Sean Carroll: Quantum Gravity | AI Podcast Clips
00:00:00.000 | - The emergence of space-time.
00:00:03.940 | - That's right.
00:00:05.040 | - Okay, so I thought space-time was fundamental.
00:00:09.260 | - Yeah, I know.
00:00:10.100 | - So this sort of dream that Einstein had
00:00:12.360 | that everybody had and everybody has
00:00:15.120 | of the theory of everything.
00:00:18.280 | So how do we build up from many worlds,
00:00:21.240 | from quantum mechanics, a model of space-time,
00:00:25.000 | a model of gravity?
00:00:26.980 | - Well, yeah, I mean, let me first mention very quickly
00:00:28.760 | why we think it's necessary.
00:00:30.360 | We've had gravity in the form that Einstein bequeathed it
00:00:34.300 | to us for over a hundred years now, like 1915 or 1916,
00:00:37.540 | he put general relativity in the final form.
00:00:40.220 | So gravity is the curvature of space-time,
00:00:42.820 | and there's a field that pervades all the universe
00:00:45.300 | that tells us how curved space-time is.
00:00:47.780 | - And that's a fundamentally classical.
00:00:49.660 | - That's totally classical, right, exactly.
00:00:51.820 | But we also have a formalism, an algorithm
00:00:55.420 | for taking a classical theory and quantizing it.
00:00:58.680 | This is how we get quantum electrodynamics, for example.
00:01:01.620 | And it could be tricky.
00:01:02.580 | I mean, you think you're quantizing something,
00:01:05.260 | so that means taking a classical theory
00:01:07.220 | and promoting it to a quantum mechanical theory,
00:01:10.120 | but you can run into problems.
00:01:11.580 | So they ran into problems when they did that
00:01:13.260 | with electromagnetism,
00:01:14.180 | namely that certain quantities were infinity
00:01:16.660 | and you don't like infinity, right?
00:01:18.020 | So Feynman and Tomonaga and Schwinger won the Nobel Prize
00:01:21.800 | for teaching us how to deal with the infinities.
00:01:23.900 | And then Ken Wilson won another Nobel Prize
00:01:26.060 | for saying you shouldn't have been worried
00:01:27.180 | about those infinities after all.
00:01:29.040 | But still, it's always the thought
00:01:31.680 | that that's how you will make a good quantum theory.
00:01:33.520 | You'll start with a classical theory and quantize it.
00:01:35.580 | So if we have a classical theory, general relativity,
00:01:38.040 | we can quantize it or we can try to,
00:01:40.640 | but we run into even bigger problems with gravity
00:01:43.880 | than we ran into with electromagnetism.
00:01:45.520 | And so far, those problems are insurmountable.
00:01:47.760 | We have not been able to get a successful theory
00:01:49.960 | of quantum gravity by starting
00:01:52.740 | with classical general relativity and quantizing it.
00:01:55.680 | And there's evidence that,
00:01:57.140 | there's a good reason why this is true,
00:01:58.580 | that whatever the quantum theory of gravity is,
00:02:02.780 | it's not a field theory.
00:02:04.660 | It's something that has weird non-local features
00:02:07.540 | built into it somehow that we don't understand.
00:02:09.980 | And we get this idea from black holes and Hawking radiation
00:02:13.820 | and information conservation
00:02:15.300 | and a whole bunch of other ideas I talk about in the book.
00:02:17.840 | So if that's true, if the fundamental theory
00:02:20.420 | isn't even local in the sense
00:02:21.860 | that an ordinary quantum field theory would be,
00:02:24.100 | then we just don't know where to start
00:02:25.700 | in terms of getting a classical precursor and quantizing it.
00:02:30.080 | So the only sensible thing,
00:02:31.760 | at least the next obvious sensible thing to me
00:02:33.840 | would be to say, okay, let's just start intrinsically quantum
00:02:36.600 | and work backwards, see if we can find a classical limit.
00:02:39.320 | - So the idea of locality,
00:02:40.760 | the fact that locality is not fundamental
00:02:43.600 | to the nature of our existence,
00:02:51.760 | I guess in that sense,
00:02:52.840 | modeling everything as a field makes sense to me.
00:02:55.040 | Stuff that's close by interacts,
00:02:57.640 | stuff that's far away doesn't.
00:02:59.960 | So what's locality and why is it not fundamental?
00:03:03.280 | And how is that even possible?
00:03:04.680 | - Yeah, I mean, locality is the answer to the question
00:03:07.040 | that Isaac Newton was worried about
00:03:08.800 | back at the beginning of our conversation, right?
00:03:10.440 | I mean, how can the earth know
00:03:12.600 | what the gravitational field of the sun is?
