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In his book `Mathematical Foundations of Quantum Mechanics', von Neumann asserted the following: the Compton-Simon experiment showed that the state vector must collapse upon measurement of any self-adjoint operator. Comparing von Neumann's account with the Compton-Simon paper, we find that von Neumann had misinterpreted the experiment as consisting of two successive measurements (which gave identical results), whereas the experiment only measured two angles on the same photographic plate. Note, however, that the state vector must collapse upon measurement of an additively-conserved quantity; otherwise the conservation law could be violated. Next, it turns out that the mathematical problem of explaining collapse is not fully defined until one specifies the nature of the apparatus. If the apparatus does not have a `classical description', the problem is insoluble, even if the measurement is only approximate (Fine, Simony, Brown, Simony and Busch); but if it does, the problem is soluble within Schroedinger dynamics (with a time-dependent hamiltonian) for additively-conserved observables. The solution, a modification of Sewell's, shows that the state vector has collapsed, but it does not reveal the eigenvalue of the collapsed state. The collapse is irreversible, and results from the interplay of additive conservation laws with the quantum measurement postulate. Indeed, the quantum measurement problem - as expounded by Wigner - may be better understood as the problem of establishing that the two are compatible with each other; it has little relevance to actual measurements., Comment: 40 pages, 1 figure

Few things in physics have caused so much hand-wringing as von Neumann's collapse hypothesis. Unable to derive it mathematically, von Neumann attributed it to interaction with the observer's brain! Few physicists agreed, but tweaks of von Neumann's measurement theory did not lead to collapse, and Shimony and Brown proved theorems establishing `the insolubility of the quantum measurement problem'. Many different `interpretations' of quantum mechanics were put forward, none gained a consensus, and some scholars suggested that the foundations of quantum mechanics were flawed to begin with. Yet, in the last ninety years, no-one looked into now von Neumann had arrived at his collapse hypothesis! Von Neumann based his argument on the experiment of Compton and Simon. But, by comparing readings from von Neumann's book and the Compton-Simon paper, we find that the experiment provides no evidence for the collapse hypothesis; von Neumann had misread it completely! We suggest that von Neumann had relied on his phenomenal memory rather than the printed Compton-Simon paper, and his memory had failed him for once. Our finding has considerable implications for physics, which -- briefly sketched here -- will be discussed elsewhere in detail. An Appendix raises some questions for historians of physics.

In 1954, Ashkin, Page and Woodward (hereafter APW) reported on the first counter experiments to measure the electron-electron and electron-positron scattering cross-sections at low relativistic energies (0.6-1.7 MeV). Their aim was to look for the spin and exchange or virtual annihilation effects predicted by the Moller and Bhabha formulae. Their experiments confirmed these effects, but the measured cross-sections at 0.61 MeV were significantly smaller than their predicted values. The authors remarked that these deviations were `presumably due to multiple scattering'. However, careful reading of the unpublished theses of Page (1950) and Ashkin (1952), Page's letter (1951) and the APW paper reveals no credible evidence of multiple scattering at 0.61 MeV; if anything, the evidence rules against multiple scattering. If multiple scattering is ruled out, the observations may indicate a departure from quantum electrodynamics. This departure may be due to a non-Coulomb central force, a weakening of the spin-statistics connection, or both. Only experiment can tell which of these possibilities holds true, and therefore we suggest the new electron-electron scattering experiments be carried out at different energies (at 0.4-1.0 MeV) and different scattering angles, as well as specific tests for multiple scattering. We consider a non-Coulomb central force to be very unlikely, and advance the hypothesis that a fraction of the electron pairs scatter as spin-zero fermions (which would lower the observed cross-section). Numerical calculations show that the hypothesis may be tested quantitatively in a Page-type experiment, even with little improvement in the accuracy he achieved in 1950., Comment: 17 pages, 5 figures

It often goes unnoticed that, even for a finite number of degrees of freedom, the canonical commutation relations have many inequivalent irreducible unitary representations; the free particle and a particle in a box provide examples that are both simple and well-known. The representations are unitarily inequivalent because the spectra of the position and momentum operators are different, and spectra are invariant under unitary transformations. The existence of these representations can have consequences that run from the merely unexpected to the barely conceivable. To start with, states of a single particle that belong to inequivalent representations will always be mutually orthogonal; they will never interfere with each other. This property, called superseparability elsewhere, is well-defined mathematically, but has not yet been observed. This article suggests two single-particle interference experiments that may reveal its existence. The existence of inequivalent irreducibile representations may be traced to the existence of different self-adjoint extensions of symmetric operators on infinite-dimensional Hilbert spaces. Analysis of the underlying mathematics reveals that some of these extensions can be interpreted in terms of topological, geometrical and physical quantities that can be controlled in the laboratory. The tests suggested are based on these interpretations. In conclusion, it is pointed out that mathematically rigorous many-worlds interpretations of quantum mechanics may be possible in a framework that admits superseparability., Comment: 24 pages, 5 figures. arXiv admin note: significant text overlap with arXiv:1002.0547

Results of the welcher Weg experiment of Duerr, Nonn and Rempe are explained by using ray representations of the Galilei group. The key idea is that the state of the incoming atom be regarded as belonging to an irreducible unitary ray representation of this group. If this is the case, interaction with an interferometer with a which-way detector must split this state into the direct sum of two states belonging to representations with different internal energies. (While the zero of internal energy is arbitrary, the difference between two internal energies is well-defined and is invariant under unitary transformations.) The state of the outgoing atom will then be a superposition of two mutually orthogonal states, so that there will be no interference. Neither complementarity nor entanglement plays a role in this explanation. Furthermore, in atom interferometry it is not enough for a quantum eraser to erase the internal energy difference; to restore interference, two copies of a representation have to be collapsed into one. In a direct sum of copies of the same representation, copies of the same state will still be orthogonal. These assertions may be testable, and two new atom interferometry experiments are suggested. One of them is an `own-goal' experiment which may decisively refute the explanation offered here, and restore the aura of mystery that this paper tries to dispel., Comment: 18 pages, 3 figures

Feynman contended that the double-slit experiment contained the `only mystery' in quantum mechanics. The mystery was that electrons traverse the interferometer as waves, but are detected as particles. This note was motivated by the question whether single electrons can be detected as waves. It suggests a double-slit interferometry experiment with atoms of noble gases in which it may be possible to detect an individual atom as a probability wave, using a detector which can execute two different types of simple harmonic motion: as a simple pendulum, and as a torsion pendulum. In the experiment, a torsional oscillation will never be induced by the impact of a probability wave, but will always be induced by the impact of a particle. Detection as a wave is contingent on the atom interacting much more strongly with the macroscopic detector as a whole than with its microscopic constituents. This requirement may be more difficult to meet with electrons, protons, neutrons or photons than with atoms., Comment: 6 pages, 1 figure

The existence of inequivalent irreducible unitary representations of the CCR suggests that two identical charged bosons may, under suitable conditions, be unable to interact with each other even though their wave functions overlap considerably in space at a fixed time. An experiment is proposed to test this possibility., Comment: 12 pages, 1 figure

Wigner had expressed the opinion that the impossibility of exact measurements of single operators like position operators rendered the notion of geometrical points somewhat dubious in physics. Using Sewell's recent resolution of the measurement problem (collapse of the wave packet) in quantum mechanics and extending it to the measurement of operators with continuous spectra, we are able to compare the situation in quantum mechanics with that in quantum mechanics. Our conclusion is that the notion of a geometrical point is as meaningful in quantum mechanics as it is in classical mechanics., Comment: 20 pages