- How does many worlds help us understand our particular branch of reality? So okay, that's fine and good, that is everything is splitting, but we're just traveling down a single branch of it. So how does it help us understand our little unique branch? - Yeah, I mean, that's a great question.

But that's the point is that we didn't invent many worlds 'cause we thought it was cool to have a whole bunch of worlds, right? We invented it because we were trying to account for what we observe here in our world. And what we observe here in our world are wave functions collapsing, okay?

We do have a situation where the electron seems to be spread out, but then when we look at it, we don't see it spread out, we see it located somewhere. So what's going on? That's the measurement problem of quantum mechanics, that's what we have to face up to. So many worlds is just a proposed solution to that problem.

And the answer is nothing special is happening, it's still just the Schrodinger equation, but you have a wave function too. And that's a different answer than would be given in hidden variables or dynamical collapse theories or whatever. So the entire point of many worlds is to explain what we observe, but it tries to explain what we already have observed, right, it's not trying to be different from what we've observed, because that would be something other than quantum mechanics.

- But the idea that there's worlds that we didn't observe that keep branching off is kind of, it's stimulating to the imagination. So is it possible to hop from, you mentioned the branches are independent. Is it possible to hop from one to the other? - No. - So it's a physical limit.

The theory says it's impossible. - There's already a copy of you in the other world, don't worry. - Yes. - Then leave them alone. - No, but there's a fear of missing out, FOMO. - Yes. - But I feel like immediately start to wonder if that other copy is having more or less fun.

- Yeah, well, the downside to many worlds is that you're missing out on an enormous amount. (laughing) And that's always what it's gonna be like. - And I mean, there's a certain stage of acceptance in that. - Yes. - In terms of rewinding, do you think we can rewind the system back?

Sort of the nice thing about many worlds, I guess, is it really emphasizes the, maybe you can correct me, but the deterministic nature of a branch. And it feels like it could be rewound back. Do you see as something that could be perfectly rewinded back? - Yeah. If you're at a fancy French restaurant, and there's a nice linen white tablecloth, and you have your glass of Bordeaux, and you knock it over, and the wine spills across the tablecloth, if the world were classical, okay, it would be possible that if you just lifted the wine glass up, you'd be lucky enough that every molecule of wine would hop back into the glass, right?

But guess what? It's not gonna happen in the real world. And the quantum wave function is exactly the same way. It is possible in principle to rewind everything if you start from perfect knowledge of the entire wave function of the universe. In practice, it's never gonna happen. - So time travel, not possible?

- Nope. At least quantum mechanics has no help. - What about memory? Does the universe have a memory of itself where we could, so not time travel, but peek back in time, and do a little replay? - Well, it's exactly the same in quantum mechanics as classical mechanics. So whatever you wanna say about that, the fundamental laws of physics in either many worlds, quantum mechanics, or Newtonian physics, conserve information.

So if you have all the information about the quantum state of the world right now, you're Laplace's demon-like in your knowledge and calculational capacity, you can wind the clock backward. But none of us is, right? And so in practice, you can never do that. You can do experiments over and over again, starting from the same initial conditions for small systems, but once things get to be large, Avogadro's number of particles, right?

Bigger than a cell, no chance. (upbeat music) (upbeat music) (upbeat music) (upbeat music) (upbeat music) (upbeat music)