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Physical Review A

Composition law of amplitudes and quantum theory

J. M. Vigoureux

Published Mar 1, 1994• 1 citation
Publisher

Abstract

Mathematical methods in optics show that the overall amplitude resulting from the infinite summation over all the various alternative paths that the light can take inside a Fabry-P\'erot interferometer can directly be obtained by using a complex generalization of Einstein's composition law for parallel velocities. Moreover, this result appears to be quite general. In fact, the reflection coefficient for any number of interfaces or quantum wells can be easily obtained by using the composition law. This suggests that a more general way to obtain the probability amplitude for an event which usually needs infinite summation on virtual paths (as in the Fabry-P\'erot interferometer) is to compose amplitudes following Einstein's addition law. I explore this problem and show how the composition law avoids divergences and guarantees the modulus of any total wave function to be less than (or equal to) unity. By examining its physical meaning in the light of probability theory, I also show that it provides an interesting expression for nonlocality and nonseparability.

DOI
10.1103/physreva.49.1602

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Mathematical methods in optics show that the overall amplitude resulting from the infinite summation over all the various alternative paths that the light can take inside a Fabry-P'erot interferometer can directly be obtained by using a complex generalization of Einstein's composition law for parallel velocities. Moreover, this result appears to be quite general. In fact, the reflection coefficient for any number of interfaces or quantum wells can be easily obtained by using the composition law. This suggests that a more general way to obtain the probability amplitude for an event which usually needs infinite summation on virtual paths (as in the Fabry-P'erot interferometer) is to compose amplitudes following Einstein's addition law. I explore this problem and show how the composition law avoids divergences and guarantees the modulus of any total wave function to be less than (or equal to) unity. By examining its physical meaning in the light of probability theory, I also show that it provides an interesting expression for nonlocality and nonseparability.
The most general mathematical law for summing bounded quantities is not the arithmetic law, but a composition law of which the summation law for velocities in special relativity is only one particular example. We believe that this composition law, should also be used in quantum theory. We present it with a few examples, we discuss its physical meaning and we show how it can be useful in quantum theory. We show in particular that its use may open the way for a new interpretation of the use of probabilities in quantum theory.
This paper is concerned with the question of the existence of composition laws in the sum-over-histories approach to relativistic quantum mechanics and quantum cosmology, and its connection with the existence of a canonical formulation. In nonrelativistic quantum mechanics, the propagator is represented by a sum over histories in which the paths move forward in time. The composition law of the propagator then follows from the fact that the paths intersect an intermediate surface of constant time once and only once, and a partition of the paths according to their crossing position may be affected. In relativistic quantum mechanics, by contrast, the propagators (or Green functions) may be represented by sums over histories in which the paths move backward and forward in time. They therefore intersect surfaces of constant time more than once, and the relativistic composition law, involving a normal derivative term, is not readily recovered. The principal technical aim of this paper is to show that the relativistic composition law may, in fact, be derived directly from a sum over histories by partitioning the paths according to their first crossing position of an intermediate surface. We review the various Green functions of the Klein-Gordon equation, and derive their composition laws. We obtain path-integral representations for all Green functions except the causal one. We use the proper time representation, in which the path integral has the form of a nonrelativistic sum over histories but is integrated over time.The question of deriving the composition laws therefore reduces to the question of factoring the propagators of nonrelativistic quantum mechanics across an arbitrary surface in configuration space. This may be achieved using a known result called the path decomposition expansion (PDX). We give a proof of the PDX using a spacetime lattice definition of the Euclidean propagator. We use the PDX to derive the composition laws of relativistic quantum mechanics from the sum over histories. We also derive canonical representations of all of the Green functions of relativistic quantum mechanics, i.e., express them in the form 〈x''\ensuremath{\Vert}x'〉, where the {\ensuremath{\Vert}x〉} are a complete set of configuration-space eigenstates. These representations make it clear why the Hadamard Green function G(1){\mathit{G}}^{(1)}G(1) does not obey a standard composition law. They also give a hint as to why the causal Green function does not appear to possess a sum-over-histories representation. We discuss the broader implications of our methods and results for quantum cosmology, and parametrized theories generally. We show that there is a close parallel between the existence of a composition law and the existence of a canonical formulation, in that both are dependent on the presence of a timelike Killing vector. We also show why certain naive composition laws that have been proposed in the past for quantum cosmology are incorrect. Our results suggest that the propagation amplitude between three-metrics in quantum cosmology, as constructed from the sum over histories, does not obey a composition law.
In learning quantum mechanics, an essential question has always been: How does one go about developing a “physical feel” for quantum phenomena? Naturally, one needs a basis or ground zero to start from, and that basis must be unlike anything with which we are already familiar in consequence of our experiences with the world of classical physics. We argue (channeling Richard Feynman) that the most elementary and the least cumbersome concept to build upon is the existence of complex probability amplitudes for physical events. An event that can take place in multiple alternative ways should be treated by adding the corresponding amplitudes when the paths are, in principle, indistinguishable, and by adding the probabilities themselves when the paths are distinguishable. Once we accept this principle and hone our intuition by examining quantum phenomena in its light, we will be on the path to "understanding" quantum mechanics. Elementary examples from the field of quantum optics demonstrate how adherence to Feynman’s principle could lead to a better, more “intuitive” appreciation for the magic of quantum mechanics.
This paper examines the geometric structure underlying amplitudes, interference, and probability in quantum mechanics. Building on the interpretation of quantum phase as holonomy, amplitudes are analyzed in terms of parallel transport in complex line bundles. Interference is interpreted as arising from the composition of alternative transport paths, and probability emerges from the invariant comparison of amplitudes under phase transformations. Under general assumptions of linearity and phase invariance, the squared-norm structure associated with probability is shown to arise naturally from consistency conditions on amplitude composition. The analysis focuses on kinematic structure and does not assume specific dynamical models.
The foundations of quantum mechanics have traditionally relied upon the probabilistic interpretation where wave function moduli determine spatial probabilities, treating measurement transitions through sudden collapse or decoherence. This paper introduces the theoretical framework of Quantum Probability Refraction (QPR), proposing that quantum probability currents bend when crossing distinct measurement environments in a manner analogous to light refraction in optics. We define the probability refractive index as n_P equal to v_{P,0} divided by v_P and derive a generalized Snell's law for probability flows expressed as n_{P1} multiplied by the sine of theta_1 equals n_{P2} multiplied by the sine of theta_2. By establishing the mathematical formulation, continuity equations, conservation laws, and the optical-like phenomena of total internal reflection and evanescent probability waves, this work presents a geometric theory of probability flows. This paradigm offers a novel continuous perspective on quantum measurement boundaries and opens new pathways for controlling quantum information transport.
I show that the reflection coefficient of any stratified planar structure can be obtained by using a complex generalization of Einstein's addition theorem for parallel velocities. This result also applies to multiple quantum wells. It provides a new mathematical tool in optics and in quantum theory and may lead to useful algorithms in computing. It may also give a new insight into special relativity. The composition law of velocities, in fact, no longer appears as a specific result of special relativity but rather as the expression, in the particular case of kinematics, of a more general law of physics. The possible use of the composition law of probability amplitude in quantum theory is also presented.