Dan said: > I found it interesting that it got to be a hot topic after I was > involved with it.
I suppose this is because Bell had just proven his celebrated theorem in 1971 and at about the same time the state of the art in quantum optics had advanced far enough that it was possible to actually perform EPR experiments, as Freedman and Clauser did in 1972. It must've been an exciting time :) > I do try to keep up. I'm also in a bit of a different position, since > I do have a philosophy degree and have illusions of semi-retiring and > spending some time writing in the area of the philosophy of QM. :-) That would be really fun. Have you read Redhead's _Incompleteness, Nonlocality and Realism_? [Fitting together QM and SR] > I'm not quite sure what you are getting at. I think they do fit > together well, and I do see basic principals that are underly they > interrelationship. Here's roughly what I meant. Quantum mechanics and special relativity both have a number of conceptual innovations with respect to the Newtonian worldview. Special relativity gives us a new view of the causal structure of the past in which past/present/future is replaced by "absolute past" / "causally disconnected" / "absolute future". In special relativity, there is no absolute standard of simultaneity and any communication that is simultaneous in a given inertial reference frame can be used to violate causality in another (actually, this is not quite true - it requires two such simultaneous communications). Quantum mechanics, in contrast, preserves Newtonian causality but introduces such ideas as objective randomness, irreducible uncertainty in measurements and non-locality. It might therefore seem that the two theories wouldn't fit together because non-locality could be used to communicate simultaneously and so violate causality. However, this isn't the case because, although QM has non-local correlations between measurements, one of the other conceptual innovations, objective randomness, steps in to prevent us from using non-locality for communication. It's this that seems to me to be surprising - that we can take these two entirely separate sets of conceptual innovations and use them all. (Indeed, we can make quantum field theory by treating them all as axioms, and get such wonderful new things as the spin-statistics theorem by imposing the need for a Minkowskian causal structure more rigorously. Not that quantum field theory is entirely free of problems...) > But, those principals are metaphysical, not physical. What are these principles? > Bell and Wigner are the ones who showed that there cannot be a local > hidden variable theory. Yes. I knew that, of course. You just used the word "interpretation" and I was wondering if they'd come up with a full interpretation of QM. I wouldn't call Bell's theorem an "interpretation" as such - I'd save that word for things like the Many Worlds interpretation or the Copenhagen interpretation. [Spin foams] > Well, we have gotten different feels on this. Since I'm a plumber, > maybe we have different ideas of nice and simple. I think that a statement like "the universe is a spin foam of such-and-such a type" is as nice and simple as "the universe is a Riemannian manifold" as a view of space and time. Indeed, it might even be simpler because it doesn't rely on stuff from analysis and differential geometry but only graph theory. Of course, they're still far short of a proper theory of quantum gravity, but it seems to me that they'll probably end up with something much simpler than superstring theory (which is, however, much more ambitious) or even than the Standard Model. I mean, I've done entire courses on QFT and I still couldn't describe the theory in simple terms. Rich
