From: mcirvin@husc8.harvard.edu (Mcirvin)
Newsgroups: sci.physics
Subject: Re: Uncertainty Principle [T.Bollinger => LONG]
Date: 5 Sep 92 19:20:55 GMT
Article-I.D.: husc8.mcirvin.715720855

jbaez@riesz.mit.edu (John C. Baez) writes:

[about "disturbed-particle" gedankenexperiments designed to extract
 the uncertainty principle from the properties of photons: why do
 they even work as well as they do?]

>Perhaps they are showing that a world with light described
>quantum-mechanically and other particles described classically is
>inconsistent or otherwise incoherent?  It would be nice if someone could
>extract the rigorous core, if any, out of these hand-wavy arguments.

I think it's something close to that.  Quantum mechanics does tend
to leak from one part of physics to another when the associated 
phenomena interact.  It's similar to the argument you posted a while
ago about the implausibility of coupling classical gravity to quantum-
mechanical matter:  unless you put in something kludgey, like coupling
gravity to expectation values and assuming a collapse that happens
before everything gets ridiculous, there doesn't seem to be a 
consistent way to do it.

[Terry Bollinger's description of Feynman's demonstration of
the irrelevance of irreversible processes deleted]

>If you look at my explanation of your angular momentum puzzle - 
>which seems essentially the same as Mcirvin's - you will see that it
>relies on the fact that measuring one component of the angular momentum
>of a particle really does affect other components of the angular
>momentum of the particle [...]

> So unless this is full of baloney, there DOES seem to be
>something to the "disturb-the-particle" view of uncertainty - although
>it doesn't tell the whole story.

This has a lot to do with problems of QM interpretation.  Whatever
happens at a measurement, whether it's a sudden collapse of the
state vector or simply the connection of dynamics of the external
world to the eigenvalue of a particular operator, it happens when
the particles go through the blocked Stern-Gerlach filters.

Feynman's demonstration that an irreversible process is not
necessary to do a "measurement," by the fact that we can remove
the block in his apparatus and recombine the base states, is
interesting to look at in the context of various interpretations.

If you think in terms of some actual collapse process, it
doesn't happen in the unblocked filter, or else we couldn't
recombine the paths.  The electron would follow only one path
or the other after the collapse.  The collapse seems to
be associated with the presence of the block.  Whether it's
something in the physics of the interaction with the block; or
with some gravitational difference between alternate block
states, as Penrose would have it; or with the ability of
conscious individuals to perceive the results, as I think
Wigner would have it, is a thing that further divides such
interpretations.  (If there really must be collapse, I'd
go with the first possibility of the three.)

In an interpretation that has no collapse postulate, the
difference is easier to understand:  when the block is
there, the eigenvalue of the particular spin operator being
measured is linked to irreversible processes in the outside
world, and interference between various eigenstates of that
particular operator becomes extremely hard to measure in a very
short time, whereas interference between eigenstates of the
other spin components is still easy to measure.  In this
picture, what's important is that the measurement disturbs
the *world*.  

In either case there's a definite difference
between measurements that can be "quantum erased" and
measurements that can't.  So I think Feynman's argument is
a little unfair, but it's also not clear to me that something
physically has to happen *to the particle*; what's necessary
for us to be able to treat the wave function as if it has
suddenly changed is for the difference between various
eigenvalues to escape irreversibly into the world-- most
likely irreversibly in the thermodynamic sense of the word.
Otherwise, we could unscramble the results of the "measurement"
process, like Feynman does in his gedankenexperiment when he
recombines the beams, and the possibilities we have neglected
after the measurement would come into play again.

-- 
Matt McIrvin, Cambridge, Massachusetts, USA