Sean Carroll: Many-Worlds Interpretation of Quantum Mechanics

00:00:00.000 | - So coming back to the textbook definition

00:00:03.800 | of quantum mechanics, this idea that I don't think

00:00:06.760 | we talked about, can you, this one of the most

00:00:10.760 | interesting philosophical points, we talked

00:00:14.000 | at the human level, but at the physics level,

00:00:17.440 | that at least the textbook definition of quantum mechanics

00:00:21.920 | separates what is observed and what is real.

00:00:26.800 | One, how does that make you feel?

00:00:29.520 | And two, what does it then mean to observe something

00:00:36.080 | and why is it different than what is real?

00:00:38.800 | - Yeah, you know, my personal feeling, such as it is,

00:00:43.160 | is that things like measurement and observers

00:00:47.060 | and stuff like that are not going to play

00:00:49.120 | a fundamental role in the ultimate laws of physics.

00:00:51.700 | But my feeling that way is because so far,

00:00:54.400 | that's where all the evidence has been pointing.

00:00:57.000 | I could be wrong and there's certainly a sense

00:00:59.600 | in which it would be infinitely cool

00:01:02.120 | if somehow observation or mental cogitation

00:01:06.760 | did play a fundamental role in the nature of reality.

00:01:10.400 | But I don't think so and I don't see any evidence for it,

00:01:12.920 | so I'm not spending a lot of time

00:01:13.960 | worrying about that possibility.

00:01:16.560 | So what do you do about the fact that in the textbook

00:01:18.820 | interpretation of quantum mechanics,

00:01:20.300 | this idea of measurement or looking at things

00:01:24.160 | seems to play an important role?

00:01:26.120 | Well, you come up with better interpretations

00:01:28.760 | of quantum mechanics and there are several alternatives.

00:01:30.720 | My favorite is the many worlds interpretation,

00:01:33.520 | which says two things.

00:01:35.320 | Number one, you, the observer,

00:01:38.440 | are just a quantum system like anything else.

00:01:40.480 | There's nothing special about you.

00:01:41.800 | Don't get so proud of yourself.

00:01:43.560 | You know, you're just a bunch of atoms.

00:01:45.520 | You have a wave function.

00:01:46.920 | You obey the Schrodinger equation like everything else.

00:01:49.560 | And number two, when you think you're measuring something

00:01:52.960 | or observing something, what's really happening is

00:01:55.440 | you're becoming entangled with that thing.

00:01:58.720 | So when you think that there's a wave function

00:02:00.520 | for the electron, it's all spread out,

00:02:02.120 | but you look at it and you only see it in one location.

00:02:04.920 | What's really happening is that there's still

00:02:07.040 | the wave function for the electron in all those locations,

00:02:09.280 | but now it's entangled with the wave function of you

00:02:12.680 | in the following way.

00:02:14.220 | There's part of the wave function that says

00:02:15.640 | the electron was here and you think you saw it there.

00:02:18.680 | The electron was there and you think you saw it there.

00:02:20.920 | The electron was over there and you think you saw it there.

00:02:22.800 | Et cetera.

00:02:23.880 | So, and all of those different parts of the wave function,

00:02:26.680 | once they come into being, no longer talk to each other.

00:02:29.920 | They no longer interact or influence each other.

00:02:32.120 | It's as if they are separate worlds.

00:02:34.640 | So this was the invention of Hugh Everett III,

00:02:37.680 | who was a graduate student at Princeton in the 1950s.

00:02:40.920 | And he said, basically, look,

00:02:43.160 | you don't need all these extra rules

00:02:45.400 | about looking at things.

00:02:46.720 | Just listen to what the Schrodinger equation is telling you.

00:02:49.300 | It's telling you that you have a wave function,

00:02:51.460 | that you become entangled,

00:02:53.040 | and that the different versions of you

00:02:54.360 | no longer talk to each other.

00:02:56.040 | So just accept it.

00:02:57.720 | It's just, he did therapy more than anything else.

00:02:59.840 | He said, it's okay.

00:03:01.280 | You don't need all these extra rules.

00:03:03.360 | All you need to do is believe the Schrodinger equation.

00:03:05.520 | The cost is there's a whole bunch of extra worlds out there.

