On 16 January 2014 03:51, Jesse Mazer <laserma...@gmail.com> wrote: > > On Wed, Jan 15, 2014 at 5:10 AM, LizR <lizj...@gmail.com> wrote: > >> On 15 January 2014 22:55, Bruno Marchal <marc...@ulb.ac.be> wrote: >> >>> >>> On 14 Jan 2014, at 22:04, LizR wrote: >>> >>> Sorry, I realise that last sentence could be misconstrued by someone >>> who's being very nitpicky and looking for irrelevant loopholes to argue >>> about, so let's try again. >>> >>> Now how about discussing what I've actually claimed, that the time >>> symmetry of fundamental physics could account for the results obtained in >>> EPR experiments? >>> >>> >>> Logically, yes. >>> >>> But you need "hyper-determinism", that is you need to select very >>> special boundary conditions, which makes Cramer's transaction theory close >>> to Bohm's theory. >>> >> >> I'm not sure what you mean by special boundary conditions. The bcs in an >> Aspect type experiment are the device which creates the photons, and the >> settings of the measuring apparatuses. These are special but only in that >> the photons are entangled ... note that this isn't Cramer's or Bohm's >> theory (the transaction theory requires far more complexity that this). >> > > > Time symmetry in the laws of physics alone, without any special > restriction on boundary conditions, can't get you violation of Bell > inequalities. Ordinary time symmetry doesn't mean you have to take into > account both future and past to determine what happens in a given region of > spacetime after all, it just means you can deduce it equally well going in > *either* direction. So in a deterministic time-symmetric theory (Price's > speculations about hidden variables are at least compatible with > determinism) it's still true that what happens in any region of spacetime > can be determined entirely by events in its past light cone, say the ones > occurring at some arbitrarily-chosen "initial" tim. This means that in a > Price-like theory where measurement results are explained in terms of > hidden variables the particles carry with them from emitter to > experimenters, it must be true that the original "assignment" of the hidden > variables to each particle at the emitter is determined by the past light > cone of the event of each particle leaving the emitter. Meanwhile, the > event of an experimenter choosing which measurement to perform will have > its own past light cone, and there are plenty of events in the past light > cone of the choice that do *not* lie in the past light cone of the > particles leaving the emitter. > > So, without any restriction on boundary conditions, one can choose an > ensemble of possible initial conditions with the following properties: > > 1. The initial states of all points in space that line in the past light > cone of the particles leaving the emitter are identical for each member of > the ensemble, so in every possible history generated from these initial > conditions, the particles have the same hidden variables associated with > them. > > 2. The initial states of points in space that lie in the past light cone > of the experimenters choosing what spin direction to measure vary in > different members of the ensemble, in such a way that all combinations of > measurement choices are represented in different histories chosen from this > ensemble. > > If both these conditions apply, Bell's proofs that various inequalities > shouldn't be violated works just fine--for example, there's no combination > of hidden variables you can choose for the particle pair that ensure that > in all the histories where the experimenters measure along the *same* axis > they get opposite results (spin-up for one experimenter, spin-down for the > other) with probability 1, but in all the histories where they measure > along two *different* axes they have less than a 1/3 chance of getting > opposite results. Only by having the hidden variables "assigned" during > emission be statistically correlated to the choices the experimenters later > make about measurements can Price's argument work, and the argument above > shows that time-symmetry without special boundary conditions won't suffice > for this. > > If you're right then Price is wrong. However I don't recall him saying that the only consequence of time symmetry is that events can be, so to speak, worked backwards equally well. In particular, I read his EPR explanation as showing that both future and past boundary conditions were relevant in explaining the violations of B's Inequality. The "forwards-and-backwards" version would prevent time symmetry having any detectable effects, as far as I can see. (Also I'd like to see an explanation of EPR which works backwards from the measurement settings to the emitter and explains the violation of B's Inequality. That would definitely be a clincher!)
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