On 1/15/2014 4:08 PM, LizR wrote:
On 16 January 2014 03:51, Jesse Mazer <laserma...@gmail.com <mailto:laserma...@gmail.com>> wrote:



    On Wed, Jan 15, 2014 at 5:10 AM, LizR <lizj...@gmail.com 
<mailto:lizj...@gmail.com>>
    wrote:

        On 15 January 2014 22:55, Bruno Marchal <marc...@ulb.ac.be
        <mailto: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!)

You can do that (in fact it may have been done). You have two emitters with polarizers and a detector at which you post-select only those particles that arrive and form a singlet. Then you will find that the correlation counts for that subset violates Bell's inequality for polarizer settings of 30, 60, 120deg.

Brent

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