McCabe, Gordon (2020) What is light? [Preprint]
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Abstract
The purpose of this paper is to answer the question, `What is light?', from a mathematical and foundational perspective. The paper begins by exploring the relevance of space-time symmetries, and the nature of polarization, before a detailed exposition of the quantized radiation field, and the difficulties created by gauge freedom.
Attention then turns to the interaction of light with matter, beginning with the coupled electromagnetic field, then progressing to the representation of scattering and virtual particles in quantum electrodynamics. This provokes an analysis of the Coulomb electrostatic field, and the question of whether longitudinal and scalar photons exist. It is argued that the longitudinal component of the electric field is associated with the state-space of a charged matter field, not the Fock space of the free electromagnetic field, hence the presence of electrostatic fields is consistent with the vacuum state of the free electromagnetic field. It is also argued that scalar and longitudinal photons do indeed exist as links in the spacelike networks into which the Coulomb interaction can be decomposed.
Consideration of the stimulated emission of light leads to a general exposition and analysis of the `coherent states' of the quantized radiation field. As a by-product of this, a novel explanation is proposed for why there is something classical rather than nothing classical. An attempt is made to develop this into a fully-fledged universe creation scenario. The role of fermions and the gravitational degrees of freedom in such a scenario are discussed, and a comparison is drawn with the inflationary cosmological scenario.
The role of coherent states in our concept of the classical world is then critically analysed. The notion that the classical states of the radiation field are emergent from the quantum states is rejected. In particular, it is argued that the classical states of light do not emerge in the limit where there are large numbers of photons, and it is pointed out that the putative emergent classical states fail the test of reference-frame independence.
The paper concludes by expounding the implications of the nature of light for `decoherence', a way of trying to reconcile quantum theory with the apparent nature of the macroscopic world.
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