What is it about?
We have all noticed macroscopic interatomic forces, which cause "stickiness" between surfaces, as in the case of adhesion and cohesion forces. These interactions, variously referred to as van der Waals, Casimir, and dispersion forces, were first correctly explained by means of the newly developed non-relativistic quantum theory of the atom by Eisenschitz and London in 1930. In that early description, which predates the development of quantum electrodynamics (QED), the electromagnetic field was treated classically. Later, in 1948, Casimir and Polder extended the theory of dispersion forces to include the effects of retardation within the framework of QED. However, in 1963, Mclachlan was able to prove that it is possible to obtain the same retarded results from a classical theory of the electromagnetic field. Of course, this does not prove that the electromagnetic field is classical but only that dispersion force theoretical predictions and experiments are incapable, by themselves, to discriminate between classical and quantum electrodynamics. In this paper, written for the Special Issue on "Symmetries in Quantum Mechanics" edited by Professor Jordan Maclay, after an extensive historical review, I describe having retooled Mclachlan's concept to apply it to "gravitational dispersion forces." These interactions, which are extremely weak, have been conjectured in the last few years and the claim has been repeatedly made that detecting them would be a conclusive proof that gravity is indeed quantized. Instead, I prove that, just as in the case of electromagnetic dispersion forces, there is no difference between the predictions one obtains by adopting a classical or a linearized quantum gravity framework. Therefore, if gravitational dispersion forces are detected, that will not represent a conclusive proof of gravity quantization. Analogously to the case of QED, gravity may well be quantized but dispersion forces are an ineffective experimental tool to demonstrate it.
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Why is it important?
Any conceptual tool - and of course any experiment - potentially capable to explore the fundamental nature of gravity is of paramount importance to the crucial challenge to determine the structure of spacetime. Several predictions have been made that gravity must be quantized in order to avoid severe inconsistencies with our understanding of universal laws. However, accessing the scales that might reveal the quantum structure of gravity is an enormous challenge. It is of critical importance, therefore, that we do not set ourselves up for a misinterpretation of any experimental results due to having created an incorrect conceptual tool to address this crucial issue. My results show that detecting gravitational dispersion forces, although a fascinating experimental result, will not clarify the ultimate structure of spacetime since predictions from two incompatible theories are indistinguishable.
Perspectives
At the theoretical level, my results are one further proof that we must proceed with extreme caution as we explore the nature of spacetime. It has become fashionable, indeed obligatory, to describe Casimir forces as "quantum forces." It is true that a quantum description of the atom was necessary to gain an understanding of electrodynamical interatomic forces. However, this fashionable terminology is now being misapplied to further the belief that any dispersion force must be a quantum force. This has been proven technologically false as purely classical dispersion forces have been demonstrated in the laboratory, as I describe in this review. Sometimes the very superficial observation is made that, since dispersion forces are proportional to the Planck constant, they would disappear in the classical limit. This is true but we must remember that London forces, in which the electromagnetic field is classical, are proportional to the Planck constant. Furthermore, Mclachlan proved that even the retarded Casimir-Polder forces can be recovered in classical electrodynamics provided that quantum mechanics be used to describe the atom. Once translated to the gravitational case, these results show that we would be simply be misguided to believe that gravitational dispersion forces, if detected, would prove gravity quantization. In this case, the two points of view yield identical results so that, according to the scientific method, we cannot use any experiment of this type to clarify the nature of the field. Since no theory of quantum gravity still exists, it is absolutely critical that we not be led astray by preconceived notions about the quantum nature of spacetime. In this paper, I offer a glimmer of hope that an experimental detection might be possible, or at least more feasible, by using mixed gravitational-electrodynamical potentials, whose existence was first speculated by Spruch in 1986, as these quantities would yield forces many orders of magnitude larger than a purely gravitational potential. However, such experiments, even if successful, would not be conclusive. I explore the possibility that discrepancies with respect to theoretical expectations might shed light on the nature of spacetime in another recent paper, "Signatures of minimal length from Casimir-Polder forces with neutrons," published in the International Journal of Modern Physics D.
Dr. Fabrizio Pinto
Izmir Ekonomi Universitesi
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This page is a summary of: Gravitational Dispersion Forces and Gravity Quantization, Symmetry, December 2020, MDPI AG,
DOI: 10.3390/sym13010040.
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