Sean Carroll

And they show up, these kinds of puzzles, again and again.

You can guess that things like the Boltzmann brain scenario, where there's random fluctuations that create observers like us,

in the far future or maybe in the far past, but ones that don't arise via thermodynamically sensible evolution from a low entropy Big Bang, like we think we did.

There's examples like the multiverse of Everettian quantum mechanics.

When I measure the spin of an electron, and it could be spin up or spin down, I'm saying, okay, now there's a spin up particle, spin down particle.

They're two separate worlds, right?

And I want to say, what's the probability that I'm in one world or the other?

Does it matter how thick the world is or the different things that come into consideration here?

So this is obviously a set of puzzles that is very relevant for cosmology and physics, you know, real things we care about, quantum mechanics, the multiverse, things like that, as well as for philosophers who want to know how should we be rational in situations like this?

How should we reason in these circumstances of uncertainty?

It's a tough one and today's podcast guest is one of the world's experts on these issues.

Where you say you flip a coin and if it's heads, you're going to wake up Sleeping Beauty on Monday.

If it's tails, you're going to wake up on Monday and Tuesday.

Do you give equal credence to all three possibilities, heads and Monday, tails and Monday, tails and Tuesday?

Or do you say, no, the coin is 50-50.

It's a fair coin.

I think that it's still 50% chance that I was in the heads universe versus the tails universe, even though I only wake up once if I'm in the heads universe.

Tough call.

Smart people disagree about this.

Super relevant, even though it's just a fun philosophical thought experiment.