One of the greatest contributors to modern physics, Albert Einstein (1879–1955) revolutionized the way scientists think about time, space, motion and gravity. He proposed the theory of relativity, reasoning that because the speed of light is constant, then distance and time, both of which define the speed of light, must be relative. His work also showed that it was possible to make an atomic bomb.
Albert Einstein was born in 1879 at Ulm in Germany. The following year his family moved to Munich. In 1895 they moved to Zurich, where the 16-year-old Albert failed his entrance examination in engineering to the Zurich Polytechnic. His early education was undistinguished, and years later he wrote with a lack of enthusiasm about the strict teaching he endured at the Luitpold Gymnasium when he was about 15 years of age:
When I was in the seventh grade at the Luitpold Gymnasium I was summoned by my home-room teacher who expressed the wish that I leave the school. To my remark that I had done nothing amiss he replied only “your mere presence spoils the respect of the class for me.” I myself, to be sure, wanted to leave school and follow my parents to Italy. But the main reason for me was the dull, mechanised method of teaching. Because of my poor memory for words, this presented me with great difficulties that it seemed senseless for me to overcome. I preferred, therefore, to endure all sorts of punishments rather than to learn to gabble by rote.
Albin Herzog, the director of the Zurich Polytechnic, urged Einstein not to give up his endeavors, but to seek entrance to the progressive Swiss Cantonal School in nearby Aargau. Einstein followed the advice, and the school turned out to be the making of him. He lodged with one of the teachers, Jost Winteler, where he was treated as one of the family. In fact he became one of the family, for one of the Winteler’s sons married Einstein’s sister Maja. One of their daughters even married his friend Michele Besso.
After completing his schooling Einstein became a part-time teacher and a private tutor until 1902, when he was offered a job at the Swiss Patent Office in Bern on the modest salary of 3500 francs a year. At the age of 23 he had still achieved nothing in the academic world, but at least he had a regular job, and also one that wasn’t too onerous—it gave him plenty of time to speculate about questions of philosophy and the nature of the universe. Einstein did not conform to the popular image of the quiet cloistered academic, however. At Bern he made two close friends, Konrad Habicht (1886–1958) and Maurice Solovine (1875–1958), and the three called themselves the “Olympian Academy.” They discussed philosophy, literature and physics in a noisy and boisterous fashion well into the night, much to the annoyance of their immediate neighbors.
In 1903 Einstein married Mileva Maric. The union was blessed with a son called Hans Albert, born in 1904, and by a second son, Eduard, born in 1910. Albert Einstein’s academic career was still progressing very slowly at this time, but in 1905, at the age of 26, he submitted a PhD thesis to the University of Zurich. The thesis was rejected as being too short. Einstein put in one extra sentence and resubmitted the thesis. This time it was accepted.
It was at Bern, while he was traveling along the high street by tram, that Einstein first began to wonder about relative motion. If the tram were to speed up to approach the velocity of light, then he realized that the time shown on the town hall clock ahead of him would appear to speed up because the light from it had a progressively shorter distance to travel. He surmised that on his return journey, when the tram was speeding away from the clock, the clock would appear to slow down, for the light took progressively longer to reach him. If the tram reached the speed of light then the clock would remain at a fixed time, he surmised—in other words, time shown on the clock would appear to stand still. He calculated the equations relating space and time in the two frames—one frame being the high street in Bern with the clock, and the other being the moving tram—and formulated what he called the principle of relativity: that the laws of physics were the same in all uniformly moving frames. He showed how time and distance could be different as seen by two or more observers in different frames, but the laws of physics were the same.
Einstein pondered the ideas of relativity for several years before giving his first findings to the world. His first published paper, in 1905, was a description of the phenomenon known as the photoelectric effect. This was a property observed in certain metals. When light shone on the metal, electrons were emitted. Einstein explained the effect in terms of particles of light striking the atoms of the metal and causing them to release electrons. This theory alone would have won Einstein fame, but it was followed very quickly by something much more radical. It was his account of the principle of relativity—now called the special theory of relativity—in which he expounded his ideas about moving and fixed frames. His famous imaginary experiment involves a frame moving at constant speed relative to a fixed frame, such as a train moving along a straight track. Now, imagine a stationary observer situated by the side of the track halfway between two signal lights. Both the lights flash at the same time, and the observer, being positioned an equal distance away from both, concludes that the lights were switched on at exactly the same time.
