SOURCE: "Harbinger of the Atomic Age," in Books That Changed the World, revised edition, American Library Association, 1978, pp. 374-82.

[Downs was an American librarian, author, and editor whose professional life was committed to championing intellectual freedom and opposing literary censorship. In the following essay, originally published in the first edition of Books That Changed the World, he discusses various concepts rooted in Einstein's special and general theories of relativity and their impact on scientific study.]

Albert Einstein is one of the rare figures in history who succeeded in becoming a legend of heroic proportions during his own lifetime. The more incomprehensible to the lay public his ideas appeared, the more intriguing they seemed, and the more Olympian their progenitor. As Bertrand Russell aptly remarked, "Everybody knows that Einstein has done something astonishing, but very few people know exactly what it is that he has done." To be told, though inaccurately, that there are scarcely a dozen men in the entire world who fully grasp Einstein's theories of the universe, challenges and intrigues. The incomprehensibility of Einstein's theories stems from the complex nature of his field of operation. One point of view was expressed by George W. Gray:

Inasmuch as the theory of relativity is presented by its author in mathematical language, and in strictness of speaking cannot be expressed in any other, there is a certain presumption in every attempt to translate it into the vernacular. One might as well try to interpret Beethoven's Fifth Symphony on a saxophone.

Nevertheless, Einstein himself, Bertrand Russell, Lev Davidovitch Landau, and others have shown that it is possible to describe the Einstein cosmos intelligibly without resort to mathematical symbolism. And a fantastic world it is, extremely upsetting to ideas firmly established for centuries, "a strange pudding for the layman to digest." We are asked, for example, to accept such incredible conceptions as these: space is curved, the shortest distance between two points is not a straight line, the universe is finite but unbounded, parallel lines eventually meet, light rays are curved, time is relative and cannot be measured in exactly the same way everywhere, measurements of length vary with speed, a body in motion will contract in size but increase in mass, and a fourth dimension—an interweaving of space and time—is added to the familiar three of height, length, and width.

Though Einstein's contributions to science have been innumerable, his fame rests primarily upon the theory of relativity, an achievement which, Banesh Hoffman concluded, "has a monumental quality that places its author among the truly great scientists of all time, in the select company of Isaac Newton and Archimedes. With its fascinating paradoxes and spectacular successes it fired the imagination of the public."

The revolution in concepts brought about by Einstein began in 1905, with the appearance in a German journal, Annalen der Physik, of a thirty-page paper carrying the unexciting title "On the Electrodynamics of Moving Bodies." At the time, Einstein was only twenty-six years of age, and serving as a minor official in the Swiss patent office. He had been born into a middle-class Jewish family at Ulm, Bavaria, in 1879. As a student, he was not precocious even in mathematics. Because of failure of the family fortune, Einstein was forced out on his own at fifteen. Emigrating to Switzerland, he was able to continue his scientific education at the Polytechnic Academy in Zurich, married a fellow student, and became a Swiss citizen. To earn a living he settled in a job making preliminary reports and rewriting inventors' applications for the patent office. His spare time was used for intensive study of the works of philosophers, scientists, and mathematicians. Soon he was ready to launch the first of a flood of original contributions to science, destined to have far-ranging repercussions.

In his 1905 paper, Einstein set forth the Special Theory of Relativity, challenging man's existing concepts of time and space, of matter and energy. The foundations for the theory were laid down in two basic assumptions. The first was the principle of relativity: all motion is relative. A familiar illustration of the principle is a moving train or ship. A person sitting in a train with darkened windows would have, if there was little commotion, no idea of speed or direction, or perhaps even that the train was moving at all. A man on a ship with portholes closed would be in a similar predicament. We conceive motion only in relative terms, that is in respect to other objects. On a vastly greater scale, the forward movement of the earth could not be detected if there were no heavenly bodies for comparisons. It should be noted that this principle was stated explicitly by Newton in his Principia, and was not a new contribution; Einstein merely affirmed its relevance to electrodynamics as well as in mechanics.

