It has long been my ambition to understand why, in mainstream physics, the speed of electromagnetic radiation is deemed to be the same for every observer. Physicists use a mix of stipulation, deduction and observation and experiment to justify what might at first sight look like an obvious impossibility. In this blog, over the next few years and decades I will attempt to identify the precise role played by these various strands, and I will explore whether there are any alternative theoretical frameworks which are consistent with observation and experiment but within which the speed of electromagnetic radiation is not deemed to be the same for every observer.
To this end I will look at a number of relevant publications in the fields of electrodynamics and special relativity (SR), including writings by Albert Einstein (1, 2), William Geraint Vaughan Rosser (3), Wolfgang Rindler (4), Vesselin Petkov (5) and Kevin Brown (6).
Specific questions to be developed in subsequent posts with reference to these writings include the following:
1) Stipulation: In (1), Einstein arrives at a definition of simultaneity in points A and B by “stipulating” that the time it takes for a light signal to travel from A to B is always the same as the time it takes for such a signal to travel from B to A. Does this mean that what Einstein variously calls the “premise” or “principle” of the constancy of the speed of light is at least in part a matter of stipulation? Are there any alternative definitions of simultaneity that do not imply that the speed of light from A to B is always the same as from B to A?
2) Deduction: The constancy of the speed of light for every observer is often presented as a consequence of the principle of relativity combined with Maxwell’s equations of electromagnetism. So, what exactly is the role of this principle and of these equations? What exactly in Maxwell’s equations is it that entails the constancy of the speed of light? And how strong is the evidence that these equations are correct? In modern-day derivations of the Ampere-Maxwell equation, for example, necessary and sufficient conditions are routinely confused. And if Maxwell’s equations are used to derive the constancy of the speed of light, where does that leave our understanding of magnetic fields? Magnetic field effects are usually interpreted as relativistic effects, but there is a logical snag if the constancy of the speed of light for every observer is derived from the equations of electromagnetism in the first place. So, are there any alternative theoretical frameworks to explain magnetic fields, and what form do Maxwell’s equations take in those theories?
3) Observation and experiment: The constancy of c for every observer is often said to be strongly suggested by a range of observations or experiments, including the Michelson-Morley experiment, double star observations, the phenomenon of stellar aberration and high-speed particle experiments. In fact the Michelson-Morley experiment discredited the idea of a single, static ether, which had originally been the basis for Maxwell’s equations. In an apparent bid to save Maxwell’s equations, SR does not abandon the ether concept but generalizes it to every observer: in SR light behaves as if every observer were at all times at rest in a static ether in which all electromagnetic field disturbances travel at c.
But instead it might make more sense to identify electromagnetic field disturbances with outwardly progressing modifications in the electric fields of the charged particles whose acceleration caused the disturbances. If this were so, it might be necessary to take account of a particle’s complete “acceleration history” – including local accelerations under the impact of a local electromagnetic force, distorting its electric field, and more global accelerations, together with its local electric field, under the influence of gravity - to fully account for its current behaviour and effects on other particles. Can all observations and experiments be explained in the framework of such a theory?
Finally, is the constancy of the speed of light for every observer only possible if four-dimensional “Minkowski space” is not just a convenient mathematical and illustrative tool to describe a three-dimensional world but an accurate representation of reality? If so, does that mean we live in a four-dimensional "block universe" in which the flow of time is just an illusion? Or is such a conclusion contradicted by observation or experiment?