Next, imagine another observer traveling on a train between the two simultaneously flashing lights. The observer on the train also sees the lights flash, but he is moving toward one light and away from the other. So he sees the light he is traveling toward before he sees the light he is traveling away from because it has less distance to travel. To this observer, the lights flash at different times. According to Einstein’s theory of relativity, different people do not therefore necessarily see the same event at the same time.
Albert Einstein believed that the speed of light was exactly the same in all frames of reference. He believed that if the observer on the train were to measure the speed of the light from the two stationary signals then he would arrive at the same result as the ground-based observer. This suggestion, taken to its ultimate, leads to some astonishing conclusions. In the moving frame of the train, for example, all lengths in the direction of motion are seen as foreshortened from the rest frame of the Earth. But according to Einstein’s theory there was really no such thing as a privileged rest frame (in other words, a frame that does not itself move while all other things are moving relative to it); every observer in a uniformly moving frame could assume that he or she was stationary while the rest of the world moved past. Space and time were related in such a fashion that time appeared as a fourth dimension. Einstein’s theory that light always traveled at the same speed was difficult to prove, but in fact the proof was already there. It was the conclusion reached by Michelson and Morley in their seemingly negative experiment conducted in the previous century.
Some of the conclusions of Einstein’s special theory of relativity were to have repercussions in the field of astronomy. Using Einstein’s hypothesis, the mass of a body increases with its speed. The closer the speed approaches to that of the speed of light the heavier the body becomes, so that if it can be accelerated to reach light speed then its mass will become infinite. It follows, therefore, that it is impossible for anything to travel faster than light.
This was a serious blow for astronomers. It meant, for example, that within a human lifetime it would be impossible to send a message to star systems and to get a message back again, except in the case of a few close systems. It also meant that the universe would be much more difficult to explore, and for humans to travel through, than was first thought.
As soon as Einstein’s paper was published he was formulating another theory destined to be just as earth-shattering. It concerned the relationship between acceleration and gravity. He was struck by the fact that these seemingly very different concepts produced some very similar results. In another thought experiment, Einstein envisaged an earthbound laboratory. In it, the experimenter could verify the laws of mechanics and conclude that all matter was drawn to the Earth by the force of gravity. In the laboratory, a body would fall with an acceleration g, just as 300 years earlier Galileo’s musket and cannon balls had fallen from the leaning tower of Pisa in his experiment on the study of gravity.
Einstein then imagined a second laboratory that was effectively a spacecraft. This craft was equipped with engines that could propel the laboratory through space with a constant acceleration of g. The experimenter in this imaginary craft could also stand in his laboratory and demonstrate that any object falling to the floor appeared to fall with an acceleration of g.
The gravitational field of the earthbound laboratory and the acceleration of the other laboratory in space gave exactly the same results for the experiments on the laws of mechanics. Then Einstein formulated what he called the principle of equivalence. He claimed that if his experimenters could not perceive the world outside their laboratories then there was no way of telling the difference between the gravitational field and the constant acceleration. The effects were equivalent.
Einstein went further. He maintained that the skeptical scientist could use something to devise an experiment to tell which type of laboratory he was in. The “something” was a beam of light. Light traveled in a straight line. Therefore if a beam of light crossed the earthbound laboratory the observer would measure its course as a straight line. If the observer in the accelerated space laboratory performed the same experiment, however, then the acceleration would cause the light to appear to be slightly deflected down to the floor of the laboratory. Imagine the light beam entering horizontally on the left-hand side of the laboratory. By the time the beam has crossed to the right-hand side the accelerating laboratory has moved, thus the light beam appears lower; its path is bent toward the floor of the laboratory. The skeptical scientist did not know that gravity could bend the light so he assumed that he had found a difference between the two laboratories.
But Einstein insisted that there was no measurable difference between the two laboratories. He explained his assertion by claiming that in the earthbound laboratory gravity would draw the light beam to the floor by exactly the same amount as measured in the accelerated laboratory. One of the basic assumptions in Einstein’s new theory was that a beam of light was deflected by a gravitational field. The deflection was very small and there was no way to measure it in an earthbound laboratory. But it could be measured during a total eclipse of the Sun. If there were stars near the rim of the Sun during an eclipse, then the light from those stars would be deflected by the Sun’s gravity and the stars would appear to be pushed outward to different positions for the duration of the eclipse.
