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Albert Einstein's Relativity: The Special and the General Theory (1920) is a cornerstone in the edifice of modern physics. With it the great scientist and humanist took his place beside other great teachers of science. Among the greatest achievements of human thinking, the theories of relativity are commonly regarded as the exclusive domain of highly trained physicists and mathematicians. Disapproving of this segregation as he was, Einstein took it upon himself to explain in this book both theories in their simplest and most down-to-earth form, intending it for "those readers who, from a general scientific and philosophical point of view, are interested in the theory, but who are not conversant with the mathematical apparatus." Indeed, within the vast literature on the philosophy of space and time, Einstein's Relativity shall remain an illuminable and intelligible exposition, highly quotable as one of the most lucid presentations of the subject matter, and a launching pad for any further inquiry on the fascinating features of our universe.
Albert Einstein (1879-1955) is one of the icons of our times, requiring almost no introduction. A Nobel laureate, the author of the special and the general theories of relativity, and a key figure in the birth of quantum mechanics, he is widely acclaimed as one of the most creative intellects of human history. The German-Jewish-born "technical-expert-third-class" in the Swiss patent office in Bern originally intended to become a secondary-school teacher - a profession for which he had a natural talent, as readers of Relativity would surely appreciate - but in 1909, having completed an astonishing range of theoreticalphysics publications, written in his spare time without the benefit of close contact with scientific literature or colleagues, he was recognized as a leading scientific thinker and two years later was appointed a full professor at the Karl-Ferdinand University in Prague. A year later he returned to Zurich to begin his work on the general theory of relativity and in 1914 accepted a distinguished research position in the Prussian Academy of Sciences together with a chair (but no teaching duties) at the University of Berlin. He was also offered the directorship of the Kaiser Wilhelm Institute of Physics in Berlin, which was about to be established. After a number of false starts, Einstein published, late in 1915, the definitive version of the general theory of relativity, and in so doing forever changed our views of the cosmos.
Einstein was first idolized by the popular press when British eclipse expeditions in 1919 confirmed his predictions on the bending of light rays near the sun. The London Times ran the headline on 7 November 1919: Revolution in science - New theory of the Universe - Newtonian ideas overthrown, and three weeks later printed Einstein's popular exposition on relativity. The exposition became a classic, and Einstein became an overnight sensation, the world's first and greatest scientific superstar. Two years later he received the Nobel Prize for his 1905 work on the photoelectric effect. By then Einstein was internationally known, and when he was offered a post in Princeton in 1932 he moved to the United States, never to return to Germany. His late career was marked by unsuccessful attempts to unify the laws of physics, and by a strong distaste for the fashionable so-called "Copenhagen interpretation" of quantum mechanics. A week before his death, Einstein signed his last letter, written to Bertrand Russell, in which he agreed that his name should go on a manifesto urging all nations to give up nuclear weapons. It is only appropriate that one of his last acts was to argue, as he had done all his life, for international peace. With Einstein's death in 1955 the world had not only lost one of its foremost thinkers but also a humanist fighter for peace and freedom.
1905 was a remarkable year for Einstein. Among his articles published that year, the paper "On the Electrodynamics of Moving Bodies" delineated the principles of the special theory of relativity. Shortly thereafter his paper "Does the Inertia of a Body Depend upon its Energy Content?" was published; this paper contained the famous equation E = mc2 stating the equivalence of energy and mass. Both papers propounded a revolutionary operational interpretation of a certain mathematical machinery, devised originally by the Dutch physicist H. Lorentz in order to square Maxwell's theory of electrodynamics with apparently contradictory experimental results. Relying as they did on the postulate of relativity and on the postulate of the constancy of the speed of light in a vacuum, they resulted in a new conception of space and time, the radical features of which were best captured in the dramatic words of Einstein's teacher, the mathematician H. Minkowski (1908): "…space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality."
The best way to understand the special theory of relativity (STR) is, according to Einstein himself, to see it as a theory of principle, its two famous principles being the relativity principle (that the laws of nature co-vary with uniformly moving reference frames, or as Bondi (1980) puts it, "that velocity does not matter"), and the light principle (that the speed of light in a vacuum is constant, independent of the speed of the source). Another famous theory of principle is thermodynamics; Einstein used to point to this theory as one of his favorites that inspired his conception of STR. In theories of principle such as thermodynamics or STR one starts from empirically observed general properties of phenomena such as the non-existence of perpetual motion machines, in order to infer general applicable results without making any assumptions on hypothetical constituents of the system at hand. Since the building blocks of these theories are "not hypothetically constructed but empirically discovered," in so doing, says Einstein, one employs "the analytic, not the synthetic method." Lorentz's contraction and dilation theory, along with statistical mechanics and its predecessor the kinetic theory of gases, are, on the other hand, examples of constructive theories. They begin, according to Einstein, with certain hypothetical elements and use these as building blocks in an attempt to construct models of more complex processes.
