This is a clip from a conversation with Sean Carroll from Nov 2019. Check out Sean's new book on quantum mechanics titled Something Deeply Hidden: https://amzn.to/2C6aCaf New full episodes are released once or twice a week and 1-2 new clips or a new non-podcast video is released on all other days. You can watch the full conversation here: https://www.youtube.com/watch?v=iNqqOLscOBY
(more links below)

Podcast full episodes playlist:
https://www.youtube.com/playlist?list=PLrAXtmErZgOdP_8GztsuKi9nrraNbKKp4

Podcasts clips playlist:
https://www.youtube.com/playlist?list=PLrAXtmErZgOeciFP3CBCIEElOJeitOr41

Podcast website:
https://lexfridman.com/ai

Podcast on Apple Podcasts (iTunes):
https://apple.co/2lwqZIr

Podcast on Spotify:
https://spoti.fi/2nEwCF8

Podcast RSS:
https://lexfridman.com/category/ai/feed/

Note: I select clips with insights from these much longer conversation with the hope of helping make these ideas more accessible and discoverable. Ultimately, this podcast is a small side hobby for me with the goal of sharing and discussing ideas. I did a poll and 92% of people either liked or loved the posting of daily clips, 2% were indifferent, and 6% hated it, some suggesting that I post them on a separate YouTube channel. I hear the 6% and partially agree, so am torn about the whole thing. I tried creating a separate clips channel but the YouTube algorithm makes it very difficult for that channel to grow. So for a little while, I'll keep posting clips on this channel. I ask for your patience and to see these clips as supporting the dissemination of knowledge contained in nuanced discussion. If you enjoy it, consider subscribing, sharing, and commenting.

Sean Carroll is a theoretical physicist at Caltech and Santa Fe Institute specializing in quantum mechanics, arrow of time, cosmology, and gravitation. He is the author of several popular books and is a host of a great podcast called Mindscape.

Subscribe to this YouTube channel or connect on:
- Twitter: https://twitter.com/lexfridman
- LinkedIn: https://www.linkedin.com/in/lexfridman
- Facebook: https://www.facebook.com/lexfridman
- Instagram: https://www.instagram.com/lexfridman
- Medium: https://medium.com/@lexfridman
- Support on Patreon: https://www.patreon.com/lexfridman

We talked a little bit about error of time last time, but in many worlds, there is a kind of implied error of time, right?

So you've talked about the error of time that has to do with the second law of thermodynamics.

That's the arrow of time that's emergent or fundamental.

So that arrow of time, is that different than the arrow of time that's implied by many worlds?

In both cases, you have fundamental laws of physics that are completely reversible.

If you give me the state of the universe at 1 moment in time, I can run the clock forward or backward equally well.

There's no arrow of time built into the laws of physics at the most fundamental level.

But what we do have are special initial conditions 14 billion years ago near the Big Bang.

In thermodynamics, those special initial conditions take the form of things were low entropy, and entropy has been increasing ever since, making the universe more disorganized and chaotic, and that's the arrow of time.

In quantum mechanics, the special initial conditions take the form of there was only 1 branch of the wave function, and the universe has been branching more and more ever since.

Okay, so if time is emergent, so it seems like our human cognitive capacity likes to take things that are emergent and assume and feel like they're fundamental.

So what, so if time is emergent, and Locality, like is space emergent?

But I didn't say time was emergent, I said the arrow of time was emergent.

What's the difference between the arrow of time and time?

Are you using arrow of time to simply mean they're synonymous with the second law of thermodynamics?

No, but the arrow of time is the difference between the past and future.

So there's space, but there's no arrow of space.

You don't feel that space has to have an arrow, right?

You could live in thermodynamic equilibrium.

There'd be no arrow of time, but there'd still be time.

There'd still be a difference between now and the future or whatever.

So if nothing changes, there's still time.

Like if the whole universe consisted of the Earth going around the sun, okay?

It would just go in circles or ellipses, right?

Things would change, but it's not increasing entropy.

If you took a movie of that, and I played you the movie backward, you would never know.

So the arrow of time can theoretically point in the other direction for brief, briefly.

that it points in different directions, it's not a very good arrow.

I mean, the arrow of time in the macroscopic world is so powerful that there's just no chance of going back.


Speaker 0: We talked a little bit about error of time last time, but in many worlds, there is a kind of implied error of time, right? So you've talked about the error of time that has to do with the second law of thermodynamics. That's the arrow of time that's emergent or fundamental. We don't know, I guess.

Speaker 1: No, it's emergent.

Speaker 0: Is everyone agree on that? Well, nobody agrees with everything. They should. They should. Okay.

So that arrow of time, is that different than the arrow of time that's implied by many worlds?

Speaker 1: It's not different, actually, no. In both cases, you have fundamental laws of physics that are completely reversible. If you give me the state of the universe at 1 moment in time, I can run the clock forward or backward equally well. There's no arrow of time built into the laws of physics at the most fundamental level. But what we do have are special initial conditions 14 billion years ago near the Big Bang.

In thermodynamics, those special initial conditions take the form of things were low entropy, and entropy has been increasing ever since, making the universe more disorganized and chaotic, and that's the arrow of time. In quantum mechanics, the special initial conditions take the form of there was only 1 branch of the wave function, and the universe has been branching more and more ever since.

Speaker 0: Okay, so if time is emergent, so it seems like our human cognitive capacity likes to take things that are emergent and assume and feel like they're fundamental. So what, so if time is emergent, and Locality, like is space emergent?

Speaker 1: Yes.

Speaker 0: Okay.

Speaker 1: But I didn't say time was emergent, I said the arrow of time was emergent. Those are different.

Speaker 0: What's the difference between the arrow of time and time? Are you using arrow of time to simply mean they're synonymous with the second law of thermodynamics?

Speaker 1: No, but the arrow of time is the difference between the past and future. So there's space, but there's no arrow of space. You don't feel that space has to have an arrow, right? You could live in thermodynamic equilibrium. There'd be no arrow of time, but there'd still be time.

There'd still be a difference between now and the future or whatever.

Speaker 0: Oh, so, okay. So if nothing changes, there's still time.

Speaker 1: Well, things could even change. Like if the whole universe consisted of the Earth going around the sun, okay? It would just go in circles or ellipses, right? Things would change, but it's not increasing entropy. There's no arrow.

If you took a movie of that, and I played you the movie backward, you would never know.

Speaker 0: So the arrow of time can theoretically point in the other direction for brief, briefly. To the extent

Speaker 1: that it points in different directions, it's not a very good arrow. I mean, the arrow of time in the macroscopic world is so powerful that there's just no chance of going back. When

See all Lex Fridman transcripts on Youtube

Sean Carroll: Arrow of Time