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.

Jesse





>
>> Those are still many-world theories, + some "ugly" selection principle to
>> get one branch. It is very not "natural", as you have quasi
>> microsuperposition (appearance of many branches), but the macro-one are
>> eliminated by ad hoc boundary conditions, which will differ depending on
>> where you will decide to introduce the Heisenberg cut. Also, QM will
>> prevent us to know or measure those boundary conditions, which makes them
>> into (local, perhaps, in *some* sense) hidden variable theory.
>>
>
> I don't understand the above. The theory is simply QM with no collapse and
> with no preferred time direction (it assumes any system which violates
> Bell's inequality has to operate below the level where decoherence brings
> in the effects of the entropy gradient). It is both local and realistic,
> since time symmetry is "Bell's 4th assumption" - it allows EPR experiments
> to be local and realistic (I am relying on John Bell for this information,
> I wouldn't be able to work it out myself). So it definitely is a "hidden
> variable theory".
>
> I think for it to work the system is kept from undergoing decoherence or
> any interaction that would lead to MWI branching. EPR experiments only
> appear to work for systems that are shielded from such effects, I think? So
> there isn't a problem with the MWI - the whole thing takes place in one
> branch, with no quantum interfence etc being relevant. (I believe that EPR
> experiments lose their ability to violate Bell's inequality once
> interactions occur that could cause MWI branching within the system under
> consideration???)
>
>>
>> Many worlds is far less ad-hoc, imo. There is no Heisenberg cut, and the
>> boundary conditions does not play any special role, and indeed they are all
>> realized in the universal wave (and in arithmetic).
>>
>
> Please explain about the Heisenberg cut. I've heard the term, but don't
> know how it relates to EPR experiments.
>
> Have you read Huw Price's book "Time's arrow and Archimedes' Point" ?
>
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