Einstein's Theories of Relativity

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Einstein's Theories of Relativity

Overview

At the dawn of the twentieth century the classical laws of physics put forth by Sir Isaac Newton (1642-1727) in the late seventeenth century stood venerated and triumphant. The laws described with great accuracy the phenomena of everyday existence. A key assumption of Newtonian laws was a reliance upon an absolute frame of reference for natural phenomena. As a consequence of this assumption, scientists searched for an elusive "ether" through which light waves could pass. In one grand and sweeping "theory of special relativity," Albert Einstein (1879-1955) was able to account for the seemingly conflicting and counter-intuitive predictions stemming from work in electromagnetic radiation, experimental determinations of the constancy of the speed of light, length contraction, time dilation, and mass enlargements. A decade later, Einstein once again revolutionized concepts of space and time with the publication of his "general theory of relativity."

Background

In the eighteenth and nineteenth centuries predominant philosophical and religious thought led many scientists to accept the argument that seemingly separate forces of nature shared a common source or absolute reference frame. Against this backdrop, nineteenth-century experimental work resulted in the unification of concepts regarding electricity, magnetism, and light by James Clerk Maxwell (1831-1879) with his four famous equations describing electromagnetic waves. Prior to Maxwell's equations, it was thought that all waves required a medium of propagation (i.e., an absolute reference frame). Maxwell's equations, however, established that electromagnetic waves do not require such a medium. Maxwell was, however, not convinced of this and he worked toward establishing the existence and properties of an "ether" or transmission medium.

The absence of a need for an ether for the propagation of electromagnetic radiation (e.g., light) was subsequently demonstrated by the ingenious experiments of Albert Michelson (1852-1931) and Edward Morley (1838-1923). The importance and implications of the Michelson-Morley experiment was lost on much of the scientific world. In many cases, the lack of determination of an ether was thought simply a problem of experimental design or accuracy. In contrast to this general dismissal, Einstein, then a clerk in the Swiss patent office developed a theory of light that incorporated implications of Maxwell's equations and demonstrated the lack of need for an ether.

In formulating his special theory of relativity, Einstein assumed that the laws of physics are the same in all inertial (moving) reference frames and that the speed of light was constant regardless of the direction of propagation and independent of the velocity of the observer.

Important components of Einstein's special theory involved length contraction and time dilation for bodies moving near the speed of light. In separate papers published in 1889, both Irish physicist George Francis Fitzgerald (1851-1901) and Dutch physicist Hendrik Antoon Lorentz (1853-1928) pointed out that the length of an object would change as it moved through he ether, the amount of contraction related to the square of the ratio of the object's velocity to the speed of light. Subsequently, this was known as a Fitzgerald-Lorentz contraction. Near the same time, French mathematician Jules-Henri Poincaré (1854-1912) pointed out problems with concepts of simultaneity and, just a year before Einstein published the special theory of relativity, Poincaré pointed out that observers in different reference frames would measure time differently.

The special theory of relativity also contained Einstein's work relating to mass and energy. Accordingly, in 1905 Einstein also published an elegantly brief but seminal paper in which he asserted and proved his most famous formula relating mass and energy (Energy = mass times the speed of light squared). Einstein's equation E = mc² is the best known—and arguably least understood—scientific formula. With ominous overtones to a nuclear age that would not dawn until the end World War II, Einstein's equation asserted that tremendous energies were contained in small masses.

A decade later, in 1915, Einstein published his general theory of relativity, which supplanted long-cherished and well-understood Newtonian concepts of gravity that had held prominence since the publication of Newton's Principia in 1687.

Impact

Einstein's theories of relativity brought about a revolution in science and philosophy, rivaled only by the contributions of Newton. In fact, although Newtonian physics still enjoys widespread utility, relativistic physics supplanted Newtonian cosmological concepts.

Einstein's papers on special relativity were revolutionary for both physics and science in general. Notable scientists, including German physicist Max Planck (1858-1947), who were in the process of developing quantum theory took notice, others set out to unsuccessfully reconcile relativity theory with Newtonian theories of gravitation. The special theory of relativity was quickly accepted by the general scientific community, and its implications for general philosophical thought were profound.

Despite this general acceptance, it is important to note that Einstein did not receive the Nobel Prize for relativity. Ironically, Einstein's 1921 Nobel Prize for Physics was awarded for his important contributions to quantum theory (a theory ultimately irreconcilable with relativity theory) via his explanation of the photoelectric effect.

The special theory of relativity challenged and eventually overturned classical concepts such as French chemist Antoine-Laurent Lavoisier's (1743-1794) conservation of mass and Prussian physicist Hermann von Helmholtz's (1821-1894) conservation of energy. According to special relativity theory, mass itself was not conserved except as part of a fusion of mass and energy. Mass was nothing more than a manifestation of energy.