In 1911 Einstein calculated that the deflection of the starlight by the Sun would be less than 1 second of arc—in fact, he calculated it to be 0.83 seconds, a tiny deflection but measurable with the techniques of the day. Unfortunately, World War I (1914–18) prevented Einstein from proving his theory at the next suitable eclipse. Einstein did not let this misfortune halt his research, however, and he sought other evidence to support his theories. He knew that in the 19th century the astronomer Urbain-Jean-Joseph Le Verrier (1811–77), who predicted the position of the planet Neptune, had also made some careful measurements of the planet Mercury. Le Verrier was able to show that the orbit of Mercury was not a simple ellipse, but one that precessed very slowly around the Sun. It was a triumph of astronomy to measure the amount of the precession—only 38 seconds of arc in a whole century. Le Verrier tried to explain the precession in terms of an unknown planet closer to the Sun than Mercury. An amateur astronomer claimed to have discovered the planet, and it was given the name Vulcan—but in fact Vulcan did not exist and the precession still remained unexplained.
Einstein knew of Le Verrier’s result and he realized that the precession could be explained by the theory of relativity—the relative bending of space under the influence of gravity. He calculated a figure of 43 seconds of arc per century for the advance of the perihelion. It was a brilliant result and was highly acclaimed, but it raised a few eyebrows. After all, the precession of Mercury had been discovered long ago by Le Verrier, and Einstein therefore already knew the answer to the question of how relativity affected the orbit of Mercury. Had he predicted the precession before the findings of Le Verrier had been known, then that would have been far more impressive. Einstein knew that the next suitable eclipse of the Sun would prove his assertions.
However, he had to wait a few more years during which time he revised his calculations. He decided his figures were out by a factor of two, and his new prediction for the bending of light by the Sun was 1.7 seconds of arc. When World War I was over, two British expeditions were sent to observe an eclipse of the Sun. One went to Sobral in Brazil and the other headed for the tiny Portuguese island of Principe off the west coast of Africa. On May 23, 1919 the sky darkened at those two places just as the astronomers had predicted. In Principe the rain came just as the eclipse was due, but observer Arthur Eddington (1882–1944) was still able to take his photographs, and he measured the star positions with enormous excitement. In Brazil the plates were also measured with great care. Both parties found that the light from the stars near the rim of the Sun were deflected by exactly the amount that Einstein had predicted earlier.
It was a great moment and the astronomers could not contain their joy. Einstein’s principle of equivalence had been vindicated. It opened up a new and far-reaching theory of relativity that became known as general relativity. Einstein was able to show that space consisted of a curved space–time medium in which the stars and the planets bent the fabric of space–time with their gravitational fields. It was as a result of general relativity that Einstein discovered a relationship between matter and energy, expressed in terms of the simple equation E = mc2.
At the time Einstein developed general relativity, the universe was thought to be static, with only random motions of other galaxies toward and away from our galaxy. His work, however, predicted the inexorable contraction of the universe due to all the galaxies exerting a gravitational pull. Einstein thus found it necessary to modify his field equations by the inclusion of a “cosmological constant” that represented an outward pressure associated with otherwise empty space. Such a force could act as a counterbalance against gravity to produce a stable, eternally unchanging universe.
Not all scientists were convinced of the need for the cosmological constant in Einstein’s equations. Alexander Friedmann (1888–1925), a Russian meteorologist and mathematician, dispensed with the cosmological constant to investigate solutions where the universe could change and expand with time. He proposed the radical idea of an evolving universe in an article in 1922, but the result was largely ignored by the establishment, and Friedmann received no recognition of his idea. Subsequently Georges Lemaitre (1894–1966), a Belgian astronomer and priest, independently experimented with the value of the cosmological constant to conclude that the universe was expanding, just two years before Edwin Hubble (1889–1953) published his correlation between the distance and velocity of galaxies. Lemaitre also extended the idea of a changing universe, by following the model through time in reverse to infer a single point of creation. This is the first suggestion of an initial “Big Bang” moment.
The work of both Friedmann and Lemaitre was strongly criticized as irrelevant by Einstein at the time, although he quickly recanted after Hubble’s discovery and thereafter publicly supported Lemaitre’s interpretation. Einstein also then discarded the cosmological constant, allegedly dismissing it as “the biggest blunder” of his life.
A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is “bent” around a massive object (such as a cluster of galaxies or black hole) between the source object and the observer. The process is known as gravitational lensing, and is one of the predictions of Albert Einstein’s general theory of relativity.
Initially Einstein only considered a form of gravitational lensing by single stars (as confirmed by Sir Arthur Eddington in 1919 during a solar eclipse). Although the Russian physicist Orest Chwolson (1852–1934) is credited as being the first to discuss gravitational lensing in print (in 1924), the effect is more usually associated with Einstein, who published a more famous article on the subject in 1936. In 1937 the American-based Swiss astronomer Fritz Zwicky (1898–1974) first considered the case where a galaxy could act as a lens, something that according to his calculations should be well within the reach of observations. However, it was not until 1979 that the first gravitational lens would be discovered. It became known as the “Twin Quasar” since it initially looked like two identical quasars; it is officially named Q0957+561.
