Sean Carroll: Time Travel in Many-Worlds

00:00:00.000 | - How does many worlds help us understand

00:00:04.520 | our particular branch of reality?

00:00:07.780 | So okay, that's fine and good,

00:00:09.320 | that is everything is splitting,

00:00:10.840 | but we're just traveling down a single branch of it.

00:00:13.260 | So how does it help us understand our little unique branch?

00:00:18.080 | - Yeah, I mean, that's a great question.

00:00:19.840 | But that's the point is that we didn't invent many worlds

00:00:22.640 | 'cause we thought it was cool

00:00:23.600 | to have a whole bunch of worlds, right?

00:00:24.920 | We invented it because we were trying to account

00:00:27.480 | for what we observe here in our world.

00:00:30.200 | And what we observe here in our world

00:00:32.120 | are wave functions collapsing, okay?

00:00:34.560 | We do have a situation where the electron

00:00:37.560 | seems to be spread out, but then when we look at it,

00:00:39.520 | we don't see it spread out, we see it located somewhere.

00:00:42.120 | So what's going on?

00:00:43.120 | That's the measurement problem of quantum mechanics,

00:00:44.920 | that's what we have to face up to.

00:00:46.400 | So many worlds is just a proposed solution to that problem.

00:00:50.120 | And the answer is nothing special is happening,

00:00:52.720 | it's still just the Schrodinger equation,

00:00:54.600 | but you have a wave function too.

00:00:56.880 | And that's a different answer than would be given

00:00:58.880 | in hidden variables or dynamical collapse theories

00:01:01.960 | or whatever.

00:01:03.000 | So the entire point of many worlds

00:01:05.480 | is to explain what we observe,

00:01:07.360 | but it tries to explain what we already have observed,

00:01:11.160 | right, it's not trying to be different

00:01:13.640 | from what we've observed,

00:01:14.600 | because that would be something other than quantum mechanics.

00:01:17.120 | - But the idea that there's worlds that we didn't observe

00:01:20.600 | that keep branching off is kind of,

00:01:25.240 | it's stimulating to the imagination.

00:01:27.040 | So is it possible to hop from,

00:01:31.480 | you mentioned the branches are independent.

00:01:33.680 | Is it possible to hop from one to the other?

00:01:37.360 | - No.

00:01:38.200 | - So it's a physical limit.

00:01:39.880 | The theory says it's impossible.

00:01:43.280 | - There's already a copy of you in the other world,

00:01:44.720 | don't worry.

00:01:45.680 | - Yes.

00:01:46.520 | - Then leave them alone.

00:01:47.760 | - No, but there's a fear of missing out, FOMO.

00:01:52.160 | - Yes.

00:01:53.000 | - But I feel like immediately start to wonder

00:01:56.080 | if that other copy is having more or less fun.

00:02:00.440 | - Yeah, well, the downside to many worlds

00:02:02.040 | is that you're missing out on an enormous amount.

00:02:04.400 | (laughing)

00:02:05.640 | And that's always what it's gonna be like.

00:02:07.280 | - And I mean, there's a certain stage of acceptance in that.

00:02:10.600 | - Yes.

00:02:11.440 | - In terms of rewinding,

00:02:13.480 | do you think we can rewind the system back?

00:02:15.880 | Sort of the nice thing about many worlds, I guess,

00:02:19.040 | is it really emphasizes the,

00:02:22.240 | maybe you can correct me,

00:02:23.480 | but the deterministic nature of a branch.

00:02:27.940 | And it feels like it could be rewound back.

00:02:31.680 | Do you see as something that could be

00:02:35.360 | perfectly rewinded back?

00:02:39.400 | - Yeah.

00:02:40.240 | If you're at a fancy French restaurant,

00:02:41.840 | and there's a nice linen white tablecloth,

00:02:44.080 | and you have your glass of Bordeaux,

00:02:45.680 | and you knock it over,

00:02:46.760 | and the wine spills across the tablecloth,

00:02:49.680 | if the world were classical, okay,

00:02:52.960 | it would be possible

00:02:54.240 | that if you just lifted the wine glass up,

00:02:56.240 | you'd be lucky enough that every molecule of wine

00:02:58.680 | would hop back into the glass, right?

00:03:01.440 | But guess what?

00:03:02.280 | It's not gonna happen in the real world.

00:03:04.080 | And the quantum wave function is exactly the same way.

00:03:07.040 | It is possible in principle to rewind everything

00:03:09.800 | if you start from perfect knowledge

00:03:11.480 | of the entire wave function of the universe.

00:03:13.880 | In practice, it's never gonna happen.

00:03:16.320 | - So time travel, not possible?

00:03:20.120 | - Nope.

00:03:21.400 | At least quantum mechanics has no help.

00:03:23.560 | - What about memory?

00:03:27.520 | Does the universe have a memory of itself

00:03:29.440 | where we could,

00:03:30.360 | so not time travel,

00:03:34.480 | but peek back in time,

00:03:37.180 | and do a little replay?

00:03:39.240 | - Well, it's exactly the same in quantum mechanics

00:03:44.120 | as classical mechanics.

00:03:45.120 | So whatever you wanna say about that,

00:03:47.360 | the fundamental laws of physics

00:03:49.160 | in either many worlds, quantum mechanics,

00:03:51.080 | or Newtonian physics,

00:03:52.480 | conserve information.

00:03:55.160 | So if you have all the information

00:03:57.360 | about the quantum state of the world right now,

00:03:59.640 | you're Laplace's demon-like in your knowledge

00:04:01.680 | and calculational capacity,

00:04:03.680 | you can wind the clock backward.

00:04:05.480 | But none of us is, right?

00:04:07.200 | And so in practice, you can never do that.

00:04:09.440 | You can do experiments over and over again,

00:04:11.580 | starting from the same initial conditions for small systems,

00:04:14.920 | but once things get to be large,

00:04:16.920 | Avogadro's number of particles, right?

00:04:18.720 | Bigger than a cell, no chance.

00:04:21.600 | (upbeat music)

00:04:24.180 | (upbeat music)

00:04:26.760 | (upbeat music)

00:04:29.340 | (upbeat music)

00:04:31.920 | (upbeat music)

00:04:34.500 | (upbeat music)

00:04:37.080 | [BLANK_AUDIO]

Sean Carroll: Time Travel in Many-Worlds

Chapters