From: scerir (scerir@libero.it)
Date: Tue Jan 15 2002 - 01:06:59 MST
> I don't think this description can be correct. You make it sound like
> changing the filter over here causes instantaneous observable changes
> in the interference fringes over there. Clearly this would contradict
> relativity, which you say the experiment does not do. Presumably there
> is some need to bring the photons (or at least information about them)
> together after the experiment in order to observe the changes in the
> interference fringes. In that case there is an ordinary causal link
> between changing the filter and the observed changes, due to particles
> which had passed through the filter.
> Hal
As they say, there is no paradox here since the determination of coincident
events can only be made _after_ the records of the 'clicks' at both near (signal)
and remote (idler) detectors are compared, and only _then_ the appearance
or disappearance of interference fringes become apparent.
Bringing together these records requires the propagation of signals through
classical channels, at a speed limited by 'c' . Hence there is no conflict
with the postulates of the theory of relativity (_informations_ cannot travel
faster than 'c', but _influences_ could do that, of course).
Perhaps it is also not correct to say that the experimenter's arbitrary choice
of the remote and space-like separated fillter (idler) _caused_ the collapse
of the (signal) photon wavepacket, on the near side.
That's because 'No elementary quantum phenomenon is a phenomenon until
it is a recorded phenomenon' (J.A. Wheeler, after Bohr). Thus we can encrease
the distance of the filter (idler remote side), from the source, as much as we can.
This way we are sure that, on the near side, the signal collapsing wave packet
could not have known _in advance_ which was the setting of that filter (idler
remote side). Nevertheless we find that the signal collapsing wave packet, on
the near side, depends on that _subsequent_ setting on the filter on the remote
idler side ('delayed choice principle'). 'No elementary quantum phenomenon is
a phenomenon until it is a recorded phenomenon'.
Only nonlocal and uncaused, out of time correlations-at-a-distance are predicted
by quantum theory. This may have something to do with the time-symmetrical
Schroedinger wave equation. Just measurements, or recordings, introduce
a time-asymmetry (irreversibility). Now, between measurements, between
recordings, we can have two opposite time arrows (quantum phenomena may
feel past and future at the same time, I mean: past measurement and future
measurement).
Regards
-s.
An interesting two-photon 'ghost' image effect was found by Shih (et. al.)
[in Experimental Metaphysics, Boston Studies in the Philosophy of
Science, vol. 193, Kluwer Academic Publ., 1997]. A pair of signal and
idler entangled photons (PDC generated) is propagated to different
directions and detected by two distant detectors. A mask (i.e. with
these letters: LOVE), is illuminated by the _signal_ beam. Surprisingly
an image of that mask, with those letters (LOVE), is observed by scanning
the _idler_ detector.
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