Oddly enough, the clearest statement of this insight came only as an afterthought, appearing in Einstein’s second paper on relativity in 1905, in which he explicitly concluded that “radiation carries inertia between emitting and absorbing bodies”. The point is that light conveys not only momentum, but inertia. For example, after a body has absorbed an elementary pulse of light, it has not only received a “kick” from the momentum of the light, but the internal inertia (i.e., the inertial mass) of the body has actually increased.

Once it is posited that light is inertial, Galileo’s principle of relativity automatically implies that light propagates isotropically from the source, regardless of the source’s state of uniform motion.

— 1.6  A More Practical Arrangement

— Reflections on Relativity

— mathpages

2013.05.06 Monday ACHK