From: scerir (scerir@libero.it)
Date: Sat May 11 2002 - 15:47:27 MDT
Hal Finney wrote:
> Actually if you read this closely, you find that it does not show the
> signalling phenomenon. The key point is that to see a difference,
> you must look at only "coincidence" events. These are cases where the
> detector at the left (beyond the slits) triggered simultaneously with
> the detector at the right. If we restrict our attention to that subset
> of the photons, then indeed changing the width of the slit at the left
> changes the pattern at the right. If you look at figure 4 you can see
> the coincidence detector, connected to the photon detectors at both sides.
> Likewise figure 5 displays the "coincidence pattern", meaning it is only
> showing events for which photons were captured by both sides.
Of course. They must discard every event wich has nothing to do with
entangled photons. I also wrote "It is not easy to find a big good
source of (just) entangled photons! There is always some noise in the SPDC.
In these experiments they have to use filters, polarizers, and detectors
of coincidences to be sure that they are recording just entangled
events."
> But the point is that we can only identify coincidence events via
> ordinary communication.
Yes. (Unless we have a perfect big source of entangled photons).
> Therefore this does not allow for remote communication, any more than
> with other similar EPR setups.
I did not say that this "ghost" effect was a FTL device. But this
effect is important because *in case of coincidence* the effect
(pattern, image) comes with probability 1. Here the randomness
resides in the source, which is not perfect.
In the usual EPR setups the randomness resides, imo, in the source
and also in the measurement. Let's have a 2-state entangled
quantum system, and observers A and B (far away). A performs some
measurement on his side of the system. From this measurement
A gets some information, in terms of density matrix, about the
other side of the system. But B, who does not know what measurement,
if any, was permormed by A, must use his usual, previous density
matrix, describing his side of the entangled quantum system.
Jarrett proved that if the 'locality condition'(the requirement
that the probability of getting an outcome at wing A of some EPR
apparatus is independent from the 'setting' chosen at wing B and
viceversa) is violated, one can send FTL signals. And Eberhard,
Ghirardi-Rimini-Weber, Page, et al. proved that QM does not
violate the 'locality condition', but violate another condition,
the 'separability condition' (which requires that the probability
of an outcome at wing A of an EPR apparatus does not depend on
the 'outcome' at wing B and viceversa). Thus QM violate the
separability (of things, of twin photons) but does not violate SR.
Ok this post is already too long but let me quote this.
P.A.M. Dirac, The Development Of Quantum Mechanics,
Conferenza Tenuta il 14 Aprile 1972, Roma
Accademia Nazionale dei Lincei, 1974
(11 pages)
' This statistical interpretation is now universally accepted as
the best possible interpretation for quantum mechanics, even
though many people are unhappy with it. People had got used
to the determinism of the last century, where the present
determines the future completely, and they now have to get used
to a different situation in which the present only gives one information
of a statistical nature about the future.
A good many people find this unpleasant; Einstein has always
objected to it. The way he expressed it was: 'The good God does
not play with dice'. Schroedinger also did not like the statistical
interpretation and tried for many years to find an interpretation
involving determinism for his waves. But it was not successful
as a general method. I must say that I also do not like indeterminism.
I have to accept it because it is certainly the best that we can do
with our present knowledge. One can always hope that there will
be tuture developments which will lead to a drastically different
theory from the present quantum mechanics and for which
there may be a partial return of determinism. However, so long
^^^^^^^
as one keeps to the present formalism, one has to have this
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
indeterminism. '
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