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).

> 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

> 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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