Sean Carroll

Yeah, it's a great question.

So the way I like to think about it is we have an equation, the Schrodinger equation.

You can read about it in my book, Quanta and Fields, by the way.

And the equation makes predictions for what's going to happen.

And the simplest reading of the prediction is that the universe branches into these many copies, slightly changed because in one the electron is spinning clockwise, in the other it's counterclockwise, but otherwise the universe is the same.

And we live in one of those possibilities, we don't see the other ones, and it seems to fit the data.

So then you say, okay, what about all those other possibilities?

Should we take them seriously as real?

And the answer is, if you take our possibility as real, then you have two choices.

Number one, either you take the other possibilities as also real, you know, you treat them equal, they're all there in the equation, I'm going to take them seriously.

Or you tell me why I shouldn't treat the other ones as real.

And there's a long history of people trying to come up with reasons why the other worlds aren't there.

Disappearing worlds, theories of quantum mechanics, as Ted Bunn has called them.

It turns out to be hard.

It turns out to be awkward.

It doesn't fit in well with modern physics.

You can try to do it.

It's a free country.

Go nuts.

Invent all the theories of quantum mechanics you want.