Sean Carroll

And that's all of the universe.

And so it's a finite dimensional Hilbert space.

Now, Tom and Willie have a complicated version of why that does not give you a Boltzmann brain problem.

Basically, they're not Everettians.

They're more Copenhagen-esque, and they have strict rules for what counts as an observer and things like that, and none of it really makes a lot of sense to me, even though I have tried, because I'm just so Everettian deep down that I have trouble wrapping my brain around what they're saying.

But as an Everettian...

It's very straightforward.

There's a quantum state.

It's evolving.

That's all you have to work with.

And so if most of the time people like me, if I conditionalized on the existence of an observer like myself, then I found that, oh, it's just a random fluctuation in empty space.

I would count that as bad.

And that's why I was not that excited about these ideas.

But what we realized, Sakshi and Nadia and myself, is that there's a loophole to this argument.

And it's kind of like a fussy technical loophole, to be perfectly honest.

But just the existence of a loophole, I think, is very interesting.

There's not, in other words, a knockdown argument that the universe can't be cyclic and described quantum mechanically in a finite dimensional Hilbert space.

And so the loophole is the following.

The way that you get the ordinary Poincaré recurrence theorem from Henri Poincaré at the end of the last century is to say, you know, there's some space of possibilities, some space of states through which the system evolves over time.

And if you don't really, really finely tune it, it will fill.