Although Einstein's special theory was limited to special cases dealing with systems in uniform non-accelerated motion, it reverberated with philosophical consequence because it dispensed with absolutes (i.e., "absolute" rest or motion). Special relativity gave rise to a plethora of paradoxes dealing with the passage of time (e.g., the twin paradox) and with problems dependent upon assumptions of simultaneity. For example, under certain conditions, it was impossible to determine when one event happened in relation to another event. Classically separate concepts of three dimensions of geometrical space and a fourth dimension of time were fused into space-time.

As a result, those unfamiliar with the mathematical underpinnings of relativity theory often classified it as bizarre and in opposition to common sense. Einstein patiently pointed out that our perceptions of "common sense" were derived only from experiences with objects of an intermediate size, between the atomic and cosmic scale, that moved at velocities far below the speed of light.

It is not possible to understate the impact of Einstein's general theory upon the scientific community. Newtonian formulations of gravity were, prior to the publication of general relativity theory, considered proven. Only the mechanism, not the effects of gravity, were still a subject of inquiry (e.g., Poincaré's gravity waves). Although a quantum theory of gravity remains elusive, Einstein's fusion and curvature of spacetime provided a predictive explanation of the mechanisms by which bodies could attract each other over a distance.

In publishing his general theory of relativity, Einstein carefully listed three potential proofs where his theory would predict phenomena differently than classical Newtonian theory. Unlike the esoteric proofs of special relativity, the proofs of general relativity could be measured by conventional experimentation.

General relativity made sensible predictions regarding the subtle shift in the position of the perihelion of a planet, not foreseen by Newtonian theory. Because of it's proximity to the strength of the Sun's gravitational field, Einstein pointed out that it was possible to test the predications of general relativity against the orbit of Mercury. Small discrepancies in the orbit of Mercury (explained by some astronomers as evidence, perhaps, for the presence of another planetoid closer to the Sun) were immediately resolved.

Einstein also asserted that light subjected to an intense gravitational field would show a red shift. Observations of the red-shift of light between Sirus and its binary star companion lent further support to general relativity.

Most importantly, general relativity demanded that light would be deflected by a gravitational field. For more than four years, through the conflicts of World War I, the scientific world awaited the opportunity to test this important prediction. Following the war, the opportunity came with the solar eclipse of 1919. The irony of the fact that the British Royal Astronomical Society sent two expeditions to confirm the work of a German physicist was not lost on a world preoccupied with war reparations. Regardless, the expeditions measured the positions of the brightest stars near the sun as it was eclipsed, and shifts in the positions of other stars established that light was bent by its passage near the Sun's intense gravitational field.

This confirmation of general relativity earned Einstein widespread fame outside the scientific world and he quickly became the most influential scientist in the world. This fame would be most profoundly manifest two decades later when he was urged to write United States President Franklin D. Roosevelt in an attempt to make Roosevelt aware of the potential uses of atomic fission. Einstein's letter influenced the establishment of the Manhattan Project, which enabled the United States to first develop atomic weapons.

Upon the rise of Adolf Hitler in Germany, Einstein, a Jew, was permitted to remain in the United States. In addition to personal fame, Einstein's work found itself transcending academic, political, and cultural borders in much the same manner as the evolutionary theory of Charles Darwin (1809-1882). Many social commentators were quick to adopt selected postulates from relativity theory to support their respective causes. The rise of relativity theory that asserted no preferred reference frames gained in popular esteem as the single perspectives of traditional political empires crumbled and worries about totalitarian rule grew.

General relativity, essentially a geometrization of physical theory, sparked a grand revision of cosmology that continues today. The fusion of space-time under the theory of general relativity dispensed with detached and measurable absolutes and made observers integral to measurement in a much broader sense than had the special theory. The general theory also described non-uniform, or accelerated, motion. The motion of bodies under general relativity is explained by the assertion that in the vicinity of mass, space-time curves. The more massive the body the greater is the curvature or attraction.

The most stunning philosophical consequence of general relativity was that space-time was not an external grid by which to measure the universe. Space-time was a creation of the universe itself. This concept was critical during debates regarding the expansion of the universe argued by Edwin Hubble (1889-1953) and others. Under general relativity, the universe was not expanding into reexisting space and time, but rather creating it as a consequence of expansion. In this regard, general relativity theory set the stage for the subsequent development of big bang theory.

General relativity sparked great excitement among astronomers who immediately grasped the significance of the theory. Shortly before his death, German physicist Karl Schwarzschild (1873-1916) proposed equations that describe the gravitational field of massive compact objects that prepared the way for prediction and—as technology improved during the course of the twentieth century—discoveries related to the evolutionary stages of stars (e.g., neutron stars, pulsars, black holes, etc.). Along with quantum theory, special and general relativity theory remain among the most influential theories in science.

K. LEE LERNER