Sean Carroll

Hello everyone, and welcome to the Mindscape Podcast.

I'm your host, Sean Carroll.

One of the things we've talked about many times on the podcast is how you update your beliefs when new evidence comes in.

That is to say, the process of Bayesian reasoning.

Bayes' formula, of course, gives you a quantitative way of saying if I have some prior credence for some claim being true and I very quantitatively measure some data and I can calculate the likelihood of that data being obtained under all sorts of different propositions being true, I can update.

my credences, to get one that takes that data into account.

We don't necessarily every time work in such a quantitative vein, but this process is basically what we do in science, right?

In science, we have different kinds of theories that propose to provide explanations for different kinds of phenomena.

And we have different feelings.

Some theories are more likely than others.

My favorite example is always is the dark matter, something like a weakly interacting massive particle, a WIMP, or something like an axion.

So these are two different particle physics candidates for the dark matter.

They're both plausible.

We don't have any idea which one is true or even if it's some other theory.

But we have favorites, right?

We don't give them equal probability because maybe it fits in better to other things we know, etc.,

So that seems like a pretty straightforward kind of process.

You have prior probabilities for theories being true or whatever, and then you get more data and you update your belief, your degree of belief, your credence.

Here's a puzzle.

What if you're a cosmologist?