Saturday, January 07, 2012

A sensible quantum reaction to Pusey et al.

On November 18th, 2011, I wrote about the invalid paper by Pusey et al.
Nature hypes anti-QM paper by Pusey et al.
It was a pleasure today to learn about one of the first replies to this paper published in the middle of December 2011,
The quantum state should be interpreted statistically
by Holger Hofmann in Japan – and not only because the author says the same things as your humble correspondent.

First, you should replace "to proof" by "to prove" twice in the paper, to avoid irritation by this imperfection of the author's English. ;-)

In the second sentence of the abstract, Hofmann already points out that Pusey et al. implicitly assume that the natural phenomena have to be reflections of some hidden variable model. They assume that the observations are reflections of some objective reality. You may remember I said the same thing: they are trying to settle an "ontic vs. epistemic" battle, not realizing that both "ontic" as well as "epistemic" camps (of hidden variable advocates) have been known to be wrong at least for half a century. Hofmann says many comments about the need to treat physics in a positivist way and only accept that the results of observations are "real".

One of the things he proves in the main text and in the final note [3] is that the authors are dogmatic. Hofmann, being perfectly polite, adds that by "dogmatic", he doesn't mean that they're narrow-minded. But he has the duty to kindly point out that their blind belief unsupported by the evidence is more similar to that of Christian bigots and Islamic fundamentalists than what some physicists are ready to admit – which may be dangerous. ;-)

Holger Hofmann seems to be a pretty successful researcher in the foundations of quantum mechanics, given the fact that what he writes is actually valid. But these features may have something to do with his Japanese affiliation (or exile?).

The short paper has a "constructive" portion, too. He shows that one may interpret the density matrix as a statistical distribution and in a similar way as Pusey et al. did – by using the methods of the Wigner function. The agreement with the quantum mechanical predictions is guaranteed and the paradox is avoided – despite this "seemingly classical interpretation" – by the fact that the probabilities encoded in the density matrix in this way may be negative. However, that doesn't lead to actual negative "frequencies" how often a certain outcome is measured because these negative probabilities only refer to joint properties of the system that can't be measured simultaneously.

So thankfully, I just learned that there are theorists who are mainly working in "quantum foundations" who are not completely deluded when it comes to the meaning of quantum mechanics.


  1. Come on, Lubos, you're being too hard on Pusey and Nature schlock magazine as well, as these eminent scientists would agree:


    HA ha ha sorry I couldn't resist

  2. A hilarious page, Brian, that explains... everything. ;-)

  3. Lumo or anybody:

    Since I am an Internet idiot and cannot figure it out all by myself, can u please advise how to make my comments appear in the Comments section, as opposed to the Slow Feedback section? Not that my comments will be particularly valuable from the theoretical physics point of view, with me being a chemist, but it seems that the comments section gets more traffic, and provides a livelier discussion. I promise not to post any dumb stuff. Thanks - Wojtek

  4. Dear Wojtek, first, click at "comments" under an article. It seems you have done so and you have seen that there exist "fast comments", too.

    At the bottom, you will see a window to "post a message". If you can't write anything into the text area, then you probably use a version of Internet Explorer that has problems with Echo - a big problem for them, I would say.

    If that's so, download Chrome or Firefox.