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I want to know if the following has been done experimentally; after the spin (or any other characteristic with a probability of 50%) of 2 entangled particles has been measured, we change the spin of one and we see the spin of the other changing instantaneously at a distance.

For example, entangled particle A is spinning up and entangled particle B is spinning down, we make A spin down and see B start spinning up at the same time.

I know entanglement has been proven experimentally but it always seems to imply this "spooky action at a distance" and I wonder if THAT has actually been proven experimentally. Maybe my question should have been has anyone seen spooky action at a distance...

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2 Answers 2

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No. What you describe is not what is meant by quantum entanglement. What you describe would allow instantaneous communication across large distances which would allow violations of causality and would violate special relativity.

Quantum entanglement occurs when you prepare two particles such that one is spin up and the other is spin down, but you don't know which is which. You let these two particles travel (at less than or equal to the speed of light) to two measuring apparatus that can each be set at any angle, not just up/down, and measure the spins. What you find, when you compare both measurements is that the results are correlated as expected by quantum theory. For example, if both apparatus measure at 45 degrees to the prepared spin axis, the measured spins will always still be equal and opposite. See this for a full explanation.

There is no communication at all between the particles once they are separated. Thus, entanglement cannot be used for faster than light communication since it is only the comparison of results that is predicted.

Update: Another great explanation of entanglement is here.

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You are welcome. It is a common misapprehension about entanglement. Many science fiction stories also claim that entanglement allows faster than light communication, but it just ain't so. I guess that is why it is called fiction. –  FrankH Nov 24 '11 at 14:09
Maybe you should clarify the Wikipedia entry "Experimental results have demonstrated that effects due to entanglement travel at least thousands of times faster than the speed of light.[20][21] In another experiment, the measurements of the entangled particles were made in moving, relativistic reference frames in which each respective measurement occurred before the other, and the measurement results remained correlated." –  Fraggle Nov 24 '11 at 14:48
There is nothing wrong with the wiki description. It really tells the same as FrankH, it does_not_suggest that information was send thousands of times faster than the speed of ligth. The effect(which is theoretically instantaneously) was measured experimentally to be at least this speed. –  Hans-Peter E. Kristiansen Nov 24 '11 at 20:54
@Fraggle, Thanks. Wikipedia is not exactly wrong, but it is misleading. So I edited as follows: "Experimental results have demonstrated that effects due to entanglement travel at least thousands of times faster than the speed of light....[22][23] Although the "effect" of quantum entanglement appears to exceed the speed of light, there is no violation of special relativity or causality which declares that information cannot be transferred faster than the speed of light. See EPR Paradox for an explanation about how causality is not violated." –  FrankH Nov 24 '11 at 20:58
I sort of wonder if the word "effect" is appropriate at all here. –  Fraggle Nov 25 '11 at 5:17

Do you mean literally see it? I guess they are working on experiments like that:

Or do you mean it is it physically real? This says it is:

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Ok those articles are confusing because again they imply a "spooky action at a distance". The way I see entanglement is like this; imagine the 2 particles are 2 aces from a deck of cards, they become entangled in such a way that if one is of hearts the other must be spades (lets pretend there's only 2 suits) If I scramble them and give one to you and keep the other it doesn't matter how far apart we are if I look at my card and it's ace of spades then I know yours is the ace of hearts without having to call you to find out. No communication needed, no spooky action between the cards etc... –  user6328 Nov 29 '11 at 15:52

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