Sean Carroll: Arrow of Time
00:00:00.000 | - We talked a little bit about era of time last time,
00:00:05.000 | but in many worlds that there is a kind of
00:00:08.920 | implied era of time, right?
00:00:14.160 | So you've talked about the era of time
00:00:16.760 | that has to do with the second law of thermodynamics.
00:00:20.800 | That's the era of time that's emergent or fundamental.
00:00:25.040 | We don't know, I guess.
00:00:26.440 | - No, it's emergent.
00:00:28.320 | - Does everyone agree on that?
00:00:30.760 | Well, nobody agrees with everything.
00:00:32.120 | - They should. - They should.
00:00:33.520 | (laughing)
00:00:35.040 | So that era of time, is that different
00:00:37.760 | than the era of time that's implied by many worlds?
00:00:40.700 | - It's not different, actually, no.
00:00:42.160 | In both cases, you have fundamental laws of physics
00:00:45.960 | that are completely reversible.
00:00:47.640 | If you give me the state of the universe
00:00:49.480 | at one moment in time, I can run the clock
00:00:51.600 | forward or backward equally well.
00:00:53.040 | There's no arrow of time built into the laws of physics
00:00:56.200 | at the most fundamental level.
00:00:57.980 | But what we do have are special initial conditions
00:01:02.000 | 14 billion years ago near the Big Bang.
00:01:04.640 | In thermodynamics, those special initial conditions
00:01:07.320 | take the form of things were low entropy,
00:01:10.240 | and entropy has been increasing ever since,
00:01:12.360 | making the universe more disorganized and chaotic,
00:01:15.400 | and that's the era of time.
00:01:17.300 | In quantum mechanics, these special initial conditions
00:01:19.980 | take the form of there was only one branch
00:01:22.800 | of the wave function, and the universe
00:01:24.320 | has been branching more and more ever since.
00:01:27.720 | - Okay, so if time is emergent,
00:01:30.940 | so it seems like our human cognitive capacity
00:01:35.040 | likes to take things that are emergent
00:01:36.760 | and feel like they're fundamental.
00:01:39.100 | So if time is emergent, and locality,
00:01:46.360 | like is space emergent?
00:01:49.400 | - Yes.
00:01:50.240 | - Okay.
00:01:51.060 | - But I didn't say time was emergent,
00:01:51.900 | I said the arrow of time was emergent.
00:01:53.560 | Those are different.
00:01:54.840 | What's the difference between the arrow of time and time?
00:01:59.360 | Are you using arrow of time to simply mean
00:02:01.880 | they're synonymous with the second law of thermodynamics?
00:02:04.840 | - No, but the arrow of time is the difference
00:02:06.840 | between the past and future.
00:02:08.240 | So there's space, but there's no arrow of space.
00:02:11.280 | You don't feel that space has to have an arrow, right?
00:02:13.640 | You could live in thermodynamic equilibrium,
00:02:15.800 | there'd be no arrow of time, but there'd still be time.
00:02:18.360 | There'd still be a difference between now
00:02:19.800 | and the future or whatever.
00:02:21.320 | - Ah, so, okay.
00:02:22.640 | So if nothing changes, there's still time.
00:02:25.760 | - Well, things could even change.
00:02:26.960 | Like if the whole universe consisted of the Earth
00:02:30.400 | going around the sun, okay?
00:02:32.760 | It would just go in circles or ellipses, right?
00:02:35.400 | - That's an equilibrium.
00:02:36.240 | - Things would change, but it's not increasing entropy.
00:02:38.480 | There's no arrow.
00:02:39.360 | If you took a movie of that,
00:02:40.760 | and I played you the movie backward, you would never know.
00:02:43.720 | - So the arrow of time can theoretically point
00:02:49.160 | in the other direction for brief, briefly.
00:02:52.400 | - To the extent that it points in different directions,
00:02:54.920 | it's not a very good arrow.
00:02:56.000 | I mean, the arrow of time in the macroscopic world
00:02:58.880 | is so powerful that there's just no chance of going back.
00:03:01.760 | When you get down to tiny systems
00:03:03.320 | with only three or four moving parts,
00:03:04.880 | then entropy can fluctuate up and down.
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