Preface

The most primitive concept of relativity is based on the evident reciprocity in the kinematic relations between objects. For example, Heraclides of Ponticus (388-315 BC) suggested that the apparent rotation of the stars around the Earth could also be interpreted as a daily rotation of the Earth, while the stars remain stationary. Similarly Aristarchus of Samos (310-230 BC) proposed a model of the solar system in which the Earth not only rotates on its axis, but moves in an immense circular orbit around the Sun. These were remarkable insights, but the purely relational concept of motion on which they were based was not really adequate to support them. Most scholars from antiquity through the middle ages regarded the idea of a moving Earth as self-evidently incompatible with observations and common sense. For example, Claudius Ptolemy (c. 140 AD) disposed of this hypothesis by pointing out that it requires us to believe the lightest essence (the ether) is stationary, whereas the heaviest essence (Earth) is in motion, precisely contrary to their respective natures. He also argued that the surface of a rotating Earth would necessarily be moving faster than the clouds floating in the air above it, so we should never see clouds moving to the east. Furthermore, the absence of any discernable parallax in the observed positions of the stars seemed to rule out the possibility that the Earth revolves around the Sun, unless the distance to the stars is literally thousands of times the distance to the Sun, which seemed absurd.

The same objections were raised eighteen centuries later when Copernicus (1473-1543) revived the heliocentric model based on essentially the same primitive kinematic concept of relativity. Nevertheless, within a century of Copernicus' death, the heliocentric model had become fully accepted by the scientific community - despite the fact that stellar parallax still had never been detected. (The first actual measurement of stellar parallax was not achieved until 1839.) The objections to relativity that had seemed so irrefutable since ancient times could now be answered quite easily, but only because of a profound re-interpretation of the relativity principle by the successors of Copernicus, including Kepler, Galileo, Descartes, Huygens, and Newton. These men developed a physically viable theory of relativity based not on purely kinematical relations, but on the dynamical principle of inertia, according to which there exists an infinite class of relatively moving coordinate systems that are all equivalent from the standpoint of mechanical dynamics.

The dynamical principle of relativity became the operational basis of the Scientific Revolution. Indeed the complete operational equivalence of uniformly moving inertial reference frames remained an unchallenged principle of physics for centuries. However, the concept of relativity once again came into doubt during the latter half of the 19th century, as scientists struggled to incorporate the propagation of light (or, more generally, electro-magnetic waves) into the conceptual framework of physics. Careful measurements of electromagnetic phenomena yielded results which were seemingly incompatible with the classical principle of relativity. During this period, serious consideration was given to the possibility that inertial frames are not all operationally equivalent, or, what amounts to the same thing, that all motions must be referred to some kind of unidentifiable "ether" in order to account for the phenomena.

Eventually these doubts led to a re-examination of our most primitive concepts of space, time, and motion. In 1905 Einstein, building on the work of Lorentz, Poincare, and many others, showed that Maxwell's laws of electromagnetism and the propagation of light actually are consistent with the principle of inertial relativity, provided we adopt a still more comprehensive understanding of that principle and its implications. Thus Einstein did not originate relativity in 1905, he restored it to its classical status by reinterpreting the elements of time and space on a more profound level. Just as a deepening of the principle and the associated concepts of space, time, and motion had been necessary to rescue relativity from the objections of Ptolemy, it was necessary to once again re-interpret the principle to assimilate the phenomena of electromagnetic wave propagation, and this in turn led to a deeper understanding of a multitude of other phenomena.

However, soon after the classical relativity principle was reconciled with electro-magnetism, a new challenge appeared. Einstein himself was among the first to realize that the special theory of relativity which he had described in 1905 was fundamentally incompatible with gravitation and the two principles of equivalence, i.e., the equivalence of inertial and gravitational mass, and the equivalence of inertia and energy. It seemed once again that relativity would have to be abandoned. Then, in 1915, Einstein extended the principle of relativity yet again, with a still more profound re-interpretation of space and time, building on the mathematical insights of Minkowski, Riemann, and others. The general theory of relativity established equivalence between the members of an even larger class of reference systems, and in so doing achieved a conceptual unification of inertia and gravity, while retaining the structure of special relativity locally at every point of spacetime. One of Einstein's contemporaries, the physicist Max Born, later said

Nevertheless, soon after Einstein's second restoration of relativity in the form of the marvelous general theory, still another class of phenomena came under study, leading to the theory of quantum mechanics, once again making it appear that the principle of relativity would have to be abandoned. Not surprisingly, Einstein was reluctant to concede this issue, having rescued relativity twice from seemingly intractable problems, both times showing that in fact relativity was the key to a deeper understanding of the very phenomena that were thought to be incompatible with it. Could those apparent successes have been illusory? Surely it's understandable that he continued to believe in (or at least hope for) one more re-interpretation of space, time, and motion that would allow the phenomena of quantum mechanics to fit naturally within the relativistic framework.

This book examines the evolution of the principle of relativity in its classical, special, and general incarnations, both from a technical and a historical perspective, with the aim of showing how it has repeatedly inspired advances in our understanding of the physical world.

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