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Can someone convince me that there aren't hidden variables? Because my puny monkey brain hears "a variable can be one of two states, but in the beginning, it's

Can someone convince me that there aren't hidden variables? Because my puny monkey brain hears "a variable can be one of two states, but in the beginning, it's in a super-position of them. When measured, the state is determined." and it's a relative easy thing to get. But.... "There are two particles entangled together such that when one becomes up or down, the other becomes the opposite, and measuring either will determine both, instantaneously, regardless of distance" is a much harder thing to swallow. Occam's Razor says that there are hidden variables that are pre-selecting state, and we just don't know about them until we measure. ESPECIALLY when one of the requirements for the system is that we can only measure, not influence the outcome of the measurement. So please, someone who knows more than I do - why can't there be hidden variables being set at entanglement that pre-determine the measurement outcome?

weinzierl 11y ago
Bell's theorem is the keyword you should look up[1]. If you are more a CS type than a physicist understanding the CHSH game might help. "Lecture 20: Bell inequalities and nonlocality" from John Watrous of the University of Calgary [2] is a short and accessible introduction to the CHSH game. fizx mentioned Scott Aaronson's "Bell inequality violation finally done right" [3] which is about the same recent experiment as the NYT article but much better. It contains a concise description of the CHSH game. [1] https://en.wikipedia.org/wiki/Bell's_theorem https://en.wikipedia.org/wiki/Bell's_theorem [2] https://cs.uwaterloo.ca/~watrous/CPSC519/LectureNotes/20.pdf https://cs.uwaterloo.ca/~watrous/CPSC519/LectureNotes/20.pdf [3] http://www.scottaaronson.com/blog/?cat=33 http://www.scottaaronson.com/blog/?cat=33
- jahnu 11y ago
  As with many of these types of things I recommend checking if there is a simple English version too. https://simple.wikipedia.org/wiki/Bell's_theorem https://simple.wikipedia.org/wiki/Bell's_theorem
  - deleted 11y ago
    [deleted]
  - bsaul 11y ago
    That's a fantastic link, thanks !
fizx 11y ago
So the real problem isn't that "when one is up, then the other will magically be up too." That could be accomplished with local hidden variables (e.g. shared seeds on a PRNG). The real problem is that when you measure A in the "up" direction, and then B in the "10 degrees east of up" direction, then B seems to know that you measured A in the "up" direction. That is to say: B's probability distribution as a function of the direction its being measured is dependent on the direction that A is measured. There's no way to construct an "A-independent" probability distribution of B's results for arbitrary directions. The probabilities won't sum to 1 and still match experimental results. It's unfortunate that "A up" therefore "B up" is a degenerate case of this reality where classicality actually works, because it leads to confusion. Also, follow the links others have provided for a more formal explanation :)
- deleted 11y ago
  [deleted]
- mkehrt 11y ago
  This is the clearest explanation I have ever seen of this; thanks.
- trhway 11y ago
  >The probabilities won't sum to 1 and still match experimental results. exactly. The probabilities are 1 only in theory. For example theoretical Malus law over all angles would give 1. On practice - i couldn't find single photon Malus law confirmation. A slight deviation from Malus law like cutting the tail at high angles off and thus decreasing the total to below 1 (ie. when/if photons at high angles being lost disproportionally much more intensively than Malus law states) would bring the experimentally measured S to values higher than 2 while still being in the realm of local realism.
- bsaul 11y ago
  But then what about a partially shared seed (sharing only a few digits) ? Couldn't you imagine a hidden variable only influencing the probablity distribution, making it only more likely to see an up on both, although still keeping it partially random ?
- 11y ago
xlm1717 11y ago
Agree with weinzierl, you want to look into Bell's theorem. He essentially set a limit (Bell's inequality) to what local variables could do to influence the measurements on both entangled particles. Bell's inequality has repeatedly been violated in experiments on quantum mechanics, which tells us that local, hidden variable cannot be the cause for the correlations in measurements of entangled particles. Bell's theorem then simply states that local hidden variables cannot ever reproduce all the experiments of quantum mechanics. It's not strictly correct to say that Occam's Razor says we should have hidden variables. Occam's Razor is the idea that in deciding between two or more similar theories (which both have to be correct), we should prefer the simpler of the theories. Bell's theorem tells us that we should reject the notion of local hidden variables. Importantly, this does allow the notion of nonlocal hidden variables. One such theory is the de Broglie-Bohm theory, which claims that the wavefunction is an actual physical entity, referred to as the pilot wave or guiding wave. It would be nonlocal because the wavefunction collapse of one particle to measure its properties would necessarily influence the other particle at superluminal speeds. Another nonlocal hidden variable theory is superdeterminism, which claims that everything that ever happened and ever will happen has already been determined, so the correlations are not a result of some spooky interaction, they were just "predestined" to be correlated. This would again be nonlocal because the universe as a whole would have knowledge of the particle properties a priori.
timeu 11y ago
On the university's hp are two nice videos that explain the Bell test: http://hansonlab.tudelft.nl/loophole-free-bell-test/ http://hansonlab.tudelft.nl/loophole-free-bell-test/
nokcha 11y ago
These experiments provide evidence against local hidden variables. The experiments are silent about theories that involve non-local hidden variables. The act of measuring one particle of an entangled pair instantly affects the other particle, instead of this effect needing to propagate at or below the speed of light; this instant interaction ("spooky action at a distance") is incompatible with local hidden variables. As other folks have mentioned, look up Bell's inequalities for more information. Also, it should be noted that quantum entanglement does not allow information to travel faster than light.

