On 1/7/2014 1:35 PM, LizR wrote:
On 8 January 2014 08:59, Jesse Mazer <laserma...@gmail.com <mailto:laserma...@gmail.com>> wrote:


    Well, most physicists already agrees physics is time-symmetric (well, 
CPT-symmetric,
    but the implications are the same for Bell's inequality and thermodynamics),


Yes, they do, but it doesn't appear to be taken into account when discussing Bell's inequality.

    but I don't see how this alone can explain violations of the Bell 
inequality.


No, you need to work out the consequences mathematically, and I dare say that is quite difficult. This is simply a /logical/ demonstration that Bell's inequality can be violated while retaining locality and realism, which is otherwise impossible.

    To explain Bell inequality violations using a time-symmetric theory like 
the one
    sketched out by Huw Price, you need to assume hidden variables (the 
particles have
    predetermined spin states along all axes the experimenters might choose to 
measure),


Yes, hence it retains realism. The variables are only "hidden" in the sense that they can't be measured half way through the experiment - e.g. by measuring the state of photons while in flight - because any interference with the experiment would destroy the correlations between the measuring apparatus and the emitter.

    *and* you must further assume that the particle emitter that creates the 
particles
    can "predict" what axes the experimenters will choose to measure on each 
trial,


That's what time symmetry means. There is no "prediction" involved in the sense you mean - the state of the measuring apparatus affects the photons, just as the emitter does. (This can of course be extended to a multiverse, with the measuring apparatus simultaneously in various states which create a superposition of emitters. But that isn't necessary.)

    so that the statistics of what combinations of hidden variables get created 
will
    depend on the experimenters' later choices. For examples on trials where 
they are
    both going to measure along the x-axis the emitter will always create 
particles that
    have opposite spins along the x-axis, whereas on trials where the 
experimenters both
    measure on some other axis, or where they each choose different axes to 
measure, the
    emitter can create particle pairs that don't have opposite spins on the 
x-axis. Is
    this the type of solution you're thinking of?


Yes, that sounds about right. The particles' states throughout the experiment are influenced by the measurement settings as well as by the emitter that creates them. From that it follows logically that information about particle A's measurement setting is available to particle B at the point of its measurement, and vice versa. (assuming the physics is local and realistic - the particles have definite states throughout).


    If so, it seems like this goes well beyond time-symmetry, since 
time-symmetry
    doesn't normally allow for systems to contain localized "records" of events 
in the
    future the way that they can for events in the past (which presumably could 
be
    explained in terms of the thermodynamic arrow of time caused by the 
universe having
    a low-entropy past boundary condition but not a low-entropy future boundary 
condition).


I'm afraid you've missed the point here, and then gone on to tie yourself in knots. There is no thermodynamics or "sensitive dependence on initial conditions" at the level of the individual photons. Entropy is a statistical, high level outcome from a lot of low-level time-reversible processes. Price assumes realism, that the photons have a real state, with spins and so on, throughout the experiment. Time symmetry simply says that this state is influenced by boundary conditions */at either end of its path/* - by the settings of the measurement apparatus it encounters, /and/ by the state of the emitter. Since the photons are prepared so their states are corellated ("entangled") this means that the state of photon A at the point of emission influences the state of photon B (and vice versa). If the relevant physics is time symmetric, then photon A's state */throughout the experiment/* is influenced by the state of measuring device A. Hence the state of measuring device A affects photon B, via the point at which they become entangled.

Which is what is observed in EPR experiments: the settings of measurement device A affect the state of photon B.

I'll take this opportunity to agree completely with Liz's explication above. :-)

Notice too that if you take everything to be deterministic, including the experimenter's choices of measurement you can violate Bell's equality. So it just appears random to the experimenters because they can't realize that their decisions were determined where their past light cones overlapped. This is t'Hooft's hyperdeterminism. It seems like taking the observed MWI branch and making that the block universe, with all other branches not realized.

Brent

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