Wednesday, May 9, 2012

Showing that the wavefunction is real

The current Nature magazine reports:
The philosophical status of the wavefunction — the entity that determines the probability of different outcomes of measurements on quantum-mechanical particles — would seem to be an unlikely subject for emotional debate. Yet online discussion of a paper claiming to show mathematically that the wavefunction is real has ranged from ardently star-struck to downright vitriolic since the article was first released as a preprint in November 2011.

The paper, thought by some to be one of the most important in quantum foundations in decades, was finally published last week in Nature Physics (M. F. Pusey, J. Barrett & T. Rudolph Nature Phys.; 2012), enabling the authors, who had been concerned about violating the journal’s embargo, to speak about it publicly for the first time. They say that the mathematics leaves no doubt that the wavefunction is not just a statistical tool, but rather, a real, objective state of a quantum system. “People have become emotionally attached to positions that they defend with vague arguments,” says Jonathan Barrett, one of the authors and a physicist at Royal Holloway, University of London. “It’s better to have a theorem.”
I commented on this in January and November. No one told me about the embargo. It claims to disprove the "view, one held by Albert Einstein: that the wavefunction reflects the partial knowledge an experimenter has about a system."
Barrett and his colleagues are following the approach of physicist John Bell, who in 1964 proved that quantum mechanics has another counterintuitive implication: that measurements on one particle can influence the state of another, distant particle, faster than the speed of light should allow. Bell’s was a ‘no-go’ theorem: its strategy was to show that theories that do not allow faster-than-light influences cannot reproduce the predictions of quantum mechanics. Similarly, the theorem proposed by Barrett and his colleagues shows that theories that treat the wavefunction in terms of lack of knowledge of a system’s physical state will also fail to reproduce those predictions. Given how well-confirmed quantum mechanics is, the theorem suggests that such epistemic theories are wrong. “I hope this will take its place alongside Bell’s theorem,” says Barrett. ...

Their theorem does, however, depend on a controversial assumption: that quantum systems have an objective underlying physical state. Christopher Fuchs, a physicist at the Perimeter Institute in Waterloo, Canada, who has been working to develop an epistemic interpretation of quantum mechanics, says that he has avoided the interpretations that the authors exclude. The wavefunction may represent the experimenter’s ignorance about measurement outcomes, rather than the underlying physical reality, he says. The new theorem doesn’t rule that out.

Still, Matt Leifer, a physicist at University College London who works on quantum information, says that the theorem tackles a big question in a simple and clean way. He also says that it could end up being as useful as Bell’s theorem, which turned out to have applications in quantum information theory and cryptography. “Nobody has thought if it has a practical use, but I wouldn’t be surprised if it did,” he says.
Bell's theorem certainly does not show that measurements can influence states faster than the speed of light, and it has certainly not had any applications to cryptography.

That "controversial assumption" is one that Bohr and Heisenberg rejected in the 1920s, and in the
Bohr–Einstein debates of the 1930s. Schrödinger's view was somewhat different, but also consistent with this PBR paper.

This PBR paper is attacking a straw man that was rejected decades ago. Einstein lost those debates, and the consensus among physicists was that he was stubbornly refusing to accept the truth of quantum mechanics. There has been no discovery of the last 80 years to change those conclusions. Every quantum mechanics advance, theoretical and experimental, has been contrary to Einstein's view.

In case you think that I am some sort of crank who does not accept mainstream physics, consider this. No Nobel prize has ever been given for any work related to Einstein's view of quantum mechanics, or to Bell's theorem, or to that controversial assumption, or for action-at-a-distance, or for quantum cryptography, or for any post-1930 interpretation of quantum mechanics. The work would have won prizes if there were any substance to it.

Update: Lumo defends the Copenhagen interpretation today:
When I read papers such as one by Buniy and Hsu, I constantly see the wrong assumption written everything in between the lines – and sometimes inside the lines – that the wave function is an objective wave and one may objectively discuss its properties. Moreover, they really deny that the state vector should be updated when an observable is changed. But that's exactly what you should do. The state vector is a collection of complex numbers that describe the probabilistic knowledge about a physical system available to an observer and when the observer measures an observable, the state instantly changes because the state is his knowledge and the knowledge changes!
Other interpretations are possible, but if a physics paper is going to assume that the mainstream interpretation is wrong, then it should explicitly make that assumption and admit the possibility that its assumption is wrong (and the mainstream interpretation may be correct). That is the problem with PBR and many other articles in this field.

Update: Now Aaronson is seeking to cut off the funding of someone who expressed skepticism about quantum computing. I added this comment, which as so far not received moderator approval:
Wow, this is getting nasty, as you try to start a boycott of FQXi. Why stop there? Let me remind you that MIT has a professor named Noam Chomsky who has endorsed X who politically supports Y. You can fill in the blanks. Therefore I am refusing any offer of an MIT professorship and urging all others to do the same, until MIT stops lending its legitimacy to Chomsky. I haven't been getting any money from MIT anyway, but maybe communication about this issue with the MIT leadership will give hope that we’ll be able to resolve it to all sane parties’ satisfaction.

Just this week, Nature Physics published a paper that was submitted under the nonsensical title, "The quantum state cannot be interpreted statistically?" That title led me to believe that the authors had some mathematical misunderstanding of some trivial concept. Fortunately the editors required the authors to change the title to something less silly, as the paper does have some merit. But papers related to Bell's theorem go downhill from there. For some reason, the whole subject causes otherwise educated people to say crazy things. Many physics professors advise their students to stay away from the subject, in the same way that they advise not to try LSD. So Joy Christian did not take the advice. He might still be right about quantum computers being impossible.

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