00:03:08.440 | - So are the worlds being created

00:03:10.840 | whether there's an observer or not?

00:03:15.640 | - The worlds are created any time a quantum system

00:03:18.000 | that's in a superposition becomes entangled

00:03:19.960 | with the outside world.

00:03:21.160 | - What's the outside world?

00:03:24.400 | - It depends.

00:03:25.360 | Let's back up.

00:03:26.640 | Whatever it really says, what his theory is,

00:03:29.460 | is there's a wave function of the universe,

00:03:32.360 | and it obeys the Schrodinger equation all the time.

00:03:35.360 | That's it.

00:03:36.200 | That's the full theory right there, okay?

00:03:38.880 | The question, all of the work, is how in the world

00:03:42.720 | do you map that theory onto reality,

00:03:45.400 | onto what we observe, right?

00:03:47.480 | So part of it is carving up the wave function

00:03:50.400 | into these separate worlds, saying, look,

00:03:52.160 | it describes a whole bunch of things

00:03:53.400 | that don't interact with each other.

00:03:54.440 | Let's call them separate worlds.

00:03:56.080 | Another part is distinguishing between systems

00:03:58.840 | and their environments.

00:04:00.080 | And the environment is basically all the degrees of freedom,

00:04:03.000 | all the things going on in the world

00:04:04.520 | that you don't keep track of.

00:04:06.320 | So again, in the bottle of water,

00:04:08.800 | I might keep track of the total amount of water

00:04:11.120 | and the volume.

00:04:12.280 | I don't keep track of the individual positions

00:04:14.600 | and velocities.

00:04:15.580 | I don't keep track of all the photons

00:04:17.260 | or the air molecules in this room.

00:04:18.980 | So that's the outside world.

00:04:20.440 | The outside world is all the parts of the universe

00:04:23.200 | that you're not keeping track of

00:04:24.620 | when you're asking about the behavior of some subsystem of it.

00:04:27.620 | - So how many worlds are there?

00:04:33.320 | - Yeah, I don't know that one either.

00:04:35.380 | There could be an infinite number.

00:04:37.100 | There could be only a finite number,

00:04:38.340 | but it's a big number one way or the other.

00:04:40.340 | - It's just a very, very big number.

00:04:42.140 | In one of the talks, somebody asked,

00:04:44.660 | well, if it's finite,

00:04:49.060 | so actually I'm not sure exactly the logic

00:04:53.100 | you used to derive this,

00:04:54.260 | but is there going to be

00:04:57.860 | overlap, a duplicate world that you return to?

00:05:05.780 | So you've mentioned, and I'd love if you can elaborate on,

00:05:08.780 | sort of idea that it's possible

00:05:10.500 | that there's some kind of equilibrium

00:05:12.320 | that these splitting worlds arrive at.

00:05:14.980 | And then maybe over time,

00:05:17.700 | maybe somehow connected to entropy,

00:05:19.920 | you get a large number of worlds

00:05:22.140 | that are very similar to each other.

00:05:23.980 | - Yeah, so this question of whether or not Hilbert space

00:05:27.620 | is finite or infinite dimensional

00:05:30.020 | is actually secretly connected to gravity and cosmology.

00:05:34.700 | This is the part that we're still struggling

00:05:36.220 | to understand right now.

00:05:37.260 | But we discovered back in 1998

00:05:38.820 | that our universe is accelerating.

00:05:41.380 | And what that means, if it continues,

00:05:43.440 | which we think it probably will, but we're not sure,

00:05:45.200 | but if it does, that means there's a horizon around us.

00:05:49.400 | Because the universe is not only expanding,

00:05:50.960 | but expanding faster and faster,

00:05:52.820 | things can get so far away from us

00:05:54.920 | that from our perspective,

00:05:57.160 | it looks like they're moving away faster

00:05:58.560 | than the speed of light.

00:05:59.400 | We will never see them again.

00:06:00.840 | So there's literally a horizon around us,

00:06:02.600 | and that horizon approaches some fixed distance

00:06:05.800 | away from us.

00:06:07.000 | And you can then argue that within that horizon,

00:06:10.160 | there's only a finite number of things

00:06:11.580 | that can possibly happen,

00:06:12.780 | the finite dimensional Hilbert space.

