djo@pbhyc.UUCP (05/26/87)
In article <766@klipper.cs.vu.nl> biep@cs.vu.nl (J. A. "Biep" Durieux) writes: >Now jump into your car and follow one of those half-wits. After some >time you overtake it, grab your magnifying glass, and (the magic word >in QM) *observe* that the particle spins left (say). At this very same >moment, at some far distance, the wave-function for the other half-wit >collapses into the description of a right-spinning particle. So the >information that you observed your particle must have travelled in >zero time to the other half-wit. FTL information transfer. <Applause> > >Perhaps one of the philosophical physicists can comment on this? >(If they aren't already bored of it, for it keeps popping up in their >newsgroup, I believe.) This is an ontological problem that I see again and again. Physicists become so used to manipulating equations that they confuse them with the particles they describe. The wave-furction for a particle is not the particle; it is not even the wave-like behavior of the particle; it is a _descriptor_ of the wave-like behavior of the particle, and exists only within the context of a system which can process the "language" in which the wave-function is written. In other words, THE MAP IS NOT THE TERRITORY. The wave-function for the half-wit contains both possibilities (left-spinning and right spinning) BECAUSE you have not observed the particle. The particle has been spinning very happily left all along. The wave-function does not describe the particle; it describes what you know (or, more exactly, what you _can_ know without observation) about the particle. The particle, however, does not care that it has been observed. It goes right on spinning as if you had observed nothing. Likewise, the other half-wit has been spinning right all along. The collapse of the wave-function, which exists only in the mind of the observer, affects it not one whit, and nothing has travelled from you to it. Nor has anything travelled from it to you; you know something about it that you did not know before -- the direction of its spin -- but you already knew that its spin was opposed to that of the half-wit you were following. To believe that its spin was determined by your observation of the primary half-wit is sheerest anthropocentrism: the belief that the universe must conform to the state of Man's knowledge. The best reply to that belief was, and remains, Sir Arthur Eddington's famous remark that "the universe is not only queerer than we imagine -- it is queerer than we can imagine." Or words to that effect; I'm quoting from imperfect memory.
drw@cullvax.UUCP (Dale Worley) (05/27/87)
According to the "standard interpretation" of QM (exposited by biep@cs.vu.nl), the imperfect knowledge of the universe implied in the wave function is an actual property of the universe, i.e., such physical variables as "position" don't have a single, definite value. djo@pbhyc gives a reasonable exposition of the contrary view, that all physical variables have a definite value at every instant, even if we those values are hard or impossible to observe at some time. This is often called the "hidden variable" model. All philosophical flaming, you say? Not so... If one makes the three assumptions: (1) information always travels slower than the speed of light, (2) physical variables always have a definite value, and (3) physical laws are only based on local interactions (true automatically for common types of differential equations) [I may have these assumption a little wrong] one can derive certain restrictions on the outcomes of certain experiments. These restrictions are called "Bell's inequality", and are independent of the details of the physical theory, as long as it satisfies the three assumptions. The important thing is that most or all hidden variable theories predict that Bell's inequality will be satisfied, but that the standard interpretation of QM requires that it will be violated in certain instances. That is, there are experiments that must have different outcomes depending on whether the standard interpretation or a hidden variable theory describes the actual physical universe. At present, the experimental evidence tends to support the standard interpretation, although I believe that no one has yet done an experiment that fully satisfies the requirements for Bell's inequality, so the jury's actually still out... Dale -- Dale Worley Cullinet Software UUCP: ...!seismo!harvard!mit-eddie!cullvax!drw ARPA: cullvax!drw@eddie.mit.edu Un*x (a generic name for a class of OS's) != Unix (AT&T's brand of such)