So, I was watching a youtube video in order to understand quantum entanglement- you know, as a non-physicist- and at one point the speaker says that it doesn't understand how the energy of one photon can be divided to several different atoms and how it can interact at the same time with each (as this photon is entangled). What I don't understand- and please excuse my lack of knowledge in the field- why are scientists so awed at the fact that a photon- which is energy- is divided? it's not as if it's a definite hard object, it's energy. Plus, isn't matter made up of energy? I've been thinking about it for a while and would really appreciate it if someone could explain what all the fuss is about.
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None of that is the problem. Entanglement has nothing to do with "divided energy" or anything: it is a form of correlation. Saying "system A and system B are entangled" means that the results of measurements on either system will be random (as all quantum mechanical systems will), but they will be correlated with each other. Moreover, entangled systems show correlations that go above and beyond what one would expect classically.
As an example, here's one way you can have systems that have correlated, random results:
Take a series of pairs of boxes. For each box, flip a coin. If it lands on heads, put one white sock in each box. If it lands on tails, put one black sock in each box. Close both boxes and ship them. Then the results of opening each box are random (you don't know what color of the sock until you open the box) but once you do open it you know the color of the sock in the other box.
This is not the type of correlation entailed by entanglement; it is strictly weaker. This kind of sock-in-a-box correlation is known technically as a Local Hidden Variables theory.
The problem with entanglement is that you can propose some specific measures of correlation for which local hidden variable models are intrinsically limited: you can prove that they can only do so well, but not better.
However, if you introduce a pair of entangled particles, you can do better than the LHV bound. This means that to model those correlations you cannot use LHV models, and all the alternatives to those models tend to weird people out.
For more details, see the many previous questions on this subject on this site.
Emilio Pisanty
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