Einstein's second major hypothesis was that the velocity of light is independent of the motion of its source. The speed of light, 186,000 miles a second, is always the same, anywhere in the universe, regardless of place, time or direction. Light travels in a moving train, for instance, at exactly the same speed as it does outside the train. No force can make it go faster or slower. Furthermore, nothing can exceed the velocity of light, though electrons closely approximate it. The speed of light is, in fact, the only constant, unvarying factor in all of nature.

A famous experiment carried out in 1887 by two American scientists, Michelson and Morley, furnished the basis of Einstein's theory on light. They attempted to determine the velocity of the earth as it passed through the ether (a hypothetical substance which was believed to pervade all space not occupied by matter). No velocity could be determined, and most physicists abandoned the ether theory.

Einstein's paper in 1905 answered the question which had puzzled Michelson, Morley, and their fellow physicists. The essential point deduced by Einstein was that light always travels at the same velocity no matter under what conditions it is measured, and the motion of the earth in regard to the sun has no influence upon the speed of light.

Newton stated, "It may be that there is no body really at rest to which the place and motions of others may be referred." Today's scientists do not say that there is or is not such a thing as absolute motion. The question, only to be determined by experiment, is whether absolute motion can be detected. Einstein asserted that every body's movement is relative to that of another. Motion is the natural state of all things. Nowhere on earth or in the universe is there anything absolutely at rest. Throughout our restless cosmos, movement is constant, from the infinitesimally small atom to the largest celestial galaxies. For example, the earth is moving around the sun at the rate of twenty miles a second. In a universe where all is motion and fixed points of reference are lacking, there are no established standards for comparing velocities, length, size, mass, and time, except as they might be measured by relative motions. Only light is not relative, its velocity remaining changeless regardless of its source or the observer's position.

Doubtless the most difficult of all Einsteinian concepts to comprehend and the most unsettling to traditional beliefs is the relativity of time. Einstein held that events at different places occurring at the same moment for one observer do not occur at the same moment for another observer moving at a speed relative to the first. For example, two events judged as taking place at the same time by an observer on the ground are not simultaneous for an observer in a train or an airplane. Time is relative to the position and speed of the observer, and is not absolute.

As speed increases, time seems to slow down. We are accustomed to the thought that every physical object has three dimensions, but Einstein maintained that time is also a dimension of space, and space is a dimension of time. Neither time nor space can exist without the other and they are interdependent. Movement and change are constant, we live in a four-dimensional universe, with time as the fourth dimension.

Thus the two basic premises of Einstein's theory, as first presented seventy years ago, were the relativity of all motion and the concept of light as the only unvarying quantity in the universe.

In developing the principle of relativity of motion, Einstein upset another firmly established belief. Previously, length and mass had been regarded as absolute and constant under every conceivable circumstance. Now, Einstein came along to state that the mass or weight of an object and its length depend on how fast the body is moving relative to uniform motion of the observer. As an example, he imagined a train one thousand feet long, traveling at four-fifths of the speed of light. To a stationary observer, watching it pass by, the length of the train would be reduced to only six hundred feet, though it would remain a thousand feet to a passenger on the train. Similarly, any material body traveling through space contracts according to velocity. A yardstick, if it could be shot through space at 161,000 miles per second would shrink to a half-yard.

Mass, too is changeable. As velocity increases, the mass of an object becomes greater. Experiments have shown indirectly that particles of matter speeded up to 86 per cent of the speed of light weigh twice as much as they do when at rest. Movement and change and relativity had been recognized long before Einstein. The novel results of his theory come from the combination of the well-known principle of relativity with the new assumption of the constancy of the velocity of light.

Einstein's original statement of 1905, known as the special theory of relativity, is limited to uniform motion of the reference frames that are used in the description. It tells us how things look from a train moving uniformly in a straight line, but they do not tell us how things look from a rotating merry-go-round. In our cosmos, stars, planets, and other celestial bodies seldom move uniformly in a straight line. Any theory, therefore, which fails to include every type of motion offers an incomplete description of the universe. Einstein's next step, accordingly, was the formulation of his general theory of relativity, a process which required ten years of intensive application. In the general theory, Einstein studies the mysterious force that guides the movements of the stars, comets, meteors, galaxies, and other bodies whirling around in the vast universe. The general theory is valid for all kinds of observers, whether their motion is uniform or accelerated.