weinzierl 11y ago

Bell's theorem is the keyword you should look up[1]. If you are more a CS type than a physicist understanding the CHSH game might help. "Lecture 20: Bell inequalities and nonlocality" from John Watrous of the University of Calgary [2] is a short and accessible introduction to the CHSH game. fizx mentioned Scott Aaronson's "Bell inequality violation finally done right" [3] which is about the same recent experiment as the NYT article but much better. It contains a concise description of the CHSH game. [1] https://en.wikipedia.org/wiki/Bell's_theorem https://en.wikipedia.org/wiki/Bell's_theorem [2] https://cs.uwaterloo.ca/~watrous/CPSC519/LectureNotes/20.pdf https://cs.uwaterloo.ca/~watrous/CPSC519/LectureNotes/20.pdf [3] http://www.scottaaronson.com/blog/?cat=33 http://www.scottaaronson.com/blog/?cat=33

jahnu 11y ago
As with many of these types of things I recommend checking if there is a simple English version too. https://simple.wikipedia.org/wiki/Bell's_theorem https://simple.wikipedia.org/wiki/Bell's_theorem
- deleted 11y ago
  [deleted]
- bsaul 11y ago
  That's a fantastic link, thanks !

fizx 11y ago

So the real problem isn't that "when one is up, then the other will magically be up too." That could be accomplished with local hidden variables (e.g. shared seeds on a PRNG). The real problem is that when you measure A in the "up" direction, and then B in the "10 degrees east of up" direction, then B seems to know that you measured A in the "up" direction. That is to say: B's probability distribution as a function of the direction its being measured is dependent on the direction that A is measured. There's no way to construct an "A-independent" probability distribution of B's results for arbitrary directions. The probabilities won't sum to 1 and still match experimental results. It's unfortunate that "A up" therefore "B up" is a degenerate case of this reality where classicality actually works, because it leads to confusion. Also, follow the links others have provided for a more formal explanation :)

deleted 11y ago
[deleted]
mkehrt 11y ago
This is the clearest explanation I have ever seen of this; thanks.
trhway 11y ago
>The probabilities won't sum to 1 and still match experimental results. exactly. The probabilities are 1 only in theory. For example theoretical Malus law over all angles would give 1. On practice - i couldn't find single photon Malus law confirmation. A slight deviation from Malus law like cutting the tail at high angles off and thus decreasing the total to below 1 (ie. when/if photons at high angles being lost disproportionally much more intensively than Malus law states) would bring the experimentally measured S to values higher than 2 while still being in the realm of local realism.
bsaul 11y ago
But then what about a partially shared seed (sharing only a few digits) ? Couldn't you imagine a hidden variable only influencing the probablity distribution, making it only more likely to see an up on both, although still keeping it partially random ?
11y ago

xlm1717 11y ago

Agree with weinzierl, you want to look into Bell's theorem. He essentially set a limit (Bell's inequality) to what local variables could do to influence the measurements on both entangled particles. Bell's inequality has repeatedly been violated in experiments on quantum mechanics, which tells us that local, hidden variable cannot be the cause for the correlations in measurements of entangled particles. Bell's theorem then simply states that local hidden variables cannot ever reproduce all the experiments of quantum mechanics. It's not strictly correct to say that Occam's Razor says we should have hidden variables. Occam's Razor is the idea that in deciding between two or more similar theories (which both have to be correct), we should prefer the simpler of the theories. Bell's theorem tells us that we should reject the notion of local hidden variables. Importantly, this does allow the notion of nonlocal hidden variables. One such theory is the de Broglie-Bohm theory, which claims that the wavefunction is an actual physical entity, referred to as the pilot wave or guiding wave. It would be nonlocal because the wavefunction collapse of one particle to measure its properties would necessarily influence the other particle at superluminal speeds. Another nonlocal hidden variable theory is superdeterminism, which claims that everything that ever happened and ever will happen has already been determined, so the correlations are not a result of some spooky interaction, they were just "predestined" to be correlated. This would again be nonlocal because the universe as a whole would have knowledge of the particle properties a priori.

timeu 11y ago

On the university's hp are two nice videos that explain the Bell test: http://hansonlab.tudelft.nl/loophole-free-bell-test/ http://hansonlab.tudelft.nl/loophole-free-bell-test/

nokcha 11y ago

These experiments provide evidence against local hidden variables. The experiments are silent about theories that involve non-local hidden variables. The act of measuring one particle of an entangled pair instantly affects the other particle, instead of this effect needing to propagate at or below the speed of light; this instant interaction ("spooky action at a distance") is incompatible with local hidden variables. As other folks have mentioned, look up Bell's inequalities for more information. Also, it should be noted that quantum entanglement does not allow information to travel faster than light.