# Everyone is speaking of it

Everyone in my field has been fascinated by a work of Pusey, Barrett and Rudolph. I did not understand it, so I wrote to the authors. Jon Barrett replied in a crystal clear way. Here are his words on what they have proved:

“Suppose there is a hidden variable model, such that a (single) system has a hidden state lambda, and when I prepare the system in a quantum state |phi>, this actually corresponds to some probability distribution rho_{phi}(lambda). If I prepared |psi> instead, then this would correspond to the probability distribution rho_{psi}(lambda). It is obvious that rho_{phi}(lambda) and rho_{psi}(lambda) cannot be the same distribution. But we ask, can it happen that the distributions rho_{psi} and rho_{phi} overlap? The answer is obviously “no” if |phi> and |psi> are orthogonal. But what if they are non-orthogonal? We show that the answer is “no” in this case too. For any pair of quantum states which are not identical, the distributions rho_{phi}(lambda) and rho_{psi}(lambda) must be completely disjoint. Hence for any lambda, there is only one possible quantum state such that Probability(lambda|psi) is nonzero. This means that if someone knows lambda, then they know the quantum state. The whole of the quantum state must be “written into” the actual physical state of the system”. Jon also pointed me to Matt Leifer’s post, which is indeed a very clear critical appraisal.

Now, why are people excited?

Well, many seem to argue that this paper is falsifying some attempts at “being moderate”, cornering people to adopt either of the following positions:

* A realism with “crazy” features (many-worlds with the existence of all potentialities; or Bohmian mechanics with potentials that change non-locally);

* A complete renunciation to realism: we cannot hold any meaningful discourse on quantum physics, just apply the theory that gives us correct predictions for our observations.

I am not sure that the alternatives are so sharp… anyway, now I have to go back to atomic physics and figure out formulas that describe reality — sorry: the perception of the experimentalists 😉

Posted on November 21, 2011, in Common knowledge, Latest topics, Philosophy. Bookmark the permalink. Leave a comment.

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