NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/sci/physics/23218

Internet Message Format | 1993-01-21 | 3.9 KB

Path: sparky!uunet!sequent!gaia.ucs.orst.edu!comphy.physics.orst.edu!preddy
From: [e-mail] ()
Newsgroups: sci.physics
Subject: Re: What does quantum have to do with it?
Date: 21 Jan 1993 17:18:19 GMT
Organization: IBM T.J. Watson Research, NY
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[e-mail] (Daryl McCullough) writes:

> If you ask most physicists to try to explain the basic
> concepts of quantum mechanics, you will find that their
> understanding is incoherent and even inconsistent. They
> have no way of describing the laws of quantum mechanics
> without reference to one of the following:  observer,
> observables, observation, measurement, measuring device.

Correct.  This is the point behind the collective philosophy of
quantum mechanics, commonly referred to as complementarity, or,
incorrectly, the "Copenhagen interpretation."  There are no
other words to use when one talks about an experiment---that
is, in order to _talk_ about an experiment, we must use the
terms of classical physics.  Failure to understand this point
is what leads the student off into the never-never land of
classical/quantum confusion.

> However, when pressed, they usually will say that there is
> nothing special about a measuring device or an observer, they
> are quantum systems, too, although complex ones.

They are quantum systems IF one is doing an experiment on them
that requires a quantum mechanical description to explain the
outcome.  They are not quantum systems in the sense they are
"sitting out there" behaving like quantum systems.  A very
common mistake in philosophical discussions of quantum
mechanics is the assumption that QM somehow rules something
that sits "out there," waiting to be tested.  QM affords us
predictions of experiments on systems, but says nothing about
a system independent of the observer.  To force it to do so
goes beyond its intent (and beyond physics in general, for that
matter).

> ... Physicists often deny that the wave function
> is an objective physical quantity, and also will deny that
> anything *other* than the wave function is an objective
> physical quantity. They will say that nothing is real except
> observation, and then say that observation is simply a
> special case of a quantum mechanical interaction.

Physicists do not say that nothing is real except observation.
They say that nothing is real _in physics_ except observation.
And as Bohr pointed out years ago, "reality" is word just like
any other in physics, and is used in whatever experimental
context requires it.  Anything more, such as the belief in a
classical separation between the observer and the observed, is
beyond the bounds of modern physics.  (If you understand tenets
of quantum theory, then you realize the dividing line between
observer and observed is fluid, depending on what is sought in
the experiment.  If you don't understand this, then you don't
understand the "Copenhagen interpretation" of quantum mechanics
and the accounting for the results of Aspect's test of Bell's
theorem.)

> The bottom line for physicists is of course prediction, and
> in spite of the incoherent mess that quantum theory is, one
> can identify a solid enough core to make uncannily accurate
> predictions.

This "incoherent mess" is nothing more than a failure to
separate a classical notion of the way things are "out there"
from what QM predicts about experiments done on systems.

> That's enough for some people, but not for everyone.

Then "everyone" will have to look elsewhere for the answer
he/she seeks (like metaphysics, perhaps, or religion).

.............................................................
Mark Preddy                 Internet: [e-mail]
Department of Physics, OSU, Corvallis, Oregon 97331-6507  USA
.............................................................