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Physics Today

The foundations of quantum mechanics

Frederick G. Werner

Published Jan 1, 1964• 33 citations
Publisher

Abstract

What are the leading problems of quantum physics today? Where does reduction of the wave-packet occur? Why single-valued wave functions? To what extent have relativity theory and quantum theory really been united consistently? Does it make sense to speak of “quantum mechanical action at a distance”? What is the significance of electromagnetic potentials in the quantum domain? What does a leading quantum physicist have to say about the physicist's picture of nature?

DOI
10.1063/1.3051373

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What are the leading problems of quantum physics today? Where does reduction of the wave-packet occur? Why single-valued wave functions? To what extent have relativity theory and quantum theory really been united consistently? Does it make sense to speak of “quantum mechanical action at a distance”? What is the significance of electromagnetic potentials in the quantum domain? What does a leading quantum physicist have to say about the physicist's picture of nature?
After eighty years of Quantum Mechanics (QM) we have learned to live with wave functions without worrying about their physical nature. This attitude is certainly justifled by the extraordinary success of the theory in predicting and explaining not only all the phenomena encountered in the domain of microphysics, but also some spectacular nonclassical macroscopic behaviors of matter. Nevertheless one cannot ignore that the wave{particle duality of quantum objects not only still raises conceptual problems among the members of the small community of physicists who are still interested in the foundations of our basic theory of matter, but also induces thousands and thousands of physics students all around the world to ask each year, at their flrst impact with Quantum Mechanics, embarrassing questions to their teachers without receiving really convincing answers. Remember that Feynman once said \It is fair to say that nobody understands
Duality in quantum mechanical wave functions is manifest through the famous measurement problem. There have been several interpretations to explain this duality, but none have seen full consensus among physicists. The Copenhagen interpretation, which is at least to some extent the most widely accepted interpretation has the 'collapse' of the wave function (or state vector reduction) during measurement, does not attribute a physical reality to the wave function. Moreover, the idea of measurement having a role in defining reality shakes the very foundation of classical physics. On the other hand, the Many worlds interpretation proposed by Everett is a very brave attempt to attribute physical significance to the wave function. Though mathematically sound and elegant, 'the splitting of the universe' in the Many Worlds Interpretation completely redefines reality as we know it. We test Everett's original thought experiment in the presence of a super observer and for sequential measurements as well. We observe that the no-clone theorem helps the Many Worlds Interpretation, yet it does not provide a consistent picture for sequential measurements, unlike the Copenhagen Interpretation.
Einstein became bothered by quantum mechanical action at a distance within two years of Schrödinger's introduction of his eponymous wave equation. If the wave function represents the "real" physical state of a particle, then the measurement of the particle's position would result in the instantaneous collapse of the wave function to the single, measured position. Such a process seemingly violates not only the Schrödinger equation but also special relativity. Einstein was not alone in this vexation; however, the dilemma eventually faded as physicists concentrated on using the Schrödinger equation to solve a plethora of pressing problems. For the next 30 years, wave function collapse, while occasionally discussed by physicists, was primarily a topic of interest for philosophers. That is, until 1964, when Bell introduced his famous inequality and maintained that its violation proved that quantum mechanics and, by implication, nature herself are nonlocal. Unfortunately, this brought the topic back to mainstream physics, where it has remained and continues to muddy the waters. To be sure, not all physicists are bothered by the apparent nonlocality of quantum mechanics. So where have those who embrace quantum nonlocality gone wrong? I argue that the answer is a gratuitous belief in the ontic nature of the quantum state.