crummer%AEROSPACE@sri-unix.UUCP (10/02/84)
From: Charlie Crummer <crummer@AEROSPACE> I think that the Aspect experiment is most important in its philosophical implications. What it seems to say is that under appropriate conditions some attributes of a quantum mechanical system don't exist in the sense that we understand existence, i.e. cannot be described by so-called hidden variables. Though it doesn't imply action at a distance, Aspect does imply correlation at a distance which is perhaps just as puzzling. We can explain correlation in two ways: either the correlated behaviors come from objects that were prepared correlated in the past and nothing has been done to disturb the correlation or the one object actually interacts with the other to produce the correlation. Neither is the case in the Aspect result. --Charlie
McNelly.ES@XEROX.ARPA (10/03/84)
Hey, could somebody explain to us non-coms what the Aspect experiment is? -- John
crummer%AEROSPACE@sri-unix.UUCP (10/04/84)
From: Charlie Crummer <crummer@AEROSPACE> The Aspect (Alain Aspect from Paris) experiment is the equivalent of a famous gedankenexperiment proposed by Einstein, Podolsky, and Rosen (EPR) in 1937. (Check Science Abstracts for the exact reference.) The EPR experiment was meant as a reductio ad absurdum of the so-called Copenhagen (Bohr) interpre- tation of quantum mechanics. The gist is this: Think of a box with impene- trable walls. Put one particle in it. The wave function spreads uniformly throughout the box. (The Copenhagen school says that the wave function contains ALL of the information about the system.) The box is like one of those magician's boxes where you can slip a divider down the middle and take the box apart into two pieces without looking inside them. Do this. (The wave function is changed but it resides equally is both boxes.) Leaving one box in Cucamonga, take the other to Newark. One box is opened. Supposing that no particle is found. The Copenhagen interpretation maintains that the observer of this box now KNOWS FOR SURE that the particle will be found in the other box because his observation changed the system by collapsing the wave function (instantaneously?) into that box. (The timing isn't part of the experiment and isn't predicted.) He calls his friend and tells him that he will SURELY (probability = 1) find the particle in his box. This goes on and on and the predictor is ALWAYS right. "Ridiculous!", says Einstein, "Absurd!", says Podolsky, "Yeah!", says Rosen, "Unfortunately, however, that's the way it is.", says Aspect. For a description of the Aspect experiment (he didn't actually use a magician's box, Newark, or Cucamonga) see Scientific American a year or so back. (Reader's Guide will tell you.) --Charlie
ethan@utastro.UUCP (Ethan Vishniac) (10/09/84)
[]
Well actually, it's not just that the 'timing' of the collapse
of the wave function is not predicted. It is predicted in the
sense that its rather peculiar properties are well defined.
Loosely stated it can be put this way:
It is impossible for two measurements to imply inconsistent
things about the state of a system even if the system was
initially prepared as a superposition of states.
In the case of a single particle this is taken to mean that if
you measure the position of the particle then future measurements
must be consistent with that measurement i.e. must not involve
the particle moving faster then the speed of light.
In the case of a correlated system like two atoms whose spins
sum to zero this means that the measurements of the spins of
the atoms (as long as they are undisturbed) must sum to zero
regardless of whether the atoms are subsequently separated and
given separate homes (oops wrong discussion).
This does not *necessarily* imply faster-than-light propagation
of wave function collapse. I'm an agnostic on this subject, but
I believe that there are alternative views. An example is the
many-worlds picture in which a superposition of states is literally
a superposition of possible future world lines you might be following.
In that case there is an objective reality to all quantities at all
times that does not entail ftl propagation of signals. Playing these
games of alternate interpretations of quantum mechanics is frustrating
because one lacks means for distinguishing between possibilities.
"I can't help it if my Ethan Vishniac
knee jerks" {charm,ut-sally,ut-ngp,noao}!utastro!ethan
Department of Astronomy
University of Texas
Austin, Texas 78712