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.
ogil@sphinx.uchicago.edu (Lord Julius) (05/26/87)
[I removed sci.physics from the newsgroups line because this topic has already been beaten to death there about two months ago]. In article <650@pbhyc.UUCP> djo@pbhyc.UUCP (Dan'l Oakes) writes: > >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. I won't comment on the Aspect experiment because I haven't studied it, but Mr. Oakes' statement quoted above is basically wrong. The universe _does_ conform to the state of man's knowledge (a very frightening thought on one level). The best example that comes to mind is an interference experiment. This example was given in _QED_, by Richard Feynman. If one takes a light source and filters it so only one photon emerges at a time, aims it at a photon detector, and places a solid barrier with two very small holes in it between the source and the detector, like this: | | (hole) source> | <detector (hole) | | the probability that the detector will detect a photon varies depending on its position (i.e. at one point it might detect the photon 4% of the time, at another point never). If the probability of detection is plotted vs. the position of the detector one gets an interference pattern. Now put two detectors in the holes, so you know which hole the photon went through on its way to the detector. Now the probability that the original detector detects the photon is _independent of its position_! Yes, just placing those two detectors in the holes changed the results, demonstrating that the universe does conform to the state of man's knowledge, at least in one sense. To make things even worse, if the two detectors in the holes are unreliable -- they don't always detect a photon -- the interference pattern is a combination of the original, no detectors pattern and the unvarying, perfectly reliable detectors pattern. This one never fails to amaze and confuse me. It just goes to demonstrate that our normal intuition is quite often (perhaps mostly) wrong when it comes to the realm of subatomic physics. If you want a good, non-technical explanation of one field in subatomic physics, I recommend _QED_. Feynman doesn't water down his explanations of quantum electrodynamics, he just leaves out the math that lets you predict things. He is delighted by the fact that nature is more bizarre than we can imagine, and he makes much of the point that modern physics doesn't explain _why_ the universe works the way it does, it just tells us _how_ it works. A wonderful book. -- __________________________________ Display me Aeolus above Brian W. Ogilvie | Reviewing the insurgent gales ...{uwvax,hao}!oddjob!sphinx!ogil| Which tangle Ariadne's hair ogil%sphinx@uchicago.BITNET | And swell with haste the perjured sails.
djo@pbhyc.UUCP (05/26/87)
In article <1828@sphinx.uchicago.edu> ogil@sphinx.UUCP (Lord Julius) writes: >Now put two detectors in the holes, so you know which hole the photon went >through on its way to the detector. Now the probability that the original >detector detects the photon is _independent of its position_! Yes, just >placing those two detectors in the holes changed the results, demonstrating >that the universe does conform to the state of man's knowledge, at least >in one sense. > >To make things even worse, if the two detectors in the holes are unreliable -- >they don't always detect a photon -- the interference pattern is a combination >of the original, no detectors pattern and the unvarying, perfectly reliable >detectors pattern. This one never fails to amaze and confuse me. It just >goes to demonstrate that our normal intuition is quite often (perhaps mostly) >wrong when it comes to the realm of subatomic physics. Forgive me for stating the obvious, but I do believe that a modified version of Heisenberg's Principle is applicable here -- it is not our knowledge of "which hole the photon went through," but the presence and activity of the detectors themselves -- and no bull about "passive detectors," please; there's no such thing when dealing with a photon-sized mass -- which changes the behavior of the photons as observed by the "original" detector. And I second the nomination of Feynman's excellent book. Dan'l Danehy-Oakes
biep@klipper.UUCP (05/27/87)
Warning: I don't know what I'm talking about!!! In article <1828@sphinx.uchicago.edu> ogil@sphinx.UUCP (Lord Julius) writes: >Now put two detectors in the holes, so you know which hole the photon went >through on its way to the detector. Now the probability that the original >detector detects the photon is _independent of its position_! Yes, just >placing those two detectors in the holes changed the results, demonstrating >that the universe does conform to the state of man's knowledge, at least >in one sense. In article <652@pbhyc.UUCP> djo@pbhyc.UUCP (Dan'l Oakes) writes: >Forgive me for stating the obvious, but I do believe that a modified version >of Heisenberg's Principle is applicable here -- it is not our knowledge of >"which hole the photon went through," but the presence and activity of the >detectors themselves. That seems better. I cannot really believe things depend on whether some physicist is looking at what the detectors detect or not. About the wave function collapse: Doesn't that boil down to "does the bathroom light burn when I don't see it?"? If one starts with saying: my only source of knowledge is my sensations, then it makes sense to say that "the universe" only exists as far as one senses it. But in such a position one cannot talk any more about "observers", there is just one observer and that's you. (This is not solipsism.), all others are only observing as far as you observe them observing. If I dream, I am a god in a sense: I create a world I am transcendent to. Whether something is true in that world really depends on whether I observe (better: think) it or not, but it does not depend on whether the people in my dream observe it or not. So if one poses an objective reality (objective to us), I think one cannot at the same time claim that reality depends on our observations. It may depend on the thoughts of God, but that's another newsgroup - which we don't get in Europe, at least the way I observe it. Well, I think I'll try and get the QED-book first. -- Biep. (biep@cs.vu.nl via mcvax) When a doctor doctors a doctor, does the doctoring doctor doctor the doc- tored doctor with the doctoring doctor's doctrine, or does the doctoring doctor doctor the doctored doctor with the doctored doctor's doctrine?
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)