Wednesday, November 5, 2014

50 Years of Misunderstanding Bell's Theorem

Precisely 50 years ago, Bell's paper "On the Einstein Podolsky Rosen Paradox", containing his famous theorem was received by the journal Physics. Today is John Bell Day.

Bell's theorem is one of the most influential result in physics, despite the fact that it is a negative result. Contrary to what many people believe, Bell was actually searching for a hidden variable theory, and he found instead some severe limitations of such theories. The limitation expressed by Bell's theorem celebrated today is that hidden variable theories have to be nonlocal. The outcome of measurements are correlated in a way which seems to ignore the separation in space. Some misunderstand this result as rejecting determinism, or as rejecting any kind of hidden variables, or at least as proving that any theory which describes the quantum world using hidden variables has to rely on instantaneous communication.

Maybe others searching for a hidden variables description of quantum phenomena hit the same wall Bell hit, but rather than having the same revelation as Bell, they ignored it and continued to search for a replacement or completion of quantum mechanics. For example, Einstein had all the data to find Bell's theorem almost 30 years before Bell. The paper coauthored by Einstein, Podolsky and Rosen, "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?" shown that entanglement allows nonlocal correlations. But Einstein disliked nonlocality because it seemed to violate special relativity. So he concluded that quantum mechanics was incomplete, by interpreting those correlations as revealing that Heisenberg's uncertainty principle can be trespassed. So Einstein and coauthors hit the same wall as Bell, only that they considered that the problem could be solved by completing quantum mechanics. Bell's theorem clarifies their findings in showing that no matter how you put it, the world is nonlocal (if Bell's inequality is violated, as it was confirmed by experiments).

Almost 30 years later, Bell understood nonlocality as the major consequence of the EPR "paradox", and expressed it in the form of his theorem. Today, at 50 years after Bell clarified the problem, there are so many who consider that Einstein was a crackpot in what concerns quantum mechanics, and Bell defeated him. Today it is easy for any student who took a class of quantum mechanics or philosophy of physics, to consider that he has a better understanding of quantum mechanics that Einstein, and to feel superior to him (true story, just search the physics blogs and forums and you will see many examples). Most often they believe (as they are taught) that quantum mechanics is so radically different because it is not deterministic, and that what Einstein searched was a deterministic theory. And that EPR suggested this, and Bell rejected it. This is so unfair for EPR, but also for Bell.

The truth is that despite the 10 hot years of discoveries in quantum mechanics, when nearly every aspect was understood, and the foundations were laid down, nobody before Einstein, Podolsky and Rosen found that "paradox", which is true and relevant. It is unfair to consider the EPR an attack against quantum mechanics, as it is seen by many since the beginning. Rather, it is a most important discovery, which could only be made because three rebels were not satisfied with Bohr's prescriptions. Moreover, in almost 30 years since the EPR paper, nobody solved their "paradox". Not even Bohr, who rushed to respond too quickly with an article bearing the same name as the EPR one. And the solution was found by Bell, who was a supporter of hidden variables, and maybe he wouldn't find it either, without the reformulation of the EPR argument due to the main exponent of hidden variable theories at that time, David Bohm.

Now, the reader may think that I am defending the hidden variables, by praising hidden variables theorists like Einstein, Bohm, and Bell. I actually don't defend hidden variables, and I don't say this just because of the witch hunt against "Bohmians". I just want to emphasize that without these "crackpots", we would not have today the understanding of entanglement and nonlocality which allows scientists to put at use the "magic" of quantum mechanics at work in quantum computing, quantum information, quantum cryptography, and other recent hot areas.

Actually, to be honest, among Einstein, Bohm, and Bell, only the first two are considered a lacking understanding of quantum mechanics, and Bell is considered as the one who defeat them, so he is celebrated, while the other two are not. But this is only because Bell is perceived as being, because of his theorem, against hidden variables, while in fact he was also searching for a hidden variable theory.

Moreover, for some reason, many consider that Bell's theorem is only about hidden variable theories, while in fact it is about any quantum theory or interpretation which describes quantum correlations as are observed in nature, and therefore violates Bell's inequality. Including therefore standard quantum mechanics. So, quantum mechanics is nonlocal too, and no Copenhagen Interpretation, no Many Worlds Interpretation, no Decoherence Interpretation can make it otherwise. Similarly, quantum mechanics is contextual too, despite the fact that the Bell-Kochen-Specker theorem is considered to apply to hidden variable theories only.

But why some tend to consider only hidden variable theories guilty of the sins of nonlocality and contextuality? Maybe because they just want to reject such theories? Or could it be because they believe that it makes no sense to think about what happens between measurements (as Bohr teaches us)? Or because nearly everyone, when first learning about quantum mechanics, has the instinct of finding a local realist explanation, and fails, of course, and then denies having this sin by throwing stones at those who seem to have it? I think this is fine, since this is what we should do, we should question everything, and that the persistence with which we should question a claim has to increase with the degree by which that claim contradicts what we learned before, as is the case of quantum mechanics.

For lack of time, for the rush of getting published, for the fear of getting rejected for having unorthodox views, we tend to eat much more than we can digest, and actually we cease digesting. This is why misunderstanding are propagated even at the top of the scientific community. Misunderstandings concerning quantum mechanics and Bell's theorem prevent us from seeing both the truth, and the amazing beauty of quantum mechanics, which is transformed into a mere tool to calculate probabilities, and any attempt at understanding it is regarded with disdain.

I find very fortunate the fact that Tim Maudlin wrote for the 50th anniversary of Bell's theorem a paper named "What Bell did", in which he explains that Bell's result is that indeed our world, hence quantum mechanics, is nonlocal. He makes a thorough and in my opinion probably the most down to earth analysis of the meaning of the EPR paper and of Bell's theorem, and how they are misunderstood. He identifies a cluster of misunderstandings that are propagated among physicists and philosophers of physics. This is one of the cases when a philosopher really can help physicists understand physics. I'll leave you the pleasure to read it.

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