Einstein's "principle" approach to physics in STR differs from the "constructive" approach of Lorentz in two major ways. As the late eminent CERN physicist John S. Bell (1987) notes, there is a difference in style, and a difference in philosophy. The difference in style is that theories of principle, as Relativity: The Special and the General Theory nicely demonstrates, are generally more elegant and concise, while constructive theories are usually complicated and cumbersome. The difference in philosophy is that since the question of which uniformly moving reference frame is really at rest is experimentally undeterminable, Einstein - later to be joined happily by logical positivists such as Schlick and Reichenbach - declares the notions "real rest" and "real motion" as meaningless. For him only the relative motion of the two or more uniformly moving objects is real, hence no reference frame is "specially marked out" (Part II, Chapter XVIII). Lorentz, on the other hand, along with Fitzgerald, Larmor, and Poincaré, preferred the view that there is indeed a state of real rest, defined by the "aether," even though the laws of physics conspire to prevent us from detecting it experimentally. And although Einstein's STR is commonly favored today over Lorentz's conspiracy theory, it is important to note that (1) the facts of physics do not oblige us to accept one philosophy rather than the other, and (2) it is not necessary to accept Lorentz's philosophy to accept, as Einstein himself did, "Lorentzian pedagogy" - that the laws of physics in any one reference frame account for all physical phenomena, including the observations of moving observers - especially when it is often simpler to work in a single frame, rather than hurry after each moving object in turn.
The birth of the general theory of relativity was more complicated and agonizing for Einstein, although he referred to the idea that marked its conception, namely the equivalence principle between inertial and gravitational mass (originally connived by Poincaré as a skeptical argument), as "the happiest idea of my life." For a long time Einstein struggled with his famous field equations that constraint of the geometry of spacetime and the distribution of matter on it, and it was only with the help of his school friend the mathematician Marcel Grossman that he finally came to grips with their definite form.
The general theory of relativity, although of very little use in building an airplane or solving the energy crisis, is a huge step toward our understanding of nature, but Einstein himself recognized in his late career that the original philosophical goal that motivated its conception was not achieved. STR "eliminated," or more precisely made relational, two Newtonian entities that were regarded as absolute, namely simultaneity and velocity. With the general theory of relativity Einstein hoped to implement Mach's principle and to eliminate another absolute entity, namely acceleration. Einstein saw Mach's principle (Part II, Chapter XXI) as a modern version of "Occam razor": unobservable theoretical entities that do no explanatory work in a physical theory are superfluous, hence should be eliminated from the theory. Newton's concept of absolute space (responsible in Newtonian mechanics for absolute acceleration) was the target of Einstein's attack, but the general theory of relativity, although explaining geometry in terms of gravity and gravity in terms of geometry, did not exorcize the ghost of absolute space. Having reconstructed accelerated motion as inertial motion on geodesics, the theory indeed changed the meaning of the term "acceleration," yet this magnitude was not made relational in any deep sense. Spacetime, moreover, rather than being overthrown, became a physical entity, albeit dynamical, with a concrete (non-Euclidean) structure. No better words exist to describe the confusions with respect to the philosophical consequences of the theory than Einstein's own: "People slowly accustomed to the idea that physical states of space itself were the final physical reality."
Yet another word of caution is in order. In the exuberance that followed Einstein's discoveries, philosophers at one time or another have proposed that his theories support virtually every conceivable moral in ontology. One, however, should avoid this overabundance. As the philosopher John Norton (2000) notes, "[w]e learn from Einstein's theories of novel entanglements of categories once held distinct: space with time; space and time with matter; and space and time with causality. We do not learn that all is relative, that time is the fourth dimension in any nontrivial sense, that coordinate systems and even geometry are conventional or that spacetime should be reduced ontologically to causal, spatiotemporal or other relations." In fact, as Minkowski himself hints in his historical address, the word "relativity" in the "relativity postulate" is nothing but a misnomer since what it comes to mean is that only the four-dimensional world in space and time is given by physical phenomena, while the projection of it into space and time may still be undertaken with some degree of freedom. The great mathematician preferred to call this famous postulate "the postulate of absolute world," or briefly, "the world postulate."
Einstein's instrumental legacy in the conception of the theories of relativity inspired generations of physicists. However, it is also interesting to note that, having understood that the motivations underlying his theories led to philosophical bankruptcy-hence his famous argument from 1935 against the completeness of quantum mechanics and his antipathy to the anti-realism of Bohr, Heisenberg, and almost all of the members of the physics community in his late career Einstein abandoned this "new fashion" in favor of realism. Einstein's colleagues - immersed as they were in the Kantian state of mind - refused to understand how, having started this fashion in 1905, Einstein could possibly reject it twenty years later. Einstein's words to his friend the physicist Philip Frank clearly reveal his insight: Yes, I may have started it, but I regarded these ideas as temporary. I never thought that others would take them much more seriously than I did ... A joke should not be repeated too often. The philosophy of spacetime physics is a fascinating field, and Relativity: The Special and the General Theory is one of its classics. Those who delve into it in search of answers to longstanding questions regarding space, time, and the universe we live in, will find not only "few happy hours of suggestive thoughts" (as Einstein expressed his hope in his preface to the book), but also an exciting and accessible interpretation of one of the world's greatest intellectual accomplishments.
Amit Hagar is a philosopher of physics with a Ph.D. from the University of British Columbia, Vancouver. His area of specialization is the conceptual foundations of modern physics, especially in the domains of statistical and quantum mechanics.