00:03:14.920 | And the answer, as spelled out by Laplace and Einstein
00:03:17.920 | and others, is that there's a field in between.
00:03:20.240 | And the way a field works is that
00:03:22.360 | what's happening to the field at this point in space
00:03:25.240 | only depends directly on what's happening
00:03:27.760 | at points right next to it.
00:03:29.720 | But what's happening at those points
00:03:31.040 | depends on what's happening right next to those, right?
00:03:33.040 | And so you can build up an influence across space
00:03:35.960 | through only local interactions.
00:03:38.220 | That's what locality means.
00:03:39.520 | What happens here is only affected
00:03:41.000 | by what's happening right next to it.
00:03:42.360 | That's locality.
00:03:43.400 | The idea of locality is built into every field theory,
00:03:47.920 | including general relativity as a classical theory.
00:03:50.760 | It seems to break down when we talk about black holes.
00:03:53.960 | And Hawking taught us in the 1970s
00:03:56.160 | that black holes radiate.
00:03:57.720 | They give off, they will eventually evaporate away.
00:04:00.200 | They're not completely black
00:04:01.280 | once we take quantum mechanics into account.
00:04:03.760 | And we think, we don't know for sure,
00:04:06.880 | but most of us think that if you make a black hole
00:04:10.800 | out of certain stuff, then like Laplace's demon taught us,
00:04:15.200 | you should be able to predict
00:04:16.280 | what that black hole will turn into
00:04:17.960 | if it's just obeying the Schrodinger equation.
00:04:20.320 | And if that's true, there are good arguments
00:04:23.040 | that can't happen while preserving locality
00:04:25.680 | at the same time.
00:04:26.600 | It's just that the information seems to be spread out
00:04:29.460 | non-locally in interesting ways.
00:04:31.860 | - And people should, you talk about holography
00:04:34.500 | with Leonard Susskind on your Mindscape podcast.
00:04:37.520 | People should listen to it. - Oh yes, I have a podcast.
00:04:38.860 | I didn't even mention that.
00:04:39.780 | This is, I'm terrible at--
00:04:40.880 | - No, I'm gonna ask you questions about that too.
00:04:43.520 | And I've been not shutting up about it.
00:04:45.680 | - It's my favorite science podcast, or not.
00:04:47.960 | It's not even a science podcast.
00:04:50.560 | It's like, it's a scientist doing a podcast.
00:04:53.800 | - That's right, that's what it is, absolutely, yes.
00:04:55.840 | - Yeah, anyway.
00:04:57.360 | - Yeah, so holography is this idea
00:04:58.960 | when you have a black hole,
00:04:59.880 | and black hole is a region of space
00:05:02.200 | inside of which gravity is so strong that you can't escape.
00:05:05.240 | And there's this weird feature of black holes
00:05:07.560 | that, again, is a totally thought experiment feature
00:05:10.160 | 'cause we haven't gone and probed any yet,
00:05:12.160 | but there seems to be one way of thinking
00:05:14.760 | about what happens inside a black hole
00:05:17.480 | as seen by an observer who's falling in,
00:05:20.200 | which is actually pretty normal.
00:05:21.480 | Everything looks pretty normal
00:05:22.480 | until you hit the singularity and you die.
00:05:24.640 | But from the point of view of the outside observer,
00:05:27.720 | it seems like all the information that fell in
00:05:30.880 | is actually smeared over the horizon in a non-local way.
00:05:35.040 | And that's puzzling.
00:05:37.080 | So holography, because that's a two-dimensional surface
00:05:39.360 | that is encapsulating the whole three-dimensional
00:05:41.240 | thing inside, right?
00:05:43.000 | Still trying to deal with that.
00:05:44.000 | Still trying to figure out how to get there.
00:05:45.720 | But it's an indication that we need to think
00:05:47.680 | a little bit more subtly when we quantize gravity.
00:05:49.840 | - So because you can describe everything that's going on
00:05:52.840 | in three-dimensional space
00:05:55.160 | by looking at the two-dimensional projection of it,
00:05:59.040 | means that locality is not necessary.
00:06:03.040 | - Well, it means that somehow it's only a good approximation.
00:06:06.600 | It's not really what's going on.
00:06:08.080 | - How are we supposed to feel about that?
00:06:10.480 | - Supposed to feel liberated.
00:06:11.920 | (laughing)
00:06:13.400 | You know, space is just a good approximation.
00:06:16.280 | And this was always gonna be true
00:06:17.320 | once you started quantizing gravity.
00:06:19.280 | So we're just beginning now to face up
00:06:23.160 | to the dramatic implications of quantizing gravity.
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