The Story of Astronomy (19 page)

The American scientist Albert Michelson (1852–1931) devised an ingenious experiment to detect the motion of the Earth through the ether. A monochromatic light source was split into two beams. The beams were positioned at right angles to each other and, using mirrors, they were both reflected and brought together again to focus on an eyepiece where they produced a set of interference fringes. (The physics of interference fringes was well known and they were easy to detect.) The whole apparatus was mounted on a table that could be rotated so that the experiment could be performed at any inclination to the ether “wind.” The theory was that one beam of light traveled in the direction of the ether wind and the other traveled transversely. As the apparatus rotated through a right angle it was predicted that the interference fringes should shift to the left or right by a fraction of a fringe.

The first experiment seemed to show a slight movement of the fringes, but after allowing for the errors of the observation the movement did not seem to be
significant. Michelson, with his colleague Edward Morley (1838–1923), built a more delicate apparatus and repeated the experiment. Although the expected movement was only a fraction of a fringe, the apparatus was sensitive enough to detect a movement of one hundredth of a fringe. The experiment seemed infallible; it must be capable of detecting the motion of the Earth through the ether. But the experiment was repeated again and again and there was still no significant movement of the fringes. The scientists were puzzled. They could come to only one conclusion—either that the Earth was always traveling with the ether or perhaps it carried the ether with it as it orbited the Sun. This was a very unsatisfactory explanation, and as the scientific community argued about the findings some came forward with other suggestions. Eventually, the conclusion was reached that there was no ether, and so there must be another way for light to travel through space.

The Michelson–Morley experiment seemed to fail. It was unable to detect any sign of the ether. There was no medium to carry the waves of light. This problem was solved a few years later when the photon was discovered, showing that light was a stream of particles and not a wave motion. But physicists had to explain why the two light paths were equal in length when there should have been a very small measurable difference. Then the Irish
physicist George FitzGerald (1851–1901) and the Dutch scientist Hendrick Lorentz (1853–1928) both independently suggested the same solution: the length of one arm of the apparatus had shrunk in the direction of motion. It defied logic, but they asserted that a body in motion, for example, a train traveling on a straight track, was seen by a stationary observer as shorter at speed than at rest. At normal speeds the contraction was only a few atoms, but at half light-speed the train would shrink to about 87 percent of its stationary length. It all pointed to something very strange in the fabric of space/time and provided evidence for Einstein's special theory of relativity.

Relativity Redefines Astronomy

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.