In a recent publication, Experimentally Faking the Violation of Bell’s Inequalities (Gerhardt 2011) (arXiv version), the statistics of quantum mechanics is faked using classical light sources. But if it is possible for physicists to fake an experiment to imitate QM, how can we be sure that nature doesn't do the same trick on us? Can it be that QM is a fake, and in the end QM turns out to be an artifact of our imperfect measurement devices?
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3$\begingroup$ I am not going to post it as a full answer because I think that this question is a provocation to punish those who dare to consider quantum mechanics as a fact by those who have irrational reasons to dislike it. However, when one insists on locality (slower-than-light propagation of signals), and locality follows both from the field-theory nature of low-energy physics as well as special relativity, then the quantum EPR correlations simply can't be "faked". That was exactly why Einstein invented EPR to protest QM in the first place. One may only fake it by violating other rules of the game too. $\endgroup$– Luboš MotlCommented Nov 16, 2011 at 6:49
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1$\begingroup$ So one may do many tricks just like David Copperfield but physics is not about tricks and "faking" things: physics is about the search for theories that agree with all the observations. There exists no non-quantum theory that would agree with the observations of elementary particles, including their wave-like properties and their respect for the Lorentz invariance. So "faking" papers are on par with various other papers trying to create "illusions" that something is moving faster than light, and so on, and so on. They're just illusions and we know how they differ from the real world. $\endgroup$– Luboš MotlCommented Nov 16, 2011 at 6:52
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4$\begingroup$ youtube.com/watch?v=P6MOnehCOUw $\endgroup$– GeorgCommented Nov 16, 2011 at 10:26
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2$\begingroup$ I am fairly sure that in the recent neutrino experiment, Nature is indeed faking faster-than-light travel, but I also believe that it's not going to stand up to as many replications of the experiment as quantum mechanics has. $\endgroup$– Peter ShorCommented Nov 17, 2011 at 17:10
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3$\begingroup$ "Subtle is the Lord. Malicious, He is not." — Albert Einstein $\endgroup$– PM 2RingCommented May 10 at 1:33
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4 Answers
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I'm not sure it makes sense to ask if Nature is "imitating" Quantum Mechanics.
Quantum mechanics is a mathematical model that gives predictions that are in excellent, well so far perfect, agreement with what we actually see.
I guess the question is whether QM is just a good approximation to the real world or whether it's an exact description of the real world. We'll never be able to prove it's an exact description, but someday someone may find an experiment where QM gives the wrong answers. If so this would prove it's just an excellent approximation.
answered Nov 17, 2011 at 11:35
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The so-called "conspiracy" variant of the idea of superdeterminism implies that nature is indeed effectively "faking" quantum statistics. But as John Rennie points out, this is simply an interpretation, not a true alternate theory, so it's pretty entirely a philosophical rather than physical question.
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The paper of Gerhardt et al is about Bell tests in the context of quantum cryptography, where you assume that there is an intelligent and well-informed human adversary trying to break your system. Even so, they are only able to break it because they give the adversary the power to fake not only the answers (measurement results) but also the questions (measurement angles). Since the questions and answers together are all of the data that goes into the analysis, they can fake any result, even a super-quantum level of correlation. It's supposed to be a cautionary tale against trusting your equipment to tell you what measurement angles you "chose".
That is not the normal situation in experimental physics. Normally, physicists assume that the universe is just doing its thing as usual and not trying to deceive them. What Bell proved is that absent certain kinds of deception, it's impossible to fake quantum statistics.
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We can't be 100% sure that quantum mechanics is exact until we are able to do a quantum computation in a real system that would be impossible to perform on a classical computer roughly of size the universe. This looks like it is possible, there is no indication that quantum mechanics is only approximate, and there is no known classical theory which limits to quantum mechanics for small systems, but becomes something else for large ones.
But our tests of quantum mechanics are limited to a certain extent by our ability to compute predictions of the theory. By definition, we cannot predict the behavior of a quantum mechanical system which takes exponential resources, because the computation is too big. We must resort to approximations which might disguise a failure of the theory. So while it is unlikely, it is possible that nature is not fundamentally quantum mechanical, and that we just have not seen the breakdown of QM yet.
This type of statement is predictive--- it requires that a quantum computer will have mysterious unaccounted for sources of decoherence which are not due to known interactions, but which are fundamentally due to the breakdown of quantum mechanics. If a quantum computer is built which can regularly factor 100,000 digit numbers, this is good enough to establish that our universe is not classical down below, because such a feat would require too big a classical computer to realistically implement inside our universe, were it classical.
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$\begingroup$ I don't think your first sentence holds up logically. Even if we were able to do a quantum computation in a real system that is impossible to perform on a classical computer roughly the size of the universe, how do we know that there's not a part of the universe invisible to us that is contributing to the computation? Also, how could the entire universe be contributing to a computation that is taking place in a small room on Earth? $\endgroup$ Commented Nov 17, 2011 at 14:44
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$\begingroup$ @Peter Shor: I am not speaking as a philosopher, but as a physicist. I agree that it is logically possible that the universe is being simulated on an enormous computer, but this would not be physical. The reason for allowing the whole universe to contribute is the holographic principle--- the room is spread out over a cosmological horizon, and in principle some fraction of the whole horizon could be contributing to the process. Any more bits than what can fit on the cosmological horizon is physically impossible. BTW, this argument is due to t'Hooft, not me personally. $\endgroup$ Commented Nov 17, 2011 at 14:48
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$\begingroup$ I would personally be convinced by factoring 1000 digit numbers, or even a few hundred. It is very implausible to get such an exponential speed up in any imaginable "beable" model even spread out nonlocally on the cosmological horizon. But Grover's algorithm, or any other square-root type speedup, could be imaginably fake for essentially any system size (although it likely isn't--- I tend to believe QM is exact--- it just would be nice to be sure). $\endgroup$ Commented Nov 17, 2011 at 15:05
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$\begingroup$ Good point about the cosmological horizon. But if the universe is expending all its computing power on this little room on earth with a quantum computer that's factoring a large number, where is it getting the computing power to power the quantum computers being used by alien civilizations (let alone processing ordinary physics)? $\endgroup$ Commented Nov 17, 2011 at 15:26
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$\begingroup$ @Peter Shor: sure, this is implausible. This is why small numbers are already enough to convince me. But I imagine that if you can factor 1000 digit numbers, there is no harm in going to 100,000, just to convince the die-hard skeptics. $\endgroup$ Commented Nov 17, 2011 at 16:00