Preface |
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The earliest ideas of relativity were based on the evident reciprocity of spatial relations and motions. For example, Heraclides suggested that the observed rotation of the stars around the Earth might just as well be interpreted as a daily rotation of the Earth while the stars remain stationary. Aristarchus went even further, proposing that the Earth not only rotates on its axis, but moves in an immense circular orbit around the Sun. However, most philosophers from antiquity through the middle ages regarded the idea of a moving Earth as self-evidently incompatible with observation as well as common sense. Ptolemy rejected the hypothesis of a moving Earth because it requires us to believe the lightest essence (the ether) is stationary and 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. Moreover, 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 implausible. |
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The same objections were raised eighteen centuries later when Copernicus revived the heliocentric model based on essentially the same primitive kinematic concept of relativity, and yet within a century of Copernicus' death the heliocentric model had been 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 old conceptual objections to relativity that had once seemed irrefutable could now be answered, but only because of a profound re-interpretation of the relativity principle brought about by the successors of Copernicus, including Kepler, Galileo, Descartes, Huygens, and Newton. The new theory of relativity was based not on purely kinematical relations, but on the dynamical concept 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. |
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The principle of relativity founded on the concept of inertia 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 principle 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, electromagnetic 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 the rest frame of some kind of "ether" in order to account for the phenomena. |
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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. |
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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 Gauss, Riemann, Minkowski 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 |
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The theory appeared to me then, and it still does, the greatest feat of human thinking about nature, the most amazing combination of philosophical penetration, physical intuition, and mathematical skill. It appealed to me like a great work of art ... |
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Nevertheless, during the same years in which Einstein was developing and extending the modern theory of relativity, 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 the 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? He agreed that this was possible, but 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. To this day the beauty and elegance of general relativity challenges the imaginations of scientists seeking to reconcile it with the latest theories of physics. |
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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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