From: aephraim@physics.Berkeley.EDU (Aephraim M. Steinberg)
Newsgroups: sci.physics
Subject: Re: Aristotle and the Modern Physicist
Date: 28 Jul 1992 03:56:37 GMT
Organization: University of California, Berkeley
NNTP-Posting-Host: physics.berkeley.edu

In article <mcirvin.712271293@husc8> mcirvin@husc8.harvard.edu (Mcirvin) writes:
%aephraim@physics.Berkeley.EDU (Aephraim M. Steinberg) writes:
%
%>Anyway, I'm not convinced that the center of mass of a system can be 
%>different in two parts of a coherent superposition.  
%
%Trivially, it can.  Just look at the different position
%eigenstates of the expanding wavefront of an electron.  As
%for magnifying the effect by using a Schrodinger's cat type
%apparatus to shove weights around, I think your objection
%about the center of mass is sound.

An amusing counterexample.  I should've been more careful about what I
was saying; I meant that if a box starts with a well-defined center of
mass, no evolution that is confined to the box and its contents will
turn it into a state with a not-well-defined center of mass, which seemed
to me to be the flavor of the original proposal.

% 
%>I'd be slightly
%>more inclined to believe that you could superpose two distributions with
%>different QUADRUPOLE moments or something like that, affecting more subtle
%>things than the simple force on a nearby object. 
%
%The aftermath of a cat-box experiment probably shouldn't be
%called a *coherent* superposition in the modern sense of the
%term, but you could indeed rig one so that the quadrupole

I'm not really sure what makes you say this.  Certainly, the cat alone is
not in a coherent superposition, nor is the radioactive nucleus, taken
alone, but together, they are.  (Assuming they have not yet interacted
with anything else.)

%moment would depend on the outcome.  Just replace the hammer
%above the poison jar with a mechanism that pushes apart two
%large weights when the decay occurs.  This *would* affect the
%fall of a nearby object, if it's close enough.  Imagine that
%it's near the end of the box where one of the weights ends up
%in the event of a decay.

Right, my mistake.

%>The reason I really respond is just because it's amusing that you come
%>up with this particular example.  I wonder if you're aware that it's exactly
%>what Aharonov uses in one of his latest proposals?  In conjunction with his
%>theory of "weak measurements," he uses such a system, in which the rate of
%>a clock depends on the state of the mass shell and can thus be placed in
%>a superposition state, to create a time machine.  
%
%It is quite an ingenious proposal.  Now that you mention it,
%let me correct a blatant error in the recent Scientific
%American article about measurement theory:  the "time machine"
%is not the usual variety in which one can travel into the
%past and violate causality.  Rather, it's a device for
%reversing the time evolution of a system, returning it to
%a younger state.  It's not so much a time machine as a
%kind of rewind button.

Am I just confusing Aharonov with Douglas Adams, or wasn't there a way
of considering the rest of the universe to be "inside" the shell, while
you sat "outside" it, so that everything except you would evolve backwards
in time?  Does seem to have some problems with boundary conditions, I
admit... but it does make for much better reading :-).



-- 
Aephraim M. Steinberg                    | "WHY must I treat the measuring
UCB Physics                              | device classically??  What will
aephraim@physics.berkeley.edu            | happen to me if I don't??"
                                         |       -- Eugene Wigner