Gravitational lenses act equally on all kinds of electromagnetic radiation, not just visible light. The phenomenon often creates streaks and arcs out of the lensed object. If the source, massive lensing object and observer lie in a straight line, the source will appear as a ring, which is often referred to as an Einstein Ring. If the “lens” is very symmetrical, then the source appears as four images symmetrically arranged around the lensing object. This is known as an Einstein Cross, or Huchra Lens, after the American astrophysicist John Huchra (born 1948), who first discovered it in 1985.
Einstein had spent a great deal of time on his theory but, like Isaac Newton before him, he found that he lacked the mathematical tools to formulate his ideas and develop them. He was able to turn to his staunch friend Marcel Grossmann (1878–1936) in his time of need. Grossmann knew about tensor calculus (used to deal with the mathematics of four dimensions) and the curvature of space. Using the shorthand notation of tensor calculus a great deal could be expressed with relatively few equations, but the field equations were formidable and solutions could only be found in a few special cases. As more and more mathematicians became involved other solutions to the equations were found. But Einstein was never satisfied with his efforts. James Clerk Maxwell had shown that the electric and magnetic fields were one and the same. Einstein wanted to unify the electromagnetic field and the gravitational field into a single, unified field theory.
In the 1920s there came many new and exciting advances in physics. Einstein attended all the important conferences in these years and he was well aware of the developments in quantum mechanics. He could not believe that the physics of the atom depended upon probabilities and uncertainties. His famous saying was that “God did not play dice with the universe.” But in this respect Einstein’s instinct was proved wrong and because of this he became isolated from many of the scientists of his time.
In the 1930s, as Hitler and the Nazis rose to power, Einstein was able to foresee all too clearly the future development of Germany. He was the world’s most famous scientist by this time and he had plenty of contacts in England, in other European countries and in the United States of America. He had no problem in finding a sinecure well away from Germany, and in 1933 he left for America accompanied by his second wife Elsa, his secretary and his collaborator Professor Walter Mayer (1887–1948). He had a choice of practically any university in America in which to work, but he chose Princeton, New Jersey, as the place to spend his later years. Here, safe from persecution, Einstein was able to speak out freely against the Nazis.
During World War II (1939–45), the possibility of building atomic weapons came to the fore. There were two major questions to be answered. First, was it possible to build a weapon that would convert matter into energy as predicted by Einstein’s famous equation E = mc2? In other words, was it possible to build an atomic bomb? Second, if it was possible to make such a bomb, then could the Nazis already be working on a similar project? The answers to both of the questions were not particularly surprising, and nor was the outcome. The US government wrote to Einstein asking him if it was indeed possible to manufacture an atomic bomb. He had no option but to reply in the affirmative, and the US government immediately set up a program to build one. The end result was the terrible destruction of Hiroshima and Nagasaki by atomic bombs. At the end of the war it was discovered that the Nazis had no plans for making an atomic bomb, and it left Albert Einstein feeling for the rest of his life that he had been somehow responsible for the development of nuclear weapons.
In the 1920s the sciences of atomic physics and nuclear physics were both destined to play a major part in the advancement of astronomy. Einstein had redefined the laws of mechanics and astronomy. Newton’s laws still held good for most terrestrial observations, however, and it was only when velocities comparable to the speed of light were involved that Einstein’s theories took precedence. Einstein displayed the portraits of three of his most admired predecessors on the wall of his study. All three were British: they were Isaac Newton, Michael Faraday and James Clerk Maxwell. He felt some guilt at having destroyed the models of the universe so painstakingly put together by his predecessors. In later life, when he was working on his autobiographical notes, he found himself making a list of the difficulties he had discovered in the Newtonian system. He suddenly stopped himself in his tracks and he addressed Newton directly and movingly:
Enough of this. Newton forgive me. You found the only way that, in your day, was at all possible for a man of the highest powers of intellect and creativity. The concepts that you created still dominate the way we think in physics, although we now know that they must be replaced by others farther removed from the sphere of immediate experience if we want to try for more profound understanding of the way things are interrelated …
The last 20 years of Einstein’s life were pleasant and peaceful. He still worked on his field equations and in 1950 he produced another set of solutions. They were never published, and validators soon discovered errors in his work. Despite this, he is remembered as the greatest scientist of the 20th century, even though he was never able to reach his final objective, to unify the forces of nature.