Sean Carroll

What there is is the wave function, which is a combination, a superposition of all of those possibilities.

OK, so that's quantum mechanics.

I hope that made sense.

There's more to be said about quantum mechanics, but that's the basic ontology, we would say, of the theory, what it says really exists.

We're working for the purposes of this podcast in a realist ontology for the quantum wave function.

We think the wave functions represent reality, OK?

And the Copenhagen interpretation goes on to say you don't measure all of reality when you do a measurement.

You see a version of it, and you can only predict the probability that you see some version of it.

Many worlds, by contrast—and we're not going to get into the worlds aspect of many worlds that much because it's actually not relevant for what we're talking about today.

I've often said that many worlds is not mostly about the worlds.

The worlds come along.

They're there, no doubt.

But what Many Worlds is really about is saying there's no such thing as collapse.

There's no such thing as a special cordoned off concept of measurement in quantum mechanics.

There's just the Schrodinger equation, letting the wave function evolve as it will.

And Everett's brilliant idea was if you count the observer as part of the quantum system,

you get a very different answer than the Copenhagen story.

And again, people like Heisenberg and Bohr, they're not dummies.

They're very, very smart people who thought about this stuff really, really deeply and carefully.

And they knew that if you treated the observer as part of the quantum system, you were going to quickly go down a very strange road.