Sean Carroll

The way that we explain what electrons or other things, everything is quantum.

The way that we explain what these things are is different than how they appear to us when we measure them.

So we teach our students something like that Copenhagen interpretation that I referred to that says that electrons behave in one way,

When you're not measuring them, that is to say they obey the Schrodinger equation and they settle into orbitals in atoms and so forth.

But then when you look at them, you need a whole other set of rules.

When you make a measurement, when you observe them, you see a dot.

You can't predict exactly where the dot's going to be.

What you can do is predict the probability of the dot being there.

And that measurement that you did radically changes the state of the electron.

We call that the collapse of the wave function.

Now, as I said in the intro, these ideas, these concepts that are playing a crucially important role here are not well-defined.

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.