On 16 January 2014 01:13, Bruno Marchal <marc...@ulb.ac.be> wrote:

> On 15 Jan 2014, at 11:10, LizR 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.
> The setting of the analyser must be predetermined. And not in the
> mechanist sense, where the choice of the analyser is still made by you,
> even if deterministically so. With only one branch, you are not just using
> irreversibility, but you are using the boundary condition selecting a
> branch among all in the universal wave.
> You mean the measuring devices? If so, of course they are predetermined -
as is the state of the emitter. Time symmetric physics would guarantee
that. I don't see that is special, however. (But I will read on :)

> 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".
> Yes, and I am willing to accept it is local. but it is "hyper-determined".
> It means that if I chose the setting of the two analyser in the Aspect
> experience by looking at my horoscope, that horoscope was determined by the
> whole future of the phsyical universe. Logically possible, you are right,
> but "ugly", as it is a selection principle based on boundary conditions. It
> is "more local" than Bohm, and it does not need a new potential, but it is
> sill using abnormal special data for the "TOE". It is no more a nice and
> gentle equation like the SWE, but that same equation together with tuns of
> "mega-terra-gigabyte of data".
> I don't follow the whole "future" business. I'm only talking about what
might be called micro-symmetry - symmetry below the level of
coarse-graining at which entropic processes emerge. This is particularly
not a problem in a block universe, or block multiverse, where there is no
moving arrow of time and the outcomes are already "there" (maybe in
multiple branches in the MWI). Where / why is all that extra data required?

> 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???)
> ?

I may be wrong on this, but I believe EPR experiments require that the
particles being measured are isolated from decoherence - and hence from
branching, in the MWI. So the MWI can be ignored for the purposes of
analysing the experiment. Any proposed explanation for violations of Bell's
inequality, therefore, should take into account what is logically possible
if time is directionally neutral at the level of the particles being

LATER NOTE - see below for me revising my ideas on this!

>> 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.
> The Heinsenberg cut is where the wave should collapse in the Copenhagen
> QM.
> Von Neumann understood well that it is largely arbitrary.
> In all "one world theory", you have to justify why the superposition works
> so well for the micro-worlds, and disappear for the macro-world.  Using
> reversiblity, cannot by itself solve that problem. What works is
> reversibility and the boundaries conditions. God needs to know all the
> detail of the big crunch to program convenably the big bang, so as making
> an Aspect result consistent with "one-world", locality and determinacy.

OK, but in a block universe any dependence on a BC is built in. (In reality
it appears there may well not be a BC, only a BB, which means that the
boundary conditions are a singularity and timelike infinity).

I should add that it is possible a universe with a BB and a BC would have
radically different, or perhaps no, entropic and radiative arrows of time.

> Have you read Huw Price's book "Time's arrow and Archimedes' Point" ?
> No. I know it,  as it is often discussed on forums.
> I am not convinced, as I tend to not believe in any primitive time and
> space, at least when I tend to believe in comp (of course I *know* nothing).

Yes, this is all speculative of course! But I feel I should bring it up
when someone says something like:

"Bell based his analysis on three assumptions..."

Because Bell himself agreed that he used four, and that this is the 4th
one. For some reason it is hardly ever included in discussions, however,
which I think is a shame because without it we get non-locality or
non-reality, and quantum theory seems deeply mysterious. (Consider it my
duty to Albert Einstein to mention it!)

> QM is indeed reversible (in large part), but using this to select one
> branch by boundary condition, is still like a form of cosmic solipsism to
> me. We can't refute it, and unlike most QM collapse theories, we can't
> criticize it from locality and determinacy, but that does not yet make it
> convincing compare to MW, and infinitely more so in the comp frame, where
> we can't avoid the many "dreams".

It isn't intended to be an alternative to the MWI, it is merely an
explanation of how Bell's inequality can be violated while retaining
locality and realism. It happens that EPR experiments are prepared in a way
that seems to stop decoherence, and hence (I think?) makes the MWI unable
to have any explanatory power in this particular case. If so, this may be
one of the few cases where time symmetry is actually manifested in a
detectable manner. Although I guess the device that sets the measurement
settings while the photons are in flight could be based on quantum
randomness, so we do have a superposition (or set of branches, depending on
the H. cut). But I can't see why that would be a problem. Each branch has a
well defined boundary condition at both ends of the photon's trajectory, so
the logic holds.

By the way, I may have this wrong but it seems to me your
"hyperdeterminism" objection is an objection to block universes generally.
I can't see how the big crunch (or timelike infinity) being a boundary
condition on the universe is a problem in a block universe (or multiverse)

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