What is quantum entanglement?

We put this question to Dave Zobel, author of "The Science of TV's The Big Bang Theory: explanations even Penny would understand"

Dave Zobel - Quantum entanglement which Albert Einstein referred to as spooky action at a distance or he actually used the German words for it which sounds like spooky action at a distance if you say it with a German accent is, this concept that we really can't tell why it happens. We can say what happens. It's a bit like some of the other things we've spoken about in the show already. If you have two particles that are created at the same instance by the same process which can happen quite frequently, they can have certain properties that are identical but opposite. One of them is a property called spin which has nothing to do with what we think of when we say the word spin, and that's why we call it spin! If those two particles are moved very far apart but no one has measured their spin, then it's not just that they have spin that is unknown. They don't yet have any meaningful spin. It's not that they have spin zero. They just have no spin that we can speak of. If you then measure the spin of one of the particles, you will find that whatever it is, when you measure the spin of the other particle, it's the exact opposite. It's as if the two particles had spoken to each other and said, "Okay, he's about measure me. I'm going to have spin plus one. You have spin minus one, right?" But, in fact, they can't communicate that way and so, we don't really know what's happening and that, I think, is why Einstein used the German word for spooky. Chris - And I think Niels Bohr said, "If you're not baffled by quantum mechanics then you just didn't understand it!"