Against Quantum Theory Without Observers
Quantum Theory Without Observers
Sheldon Goldstein had the temerity to write an article with this title that Physics Today unfortunately printed . In this article, Goldstein asserts that while "many physicists pay lip service to the Copenhagen interpretation ... hardly anyone truly believes" that "quantum mechanics is about observations or results of measurement." Goldstein is wrong about that in my case. It seems very simple to me because the concepts of physics are mutable. I admit that physics is conceptually very beautiful, and it is hard to maintain a properly skeptical attitude. However, theoretical physics must occasionally adjust to new observations that are inconsistent with old concepts. I hope it happens again in my lifetime. Goldstein's admonition that "quantum mechanics is fundamentally about atoms and electrons ..." doesn't specify what aspect of the existing conceptual framework should be considered salvageable. For example, Feynman gave a pretty good account of electrons, but there are still some fundamental constants in his theory. Those fundamental constants can be considered one measure of our ignorance about the way nature works. I believe Feynman himself once said that he had only "swept a great problem under the rug."
Schreoedinger's Cat Paradox
Goldstein quotes Schroedinger's famous paper  on the "measurement problem" of macroscopic superpositions. Schroedinger poses his famous cat paradox in a very off-hand way in this paper. He says it is "quite ridiculous," but he does not describe in any detail just what is ridiculous about it. Even if the cat were to be conscious, so what? Goldstein is not a materialist if he thinks this makes any difference. The quantum description is correct, in principle, but probably no significant error will be introduced by assuming that the cat is always either alive or dead. If somebody could prove otherwise, his or her contribution would have merit equivalent to that of Bell's theorem (see below). By the way, Schroedinger made at least one other error in his paper. He asserts that Dirac's theory "very strongly transcend(s) the conceptual plan of Q.M." because of the "stubborn resistance to go forward to the problem of several electrons." Goldstein, too wants to press forward with heterodox views even though they "present formidable difficulties for the development of a Lorentz-invariant formulation." The reader must be aware that the combination of quantum mechanics and general relativity is where the action is now, and Goldstein is therefore at least 60 years behind the times.
Mermin  gave an especially lucid account of Bell's theorem. Einstein, Podolksy and Rosen had argued that the quantum description of the decay of a singlet resonance into two identical particles that have spin is necessarily incomplete. One horn of the dilemma, as they saw it, is that the spin components along various directions must have been determined while the two particles were still close together. In this case, the quantum description would be incomplete. Bell disproved this. The alternative for EPR was that, against intuition, the two particles somehow communicate, even if the two measurements are made simulataneously. This is because the correlation is perfect even when the two measurements are separated by a spacelike interval. Fortunately, this conceptual difficulty does not suggest any observations or measurements that would violate relativistic causality. This is why it is necessary to say that quantum mechanics and physics in general is about observation and measurement and not about "stuff," which may turn out to be even stranger than current theories suppose. As they say, truth is often stranger than fiction.
The Feynman Intepretation
Feynman  was very straightforward about the conceptual problem of quantum mechanics. He said "we cannot make the mystery go away we will just tell you how it works." He uses the double slit experiment as an example and discusses it in great detail because "it contains the only mystery." He gets to the classical limit by allowing the mass of the particles to increase, and nothing conceptually different happens as the wavelength of the particles decreases. However, the interference fringes become so fine that you cannot see them as a practical matter. Hence real measurements with macroscopic particles show the expected smooth curve. I think Feynman would say that you don't have to "shut up and calculate," if that is how you characterize the Copenhagen interpretation, but you do have to calculate. If your detector can resolve the fringes, you aren't justified in using classical terms with their implicit assumptions.
A Superposition Experiment You Can Do
Spin states of photons are describable in terms of polarization, and very good polarizers are available for visible light. They work by absorbing almost all the photons that are polarized in one direction and transmitting almost all the photons polarized at right angles to that direction. Get three polarizers and cross two of them so that no light comes through. Then insert the third one between the first two at approximately 45 degrees. Light comes through again, showing that the middle polarizer erases the initial polarization and creates a new one. How does a polarizer alter the polarization of a photon without absorbing it? That is the mystery. I have heard people who should know better explain it by saying polarizers are highly birefringent. You can see strain birefringence in a glass object by putting it between crossed polarizers, but that is not what is going on here. It is just superposition, pure and simple.
1. Sheldon Goldstein, "Quantum Theory Without Observers" Physics Today March 1998, pp. 42-46 and April 1998 pp. 38-42.
2. J. D. Trimmer, A Translation of Shrodinger's Cat Paradox Paper as of April 19, 1998.
3. N.D. Mermin, "Is the Moon there when Nobody Looks? Reality and the Quantum Theory," Phys. Today 38(4), 38-47 (1985).
4. R. P. Feynman, R. B. Leighton and M. Sands, "The Feynman Lectures on Physics," Volume III, Addison Wesley, Reading, MA (1965).
(last updated April 19, 1998)
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