Sean Carroll

Imagine a cosmology where the dark matter particle is an axion.

And it's another cosmology where the dark matter particle is a weakly interacting massive particle.

But as far as every experiment we've ever done and every observation we've ever made right now, the universe looks the same in those two theories.

Isn't that a kind of self-locating uncertainty between possible worlds rather than locations in the real world?

I guess all that is perfectly fair, and I certainly wouldn't want to think that one must assign some symmetry to these two cases of the different cosmologies and therefore give them equal credences.

I guess all I'm trying to get at is the idea that we need to have some credences in these situations, just as a matter of practical rationality, right?

Some of the pushback I've gotten—

the notion that we need to assign these credences is just, no, I don't.

Like, what do I just don't have an attitude, just don't have an opinion about it.

And I want to say, well, but to get through life, you kind of implicitly do have opinions about all sorts of uncertainties.

And this is just one of them.

Yeah, no, I'm 100% on board.

I guess I didn't explain my example well enough because I didn't want to use that as a case of we should give equal credences.

I'm just invested in the idea that we should have credences.

Yeah, no, no, I didn't want to argue that it was 50-50 in that case.

I certainly don't think it's 50-50 in that case, because there's some, like you say, there might be different theoretical virtues that the different theories have.

They're not really symmetric with each other.

Absolutely.

But there is some uncertainty.

That's a great question.