elt@astrovax.UUCP (Ed Turner) (05/13/85)
The April 1985 issue of PHYSICS TODAY has a very stimulating article on the physical interpretation, history, metaphysical implications, and recent experimental verification of Bell's Inequality. Check it out! If there was ever a suitable subject for confused and confusing postings to net.physics, this is it in my opinion. So what do you think? Is there a "real problem" with Quantum Mechanics (as Feynmann puts it)? Is the moon really there when you aren't looking at it (to paraphrase Einstein)? My own opinion is that QM is at best an incomplete theory since it must rely on classical (non-quantum) descriptions of the act of measurement to obtain "the collapse of the wave function". If one sticks entirely to QM wave functions, there is no collapse to particular eigen states. Ed Turner astrovax!elt
ethan@utastro.UUCP (Ethan Vishniac) (05/14/85)
> > My own opinion is that QM is at best an incomplete theory since it must rely > on classical (non-quantum) descriptions of the act of measurement to obtain > "the collapse of the wave function". If one sticks entirely to QM wave > functions, there is no collapse to particular eigen states. > > Ed Turner > astrovax!elt This is the crucial point. Suggestions I have heard to solve this problem range from 1) Quantum gravity will lead to an understanding of wave function collapse once the gravitational energy of the wave function is included. [This argument is due to Penrose.] to 2) The spread of the wave function describes the branching of the universe. This is the "many-worlds" interpretation of QM due to Evrett and Wheeler (with some substantial contribution by Bryce Dewitt). Neither strikes me as entirely satisfactory. Quantum gravity may solve everything, but until we know what it is (1) is no more than a pious hope. (2) is an extravagant solution in the sense that we resolve our difficulties by creating an infinity of universes branching out from every quantum interaction. "Don't argue with a fool. Ethan Vishniac Borrow his money." {charm,ut-sally,ut-ngp,noao}!utastro!ethan Department of Astronomy University of Texas
brooks@lll-crg.ARPA (Eugene D. Brooks III) (05/17/85)
> > My own opinion is that QM is at best an incomplete theory since it must rely > > on classical (non-quantum) descriptions of the act of measurement to obtain > > "the collapse of the wave function". If one sticks entirely to QM wave > > functions, there is no collapse to particular eigen states. > > > > Ed Turner > > astrovax!elt > > This is the crucial point. Suggestions I have heard to solve this problem > range from What problem? Wavefunction collapse is part of the theory of measurement. The theory works. That the theory might not be intuitively appealing at first sight does not imply that the theory is wrong or incomplete. It might imply that there is something wrong with your intuition.
gwyn@brl-tgr.ARPA (Doug Gwyn <gwyn>) (05/17/85)
> What problem? Wavefunction collapse is part of the theory > of measurement. The theory works. (1) Where can we find a clear exposition of this "theory of measurement"? (2) I know some computer programs that also appear to work but that in reality are full of bugs. Apparent working is not good enough!
brooks@lll-crg.ARPA (Eugene D. Brooks III) (05/18/85)
> > What problem? Wavefunction collapse is part of the theory > > of measurement. The theory works. > > (1) Where can we find a clear exposition of this "theory of measurement"? Just about any serious book on QM will explain the details, the topic can't be studied lightly however and requires substatial mathemtical sophistication. See: Quantum Mechanics, by Eugen Merzbacher, Look up Measurement of observables in the index, pp 289-293 is useful. See: E. P. Wigner, Am. J. Phys, 31, 6(1963) for a review article on the topic See: Quantum Mechanics, by Albert Messiah, Look up Measurement in the index I can also dig up some papers describing Quantum NonDemolition Measurement that are relatively recent works dealing with gravity wave detectors etc where the object is to measure without disturbing the state being measured and the constraints that must be placed on the measuring device etc I you want to read them. Measurement theory in QM is well developed but is not well grasped by the physics community in general. > (2) I know some computer programs that also appear to work but that in > reality are full of bugs. Apparent working is not good enough! If you know that a program has a bug, and you can demonstrate it, then the program does not appear to work. Of course the program works with some data and your analogy to physics is a rather good one. In the case of QM and the theory of measurement lets demonstrate the bug before you claim that the theory is broken. Claiming that the theory is not intuitive or asthetic is not good enough.
