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.
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 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.
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.
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).
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".
Bruno
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