Relativity by Albert Einstein

First published:Über die spezielle und die allgemeine Relativitätstheorie (Gemeinverständlich), 1917 (English translation, 1920)

Type of work: Science

The Work:

By 1916, Albert Einstein was a world-famous professor of physics at the University of Berlin, and he had seen several scientists and popularizers write books about his special theory of relativity, which he had first published in 1905. Other such books, published between 1913 and 1916, even included some of his preliminary ideas on the general theory of relativity. With the success of several of these popularizations, he knew that a large and willing audience was available to learn about the theories of relativity from the man who created them. Having spent the majority of his scientific career communicating with colleagues via journal articles that made heavy use of advanced mathematics, Einstein realized that he would need a different way to convey his discoveries to a lay readership. When, in the fall of 1915, he started thinking about how to make his ideas understandable to nonscientists, his work on the general theory of relativity was effectively complete, but his personal life was in turmoil, and all of this would play a role in the book’s composition.

In 1914, Einstein had separated, not amicably, from his first wife Mileva and their two sons, but he was not formally divorced until 1919. During the interim, he formed an intimate relationship with his cousin Elsa Löwenthal, who had been married and had two teenaged daughters, Margot and Ilse. Einstein became friendly with the young girls, and in papers kept secret for many years scholars would later discover correspondence from Ilse in which she wrote that Einstein loved her “very much, perhaps more than any other man ever will,” and that he was “prepared to marry” either her or her mother. After Einstein’s marriage to Elsa in 1919, Margot and Ilse became his stepdaughters.

As Einstein wrote Relativity, he read aloud the completed pages to Margot, in the hope that if a teenager could understand his book then other nonscientists would as well. Scholars have compared surviving fragments of this manuscript with corrections in later editions and have detected the handwriting of Ilse. Einstein experienced some difficulty in getting started, but, driven by a desire to make his ideas widely understandable, he completed the manuscript by December, 1916. At the time, he told a friend that his presentation was so simple that a high school student could understand it, though he was unhappy with the book’s style, which he characterized as “wooden.” Nevertheless, German publishing house Vieweg in Braunschweig published Relativity in the spring of 1917, and it enjoyed rapid success. Several other German editions and many translations followed. Einstein sometimes added new prefaces to these translations, and he also made corrections and additions to both the German and the foreign versions. The book passed through fifteen German editions during Einstein’s lifetime (though there was no fifteenth, and the last one, labeled the sixteenth, was published in 1954).

Einstein structured his book in three parts: The first part, comprising seventeen chapters, is on the special theory of relativity; the second, comprising twelve chapters, is on the general theory; the third, comprising three chapters, is titled “Considerations on the Universe as a Whole.” Einstein warns readers that, despite the book’s brevity, patience and willpower will be required to grasp the main ideas and their justifications. He first reviews the basic concepts of Euclidean geometry and Newtonian mechanics before beginning his treatment of special relativity in the sixth chapter, “The Apparent Incompatibility of the Law of Propagation of Light with the Principle of Relativity.” Einstein had become aware of this incompatibility as a teenager, when he wondered what he would experience if he could travel as fast as a light wave. This thought experiment led to a dilemma that necessitated the abandonment of either the classical principle of relativity, which was based on the laws of motion discovered by Galileo and Sir Isaac Newton, or the classical laws of the propagation of light, based on the work of James Clerk Maxwell and others. Einstein discovered a way out of the dilemma by reasoning from two postulates of invariance: the constancy of light’s speed in uniformly moving systems and the immutability of physical laws in such systems. In the next ten chapters, Einstein guides readers through some of the surprising consequences for scientists’ understanding of space, time, simultaneity, mass, and energy when thought experiments are performed based on these two postulates.

A good example of this process involves Einstein’s use of a moving train to deduce the relativity of simultaneity. He imagines a train passing an embankment: At the instant an observer in the middle of the train faces an observer on the embankment, lightning strikes the front and the back of the train. The observer on the train interprets this event as nonsimultaneous since he sees the flash from the front before the one from the back, whereas the observer on the embankment sees the flashes occurring simultaneously, since both flashes reach him at the same time. In this way, Einstein shows that Newton’s idea of absolute simultaneity for all observers is wrong and that time does not have absolute significance but depends on the state of motion of the observer. Using similar reasoning, he shows that observers in different but uniformly moving systems will make dissimilar observations about an object’s length and clock times. For example, moving clocks appear to run more slowly than clocks at rest, a phenomenon that came to be called “time dilation.”

The most important and influential consequence of the special theory of relativity is Einstein’s equation expressing the equivalence of mass and energy, which states that energy equals the mass of an object multiplied by the square of the speed of light. Because of the enormous size of light’s speed, Einstein states that it is not yet possible to test this equation through experimentation, but with the development of advanced particle accelerators, Einstein would report in later editions that accelerations of subatomic particles confirmed his equation. Toward the end of his life, Einstein was distraught at the part his equation played in the development of the atomic bomb.

