The race to test relativity
Albert Einstein published his theory of special relativity in 1905, but it wasn’t until 1919, after British astronomers announced that they had verified a prediction from his generalized theory that the sun’s gravity bends light, that he became a supersta
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Einstein wrestled with his own adversary — mathematics. His 1905 theory of relativity rested on the idea that the laws of physics are the same for everyone. Restricting himself to uniform motion in a straight line, Einstein stated that no experiment you perform could tell you whether you’re moving or stationary. That simple requirement led to a revolution in our conception of space and time. Observers moving past each other disagree about length and time measurements; about whether or not two events occur at the same moment; about mass. Absolute space — the container in which we live — no longer exists. Absolute time — which inexorably charts our existence — is a fiction.
In 1907, Einstein had the “ happiest thought of my life” — a falling person has the sensation that gravity has disappeared. Put a closed box around a person in free fall and let the box fall freely. Since all objects fall at the same rate, the person floats inside the box. It’s as if the box is not moving and gravity is turned off. Accelerated motion ( falling) is undetectable. Einstein made a generalization that gravity and acceleration are equivalent. In principle, you can’t tell the difference.
Einstein’s “ equivalence principle” revealed that light bends in agravitational field. If you accelerate upwards in a rocket ship and shine a light beam from one wall to the other, the light will hit the opposite wall slightly lower, because your spacecraft is moving upwards as the light crosses the ship. So, inside a stationary spacecraft in a gravitational field, the light from one wall will also hit the opposite wall slightly lower. Light bends in the direction of gravity.
In 1912, Einstein’s mathematician friend Marcel Grossmann introduced him to a powerful form of mathematics. Called differential geometry, its equations could describe the shape of a surface independently of any observer. Einstein and Grossmann developed equations relating space and time to matter and energy. Gravity turned out to be a geometrical entity — a curvature in space-time. The
Wequations elegantly described the physics.
Alone in Germany during the war, Einstein continued his work. By 1915 he had found equations for the gravitational field that were independent of the observer’s motion or position. He sat down to calculate the motion of Mercury’s orbit. Newton’s theory of gravity was off by 43 seconds of arc per century. Crunching the numbers was a painstaking task, taking days. When Einstein finished, his heart started to palpitate. The resulting motion was precisely the observed amount. He had outdone Newton!
Einstein published a comprehensive paper on his general theory of relativity and new theory of gravitation. The momentous news might have stayed inside war- torn Germany, except that two of Einstein’s closest friends lived in neutral Holland. Hendrik Antoon Lorentz and Paul Ehrenfest received proofs of Einstein’s paper and shared it with their astronomy colleague, Willem de Sitter. Knowing that German periodicals were not reaching England, de Sitter sent Einstein’s paper to Arthur Stanley Eddington, secretary of the Royal Astronomical Society. Eddington was flabbergasted. “ Hitherto I had only heard vague rumours of Einstein’s new work,” he replied. “ I do not think anyone in England knows the details of his paper.”
Unable to publish a German paper, Eddington asked de Sitter to write a series of articles. In this way, the English- speaking astronomy community heard about Einstein’s breakthrough during the darkest years of the war. The exact accounting for Mercury’s orbital motion impressed astronomers. Einstein’s new theory also retained his earlier light-bending prediction, but the amount of bending is doubled. The slowing of clocks also remained, causing light emitted in the sun to be slightly redder than light emitted here on Earth.
At Mt. Wilson Observatory in Pasadena, Charles St. John tried to determine the cause of light reddening in the sun. Most astronomers believed that pressure in the solar atmosphere was six to seven times greater than on Earth. Lab experiments showed that pressure tends to shift light toward the red end of the spectrum; so astronomers generally believed that the observed solar redshifts were due to pressure. John Evershed in India argued that pressure in the sun is much lower and could not cause the redshifts. For some parts of the spectrum, he measured the amount predicted by relativity. For others, he got different amounts.
