This Day In History: Relativity

On this day in history, March 20, 1916, Albert Einstein published his general theory of relativity.  To help you learn more about relativity we turned to The Oxford Companion to the History of Modern Science, which we found through Oxford Reference Online. The article about relativity below is by J. L. Heilbron.

Relativity, a theory and program that set the frame of the physical world picture of the twentieth century…

…In the thought experiment Einstein often described, an observer on an embankment sees a light flash from the middle of a passing railroad carriage equipped with a mirror at either end. According to relativity, an observer seated at the center of the car would see the light returned from the mirrors simultaneously. But the observer on the embankment would see the flash from the forward mirror after that from the rear: the light has farther to go to meet and return from the forward than from the rear mirror, and the speed of light by hypothesis is the same in both directions. On a ballistic theory of light, the return flashes occur simultaneously for both parties: to the traveling observer, the light has the same speed in both directions; to the stationary one, the speed in the forward direction exceeds that in the opposite direction by twice the speed of the carriage. Further arithmetic shows that holding to the absolute and the relative simultaneously required that meter sticks and synchronized clocks moving at constant velocity with respect to a stationary observer appear to him to be shorter and tick slower than they do to an observer traveling with them. Einstein showed that the form and magnitude of these odd effects follow from the stipulation that the coordinates of inertial systems are related by a set of equations that he called the Lorentz transformation. Hendrik Antoon Lorentz had introduced them as a mathematical artifice to make the principle of relativity apply to Maxwell’s equations of the electromagnetic field… Einstein now insisted that the Lorentz transformation apply also to mechanics in place of what came to be called the “Galileo transformation” that guaranteed relativity to Newton’s laws of motion…Since the Galilean transformation supports the Newtonian expressions for force, mass, kinetic energy, and so on, replacing it required reworking the formalism of the old mechanics. This labor, in which Max Planck and his student Max von Laue played leading parts, produced expressions differing from the Newtonian ones by factors of γ. A new form of Newton’s second law emerged that satisfied the demand of relativity (“remained invariant”) under the Lorentz transformation.

As an afterthought, also in 1905, Einstein argued that energy amounts to ponderable mass and vice versa. The relativistic expressions for the momentum and kinetic energy require that, for conservation of momentum to hold, the mass m of an isolated system of bodies must increase when the system’s kinetic energy E decreases. The increase occurs at a rate (change of mass) = (change of energy)/c2, Δm = ΔE/c2. Einstein thus united the previously distinct principles of the conservation of energy and of mass. From a practical point of view, the equivalence of mass and energy as applied to nuclear power is by far the most important consequence of relativistic mechanics.

Relativity had an enormous appeal to people like Planck, who regarded the surrender of common-sense expectations about space, time, and energy as a major step toward the complete “deanthropomorphizing of the world picture” begun by Copernicus. The mathematician Hermann Minkowski declared in a famous speech to the Society of German Scientists and Physicians in 1908 that “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.” He interpreted the Lorentz transformation as a geometrical rotation in his four-dimensional space, whose points represented “world events” and whose lines represented the histories of all the particles in the universe. “The word relativity postulate … seems to me very feeble,” he said, “to express the postulate of the absolute world,” that is, Einstein’s theory as geometrized by Minkowski.

Einstein’s compulsion to remove the “all too human” from physics and his desire to overcome the limitation of SRT brought him to a more profound generalization than Minkowski’s. Taking the equivalence of inertial and gravitational masses as his guide, he worked out by 1911 that gravitational forces should affect electromagnetic fields (for example, by giving radiation potential energy) and deduced that the sun would bend the path of a ray of starlight that passed close to it. But the major conquest wrested from the equivalence of the masses was the elimination of gravity: all freely falling bodies in the same region experience the same acceleration because the presence of large objects distorts the space around them and the bodies have no alternative but to follow the “geodesics”—straight lines in the curved space in which they find themselves. In “flat space-time,” without distorting masses, bodies move in inertial straight Euclidean lines. Falling bodies apparently coerced to rejoin the earth under a gravitational force in Euclidean space in fact move freely along geodesics in the local warp in the absolute four-dimensional space-time occasioned by the earth’s presence. By 1915 Einstein had found the mathematical form (tensors) and the field equations describing the local shape of space-time that constituted the backbone of GRT, and had added two more tests: an explanation of a peculiarity in the orbit of Mercury and a calculation of the effect of gravity on the color of light.

By 1910 or 1911 German theorists had accepted SRT and a few physicists elsewhere recognized its importance…Einstein traveled, quipped, became a favorite of newspaper reporters around the world and the bête-noire of anti-Semites back home. He spent the rest of his life, in Berlin and in Princeton, trying to generalize the general by bringing electromagnetism within GRT. But neither his efforts nor those of the few other theorists who thought the game worth the candle managed to reduce electricity and magnetism to bumps in space.

…Although gravity remains outside the unified forces of the Standard Model, the pursuit of the vanishingly small and the ineffably large depend upon GRT for clues to the origin and evolution of the universe…