Re: TECH: Quantum Head Job

From: scerir (
Date: Fri Oct 12 2001 - 01:30:22 MDT

Hal Finney wrote:

> Scerir writes:
> > Most physicists would say that the electron observed in Paris
> > at T_0 was *already* there. And that those 2 w.f. represented
> > just our *knowledge* prior to that observation.

> I don't believe this is true, although I'm not sure about the exact
> details of this particular experiment. In the case of separated particles
> with correlated spins, most physicists today would *not* say that the
> particles actually had well-defined spins but that the wave function
> represented our lack of knowledge of those spins prior to observation.

The question is: does the wave function represent reality or
knowledge (lack of knowledge) ? I think that QM is a double-speaking
theory. Sometimes speaks about things (Schroedinger's cat is real,
experiments performed), sometimes about informations (Rothstein,
Brillouin, Heisenberg, Adami, Cerf, Zeilinger, Mermin, etc.).

Have a look. 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 continue to use his (usual, previous) density matrix,
describing his side of the entangled quantum system.

That's all? No, of course. We must add the *physical* condition.
The product of the 2 density matrices (observer A and B) should
not be zero. Otherwise those 2 observers may contradict each other.

> The reason they don't say this is because you can in principle bring the
> particles back together and show interference effects which would not be
> possible if they had well defined spins. If physicists had adopted the
> stance you describe, they would be forced to say that once the boxes were
> brought near each other the particles had lost their well-defined spins
> (or positions), which is absurd.

When boxes are separated interference effects are lost (definitively).

> There is a fundamental difference between these quantum effects and the
> classical phenomenon of two boxes only one of which contains a particle.
> With the quantum case it is clear that there is no well-defined state
> until the measurement is done. In the classical case this is not true.
> It confuses the issue to suggest that these two situations are similar.

Perhaps. But the *single* particle issue is interesting, also
in connection to the EPR - Bell argument, and possible
experiments. References below.


Nonlocality of a single photon revisited
Hardy Lucien
PHYS REV LETT 73: (17) 2279-2283 OCT 24 1994

The EPR argument and nonlocality without inequalities for a single photon
Hardy Lucien
ANN NY ACAD SCI 755: 600-615 1995

Nonlocality of a single photon - Reply
Hardy Lucien
PHYS REV LETT 75: (10) 2065-2066 SEP 4 1995

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