Do two particles far apart know about each other?

That time, that place

Einstein settled at Princeton in 1933, having left Nazi Germany. Eight years on, he had not let the argument go.

His objections had always taken one form: imagine a device that gets around uncertainty by some technical trick. And every time, Bohr found the flaw. That route was not going to win.

Why this question

In 1935 he changed tack, working with two younger colleagues, Podolsky and Rosen. No apparatus this time — pure logic.

Let two particles interact once and fly apart. At the moment of contact the sum of their momenta is fixed. So is the difference of their positions.

Long afterwards, with the two far apart, measure this particle's position and you know that particle's position at once, by calculation. Or measure this one's momentum and you know that one's momentum.

Either way, the far particle was never touched. Yet its position and its momentum are both available to you, depending on what you decide to do here.

What was found

The paper's title was itself a question: can quantum mechanics' description of physical reality be considered complete?

The authors answered no. Logically it must be one of two things.

One: the far particle's values were fixed all along and quantum mechanics simply fails to record them. Then the theory is not wrong but incomplete — there are hidden variables we have not found.

Two: what you do here instantly affects what is there. Einstein called this spooky action at a distance. It collides head-on with relativity's ban on anything outrunning light.

To him the second was absurd. Therefore the first: hidden variables exist.

Bohr replied six weeks later under the same title, and his answer is hard to follow even now. More to the point, the dispute looked untestable — both positions predicted identical experimental results. Physicists filed it under philosophy and got on with other things for almost thirty years.

Terms on this card

EntanglementLocality