QUANTUM THEORY WITHOUT OBSERVERS III

 

​I recently attended this conference in Bielefeld, Germany. Some of the talks can be viewed at the conference website

http://www.mathematik.uni-muenchen.de/~bohmmech/bielefeld/index.html

 

There are also interviews with certain participants and videos of some of the discussions which made up a large part of the meeting. Topics include quantum nonlocality and its difficult coexistence with relativity theory, but also the deterministic theory of Bohmian mechanics and the so-called collapse theories. A user-friendly presentation of Bohmian mechanics is given by Stefan Teufel at

Teufel talk

 

Several talks focused on the contributions of J.S. Bell, such as 

Bricmont talk and Norsen talk

 

I met Nicolas Gisin who talked about his work on nonlocal correlations

Gisin talk

 

Gisin has recently written a popular account of quantum nonlocality in French, under the title L'impensable hasard. Non-localité, téléportation et autres merveilles quantiques, published by Odile Jacob in Paris. The book explains the import of Bell's work and describes several experiments which show that nonlocal correlations do exist. An English translation has just been published by Springer under the title Quantum Chance. Nonlocality, Teleportation and Other Quantum Marvels. Click on the cover pictures below to reach the publishers.

 

The author claims to show that there is no hope for determinism in modern physics and implies that this is crucial for us to be able to claim that we have free will. A particularly important paper here would be N. Gisin: Impossibility of covariant deterministic nonlocal hidden-variable extensions of quantum theory, Phys. Rev. A 83, 020102 (2011). Gisin's book has the merit of being simply written and exceptionally clear. A short review can be downloaded here. It contains some criticisms of the author's discussion of Bohmian mechanics, indeterminism, and free will. Short commentaries on the Satigny-Jussy experiment and the before-before experiment are also available in pdf form.

 

 

 

The best book by far on quantum nonlocality, this time for the less casual reader with some determination, is undoubtedly the one by T. Maudlin (click on the book cover below to reach the publisher). Here you have the state of the art and written with the reader in mind. A lot of publications on quantum theory become obsolete with the existence of this book. It focuses precisely on what is advertised in the title: quantum nonlocality and relativity. It discusses the theory behind the Bell inequalities, superluminal energy transmission, signalling, causation, information transmission and the possibility of simulating the quantum correlations without information transmission (an idea inspired by the so-called detection loophole). It then discusses what relativity really requires and asks whether there is a problem. It’s a very thorough investigation and reader friendly, in the sense that it contains very few philosophers’ wrangles and focuses on the physical problem. 

 

Wave collapse in ‘standard’ QT and in dynamical collapse theories (GRW) is contrasted with the Bohmian picture in a perfectly objective way. You are allowed to judge for yourself. He comes to ontology only in the last chapter. You are only asked to accept this: the macroscopic objects mentioned in our description of any experimental situation (or just any situation) are in fact merely collections of microscopic objects, so if the latter have no spacetime properties, neither do the former. 

 

He discusses the spacetime properties that microscopic objects might have, and thereby effectively ‘specifies’ what those objects might be, which is the aim of ontology. His basic premiss here is that the wave function is never enough. One must have Bell’s local beables because, although it is fine to talk about the dynamics of the wave function, we also need to say what connection there might be between the behaviour of the wave function and the disposition of matter in spacetime. So the real problem for Schrödinger with his cat was that he did not provide any spacetime account of the cat.

 

He discusses ontology for Bohmian mechanics (there’s not much to be said, it’s so obvious that one wonders why people aren’t more interested), and for GRW. I don’t know enough about GRW to be able to make serious claims that I could back up. However, as far as I do understand it, one can have a matter density ontology (matter distributed in proportion to the square of the wave function), but one can also have a ‘flash’ ontology, in which the only things that actually exist (i.e., are on the list provided by the ontology) are precisely the dynamic collapse events postulated by GRW. These events, as I understand it, are intrinsically stochastic. 

 

As far as I have grasped the relativistic flash story, it is no more contrived than the GRW theory itself. This theory serves a very important logical role here: it shows that one can have a completely relativistic version of the GRW theory and more importantly, a completely relativistic implementation of nonlocality. It’s really the last thing that counts. Note that one could not even discuss that issue without introducing the local beables, i.e., the flashes. Note also that the matter density ontology cannot be made relativistic. On the other hand, personally I think the GRW is itself rather contrived, by which I suppose I mean ad hoc. If Gisin is right, it would not be possible to make a deterministic flash-type story relativistic. He may well be ‘wrong’, however. Impossibility proofs often rule out interesting possibilities among their premisses.

 

GRW ‘solves’ the measurement problem, but at a price: introducing more constants of nature (assuming I understand that, but I don’t know the full theory). In BM there is no measurement problem. The particle ontology allows one to escape entirely from that. This should be a strong enough argument to get everyone at least interested. Decoherence alone still leaves a measurement problem, because it doesn’t establish what one is talking about. One might say that the measurement problem arises precisely for those people who refuse to have local beables, and it is unlikely to go away.

 

The only problem I find in Maudlin's book is the formulation of Maudlin's scheme B simulation (see p. 160 ff). I could not generate his formulas and he no longer has the original handwritten calculations. My query can be accessed as a pdf here. And Maudlin's talk entitled What theories qualify as quantum theories without observers? at the QTWO III conference can be viewed at

 

Maudlin talk