Re: Layman Q: wave funtion and measurement

From: Theo Wollenleben (alpha0x89_at_yahoo.de)
Date: 09/04/04 

Date: Sat, 04 Sep 2004 10:41:23 +0200

Fernando Cacciola wrote:
> However, whenever and however we measure P, we find it completely and
> absolutely at either X or Y, exclusively. Therefore, upon measurement, the
> wave function 'collapses' into a form where X (or Y) is 1 and Y (or X) is 0.
> (2) We say that "the state of the system is completely defined only after we
> measure it, definition expressed by the collapsed wave function".
>
> Now my questions:
>
> (a) Saying that the state of the system is or is not defined is only
> expressing our knowledge of it right? I mean, it doesn't say that the system
> itself, as an objective reality, doesn't have a definite state right?

I think, that is the old dicussion about hidden variables. When the
system is in one of the states X or Y before messurement, then there
must be some hidden variables which we don't know and which determine
the state. From that idea Bell worked out some inequalities that have
been checked experimentally. It came out that there are no such hidden
variables. (Other key words: EPR-paradox, entanglement, quantum
teleportation)

> (b) Saying that the wave function collapses, which is saying that
> something -the wave function- changes, is telling nothing about an objective
> change in the system itself right?

No, the Kopenhagen interpretation of quantum mechanics says that the
wave function really collapses. This change is not described by the
Schrödinger equation. This subject is still under discussion
(many-worlds-interpretation, decoherence).

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