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Why the Quantum Absorber Condition is Not a Light-Tight Box

Ruth E. Kastner

Published Feb 23, 2020• 1 citation
OpenAlex

Abstract

This paper discusses the nature of the boundary condition applying to the direct-action theory of fields, also known as the 'absorber' theory, first developed in a classical version by Wheeler and Feynman. The traditional understanding of these authors and others who followed is that the direct-action theory requires that 'all emitted radiation be absorbed,' or that 'the universe is a perfect (or complete) of absorber of radiation,' implying the need for a specific sort of cosmological boundary condition that may or may not obtain in our universe. It is pointed out that this interpretation requires critical scrutiny even at the classical level, and in any case does not apply to the fully quantum form of the direct-action theory (QDAT), which involves discrete energy transfers by way of photons. In the latter case, the absorption boundary condition describes a specific interactive relationship between emitters and absorbers that is independent of cosmological boundary conditions. Thus, the quantum direct-action theory does not require any special cosmological boundary condition in order for it describe the physical world.

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This paper discusses the nature of the boundary condition applying to the direct-action theory of fields, also known as the 'absorber' theory, first developed in a classical version by Wheeler and Feynman. The traditional understanding of these authors and others who followed is that the direct-action theory requires that 'all emitted radiation be absorbed,' or that 'the universe is a perfect (or complete) of absorber of radiation,' implying the need for a specific sort of cosmological boundary condition that may or may not obtain in our universe. It is pointed out that this interpretation requires critical scrutiny even at the classical level, and in any case does not apply to the fully quantum form of the direct-action theory (QDAT), which involves discrete energy transfers by way of photons. In the latter case, the absorption boundary condition describes a specific interactive relationship between emitters and absorbers that is independent of cosmological boundary conditions. Thus, the quantum direct-action theory does not require any special cosmological boundary condition in order for it describe the physical world.
This paper presents key aspects of the quantum relativistic direct-action theory that underlies the Relativistic Transactional Interpretation. It notes some crucial ways in which traditional interpretations of the direct-action theory have impeded progress in developing its quantum counterpart. Specifically, (1) the so-called 'light tight box' condition is re-examined and it is shown that the quantum version of this condition is much less restrictive than has long been assumed; and (2) the notion of a 'real photon' is disambiguated and revised to take into account that real (on-shell) photons are indeed both emitted and absorbed and therefore have finite lifetimes. Also discussed is the manner in which real, physical non-unitarity naturally arises in the quantum direct-action theory of fields, such that the measurement transition can be clearly defined from within the theory, without reference to external observers and without any need to modify quantum theory itself. It is shown that field quantization arises from the non-unitary interaction.
One of the long term interests of George Series was the construction of a theory of spontaneous emission which does not involve field quantisation. His approach was written in terms of atomic operators only and he drew a parallel with the Wheeler-Feynman absorber theory of radiation. By making a particular extra postulate, he was able to obtain the correct spontaneous emission rate and the Lamb shift reasonably simply and directly. An examination of his approach indicates that this postulate is physically reasonable and the need for it arises because quantisation in his theory occurs after the response of the absorber has been accounted for by means of the radiative reaction field. We review briefly an alternative absorber theory approach to spontaneous emission based on the direct action between the emitting atom and a quantised absorber, and outline some applications to more recent effects of interest in quantum optics.
The classical Wheeler-Feynman absorber theory with a postulate of Lorentz-invariant zero-point electromagnetic radiation is proposed to explain quantum phenomena in a similar manner to that of stochastic electrodynamics. For this purpose, Cramer's model of a 'Minimum emitter-absorber transaction' is extended to the case where zero-point radiation exists, in the context of the classical Wheeler-Feynman theory. The Einstein-Podolski-Rosen paradox and the quantum levels of a charged simple harmonic oscillator are derived from the theory. It is shown that the condition for a transaction plays an essential role, such as in the radiative balance of a simple harmonic oscillator.
Mainstream physics says yes: the electromagnetic field is a quantum system, and even the vacuum fluctuates. But “quantized” can mean two things. Either the field itself carries quanta in flight, or only matter is quantized, with discreteness appearing in what emitters and absorbers exchange, as in direct-action electrodynamics. We sort the evidence. The photoelectric effect, Compton scattering and spectra need only quantized matter. Antibunching, two-photon interference, squeezing, non-destructive photon counting, vacuum sampling, Bell tests and light-by-light scattering exclude a classical field, but none excludes quantum direct action. Two-photon interference follows from summing over emitter–absorber pairings, a permanent, with no quanta in flight. In its established constructions, direct action reproduces QED only with a complete future absorber, which Hoyle and Narlikar found impossible in open Friedmann models. Where light is never absorbed, the readings therefore differ. A history weight on free-ending photons acts as a photon chemical potential, and the microwave background’s blackbody spectrum bounds it at about 2×10⁻⁴. We classify delocalized radiation and propose tests: directional vacuum sampling toward the night sky, where QED predicts no difference and exchange a loss of half the vacuum term or more; a cavity photon with a sky-facing port; and a positron source between open and blocked directions.
The absorber theory of radiation is discussed with particular reference to its application to quantum optical phenomena. The essential feature of the theory is that the absorber is the source of the radiative reaction force. This is an advanced action of the absorber on the emitting atom and, in the quantum version of the theory, is highly directional and polarized. Direct action absorber theory, without fields, can reproduce effects such as spontaneous emission, the Lamb shift, the dynamic Stark effect and photoelectron antibunching, which are often described as properties of the quantized field. Its most interesting application, however, is in calculating the detailed dynamics involved in the Kocher-Commins experiment. It is this experiment which most clearly displays the anti-causal nature of the radiative reaction in absorber theory and shows the advantage of absorber theory over the conventional quantum electrodynamic model in explaining the effect.
The quantum revolution began in 1900 with a novel solution to what might seem to be one of the minor puzzles of theoretical physics, the problem of black-body radiation: the problem of the interaction of those two sorts of entity which figure in the dualist ontology of late classical physics. The trouble for classical physics was that the facts about a black body's capacity to absorb and emit radiation defied thermodynamic explanation. The best that classical physics could do was to get the facts wrong and to generate paradox. So what was the problem of black-body radiation and why was it considered to be so puzzling? First, what is a black body? Some bodies absorb and emit radiation more readily than others. A mirror is a poor absorber of radiation in the form of light, a piece of coal is a good one. A good absorber of radiation in the form of light is ‘blacker’ than one that is not. One can extend the metaphor to radiation outside the visible range. Clearly, a body that is ‘blacker’ than another for one range of radiation frequencies need not be ‘blacker’ for all frequencies. A body's ‘blackness’ will, in general, depend on its chemical structure. But we can imagine a body whose blackness is maximal for all wavelengths. Such a body, if it exists, has a blackness which is independent of its chemical structure. We call it a black body. Two interesting questions are: first, are there bodies in Nature which are uniformly and maximally black for all wavelengths? and second, how does such a black body distribute the energy that it radiates as a function of the wavelength of the radiation it emits?