00:06:14.540 | In fact, we even have a guess for what the dimensionality is

00:06:17.720 | is 10 to the power of 10 to the power of 122.

00:06:22.720 | That's a very large number.

00:06:24.880 | Just to compare it, the age of the universe

00:06:26.660 | is something like 10 to the 14 seconds,

00:06:29.500 | 10 to the 17 or 18 seconds maybe.

00:06:31.660 | The number of particles in the universe is 10 to the 88th.

00:06:34.420 | But the number of dimensions of Hilbert space

00:06:36.900 | is 10 to the 10 to the 122.

00:06:39.580 | So that's just crazy big.

00:06:41.540 | If that story is right, that in our observable horizon,

00:06:45.020 | there's only a finite dimensional Hilbert space,

00:06:47.840 | then this idea of branching of the wave function

00:06:50.460 | of the universe into multiple distinct separate branches

00:06:53.860 | has to reach a limit at some time.

00:06:56.020 | Once you branch that many times,

00:06:57.840 | you've run out of room in Hilbert space.

00:07:00.020 | And roughly speaking, that corresponds to the universe

00:07:02.820 | just expanding and emptying out and cooling off

00:07:06.060 | and entering a phase where it's just empty space,

00:07:09.160 | literally forever.

00:07:10.440 | - What's the difference between splitting and copying,

00:07:15.820 | do you think?

00:07:16.940 | Like in terms of, a lot of this is an interpretation

00:07:21.780 | that helps us sort of model the world.

00:07:26.780 | So perhaps shouldn't be thought of as like,

00:07:30.520 | philosophically or metaphysically,

00:07:35.000 | but even at the physics level,

00:07:38.220 | do you see a difference between sort of generating

00:07:41.820 | new copies of the world or splitting?

00:07:45.340 | - I think it's better to think of in quantum mechanics,

00:07:48.220 | in many worlds, the universe splits rather than new copies,

00:07:51.400 | because people otherwise worry about things

00:07:53.020 | like energy conservation.

00:07:54.860 | And no one who understands quantum mechanics

00:07:57.140 | worries about energy conservation

00:07:58.640 | 'cause the equation is perfectly clear.

00:08:00.660 | But if all you know is that someone told you

00:08:02.340 | the universe duplicates, then you have a reasonable worry

00:08:04.500 | about where all the energy for that came from.

00:08:06.940 | So a preexisting universe splitting

00:08:09.380 | into two skinnier universes

00:08:11.100 | is a better way of thinking about it.

00:08:12.820 | And mathematically, it's just like,

00:08:14.540 | if you draw an X and Y axis,

00:08:16.460 | and you draw a vector of length one, 45 degree angle,

00:08:20.740 | you know that you can write that vector of length one

00:08:23.460 | as the sum of two vectors pointing along X and Y

00:08:27.100 | of length one over the square root of two.

00:08:29.340 | So I write one arrow as the sum of two arrows,

00:08:33.220 | but there's a conservation of arrow-ness, right?

00:08:35.280 | Like there's now two arrows,

00:08:36.620 | but the length is the same.

00:08:37.900 | I just am describing it in a different way.

00:08:40.300 | And that's exactly what happens when the universe branches.

00:08:42.660 | The wave function of the universe is a big old vector.

00:08:45.640 | - So to somebody who brings up a question of saying,

00:08:49.740 | doesn't this violate the conservation of energy?

00:08:53.660 | Can you give further elaboration?

00:08:57.040 | - Right, so let's just be super duper perfectly clear.

00:09:00.300 | There's zero question about whether or not many worlds

00:09:03.780 | violates conservation of energy.

00:09:05.220 | It does not.

00:09:06.340 | And I say this definitively because there are other

00:09:08.860 | questions that I think there's answers to,

00:09:10.540 | but they're legitimate questions, right?

00:09:12.540 | About where does probability come from

00:09:14.300 | and things like that.

00:09:15.500 | This conservation of energy question,

00:09:17.220 | we know the answer to it,

00:09:18.620 | and the answer to it is that energy is conserved.

00:09:21.140 | All of the effort goes into how best to translate

00:09:24.740 | what the equation unambiguously says

00:09:26.780 | into plain English, right?