In part 2 of Relativity, Einstein confronts the much more demanding task of explaining general relativity to readers with modest mathematical backgrounds. What makes this theory “general” is its applicability to all systems, those moving nonuniformly as well as those moving uniformly. As he did in discussing the special theory, Einstein begins the second part of the book with a general principle—the equivalence of gravity and acceleration. During the decade between the publication of the two theories of relativity, Einstein had what he later called “the happiest thought of his life.” He realized that a person falling from a building would have no experience of his weight.

In yet another thought experiment, Einstein analyzes what a person enclosed in a large box would experience if the box were resting on the Earth, and he compares that experience to the same person’s experience if the box were being pulled by a rope in outer space. He concludes that the acceleration of the person toward the floor of the box is always of the same magnitude, whether the box is in a gravitational field or in an accelerated system. From this equivalence of gravity and acceleration, Einstein is able to ascertain some surprising consequences. For example, light rays travel curvilinearly in gravitational fields. This leads Einstein to urge physicists to test his idea during a solar eclipse (in later editions, he was able to report on the 1919 solar eclipse expeditions that successfully confirmed his predictions).

Most significant, the general theory of relativity explains something that Newton was never able to explain—the cause of gravity. For Einstein, matter is not contained in absolute space as Newton claimed; instead, matter modifies the space around it. This curved space then modifies the way that material objects behave. For example, the curvature of space around the Sun accounts for the elliptical motions of the planets. Einstein is even able to explain the anomalous behavior of the innermost planet of the solar system. Astronomers had observed that the point in Mercury’s elliptical orbit nearest the Sun (the perihelion point) changes over time, something inexplicable in classical physics but accurately accounted for by general relativity.

In part 3 of Relativity Einstein analyzes such basic questions as whether the universe is finite or infinite, bounded or unbounded. He begins by explaining why Newton’s conception of the stellar universe as a “finite island in the infinite ocean of space” is unsatisfactory, since the destiny of Newton’s universe would be the systematic “impoverishment” of light and energy. The development of non-Euclidean geometries led to the discussion of spherical spaces that are finite but unbounded, and Einstein uses the two-dimensional surface of a sphere to give his readers an insight into the nature of three-dimensional spherical space. Beings existing on a sphere’s surface discover that every line turns into a curve, which then leads to a recognition that this two-dimensional universe is finite but with no limits. Furthermore, these spherical-surface beings have no way of discovering whether they are in a finite or an infinite universe, because their “piece of the universe” is planar. Einstein uses the three-dimensional spherical analogue to this two-dimensional universe in his development of general relativity. He accepts astronomers’ observations that matter in the universe is not distributed uniformly, leading him to conclude that the universe is “quasi-spherical” and finite. He concludes his book with the statement that his theory explains the connection between the “space-expanse” of the universe and the average density of matter in it.

Einstein’s Relativity has an advantage over the many other popularizations of relativity theories produced during and after his life, since his book draws on the personal experiences that led him to formulate these theories. Reviewers have found his presentation of complex ideas clear and understandable, and his thought experiments have become standard in textbooks as well as other popular accounts. Historians of science have found Einstein’s ongoing modifications and additions to his book helpful in understanding the evolution of his thinking about relativity, particularly its philosophical implications. In later editions, he shared with readers his skepticism about the new quantum mechanics, whose uncertainty principle challenged his profound belief in determinism. He famously said that God does not play dice with the universe. Despite many years of effort, Einstein was unable to successfully unify his general relativity and quantum mechanics, a task that continues to be the noble goal of many modern would-be Einsteins.

Bibliography

Fölsing, Albrecht. Albert Einstein: A Biography. New York: Penguin Books, 1998. This massive and detailed work was praised on its first appearance in German and in its later English (abridged) version. Scientists and general readers have found his treatment of Einstein’s life and work balanced and insightful. Extensive notes and bibliography, a helpful chronology, and an index.

Isaacson, Walter. Einstein: His Life and Universe. New York: Simon & Schuster, 2007. Isaacson, a former managing editor of Time magazine, uses many new documents in creating his portrait of “the complete Einstein”—scientist, humanist, husband, and father. The book, lauded by scientists and nonscientists, became a best seller. Notes to primary and secondary sources, brief biographies of the main characters, and an index.

Kox, A. J., Martin J. Klein, and Robert Schulmann, eds. The Berlin Years: Writings, 1914-1917. Vol. 6 in The Collected Papers of Albert Einstein. Princeton, N.J.: Princeton University Press, 1996. This volume of the ongoing protect to publish all of Einstein’s extant writings contains the German text of his popularization of the special and general theories of relativity. The introductions and notes are in English, as is a softcover companion that contains English translations of all of the German writings of volume 6. Includes a section on literature cited, as well as an index of subjects and another of citations.

Levenson, Thomas. Einstein in Berlin. New York: Bantam Books, 2003. This biographical study of Einstein’s life and work from 1914 to 1933 also contains much background material on the composition and publication of his popularization on relativity. Illustrated with photographs. Notes, bibliography, and index.