elt@astrovax.UUCP (Ed Turner) (05/20/85)
> > (2) I know some computer programs that also appear to work but that in > > reality are full of bugs. Apparent working is not good enough! > > If you know that a program has a bug, and you can demonstrate it, then the > program does not appear to work. Of course the program works with some data > and your analogy to physics is a rather good one. In the case of QM and the > theory of measurement lets demonstrate the bug before you claim the theory > is broken. Claiming that the theory is not intuitive or asthetic is not good > enough. This has the ring of a high minded scientific principle but is in fact a dangerous (if taken seriously) narrow mindedness about the proper roles of intuition and aethetics in science. Historically they have *guided* the development of many of science's most successful theories, including QM. They are clearly indespensable tools of the theoretical physicist. Of course, they are not the final arbiters of good theory; experiment is. Ed Turner astrovax!elt
brooks@lll-crg.ARPA (Eugene D. Brooks III) (05/21/85)
> > > (2) I know some computer programs that also appear to work but that in > > > reality are full of bugs. Apparent working is not good enough! > > > > If you know that a program has a bug, and you can demonstrate it, then the > > program does not appear to work. Of course the program works with some data > > and your analogy to physics is a rather good one. In the case of QM and the > > theory of measurement lets demonstrate the bug before you claim the theory > > is broken. Claiming that the theory is not intuitive or asthetic is not good > > enough. > > This has the ring of a high minded scientific principle but is in fact a > dangerous (if taken seriously) narrow mindedness about the proper roles of > intuition and aethetics in science. Historically they have *guided* the > development of many of science's most successful theories, including QM. > They are clearly indespensable tools of the theoretical physicist. Of course, > they are not the final arbiters of good theory; experiment is. > > Ed Turner > astrovax!elt As you note the final arbiter of a theory is experiment and not intuition or asthetics. It is usually the case that a theory is developed in the first place to explain anomolous behavior of a new experiment. In the case of QM, the theory is well supported by experiment (although not by ones usual intuition). The theory was developed in the first place to correctly account for experiments that could not be explained by classical physics. The theory also reduces to classical physics in the appropo limit. The theory is not broken! Lets not fix it until experimental evidence points out a need.
elt@astrovax.UUCP (Ed Turner) (06/06/85)
>intuition). The theory was developed in the first place to correctly account >for experiments that could not be explained by classical physics. The theory >also reduces to classical physics in the appropo limit. The theory is not >broken! Lets not fix it until experimental evidence points out a need. > > >>I believe that this problem is recognized to be a real one by at least some of >>the experts (e.g., Feynman, Dyson, Penrose, Hawking, not to mention Einstein). >Why are you mentioning your beliefs here? Is this net.religion? Einstein >refused to accept the basic postulates of QM to day he died. I have had many >conversations with Feynman while at Caltech and I can assure you that he finds >the basic postulates of QM entirely workable. > This is my belated and probably final contribution to an exchange between Eugene Brooks and myself about QM which arose after an article I posted about the recent experimental verification of Bell's Inequality. Our discussion (or debate), a small part of which is reproduced above, has not actually had much to do with Bell's Inequality. 1) Basically, Dr. Brooks seems to be arguing that it is not wise, necessary, scientific, justified, or something to that effect to think about extensions, ellaborations, or alternatives to a physical theory which is in essentially perfect agreement with experiment (which I am willing to grant that QM is), particularly if one only seeks a more intuitive or aesthetically appealing theory. This "pure empiricism" is a common but (I think) minority philosophy of working scientists. In any case, I certainly disagree. Evidence for the effectiveness of intuition and aesthetics in the advancing science is very abundant in the history of science. To quote the most familiar (or even cliched) examples: Ptolemy's geocentric solar system explained the existing data much more accurately than Copernicus's prettier heliocentric model, and Poincare's aether drag compression theory explained the existing data exactly as well as Einstein's later Special Relativity. There was a very nice article in Physics Today several years ago discussing the role of beauty in physical theories which gave many less well known examples; I believe it was by S. Chandrasekhar. 