Transcription
In order to understand the timing of collapse of the wave function and that sort of thing, you have to have, what I recall, two kinds of reality. There's what I call quantum reality and classical reality.
In classical reality, you can ascertain. I can say, "Hello glass, what's your shape?" and it says, "I've got a circle at the top and I'm pretty well parallel all down there." Classical reality, you can ask a thing a question about it and it can answer directly. Quantum reality, you can only confirm.
So it's a distinction. And there was a thing due to Einstein, actually, when we're talking about Einstein here, is questions: How real is the wave function? And you can think about the simplest quantum system, really, is the spin of a spin-half particle. And it can spin right-handed about the up direction or right-handed about the down direction. And you usually talk about the direction in which it's spinning. It really needs right-handed about that direction. And the different directions, there are really two states, up and down. And then all the others are superpositions of those two.
Can you ascertain I've got a free electron here, which is just on its own here? Can you ascertain what its spin is, where its spin is pointing? No, you can't. But if you had somehow, in the way that was produced, some machinery and all that stuff, by now you'd say the electron would be spinning about that axis. And you can confirm that. You can do an experiment. It says, "Hello, hello electron, is your spin about that axis?" And you, 100% certainty, let's just say you can do, repeat the experiment many, many times, 100% certainty, you get it right. So you can confirm it. If you didn't have it right, it would say yes sometimes, no other times.
So this quantum state that can be confirmed, but it cannot be ascertained. And this is how you get round the various paradoxes that are involved in special relativity. Can you send signals into the past and all that kind of stuff? It sounds pretty crazy when you look at it, but it doesn't lead to contradiction because it's quantum reality. And quantum reality, you cannot ascertain. We can only confirm.