00:09:29.340 | So this idea that there's a universe that has,

00:09:31.220 | that the universe comes equipped with a thickness,

00:09:34.100 | and it sort of divides up into thinner pieces,

00:09:36.380 | but the total amount of universe is conserved over time

00:09:39.960 | is a reasonably good way of putting English words

00:09:43.820 | to the underlying mathematics.

00:09:45.660 | - So one of my favorite things about many worlds is,

00:09:49.160 | I mean, I love that there's something controversial

00:09:52.460 | in science, and for some reason it makes people

00:09:57.060 | actually not like upset, but just get excited.

00:09:59.920 | Why do you think it is a controversial idea?

00:10:04.240 | So there's a lot of, it's actually one of the cleanest

00:10:08.560 | ways to think about quantum mechanics.

00:10:10.680 | So why do you think there's a discomfort a little bit

00:10:14.500 | among certain people?

00:10:15.960 | - Well, I draw the distinction in my book

00:10:17.760 | between two different kinds of simplicity

00:10:20.360 | in a physical theory.

00:10:21.280 | There's simplicity in the theory itself, right?

00:10:24.560 | How we describe what's going on according to the theory

00:10:27.080 | by its own rights.

00:10:28.520 | But then, you know, a theory is just some sort

00:10:30.040 | of abstract mathematical formalism.

00:10:31.760 | You have to map it onto the world somehow, right?

00:10:34.860 | And sometimes, like for Newtonian physics,

00:10:38.600 | it's pretty obvious, like, okay, here is a bottle,

00:10:41.960 | and it has a center of mass, and things like that.

00:10:44.200 | Sometimes it's a little bit harder,

00:10:46.040 | with general relativity, curvature of space-time

00:10:48.800 | is a little bit harder to grasp.

00:10:51.360 | Quantum mechanics is very hard to map

00:10:54.120 | what the language you're talking in of wave functions

00:10:56.600 | and things like that onto reality.

00:10:58.560 | And many worlds is the version of quantum mechanics

00:11:01.400 | where it is hardest to map on the underlying formalism

00:11:05.040 | to reality.

00:11:06.160 | So that's where the lack of simplicity comes in,

00:11:09.320 | not in the theory, but in how we use the theory

00:11:12.180 | to map onto reality.

00:11:13.280 | And in fact, all of the work in sort of elaborating

00:11:17.240 | many worlds quantum mechanics is in this effort

00:11:20.480 | to map it on to the world that we see.

00:11:22.720 | So it's perfectly legitimate to be bugged by that, right?

00:11:26.680 | To say like, well, no, that's just too far away

00:11:29.840 | from my experience.

00:11:31.240 | I am therefore intrinsically skeptical of it.

00:11:34.880 | Of course, you should give up on that skepticism

00:11:36.600 | if there are no alternatives,

00:11:37.920 | and this theory always keeps working,

00:11:39.360 | then eventually you should overcome your skepticism.

00:11:41.560 | But right now, there are alternatives that are,

00:11:44.160 | that people work to make alternatives

00:11:46.520 | that are by their nature closer

00:11:48.480 | to what we observe directly.

00:11:50.040 | - Can you describe the alternatives?

00:11:51.600 | I don't think we touched on it.

00:11:52.800 | So the Copenhagen interpretation and the many worlds,

00:11:57.800 | maybe there's a difference between the Everettian

00:12:02.440 | many worlds and many worlds as it is now,

00:12:05.520 | like has the idea sort of developed and so on.

00:12:08.080 | And just in general,

00:12:08.960 | what is the space of promising contenders?

00:12:12.680 | We have democratic debates now,

00:12:14.200 | there's a bunch of candidates.

00:12:15.640 | - 12 candidates on stage. - 12 candidates on stage.

00:12:17.760 | What are the quantum mechanical candidates

00:12:19.640 | on stage for the debate?

00:12:20.760 | - So if you had a debate

00:12:23.240 | between quantum mechanical contenders,

00:12:25.480 | there'd be no problem getting 12 people up there on stage,

00:12:28.600 | but there would still be only three front runners.

00:12:30.600 | (both laughing)

00:12:32.280 | And right now the front runners would be Everett.

00:12:35.440 | Hidden variable theories are another one.

00:12:37.760 | So the hidden variable theories say

00:12:39.240 | that the wave function is real,

00:12:41.400 | but there's something in addition to the wave function.

