it was a really great conversation.

02:18:48.120 | I really enjoyed it.

02:18:49.240 | You really covered a lot.

02:18:52.000 | I can't believe you're able to discuss at this level

02:18:56.480 | on so many different topics, so it's a pleasure.

02:19:00.520 | - It was super fun, thank you.

02:19:02.180 | Thanks for listening to this conversation

02:19:04.560 | with Andrew Strominger.

02:19:06.000 | To support this podcast,

02:19:07.360 | please check out our sponsors in the description.

02:19:10.160 | And now, let me leave you with some words

02:19:12.120 | from Werner Heisenberg.

02:19:14.120 | Not only is the universe stranger than we think,

02:19:18.580 | it is stranger than we can think.

02:19:21.580 | Thank you for listening, and hope to see you next time.

02:19:25.400 | (upbeat music)

02:19:27.980 | (upbeat music)

02:19:30.560 | [BLANK_AUDIO]

Andrew Strominger: Black Holes, Quantum Gravity, and Theoretical Physics | Lex Fridman Podcast #359

Chapters