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Contemporary Physics

Illusion or reality: The measurement process in quantum optics

Axel Schenzle

Published Jul 1, 1996• 11 citations
Publisher

Abstract

Whereas a measurement in the domain of classical physics merely collects information on the examined object, measurements on the microscopic scale unavoidably leave their traces on the measured system. Experiments with single atoms and few photons contribute to our understanding of this fundamental property of quantum mechanics.
DOI
10.1080/00107519608222156

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Editor's Foreword

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If Richard Feynman's path integral formulation had been proposed not merely as a computational tool for probability amplitudes, but as a fundamental physical ontology—namely, that "all histories collectively constitute reality"—the foundational paradigm of physics would have undergone a profound transformation. Traditional physics treats a single trajectory of a system in spacetime as the primary object and path integration as a summation technique over possible paths. This paper systematically develops an alternative framework: Physics of History Space, where the ensemble of all possible histories serves as the irreduceable primary object of physical description. Within this paradigm, physical reality is defined by the topology, measure, and algebraic structures over the space of histories, rather than being restricted to a single actualized trajectory. We establish the mathematical formulation of history space and re-examine the measurement problem in quantum mechanics, the problem of initial conditions in cosmology, and the philosophy of time. We demonstrate that classical single-trajectory spacetime emerges merely as a low-dimensional projection of history space under decoherence and stationary phase limits.
The quantum theory of measurement has been widely regarded, since the 1940s, as an esoteric subject of little relevance to “real” physics. Quite the opposite was true in the 1920s and 1930s when Niels Bohr, John von Neumann, and others were struggling, by means of gedankin experiments, to develop an understanding of how measurements on single quantum objects were to be incorporated into quantum theory. However, the understanding they achieved turned out to be of little use in the quantum mechanical applications that commanded the attention of physicists between 1940 and 1980: the interaction of photons, atomic nuclei, and elementary particles, the theory of masers and lasers, the properties of matter (superfluidity, superconductivity, semiconductors), etc. In each of these applications, although the phenomena studied were quantum mechanical at heart, the measurements used to probe them were so far removed from the quantum domain that the only feature of the quantum theory of measurement which entered into the experiments was the probability interpretation of squared amplitudes. There was no need to invoke what soon came to be regarded as an esoteric, problematic, dubious “collapse (reduction) of the wave function.” Fundamentally, the reason for this irrelevance of the quantum theory of measurement was technological.