On 8 January 2014 08:59, Jesse Mazer <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.

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