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arXiv:quant-ph/0603001

Does the wavefunction describe individual systems?

Antonio Di Lorenzo

Published Feb 28, 2006• 0 citations
arXivHTMLPDF

Abstract

We analyze the issue of the interpretation of the wavefunction, namely whether it should be interpreted as describing individual systems or ensembles of identically prepared systems. We propose an experiment which can decide the issue, based on the simultaneous measurement of the same observable with different detectors, and we discuss the theoretical implications of the possible experimental outcomes.

DOI
10.48550/arxiv.quant-ph/0603001

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We analyze the issue of the interpretation of the wavefunction, namely whether it should be interpreted as describing individual systems or ensembles of identically prepared systems. We propose an experiment which can decide the issue, based on the simultaneous measurement of the same observable with different detectors, and we discuss the theoretical implications of the possible experimental outcomes.
Abstract There is a consensus among physicists about the outcome of any experiment. However, there continues to be discussion about the interpretation of these results. At the heart of this discussion is the so-called measurement problem. The interaction of the particles with the measurement apparatus forms the measurement problem that has haunted physicists since the very early days of quantum mechanics. The problem is that quantum mechanics only describes the evolution of the wavefunction with time, but does not answer about the interaction with the measurement apparatus. These issues, related to the measurement problem, have led physicists to come up with different interpretations. Many possibilities are discussed. One of the most amazing resolutions of the measurement problem is via the possibility of the existence of multiple universes. This many-worlds approach is a desperate attempt to solve the measurement problem.
The basic postulates The statistical interpretation of the wavefunction is in accord with experimental facts. An interference pattern on a screen is built up by a series of apparently random events, and the wavefunction correctly predicts where the particle is most likely to land over an ensemble of trials (cf. the frontispiece ). Yet the interpretation of the wavefunction which ascribes to it a purely statistical significance is not forced on us by the experimental results – the latter show only that it is consistent to regard the wavefunction as containing information on probability; they do not exclude the possibility that it may possess other properties. On the contrary, one may take the view that the characteristic distribution of spots on a screen which build up an interference pattern is evidence that the wavefunction indeed has a more potent physical role than a mere repository of information on probabilities, for how are the particles guided so that statistically they fall into such a pattern? Such a question is naturally ruled out by the purely probabilistic interpretation. But the latter is appropriate only if we wish to reduce physics to a kind of algorithm which is efficient at correlating the statistical results of experiments. If we wish to do more, and attempt to understand the experimental results as the outcome of a causally connected series of individual processes, then we are free to enquire as to the possible further significance of the wavefunction (beyond its probabilistic aspect), and to introduce other concepts in addition to the wavefunction.
An alternative calculational procedure for quantum theory is presented, employing three features which differ from the corresponding features of orthodox quantum theory. These are: (1) the state vector is never to be “reduced”; (2) quantum theory only makes predictions about ensembles of identical experiments (not about individual experiments); and (3) the measuring apparatus must always be included in the state vector. What is new about this alternative theory is the demonstration that Features 2 and 3 (which have hitherto been discussed separately) can be consistently combined to yield an interpretation of quantum theory which has the additional advantage of Feature 1. It is shown that alternative and orthodox theories yield identical predictions for any practical experiment. However, two classes of experiments (which are impractical) for which the orthodox and alternative theories yield different predictions are described. Therefore, it is, in principle, possible to decide which theory Nature has chosen to obey. It is concluded that there is no reason, based upon experimental evidence, for preferring one theory over the other.
The Stern–Gerlach experiment can play an important role in teaching the formalism of quantum mechanics. In the context of a finite-dimensional Hilbert space students can learn how to prepare a specific quantum state starting from an arbitrary state, issues related to the time evolution of the wave function and quantum measurement. The Stern–Gerlach experiment can also be used to teach the distinction between the physical space where the experiment is performed and the Hilbert space where the state of the system lies, and how information about the state of the system in the Hilbert space can be exploited to interpret the possible outcomes of the experiment in physical space. Students can learn the advantages of choosing an appropriate basis to make predictions about the outcomes of experiments with different arrangements of Stern–Gerlach devices. The latter can also help students understand that an ensemble of identically prepared systems is not the same as a mixture. We discuss student difficulties with the Stern–Gerlach experiment based on written tests and interviews with advanced undergraduate and graduate students in quantum mechanics courses. We also discuss preliminary data which suggest that the Quantum Interactive Learning Tutorial on the Stern–Gerlach experiment is helpful in improving student understanding of these concepts.