Sean Carroll

but they're well-defined enough to get us through the 20th century, basically.

In other words, when do you do a measurement?

What counts as a measurement?

You know it when you see it, right?

We have still not developed, in the Copenhagen view, a perfectly well-defined notion of when and how and why these wave functions collapse, okay?

But it works.

If you just say, well, I'm just going to say that when I look at it, it collapses, that turns out to work.

It works really, really well.

So that's why physicists didn't spend a lot of time thinking about the foundations of quantum mechanics throughout the 20th century.

They had a version of the theory that worked pretty well.

Now, it becomes a little more complicated when you go from one electron to two electrons.

You might not think it's that complicated.

Isaac Newton went from the gravitational field of one planet to the gravitational field of two planets, right, without that much difficulty.

At least in the world of Newton or Einstein or Maxwell or any of the giants of pre-quantum physics, there's no special difficulty in going from a theory explaining one object to a theory explaining two objects.

You just now have two things happening.

You have one electron doing its thing.

It has a wave, a wave function.

And you have the other electron.

It should have its own wave function.

It turns out that's just not how it works, and we can go into justifications for this, but we have other fish to fry, so I'm just going to tell you the answer.