In his general theory of relativity, published in 1915, Einstein advanced a new concept of gravitation, making fundamental changes in the ideas of gravity and light which had been generally accepted since the time of Sir Isaac Newton. Gravity had been regarded by Newton as a "force." Einstein proved, however, that the space around a planet or other celestial body is a gravitational field similar to the magnetic field around a magnet. Tremendous bodies, such as the sun or stars, are surrounded by enormous gravitational fields. The theory also explained the erratic movements of Mercury, the planet nearest the sun, a phenomenon that had puzzled astronomers for centuries and had not been adequately covered by Newton's law of gravitation. So powerful are the great gravitational fields that they even bend rays of light. In 1919, a few years after the general theory was first announced, photographs taken of a complete eclipse of the sun conclusively demonstrated the validity of Einstein's theory that light rays passing through the sun's gravitational field travel in curves rather than in straight lines.

There followed from this premise a statement by Einstein that space is curved. Revolving planets follow the shortest possible routes, influenced by the sun's presence, just as a river flowing toward the sea follows the contour of the land, along the easiest and most natural course. In our terrestrial scheme of things, a ship or airplane crossing the ocean follows a curved line, that is the arc of a circle, and not a straight line. It is evident, therefore, that the shortest distance between two points is a curve instead of a straight line. An identical rule governs the movements of a planet or light ray.

It is not a logical deduction from Einstein's theory that space is finite. If the universe does not extend forever into space, but has finite limitations, no definite boundaries can be established. There are many cosmological models presently being considered with all sorts of different geometrical properties.

Of all the great scientific discoveries and findings coming from Einstein, his contributions to atomic theory have had the most direct and profound effect on the present-day world. Shortly after his first paper on relativity was published in 1905, in the Annalen der Physik, the same journal carried a short article by Einstein projecting his theory further. It was entitled "Does the Inertia of a Body Depend on Its Energy?" The mass-energy relation is a corollary of the special theory of relativity. At the time, he did not state that the use of atomic energy was possible. Later, he pointed out that the release of this tremendous force could be achieved according to a formula he offered, the most celebrated equation in history: E = mc 2. To interpret, energy equals mass multiplied by the speed of light and again by the speed of light. If all the energy in a half pound of any matter could be utilized, Einstein held, enough power would be released to equal the explosive force of seven million tons of TNT. Without Einstein's equation, as one commentator pointed out, "experimenters might still have stumbled upon the fission of uranium, but it is doubtful if they would have realized its significance in terms of energy, or of bombs."

In the famous equation E = mc 2, Einstein demonstrated that energy and mass are proportional to each other, differing only in state. Mass is actually concentrated energy. The formula, wrote Lincoln Barnett in a brilliant evaluation, "provides the answer to many of the long-standing mysteries of physics. It explains how radioactive substances like radium and uranium are able to eject particles at enormous velocities and to go on doing so for millions of years. It explains how the sun and all the stars can go on radiating light and heat for billions of years, for if our sun were being consumed by ordinary processes of combustion the earth would have died in frozen darkness eons ago. It reveals the magnitude of the energy that slumbers in the nuclei of atoms, and forecasts how many grams of uranium must go into a bomb in order to destroy a city."

Einstein's equation remained a theory until 1939. By that time, its author had become a resident, and was shortly to become a citizen, of the United States, for he had been driven out of Europe by the Nazis. Learning that the Germans were engaged in importing uranium and were carrying on research on an atomic bomb, Einstein wrote President Roosevelt a highly confidential letter:

Some recent work by E. Fermi and L. Szilard, which has been communicated to me in manuscript, leads me to expect that the element uranium may be turned into a new and important source of energy in the immediate future.… This new phenomenon would also lead to the construction of bombs, and it is conceivable… that… a single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory.

As an immediate result of Einstein's letter to Roosevelt, construction of the Manhattan atom-bomb project was started. About five years later, the first bomb was exploded at the Alamogordo reservation in New Mexico, and shortly thereafter the dreadful destruction caused by a bomb dropped on Hiroshima was instrumental in bringing the war with Japan to a quick end.

