From: mcirvin@husc8.harvard.edu (Matt McIrvin)
Newsgroups: sci.physics
Subject: Re: What does quantum have to do with it?
Date: 25 Jan 93 19:49:53 GMT
Article-I.D.: husc.mcirvin.727991393

daryl@oracorp.com (Daryl McCullough) writes:

>Maybe the situation is analogous to using coordinates to describe
>locations in space. The coordinate description gives a special status
>to this one location, the origin. However, the laws of physics don't
>really treat the origin any different from any other point. Maybe
>measurement is in the same position; Quantum mechanics doesn't really
>treat measurement devices any different, but nevertheless it is
>necessary to decide what are the measurement devices.

This is not a bad description of the sort of interpretation John Baez
talked about a while ago; I suspect that the historic Copenhagenites and
those who go on about "questions being outside the realm of physics" are
really thinking about something similar.  The idea is that *all* QM
really predicts are conditional probabilities: the probability of
B given A.  How you set up A and how you find out the system is in B
are not described within the QM calculation; in that sense
measurements are treated as special.  But measurements are *not* treated
as special in the sense that you can perform other "measurements" with
the *same* apparatus in between A and B, and as long as you take everything
into account, *those* processes are treated in the same way as everything
else, using quantum mechanics.  There's no physical difference between
the apparatus in its capacity as the maker of observations and the
apparatus in its capacity as a physical object; the only difference is
in which calculation you're doing, which question you're trying to
answer.

Some people have expressed this as "treating the measuring device
classically" when it's outside the quantum-mechanical evolution in
the calculation.  I think that is misleading if only because many
measuring devices wouldn't work classically-- suppose it has integrated
circuits in it!  For that matter, if it were really treated classically
you'd calculate that its atoms would collapse, yielding no measurement
at all.  A better statement is that you don't "treat" the measuring
device at all in the calculation when it's used to make the
measurements.  Whether the measuring device works or not is a
different question, one just as accessible to quantum mechanics.

I think this is what Wheeler means when he talks about "the distinction
between the probe and the probed" as the important thing in QM
measurement.  The distinction is arbitrary, but it's the first thing
you have to do when finding something out.

When taken to an extreme this becomes a kind of operationalist view:
physics then concerns itself *only* with predicting laboratory 
probabilities.  I have a little of the same problem you do when swallowing
this kind of operationalism.  You gave the example of black-hole
quantum mechanics; I'm bothered by something comparatively mundane,
namely particle physics.  An *extreme* operationalist might well regard
particle physics as the science of what detector readings one can expect
to find when one builds a certain billion-dollar machine and turns it on.
If this is the case, the whole enterprise seems pointless, since if you
don't build the machine in the first place, you know all there is to
know!  (I suppose this could be taken as a knockout argument against
SSC funding. :')

I have a certain need to believe that the repeatability of
these experiments says *something* about the structure of the world
"out there."  It doesn't have to be anything so concrete as a reification
of the wave function; I'm imagining something more like a description of
the world as what you get when you take the predicted results and
mod out the possible variations in experimental schemes.  The physical
description of the universe would have something to do with the totality
of probabilities P(b,a), which is expressed independently of the 
experimental setup-- the simplicity of the world would be related to
the simplicity of relating probabilities to each other.

This very well may be better described as metaphysics than as physics,
but it's at the very least physically motivated metaphysics.  
-- 
Matt McIrvin