2) I mentioned the list of emminent theorists quoted above not as an attempt to settle the issue by reference to authority (clearly inappropriate to net.physics) but rather to indicate that QM offends to some extent the intuition and physical aesthetics of at least some people who understand the theory very well and who could not be accused of having unsophisticated physical intuition. Note that I did not say that these people thought that "the basic postulates" of QM are unworkable (not likely since all of them have "worked" these postulates very effectively), merely that they had some intuitive or aesthetic misgivings about the theory. Since Dr. Brooks mentions personal conversations with Feynman at Caltech, perhaps I should add that my attribution of these views to the mentioned theorists is based on readings in two cases (Einstein and Penrose) and personal conversations in the other three (Dyson, Feynman, and Hawking). 3) One could easily cite other (and perhaps better) evidence that QM is not aesthetically or intuitively satisfactory. For example, many papers have been published about and whole conferences have been devoted to the physical interpretation of QM with authors/participants among the very best of the physics community. Great interest attends experiments such as the quantum nondemolition work at Caltech or Aspect's demonstration of Bell's Inequality that seem to throw further light on the fundamental interpretation of QM. Real disagreements about QM paradoxes (e.g., Schrodinger's cat) have remained unresolved for decades; a clear understanding of the mathematical and empirical content of the theory is not sufficient to resolve these paradoxes (unlike the Twin "paradox" in Special Relativity, for example). 4) It is still my opinion that the quantum theory of measurements (i.e., the collapse of wave functions phenomenon) is at the heart of the wide spread unhappiness of physicists with QM and that in some fundamental sense the theory is not complete. The Caltech work on quantum nondemolition which Dr. Brooks cites is fascinating and important but does not resolve the fundamental difficulty, in my opinion. Perhaps it is a step in the right direction though. My guess is that QM will eventually be superceded by a theory which has a more unambiguous physical interpretation with regard to measurements and perhaps other matters. It would be nice if this new theory were experimentally distinguishable from QM, but this might be impossible in practise or even in principle. Ed Turner astrovax!elt
gwyn@brl-tgr.ARPA (Doug Gwyn <gwyn>) (06/07/85)
"Collapse of the wave function" when a measurement is made has been "explained" in several different ways, none of them universally accepted. To my mind, there are equally severe problems: (a) Quantum theory is inherently linear; interactions aren't. (b) Quantum theory calls for its own strange logic about probabilistic matters, but it uses conventional mathematics that is based on the normal Boolean lattice. (c) Spinors are not generally covariant and cannot be made so.
brooks@lll-crg.ARPA (Eugene D. Brooks III) (06/08/85)
> 3) One could easily cite other (and perhaps better) evidence that QM is not > aesthetically or intuitively satisfactory. Give me a break, aesthetics and intuition like beauty are in the the eye of the beholder! The bottom line for physical theory is whether or not it is in agreement with experimental data, not just one part of it but ALL if it. The point where QM is found to disagree with experimental data defines the point where fixing need to be started, the number one problem with QM is not being able to solve it in areas of high importance. Has anyone solved QCD lately to find out whether or not it predicts protons? > It would be nice if this new theory > were experimentally distinguishable from QM, but this might be impossible > in practise or even in principle. If the new "correct" theory is not experimentally distinguisable from QM then it has nothing to offer other than to allow those that are currently living in the dark ages to catch up with those who already intuitively understand what is going on. IF ITS NOT BROKEN DON'T FIX IT. IF IT IS BROKEN (and I am not going to claim that is isn't as I would not be that foolish) THEN FIND THE BREAKAGE! To find the breakage you start with with a prediction of the theory and find an experiment that does not agree. To do anything else is to bark up the wrong tree.
elt@astrovax.UUCP (Ed Turner) (06/10/85)
> To find the breakage you start with with a prediction of the theory and > find an experiment that does not agree. To do anything else is to bark up > the wrong tree. We're getting repetitive here and, no doubt, boring our readers. Nevertheless, I'll restate (one of) my points in one more way. The above quote is a statement of a *theory* of how science is or ought to be done. It is inconsistent with the "experimental" (really historical) facts concerning many of the important advances in science (see my earlier, long posting, the Chandrasekhar article cited in it, or any good history of science for details). The odd thing about this dispute is that the above quoted "theory of science" is mainly appealing on intuitive and aesthetic grounds and thus persists despite its disagreement with fact, in other words it is an example of what it would itself classify as an unsatisfactory theory! Ed Turner astrovax!elt