Sean Carroll

And, of course, that naturally will give a boost to stochastic theories that are big and have many, many observers.

Then the total probability that one of them will be like me goes up.

But it doesn't keep giving an infinite boost when there's more and more observers just like me because all I care about is that there's at least one.

Yeah, I think that if the second theory had 100% chance that there was someone who saw red and someone who saw green, the first theory only has one person with a 50-50 chance that they see either red or green.

Then no matter what I see, I'm going to say that the data increases my credence in theory B. Because there's 100% chance that someone like me exists in theory B, only 50% chance that a person like me exists in theory A.

To be super clear, I'm a student here at Hopkins, Isaac Wilkins, and I are trying to write a paper about this.

So we haven't thought it all the way through, but I'm saying it out loud here in public as motivation to us to get the paper written.

Yeah.

Yeah.

No, that's a very, very good one.

Okay.

But now, all right, we've, uh, eaten our vegetables.

We've, we've laid the groundwork and now we can talk about Boltzmann brains.

Um, do you want to tell the audience member who's never heard of a Boltzmann brain, what that weird phrase refers to?

And I think, tell me what you think about this.

Here's a case where my fellow cosmologists let me down a little bit, because many of them just think, well, but that's silly, therefore I'm not gonna think about it.

Okay, that's an attitude.

But there's another attitude that says, no, I take this very, very seriously, the Boltzmann brain problem, and my attitude is the following.

If I lived in one of these eternal universes with random fluctuations, then I would be a Boltzmann brain.

But I look around and I see I am not, therefore I do not live in such a cosmology.