From: jgk@osc.COM (Joe Keane)
Newsgroups: sci.physics
Subject: Re: Defining local, causal etc., was: Hidden variable theories
Summary: QM is local.
Keywords: collapse
Date: 6 Oct 92 06:24:55 GMT
Reply-To: Joe Keane <jgk@osc.com>
Organization: Versant Object Technology, Menlo Park, CA
Weather: cloudy, high 73, low 57
Moon-Phase: waxing gibbous (74% of full)

In article <1992Oct1.224027.8421@lmpsbbs.comm.mot.com>
bhv@areaplg2.corp.mot.com (Bronis Vidugiris) writes:
>However, there is a undesirable (IMO) feature of QM with the current
>formalism, a problem that can be avoided if the formalism is modified
>only very slightly (i.e. the formalism is modified so that the hypothetical
>wavefunction collapses in a different manner).

Or there's no collapse at all; after all it gives the same results.  It is
well known that collapsing too early gives `wrong' results, which of course
have never been observed.  You'd think that physicists would prefer no
collapse based on parsimony alone, but obviously many people don't like it.

>This feature is the 'non-locality' of QM.

QM is not non-local!

On the issue of locality i totally agree with Paul Budnik.  I take it as an
axiom that the laws of physics are local and Lorentz invariant.  I would
change my mind if there were really good evidence against it, but IMHO
people's misunderstanding of the Aspect experiment isn't good evidence.  I
agree it's preposterous to say that a measurement here `instantaneously' has
an effect billions of light years away.

But people who believe in collapse and also want to preserve locality are in a
tight spot.  Paul Budnik is the best example of this.  It seems like the only
way to have both is to deny that the Aspect experiment works as QM predicts.
I agree that currently there are too many holes for it to definitively answer
the question.  I hope we will get some better experiments to prove Paul Budnik
right or wrong.  I think the best experiment would use people on different
planets, but this is obviously somewhat impractical at the moment.

>However, a more important question is whether reality exhibits these
>non-local effects.

Ahem, how about `space-like separated interference effects'?

>This hasn't been totally answered, but no prediction of QM has been proven
>wrong - yet.  However, QM hasn't been totally proven right, and the
>modification to it is very small.

But this `small' modification changes the outcome of the Aspect experiment!

>you get the following result from QM
>
>p(theta1,theta2) = p1(delta) = cos^2(theta2-theta1)

Of course this formula is straight from classical electromagnetism (except
that the angle is halved due to the different spin).  Interference of light
has been known for hundreds of years, and people didn't seem to mind so much.
But when you show that the exact same effect occurs for electrons, they start
to get nervous.  And then when you claim it works over space-like intervals,
they start making up reasons why it won't work.

>However, Bell has shown that any local realistic theory must follow
>the relationship.
>
>1-p2(2*delta) <= 1 - 2*p2(delta)

I don't like the use of the word `realistic' here, since i think QM is a very
realistic theory, in fact the best we've got.  I would prefer to say `if we
assume that probabilities are additive'.  That seems to be Bell's main
assumption.

Of course we know that probabilities aren't additive at the quantum level;
that is only a macroscopic approximation.  Feynman tells us that for
`indistinguishable' events we must add amplitudes, or we'll get the wrong
answer.  In case you're wondering, `distinguishable' events are where some
fool experimenter injects a random phase along one of the paths.

A simple electron interference experiment demonstrates this effect.  Close one
slit and more electrons will show up at some places.  Sometimes providing more
ways to make something happen makes it less likely to happen.  This is a bit
counterintuitive, but that's the way it works.  Of course the interference
effect goes away if you add a measuring device and mess up the phase.

>Anyway, there *is* a way to distinguish between the two condtions I
>was describing.  Local Realistic theories predict that you will have a
>constraint on p1(delta).  A simple 'speed-of-light-collapse' theory predicts
>that p1(delta) *will* change its value when the spins are measured in a
>'timelike' manner (i.e within a lightcone) vs a 'spacelike' manner.

Right.  It's important to note that local collapse gives a different answer
than `instantaneous' collapse and no collapse.  I expect that experiment will
rule out the local collapse option.  Then, it's important to note that the
other two options predict the same results.  They can't be differentiated
experimentally, so you can pick the one you like better.

--
Joe Keane, amateur physicist
jgk@osc.com (uunet!amdcad!osc!jgk)
Newsgroups: sci.physics