Swans on Tea

Even if we don’t practice to deceive.

zapperz has a post up which points to an article in the WSJ on quantum entanglement: Science, Spirituality, and Some Mismatched Socks

zz marks it as a good layman’s review, but I don’t agree.

Stranger still is entanglement. When two photons get “entangled” they behave like a joint entity. Even when they’re miles apart, if the spin of one particle is changed, the spin of the other instantly changes, too. This direct influence of one object on another distant one is called non-locality.

This is a common summary of entanglement, and it’s wrong. The entangled particles are in indeterminate states — the only thing you know is that the states have a particular relationship, e.g. one is spin up and one is spin down, or the polarizations are perpendicular, depending on how you entangled them. But the notion that one of the particles has a definite state before it’s measured is a classical interpretation, not a quantum mechanical one, and that’s where the analogies that are often used fail to work. That is, a particle prepared in this fashion does not have a state until it is measured — the state of the particle does is not “hidden.”

So if you don’t know what the state of the particle is, you can’t say that it has changed. What you can say is that when you measure the state of one particle, you instantly know the state of its entangled partner, but at the instant you do this measurement, the particles are no longer entangled. Further interactions affecting that attribute will result in no effect on the other particle. And I think this is where the quantum wheels come off the wagon, because this classical misconception has not been dispelled. There’s still this idea that the two particles communicate, and do so instantly. The description given gives the implication that this is so, and then you have the contradiction when you are told that faster-than-light communication isn’t possible with entanglement.

The amusing story of Bertlmann’s socks harms the explanation.

Mr. Bell noted that if he saw one of Mr. Bertlmann’s feet coming around the corner and it had a pink sock, he would instantly know, without seeing the other foot, that the second sock wouldn’t be pink. To the casual observer that may seem magical, or controlled by “hidden variables,” but it was no mystery to Mr. Bell because he knew that Mr. Bertlmann liked to wear mismatched socks.

For the story to work properly you have to also include the notion that which foot was sporting the pink sock wasn’t known until you measured it. All you knew was that one foot had a pink sock, and that if you measured it on Mr. Bertlmann’s left foot, you cannot say that it was on his left foot at any previous point. Thus is the weirdness of quantum mechanics and entanglement.

My take is that any article that puts forth such a basic misconception can’t be a good layman’s guide.

A better treatment