00:12:44.200 | The wave function is not everything,

00:12:45.440 | it's part of reality, but it's not everything.

00:12:47.600 | What else is there?

00:12:49.080 | We're not sure.

00:12:50.440 | But in the simplest version of the theory,

00:12:52.680 | there are literally particles.

00:12:54.240 | So many worlds says that quantum systems

00:12:58.200 | are sometimes are wave-like in some ways

00:13:00.920 | and particle-like in another

00:13:02.160 | because they really, really are waves,

00:13:05.160 | but under certain observational circumstances,

00:13:07.140 | they look like particles.

00:13:08.680 | Whereas hidden variable says

00:13:10.880 | they look like waves and particles

00:13:12.560 | 'cause there are both waves and particles

00:13:14.640 | involved in the dynamics.

00:13:16.760 | And that's easy to do if your particles

00:13:19.240 | are just non-relativistic Newtonian particles moving around,

00:13:22.880 | they get pushed around by the wave function roughly.

00:13:26.080 | It becomes much harder when you take quantum field theory

00:13:29.400 | or quantum gravity into account.

00:13:31.600 | The other big contender are spontaneous collapse theories.

00:13:36.040 | So in the conventional textbook interpretation,

00:13:38.560 | we say when you look at a quantum system,

00:13:40.920 | its wave function collapses

00:13:42.120 | and you see it in one location.

00:13:44.320 | A spontaneous collapse theory says that

00:13:46.960 | every particle has a chance per second

00:13:51.800 | of having its wave function spontaneously collapse.

00:13:54.760 | The chance is very small.

00:13:55.920 | For a typical particle,

00:13:57.000 | it will take hundreds of millions of years

00:13:58.460 | before it happens even once,

00:14:00.100 | but in a table or some macroscopic object,

00:14:02.360 | there are way more than a hundred million particles

00:14:05.040 | and they're all entangled with each other.

00:14:06.440 | So when one of them collapses,

00:14:08.080 | it brings everything else along with it.

00:14:10.080 | There's a slight variation of this,

00:14:12.520 | that's a spontaneous collapse theory.

00:14:13.900 | There are also induced collapse theories

00:14:15.660 | like Roger Penrose thinks that

00:14:17.520 | when the gravitational difference

00:14:19.140 | between two parts of the wave function becomes too large,

00:14:21.400 | the wave function collapses automatically.

00:14:25.120 | So those are basically in my mind,

00:14:27.320 | the three big alternatives.

00:14:28.800 | Many worlds, which is just,

00:14:30.080 | there's a wave function

00:14:30.960 | and always obeys the Schrodinger equation.

00:14:33.080 | Hidden variables, there's a wave function

00:14:35.160 | that always obeys the Schrodinger equation,

00:14:36.380 | but there are also new variables

00:14:38.640 | or collapse theories, which the wave function

00:14:41.360 | sometimes obeys the Schrodinger equation

00:14:43.360 | and sometimes it collapses.

00:14:44.880 | So you can see that the alternatives

00:14:46.120 | are more complicated in their formalism

00:14:48.740 | than many worlds is,

00:14:50.060 | but they are closer to our experience.

00:14:52.640 | - So just this moment of collapse,

00:14:55.420 | do you think of it as a,

00:14:57.480 | so is a wave function fundamentally

00:15:00.460 | sort of a probabilistic description of the world

00:15:03.620 | and is collapse sort of reducing that part of the world

00:15:08.080 | into something deterministic,

00:15:09.780 | where again, you can,

00:15:10.860 | and I'll describe the position and the velocity

00:15:12.900 | in this simple classical model.

00:15:15.220 | - Well, there is--

00:15:16.060 | - Is that how you think about collapse?

00:15:17.260 | - There is a fourth category,

00:15:18.780 | is a fourth contender.

00:15:19.780 | There's a Mayor Pete of quantum mechanical interpretations,

00:15:23.820 | which are called epistemic interpretations.

00:15:26.420 | And what they say is,

00:15:28.140 | all the wave function is,

00:15:29.720 | is a way of making predictions for experimental outcomes.

00:15:32.220 | It's not mapping onto an element of reality

00:15:36.140 | in any real sense.

