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Physical Review A

Neoclassical physics and blackbody radiation

J. P. Gordon

Published Dec 1, 1975• 7 citations
Publisher

Abstract

The neoclassical theory of the interaction of radiation with matter, introduced by its authors as a potentially viable alternative to quantum electrodynamics, is examined in the case of a multilevel atom interacting with blackbody radiation. The stationary statistical states of the atom and the field are derived, and two independently untenable results emerge. First, the stationary blackbody law turns out to be that of Rayleigh and Jeans, rather than that of Planck. Second, the mean energy of a harmonic oscillator, which interacts with the field only near a single frequency, turns out to be infinite. These results are discussed in the light of the neoclassical description of spontaneous emission.

DOI
10.1103/physreva.12.2487

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The neoclassical theory of the interaction of radiation with matter, introduced by its authors as a potentially viable alternative to quantum electrodynamics, is examined in the case of a multilevel atom interacting with blackbody radiation. The stationary statistical states of the atom and the field are derived, and two independently untenable results emerge. First, the stationary blackbody law turns out to be that of Rayleigh and Jeans, rather than that of Planck. Second, the mean energy of a harmonic oscillator, which interacts with the field only near a single frequency, turns out to be infinite. These results are discussed in the light of the neoclassical description of spontaneous emission.
We discuss some implications of the description of spontaneous emission of radiation which has been offered by Jaynes and his collaborators in their "neoclassical" extension of semiclassical electrodynamics. In particular, we examine the thermal-equilibrium condition of radiation interacting with a tenuous gas of atoms. We argue that rate equations may be used to describe the interaction of such atoms with the chaotic thermal radiation field. For this situation the neoclassical spontaneous emission rate is incompatible with the well-secured laws of Boltzmann and Planck. Experimental evidence bearing on the accuracy of those laws as well as on the accepted level population dependence of the induced emission rate is reviewed.
The idea about a quantum nature of Planck's blackbody radiation law is deeply rooted in minds of most physicists. Einstein's work, in which the coefficients of spontaneous and induced emission were introduced, has always been regarded as a proof that quantum energy discreteness of an atom plays a crucial role in the derivation of this law. In our paper we avoid this standpoint. It may be shown that the de Broglie wave assigned to every material particle is a result of interaction of the particle with zero-point vibrations of electromagnetic ground field. The energetic spectrum of a harmonic oscillator is obtained from this fact within classical physics which coincides with the quantum result. Thus, it is explained here how the energy discreteness came into existence in stochastic electrodynamics (SED) --- the classical electrodynamics with classical electromagnetic zero-point radiation. Next we reconsider the Einstein work from the viewpoint of SED and derive the Planck formula.
The fluctuation properties of a system of two level atoms are investigated by introducing a random precession term into the equations of motion of the atomic super Bloch vector. Starting from the assumptions of neoclassical radiation theory a general formula for the dipole field of a small system of atoms is derived. The intensity fluctuations are calculated in the narrowing limit for two extreme cases of very large and very small atomic to thermal energy ratios. The former case, describing a true superradiant emission, involves asymptotic solutions to the Fokker-Planck equation. The latter case, treated by perturbation theory, corresponds to spontaneous emission in the presence of incoherent pumping.
We use a self-field approach to quantum electrodynamics (QED) to show how one may obtain spontaneous emission and the Lamb shift in a two-level atom without second quantization of the radiation field. In addition, we compare the self-field formalism to that of the neoclassical theory of electrodynamics advanced by Crisp and Jaynes [Phys. Rev. 179, 1253 (1969)]. We show that the neoclassical model can be obtained from the self-field approach used here, but that the two are not equivalent. In particular, the self-field approach appears to give a more complete description of radiative processes. Finally, we show that the neoclassical theory's prediction of a nonexponential ``chirruped'' decay is most likely a mathematical artifact of the improper application of the superposition principle in a nonlinear model where such a principle does not hold. A correct treatment with self-field QED yields the usual exponential decay dynamics.
Introduction The preceding chapters have focused on atomic transitions induced by applied radiation. That is, there is an optical field illuminating the atoms, and their response is calculated. It is implicitly assumed that an excited atom left alone will spontaneously return to the ground state, and conserve energy by emitting light that satisfies E 2 − E 1 = ħ ω 21 in accordance with the Planck and Bohr pictures. But this spontaneous emission process is not addressed quantitatively in these earlier chapters. Spontaneous emission is one of the most pervasive phenomena of atomic physics, and yet its origins remain among the least well-understood. The number of misconceptions is enormous, and they are to be found in textbooks, journal articles, and lecture notes. Because the natural decay of an excited atom adds energy to what is usually an empty mode of the electromagnetic field, the process is fundamentally quantum mechanical. Classical or even semi-classical descriptions cannot be assured to give correct answers. This is not to say that such discussion is to be avoided, but only that caution is needed in interpreting the results. Toward the end of the nineteenth century, many experimental results made it clear that classical mechanics needed to be supplemented by new theories. The first venture into the new physics was Planck's hypothesis that the energy of classical radiating oscillators like those discussed in Chap. 1 would have to be quantized into integer multiples of ħ ω. This notion led to his famous formula for the spectrum of thermal radiation from a non-reflective object (black body), and this formula describes a radiation field in thermodynamic equilibrium with its environment. The result of Planck's hypothesis in the context of thermal equilibrium plays a vital role in the discussion of spontaneous emission. Einstein A - and B -coefficients Soon after Bohr proposed a model of atomic structure consisting of discrete energy levels, connected by radiation whose energy satisfied Δ E = ħω when atoms made transition between them, Einstein published the first serious attempt to connect the absorption and emission processes. Absorption, he reasoned, could occur only in the presence of applied radiation, whereas spontaneous decay occurred without the application of any field.
The Planck’s law of blackbody radiation is derived in several methods in this paper. The original Planck’s derivation in his 1900 paper, the derivation by Einstein, and another derivation based on the Bose-Einstein statistics are reviewed with comments on each derivation. Statistical properties of radiation are briefly reviewed in the second part of the paper. Different concepts of photon statistics are applied on the thermal (chaotic) light and laser light. Nonclassical properties of light that manifest its quantum nature are briefly highlighted with a short review of the experimental evidence embodied in ……….. 1-Introduction and historical perspective It was Robert Kirchhoff who first argued in 1859 that the thermal radiation of the blackbody was of a fundamental nature.1 Since that time, several attempts were made to derive a precise relation that matches the experimental results. Wien, suggested that the radiation density per