On 05 Feb 2014, at 20:30, LizR wrote:
On 6 February 2014 00:07, Bruno Marchal <marc...@ulb.ac.be> wrote:
OK. But time symmetry still asks fro special boundary condition, and
seems to me to still look like using ad hoc information to select
one reality against others. I agree with Deutsch's idea that Cramer
transactional theory is still a MWI, + initial conditions selecting
a reality.
Which special boundary conditions? The only ones in an EPR
experiment are the emitter and measuring device settings.
That will not be enough, I think. You need the state of the prepared
particle (say), and the measuring device + the result you measure on
the particle. Or worst, the end state of the physical universe. You
can't select one reality among all, and keep the interference right
for arbitrary measurement, without selecting the whole reversible
unique history.
The reality selected is the one with those settings
The setting by itself does not determine that reality.
- (anyway, I don't think the components of an EPR system can
decohere during the experiment, or the effect would be lost - so
there is only one reality involved).
In all other cases - where the system isn't carefully prepared to
ensure time symmetry is preserved - the effects of time symmetry
will be washed out by the entropy gradient and isn't noticable at
the macro level.
I'm not talking about Cramer. I found his theory too baroque to
understand. This is just Huw Price et al saying that logically,
given that physics is time symmetric, we can expect certain things
to be possible - like EPR.
EPR is not just possible, it is necessary, in QM.
The mechanics of how or if it works I leave to people able to do the
calculations.
We can come back on this, but my personal understanding of retrieving
unicity of outcome in the quantum description relies on fixing the
particles positions (say) completely at *some* time (by
irreversibilty, we might recover the initial and final state defining
the whole unique history). I don't like this, because it is like QM +
conspiratorial hidden positions (perhaps even depending on a
particular base). I have not read Huw Price, and my reading of Cramer
is very old.
Bruno
http://iridia.ulb.ac.be/~marchal/
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