Though the atomic bomb was the most spectacular of all practical applications of the theories of Einstein, his fame was also established by another remarkable accomplishment. Almost simultaneously with his special theory of relativity in 1905, there was developed Einstein's photoelectric law, explaining the mysterious photoelectric effect. Einstein's finding was extremely important, as being one of the fundamental particles of nature—the "light-quantum" or photon—and it was also extremely important as a step toward the discovery of quantum mechanics several decades later. It was for the discovery of the photoelectric law that Einstein was awarded the Nobel Prize in physics in 1922.

In his later years, Einstein labored indefatigably on what is known as the unified field theory, attempting to demonstrate the harmony and uniformity of nature. According to his view, physical laws for the minute atom should be equally applicable to immense celestial bodies. The unified field theory would unite all physical phenomena into a single scheme. Gravitation, electricity, magnetism, and atomic energy are all forces that would be covered by the one theory. In 1950, after more than a generation of research, Einstein presented such a theory to the world. He expressed the belief that the theory holds the key to the universe, unifying in one concept the infinitesimal, whirling world of the atom and the vast reaches of star-filled space. Because of mathematical difficulties, the theory has not yet been fully checked against established facts in physics. Einstein had unshaken faith, however, that his unified field theory would in time produce an explanation of the "atomic character of energy," and demonstrate the existence of a well-ordered universe. He did not regard the theory as anything that could be tested experimentally, however, and present-day physicists are not inclined to believe that the theory will prove fruitful.

The philosophy which inspired and guided Einstein through decades of intense intellectual effort, and the rewards therefrom, were described by him in a lecture on the origins of the general theory of relativity, at the University of Glasgow in 1933.

The final results appear almost simple; any intelligent undergraduate can understand them without much trouble. But the years of searching in the dark for a truth that one feels, but cannot express; the intense desire and the alternations of confidence and misgiving, until one breaks through to clarity and understanding, are only known to him who has himself experienced them.

On another occasion, Einstein gave evidence of the deeply spiritual side of his nature by this statement:

The most beautiful and most profound emotion we can experience is the sensation of the mystical. It is the sower of all true science. He to whom this emotion is a stranger, who can no longer wonder and stand rapt in awe, is as good as dead. To know that what is impenetrable to us really exists, manifesting itself as the highest wisdom and the most radiant beauty which our dull faculties can comprehend only in their most primitive forms—this knowledge, this feeling is at the center of true religiousness.

Innumerable scientists have paid tribute to Einstein. Quotations from two recent reviews of his career will illustrate his unique hold on the scientific world. Paul Oehser wrote:

Influence is a weak word for the work of Albert Einstein. The theories he advanced were revolutionary. In them was born the Atomic Age, and where it leads mankind we know not. But we do know that here is the greatest scientist and philosopher of our century, who has become almost a saint in our eyes and whose achievement is a justification of our faith in the human mind, a symbol of man's eternal quest, his reaching for the stars.

Another scientist, Banesh Hoffman, concluded:

The importance of Einstein's scientific ideas does not reside merely in their great success. Equally powerful has been their psychological effect. At a crucial epoch in the history of science Einstein demonstrated that long-accepted ideas were not in any way sacred. And it was this more than anything else that freed the imaginations of men like Bohr and de Broglie and inspired their daring triumphs in the realm of the quantum. Wherever we look, the physics of the 20th century bears the indelible imprint of Einstein's genius.

Concerning the present status of relativity, an international conference of theoretical physicists, at Berne, Switzerland, recently agreed that the foundations of the special and general theory have been universally accepted. Experiments have conclusively confirmed the special theory and are convincing for the general theory. The special theory has been incorporated into general physics and is used continually in atomic and nuclear physics.

For a number of years, the general theory was applied mainly to cosmology and cosmogony, but lately relativity is being applied to microphysical problems. The relationship to the quantum theory is still quite undetermined. It is apparent that general relativity provides a new approach to the ultimate properties of space and time. If true, the theory may have as much bearing on the physics of the very small as of the very large. The increasing worldwide interest in general relativity indicates that scientists believe the theory may add further to our understanding of the universe as an organic whole.