00:15:37.500 | And in fact, two different people

00:15:38.740 | might have two different wave functions

00:15:40.380 | for the same physical system

00:15:41.500 | because they know different things about it, right?

00:15:43.660 | The wave function is really just a prediction mechanism.

00:15:46.220 | And then the problem with those epistemic interpretations

00:15:48.900 | is if you say, okay,

00:15:50.700 | but it's predicting about what,

00:15:53.900 | like what is the thing that is being predicted?

00:15:56.620 | And they say, no, no, no.

00:15:58.580 | That's not what we're here for.

00:15:59.700 | We're just here to tell you

00:16:00.700 | what the observational outcomes are gonna be.

00:16:02.700 | - But the other interpretations kind of think

00:16:05.300 | that the wave function is real.

00:16:07.060 | - Yes, that's right.

00:16:09.040 | So that's an ontic interpretation of the wave function,

00:16:12.860 | ontology being the study of what is real, what exists,

00:16:15.900 | as opposed to an epistemic interpretation

00:16:17.900 | of the wave function,

00:16:18.820 | epistemology being the study of what we know.

00:16:20.780 | - I would actually just love to see that debate on stage.

00:16:24.860 | - There was a version of it on stage

00:16:26.420 | at the World Science Festival a few years ago

00:16:28.700 | that you can look up online.

00:16:29.780 | - On YouTube?

00:16:30.620 | - Yep, it's on YouTube.

00:16:31.740 | - Okay, awesome.

00:16:33.020 | I'll link it and watch it.

00:16:34.460 | - Many words. - Who won?

00:16:35.780 | - I won.

00:16:36.620 | (laughing)

00:16:38.200 | I don't know, there was no vote.

00:16:39.400 | There was no vote.

00:16:40.360 | But Brian Green was the moderator

00:16:43.840 | and David Albert stood up for spontaneous collapse

00:16:46.960 | and Shelley Goldstein was there for hidden variables

00:16:49.860 | and Rüdiger Schock was there for epistemic approaches.

00:16:52.600 | - Why do you, I think you mentioned it,

00:16:54.600 | but just to elaborate,

00:16:55.680 | why do you find many worlds so compelling?

00:16:59.220 | - Well, there's two reasons, actually.

00:17:01.620 | One is, like I said, it is the simplest, right?

00:17:03.820 | It's like the most bare bones,

00:17:05.400 | austere, pure version of quantum mechanics.

00:17:08.060 | And I am someone who is very willing

00:17:10.980 | to put a lot of work into mapping the formalism

00:17:13.440 | onto reality.

00:17:14.280 | I'm less willing to complicate the formalism itself.

00:17:17.340 | But the other big reason

00:17:18.540 | is that there's something called modern physics

00:17:21.360 | with quantum fields and quantum gravity

00:17:23.780 | and holography and space-time, doing things like that.

00:17:27.220 | And when you take any of the other versions

00:17:30.180 | of quantum theory, they bring along classical baggage.

00:17:33.780 | All of the other versions of quantum mechanics

00:17:36.060 | prejudice or privilege some version of classical reality

00:17:41.580 | like locations in space, okay?

00:17:44.200 | And I think that that's a barrier

00:17:46.720 | to doing better at understanding the theory of everything

00:17:49.640 | and understanding quantum gravity

00:17:50.800 | and the inversions of space-time.

00:17:52.540 | Whenever, if you change your theory from,

00:17:55.120 | here's a harmonic oscillator, oh, there's a spin,

00:17:57.780 | here's an electromagnetic field,

00:17:59.640 | in hidden variable theories or dynamical collapse theories,

00:18:02.460 | you have to start from scratch.

00:18:03.580 | You have to say like,

00:18:04.420 | well, what are the hidden variables for this theory?

00:18:05.800 | Or how does it collapse or whatever?

00:18:07.540 | Whereas many worlds is plug and play.

00:18:09.200 | You tell me the theory

00:18:10.140 | and I can give you as many worlds version.

00:18:12.160 | So when we have a situation like we have with gravity

00:18:14.820 | and space-time, where the classical description

00:18:18.060 | seems to break down in a dramatic way,

00:18:21.320 | then I think you should start from the most quantum theory

00:18:23.540 | that you have, which is really many worlds.

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Sean Carroll: Many-Worlds Interpretation of Quantum Mechanics

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