On Mon, Apr 4, 2022 at 12:43 AM smitra <smi...@zonnet.nl> wrote: > On 25-03-2022 23:01, Bruce Kellett wrote: > > On Sat, Mar 26, 2022 at 6:37 AM smitra <smi...@zonnet.nl> wrote: > > >> One can indeed write down non-local effective Hamiltonians, but the > >> fundamental laws of physics are strictly local. > > > > God told you this, did he? If one needs non-local physics to describe > > certain experiments, then locality is not universal. > > The Standard Model is local.
The standard model, and quantum field theory, are local because they were made that way. A fundamental assumption of QFT is micro-causality, namely, the assumption is that all field commutators vanish outside the light cone. This assumption imposes strict locality. But it is an assumption -- not a God-given truth. There are clear reasons for supposing that this assumption cannot be universally valid, since non-local effects, as evidenced by non-separable states, are clearly present in the world. If you can experimentally verify that for a > certain physical system the Standard Model (extended to take into > account finite neutrino masses) is not adequate, then you'll win the > Nobel prize for that discovery. We know that the Standard model has to > fail when quantum gravity will become important. But we cannot probe > this regime experimentally, and for the sort of things we're discussing > here, it's not relevant. > > > > > What matters is the time evolution step by step proceeds in a manifestly > local way. The entangled spin system is created locally, the two > particles travel to the locations of Alice and Bob. Alice and Bob and > their polarizers considered as large collections of particles interact > locally with the spins. While you then do find correlations that exhibit > nonlocal properties, there is then a local account of how these > correlations arise, namely the time evolution via the Schrodinger > equation with a manifestly local Hamiltonian. Such a Hamiltonian is > always available because the well established low energy physics won't > violate the Standard Model. > The philosophical position that you adopt here is known as "Humean supervenience". It derives from the empiricism of David Hume, and has been given its modern form by the philosopher David Lewis. "A broadly Humean doctrine holds that all the facts there are about the world are particular facts...it might better be taken as a doctrine of supervenience: if two worlds match perfectly in all matters of particular fact, they match in all other ways too -- in model properties, laws, causal connection, chances...." (David Lewis, Philosophical papers, 1986) Tim Maudlin (in 'The Metaphysics within physics', 2007) discusses this in detail. He summarizes Humeanism as follows: (Separability): "The complete physical world is determined by (supervenience on) the intrinsic physical state of each spacetime point (or each pointlike object) and the spatio-temporal relations between these points. Separability posits, in essence, that we can chop up space-time into arbitrarily small bits, each of which has its own physical state... so the world as a whole is supposed to be decomposable into small bits laid out in space and time." Since it has its origin in 18th century empiricism, it is not surprising that Humean supervenience (and separability) are true for classical Newtonian physics. The point that Maudlin is stressing is that although true for classical physics, Humeanism is not true for quantum physics. The crucial difference is that quantum physics allows non-separable states, for which the locality assumption does not hold. The phenomenon of quantum entanglement is the manifestation of this non-separability, and it is now eminently clear that entanglement lies at the heart of quantum physics. So QM is not Humean, and not intrinsically local. I still read papers in which Humean supervenience is simply assumed, and taken to be non-negotiable -- as, indeed, you do. But these papers are taking an outmoded philosophical position, based largely in classical intuitions, so not appropriate for quantum physics. And it is important to stress that this is a philosophical position (or dogma); it is not a scientific position, because the scientist must always remain open to the possibility that previous philosophical assumptions might well be found to be wrong or inappropriate. This is the case with Humean supervenience. Quantum physics, because of the phenomenon of entanglement (non-separability), shows that the world cannot, in fact be chopped up into arbitrarily small bits, each of which has its own physical state, independent of all the other bits.... Consequently, locality as the Humean would want, can no longer simply be assumed. To make this point even more strongly, I would urge you to come up with an actual local account of the Bell correlations -- and that means that you can no longer simply assume that it must be local because everything is local -- you have to give the details of the local mechanisms that make it all happen, in which "time evolution step by step proceeds in a manifestly local way". I am completely confident that you cannot do this -- if you could, you would have produced this local account long ago, rather than relying on some unspecified magic to make everything work out. Bruce -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to everything-list+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/CAFxXSLRt09FDcZALzk_DkfTjCwKJohb6H3tL3zzJ%2B07wscS88A%40mail.gmail.com.
