The Story of Astronomy (20 page)

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
=
mc
²
.

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
=
mc
²
? 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.

By the time Albert Einstein's ground-breaking theory of relativity was published in 1905, astronomy had made considerable progress in several other important areas. The development of new techniques and more powerful telescopes was beginning to enable astronomers to measure the distances of far-away stars and galaxies more accurately, indicating that the universe was much larger than anyone had previously imagined, and was expanding and evolving.

America's contribution to astronomy was minimal until the period after their civil war. The stability that followed the American Civil War (1861–5) allowed the US economy and the universities to expand rapidly, and more money became available for research. The contribution of the Americans then became very significant.

Henrietta Leavitt (1868–1921) was born in Lancaster, Massachusetts. From 1886 to 1888 she attended the Society for the Collegiate Instruction of Women, later known as Radcliffe College, where she graduated in 1892. In 1895 she became a volunteer assistant at Harvard Observatory and in 1902 she was given a permanent staff appointment. The observatory's great astronomical project, started by Edward Pickering (1846–1919), was to measure the brightness of all the known stars as accurately as possible. Leavitt was appointed head of the photographic stellar photometry department, and with rapidly improving photographic techniques the observatory was soon measuring stellar magnitudes to a greater accuracy than anyone had achieved before.

A new phase of the work began at Harvard in 1907 when Edward Pickering set up an ambitious plan to ascertain the stellar magnitudes photographically. Unlike earlier measurements by eye, which could be subjective, the photographic plates gave a truer reading of the colors of the stars. Henrietta Leavitt began by studying a sequence of 46 stars in the vicinity of the North Celestial Pole. With the new methods of analysis now available she determined all the magnitudes, and then followed up her work with a much larger sample in the same region of the sky. She extended the scale of standard brightness from the
3rd magnitude right down to the 21st magnitude, with her work being published progressively between 1912 and 1917. By the time of her death, Henrietta Leavitt had determined the magnitudes for stars in 108 areas of the sky. She had also discovered four new stars and 2400 variable stars. In fact, during the 1920s, she had discovered more than half of all the known variable stars in the sky.