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Physics World

Measuring the wave function of light

Stephen Mark Barnett

Published Jul 1, 1993• 2 citations
Publisher

Abstract

It is not, in general, possible to measure the state of a single quantum system. Any observable we might measure will usually have an intrinsic uncertainty associated with it, the extent of which cannot be determined by a single observation. Moreover, complementary observables cannot, even in principle, be determined with unlimited accuracy because of Heisen-berg's uncertainty principle. These features are as true for the electromagnetic field as for any other entity in quantum theory.

DOI
10.1088/2058-7058/6/7/32

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•Jun 1, 2013•0 citations
It is not, in general, possible to measure the state of a single quantum system. Any observable we might measure will usually have an intrinsic uncertainty associated with it, the extent of which cannot be determined by a single observation. Moreover, complementary observables cannot, even in principle, be determined with unlimited accuracy because of Heisen-berg's uncertainty principle. These features are as true for the electromagnetic field as for any other entity in quantum theory.
Since the publication of Heisenberg’s paper1 on the “anschaulichen Inhalt” of quantum mechanics, discussions of the fundamental limitations on the accuracy of physical measurements have been much in the foreground. According to Heisenberg, the quantum mechanics implies that it is impossible to measure simultaneous values of a coordinate and its conjugate momentum with unlimited precision. Instead, if Δp be the estimated error or uncertainty in a momentum and Δq that in the associated coordinate one must have the inequality, ΔpΔq>h2π \Delta p\Delta q>\frac{h}{{2\pi}} ΔpΔq>2πh​ (1) This inequality has come to be known quite generally as Heisenberg’s uncertainty relation.
The uncertainty principle, formulated by Heisenberg in 1927, is a fundamental principle in quantum mechanics. It forces us to review key concepts of classical physics, and from this analysis are born the concepts of quantum physics. Consequences of the Uncertainty principle also introduces the concept of incompatible sizes, a completely new concept compared to classical physics. A state of the system, which we call a quantum state, will be determined from the knowledge of a set maximum of certain quantities that are independent and compatible with each other. A measurable physical quantity is called an observable. If at a given moment a measurement is performed by one or more compatible observables it is said they are performing an observation of the system. The basic principles of quantum mechanics are expressed by a number of postulates. Their numbers and can vary from one text to another.
In this paper, we shall show how the theory of measurements is to be understood from the point of view of a physical interpretation of the quantum theory in terms of "hidden" variables, developed in a previous paper. We find that in principle, these "hidden" variables determine the precise results of each individual measurement process. In practice, however, in measurements that we now know how to carry out, the observing apparatus disturbs the observed system in an unpredictable and uncontrollable way, so that the uncertainty principle is obtained as a practical limitation on the possible precision of measurements. This limitation is not, however, inherent in the conceptual structure of our interpretation. We shall see, for example, that simultaneous measurements of position and momentum having unlimited precision would in principle be possible if, as suggested in the previous paper, the mathematical formulation of the quantum theory needs to be modified at very short distances in certain ways that are consistent with our interpretation but not with the usual interpretation.We give a simple explanation of the origin of quantum-mechanical correlations of distant objects in the hypothetical experiment of Einstein, Podolsky, and Rosen, which was suggested by these authors as a criticism of the usual interpretation.Finally, we show that von Neumann's proof that quantum theory is not consistent with hidden variables does not apply to our interpretation, because the hidden variables contemplated here depend both on the state of the measuring apparatus and the observed system and therefore go beyond certain of von Neumann's assumptions.In two appendixes, we treat the problem of the electromagnetic field in our interpretation and answer certain additional objections which have arisen in the attempt to give a precise description for an individual system at the quantum level.
The mathematics of quantum theory is based on operators rather than numbers. In optics this simple fact allows statistical quantities, such as the fluctuations in the electric field of a light beam, to be smaller than is allowed by classical physics. Experiments that measure these quantities can generally be divided into two types, depending on whether they measure the wave or the particle properties of light. This is in line with the "complementarity principle" of quantum mechanics, which states that the wave and particle aspects of a system cannot be observed at the same time.
Quantum theory treats rigorously of observables while the term “observer” is hardly itself a rigorous notion. We argue that the uncertainty principle must be understood in the Godel sense that: any given observer constitutes own practical definition of “the unobservable” (if superposition) or namely length scale, phase space, fundamental frequency etc. Conversely, the observable is definable strictly only in inverse-observer values as, that is, the “wave function collapse” or respectively, inverse-length, phase-point, harmonics. One has thus a picture of the observer that in being participant is also non-local as in Godel’s “consistency-is-undecidable” or Planck’s “the-constant-is-the-uncertainty” or indeed Einstein’s “speed-of-light-is-information-speed-limit”. Definitive of these three cases, we assert, is Peano’s (and Noether’s?) notion of the constant (our “observer”) as being for any gamut of events the meta-state (“conserved current”). Meaning now, it is the observer per se, and not his observables, which should constitute violation of Bell’s inequality—say, as the infinitesimal/imaginary axis or as the dimensionless/infinite-dimensional etc. We posit: any given observer signifies the virtual exchange of standard model or space-time of GR or just the metric (norm)—defined by the singular trait that it is the de facto “superposition” i.e. natural unit and natural limit of physical information. Observables emerge quite directly thus as the perturbations if “decoherence” or “spontaneous symmetry breaking” of the observer. Now this status of/or the observer we call rather the entity as against hitherto the uncertainty; difference is that we have an ontic as the uncertainty per se. For a prediction we show here an exact value demonstrating man as the entity or “natural unit” for quantum gravity. The idea is that in being to ourselves the most authentic sample of the term observer we should also represent to ourselves the purest sample possible of the term wave function.