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This article at NewScientist magazine (subscription required) describes entangling photons by passing them through a half silvered mirror.

It talks about the "weirdness" of entanglement and how it means that either information is traveling faster than the speed of light or other fundamental assumptions about reality are wrong because the spins of the emerging photons are always correlated when measured.

My question is just what's so strange about it? I don't get it... It seems to me to be just another variation of conservation laws. It would be like saying that it's "weird" for a photon in pair production to always produce particles of opposite charge. That's a given... Is there something to the timing of it that ruffles everybody's feathers?

I think I'm missing something.

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I think the thing you're missing is that there are no local hidden variables. That means that an entangled particle's spin is not just unknown, but it isn't set until someone measures the spin of its pair. This is feather-ruffling because it means there would be a communication between entangled particles at faster than light speed saying that one has been measured and setting the spin of the other. – Jim Aug 6 '13 at 20:46
How do we know it's not set if we can't tell what value it has until we measure it? – John Aug 6 '13 at 21:10
IIRC, usually we interpret an observable as not having a meaningful value until we do measure it. Delayed quantum erasers and suchlike are difficult to explain I otherwise. – Chay Paterson Aug 6 '13 at 21:32
@Jim one could say the exact same thing for pair particle creation. Until a magnetic field is applied,for example, one cannot tell a positron from an electron. So? I am with John on this. There is nothing mysterious within the rules of QM. Too much philosophy.Could you be more explicit about the need of hidden variables to explain these simple situations? – anna v Aug 7 '13 at 4:37
@annav Frankly, I find myself drawn to your line of thinking, however, research has told me that the main problems arise from a lack of local hidden variables. In most respects it is the same, but unlike charge, spin is a local variable. So when you use entangled particles in situation's like the experiment to prove Bell's inequality, you wind up with a non-classical result. – Jim Aug 7 '13 at 13:11
up vote 3 down vote accepted

"What's so strange about it? " This was essentially the point of the EPR argument in 1935, which says that it's silly to postulate all sorts of quantum weirdness when correlations could be explained simply by assuming that particles had definite properties. If I put each of a pair of socks in two boxes and move the boxes far apart, then it is no surprise that the socks match when the boxes are opened - it doesn't involve any weirdness or superluminal speeds.

The EPR argument was largely ignored, not so much it seems because people had any good counter-argument, but because people really, really like quantum weirdness. In the early 1960's John Bell took up the EPR argument again. What he found was that although the EPR argument worked for simple cases, like the socks, for slightly more complicated setups it ran into problems.

Two entangled particles are emitted by a central source, to be measured by Alice an Bob, who are some distance apart, and who each have a measuring device with a knob with three settings A, B and C and which gives a zero or a one as a result of the measurement.

When comparing there results for long runs of join measurements, Alice and Bob find that if the settings are the same then the results are always the same. If the settings differ by one position (so one has A and the other B, or one has B and the other C) then the results differ 1/7 of the time. A bit of thinking then leads to the result if one has setting A and the other setting C, then the results can differ at most 2/7 of the time.

The trouble is that this isn't what happens in reality. A quantum experiment can be set up such that the results differ half of the time. It is this result that requires quantum weirdness as an explanation

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