Newsgroups: sci.physics
Subject: Re: Uncertainty Principle [T.Bollinger => LONG]
From: mcirvin@husc8.harvard.edu (Mcirvin)
Date: 6 Sep 92 16:47:30 GMT

prim!dave@germany.eu.net (Dave Griffiths) writes:

>Can you (hopefully without getting too technical!) explain what is meant by
>"hidden variables"? I have seen it mentioned many times, but without a
>precise definition of the class of theories it represents.

The basic idea behind hidden variables is quite simple: it's that some or
all of the apparent randomness in quantum phenomena is actually a
manifestation of deterministic dynamics on a lower level, which we
don't see.

Such a model was proposed by Einstein, Podolsky, and Rosen as a way
of explaining the correlations between distant measurements that can
occur in QM.  Such correlations can occur when, for instance, particles
that were once associated with each other travel to widely separated
points; though QM says the the results of certain measurements are
completely random, the results for the two particles end up precisely
related to each other in some way.  It seemed to E,P, and R that this
violated the rule that information must travel at speeds less than c.
They proposed instead that, rather than the particles knowing
how to correlate instantaneously as soon as the measurement occurs,
they are just responding to internal "hidden variables" which were
correlated to begin with, when the particles were close together.

This is a "local hidden variable" theory, and for a long time nobody
knew of any way to distinguish it experimentally from QM; then John
Bell proposed a test, and the attempts to perform it so far seem to
have come up on the side of QM.  The results at least exclude many
local hidden variable theories, though some on this group argue that
they don't yet exclude other things, like wavefunction collapse at
finite speed.

Another thing the results don't exclude is a "nonlocal hidden variable"
theory like Bohm's, in which everything is deterministic but information
can travel as fast as it likes.  This defeats the purpose of the EPR
construction, but it is a possible means of going beyond QM.
 
>I don't think it's enough to just accept QM the way it is. It seems to have
>horrible flaws (the collapse of the wave function when "observed", whatever
>"observed" means) and is probably a bad model. There is surely room for a
>better theory that explains our observations but without quite so much
>wierdness.

The collapse postulate, and the associated vagueness of the term "observed,"
are properties of the Copenhagen interpretation of quantum mechanics.
This is the interpretation of QM that is usually taught as a working
model in physics classes, because it's an easy one to think in terms of
if you want to do calculations.  Others exist in which "observation" is
better defined, and in which there isn't even an extra collapse postulate.
They still use the same mathematics to describe what goes on between
measurements, so it's still QM unless your definition of quantum
mechanics is very narrow.  The question of how to interpret "QM the
way it is" is far from settled.

I doubt, though, that there's ever going to be a theory of what goes
on at atomic and smaller scales that resembles the behavior of large
objects.  All of the alternatives visible today seem "weird" in that sense.

-- 
Matt McIrvin, Cambridge, Massachusetts, USA