The following is an experiment that shows "spooky action at a distance" of quantum entanglement; that is, when we measure some properties of two entangled photons, we find that measuring property of one of the electrons determines the property of the other:


How, exactly, does this experiment show the entanglement of photons? Please explain it to me in a simple way; I cannot find an answer to this question in Wikipedia or on the whole internet because they contain many complicated words, explaining things in a very difficult way.

what i wanted to know is that why or how does an particle having predefined characteristics violates the law of angular momentum. I have watched this Veritasium/Minutephysics video (http://www.youtube.com/watch?v=ZuvK-od647c) you suggested to me. In particular, what I am trying to figure out is that, how does hidden variable theory is shown to be impossible. what's the reason behind this. explain it in simple word.


closed as too broad by Phoenix87, ACuriousMind, DanielSank, Martin, Emilio Pisanty May 23 '15 at 23:05

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  • $\begingroup$ en.wikipedia.org/wiki/Bell_test_experiments $\endgroup$ – Phoenix87 May 23 '15 at 19:17
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    $\begingroup$ Try looking at this video for a discussion on quantum entanglement: youtube.com/watch?v=ZuvK-od647c $\endgroup$ – Joshuah Heath May 23 '15 at 19:50
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    $\begingroup$ Please explain explicitly what you don't understand. It would be best if you can describe the experiment in words; youtube links are often not clicked on here. $\endgroup$ – DanielSank May 23 '15 at 21:30
  • $\begingroup$ Hidden variables aren't ruled out - as a matter of fact, the wave-function is something like a hidden variable. Local (!) hidden variable theories are different - they are (more or less) ruled out. Also, note that entanglement and local hidden variables are two different concepts. There is some overlap, but there are entangled particles admitting local hidden variable descriptions. $\endgroup$ – Martin May 24 '15 at 10:15

It all goes back to EPR experiment. In a paper published by Einstein, Podolsky and Rosen the authors argued that quantum mechanics is "incomplete". They argued that principles (such as the principle of locality) needed to be restored in order for it be a complete theory. The problem is, ultimately quantum theory is a "nonlocal" theory. What this means is that when you make a measurement on one system, a distant system is instantaneously affected. For example:

A pair of electrons are prepared in a singlet spin state. If either particle is measured with spin up (+1/2), then the corresponding particle is always measured with spin down (-1/2).

This "spooky action at a distance" as Einstein referred to it, seemed to imply information being propagated faster-than-light. However, it seemed to Einstein that this can't be right (as it violated Special Relativity).

One solution was to resort to a so-called hidden variables theory. Essentially, a hidden variables theory states that there must be "hidden variables" that would allow you explain the entanglement of the pair of particles without resorting to this "instantaneous action-at-distance". This ultimately was what Einstein was holding out for. The problem with hidden variable theories is that there is no experimental evidence for them, and certain local hidden variable theories have been shown to be impossible.

Thus, it seems we must simply live with quantum entanglement. These spin correlations have been observed in experiment time and time again, and there is no reason to believe they are not a fundamental consequence of the theory. Incidentally, while quantum entanglement certainly exists, it does not require propagation of information faster than light, and thus doesn't violate SR.




  • $\begingroup$ i was searching for that how does hidden variable theory is shown to be impossible. what's the reason behind this. explain it in simple words. $\endgroup$ – Pushkar Soni May 24 '15 at 4:13

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