St. John used the superior equipment at Mt. Wilson to observe areas of the solar spectrum caused by cyanogen gas in the solar atmosphere. Lab experiments showed that the wavelengths of light from cyanogen are independent of the gas pressure, so St. John could eliminate any pressure effects. He published preliminary results in 1917. There was no appreciable redshift. Relativity could not be correct.
Eddington was horrified. He had studied Einstein’s new theory and decided that it was beautiful, elegant, powerful — and most likely correct. “St. John’s latest paper has been giving me sleepless nights,” he complained, “chasing mare’s nests to reconcile the relativity theory with the results, or vice versa. I cannot make any headway.” Many astronomers were delighted that the prestigious Mt. Wilson astronomers had found Einstein wrong. They felt his theory was too complicated. At Lick Observatory near San Jose, Calif., astronomer Heber Curtis admitted in a publication that Einstein’s theory was attractive for many reasons but added, “ Many will feel that the idea of a four- dimensional time- space is fully as difficult of comprehension as was the mystery of gravitation, all- pervading, inexplicable, in our classical physical theories.” Curtis would later become an outspoken critic of the theory.
In 1918, a total solar eclipse was visible from the United States. Lick Director William Wallace Campbell and Curtis went to nearby Goldendale, Wash., but they had left their good equipment in Russia when war broke out. Curtis cobbled together an Einstein program using borrowed equipment, but clouds threatened to repeat the failure in Russia. At the last minute, a break in the clouds allowed him to get photographs. Soon after the eclipse party returned, Curtis went off to Washington to do war work, and the plates lay unmeasured.
In Britain, Astronomer Royal Frank Dyson initiated plans to observe an eclipse in May 1919 — if the war ended in time. He wanted Eddington to lead one of two expeditions. The War Office tried to draft Eddington, but he objected on religious grounds. Dyson appealed to national pride, arguing that Eddington was the only man who could determine whether or not a German theory would supplant the great Sir Isaac Newton. The War Office gave Eddington a reprieve on the condition that he observe this important eclipse and settle the matter.
Campbell got wind of the British plans and the race was on. When the war ended, he urged Curtis to return immediately and start measuring his plates. Meanwhile, Eddington and colleagues set sail to observe the eclipse.
Campbell headed an American delegation to Europe to help establish the International Astronomy Union — the first healing of international relations since the war. Days before he left Washington, he received word from Curtis: “I am confident that there is no Einstein effect whatsoever. . . you can make it as strong as you like.”
Campbell announced Curtis’s results at a special meeting of the Royal Astronomical Society. He cautioned that probable errors were large, but he felt Einstein’s double value was ruled out, though the smaller value might exist. At the same meeting, Dyson read a cable from Eddington — he had poor results, but one plate seemed to indicate the full Einstein deflection.
Campbell suppressed publication of Curtis’s results until the plates could be remeasured. The British had three sets of data. Eddington threw out one, because heating of the mirror had blurred the star images. The average of the other two agreed with Einstein. A special meeting of the Royal Society and the Royal Astronomical Society announced the results in November 1919. Here was one of the most surprising turns of history. A combination of war weariness, fascination with the universe, and intrigue that British scientists had verified a German theory just after the war catapulted Einstein to world fame.
In London, the Times pronounced a “ Revolution in Science” and “ Newton Overthrown.” The New York Times declared “ Lights Askew in the Heavens” and “ Men of Science Agog.” Dyson admitted to Ellery Hale, the astronomer and founder of Mt. Wilson observatory, that he had initially been a skeptic. “ Now I am trying to understand the principle of relativity and am gradually getting to think I do.”
Hale confessed: “ The complications of the theory of relativity are altogether too much for my comprehension. If I were a good mathematician I might have some hope of forming a feeble conception of the principle, but as it is I fear it will always remain beyond my grasp.” Hale was not alone among American astronomers in having difficulty with Einstein’s new theory. Their inability to master the details of general relativity bolstered their resistance to it for another decade. Jeffrey Crelinsten is a science writer and historian based in Toronto. His book, “Einstein’s Jury: The Race to Test Relativity” (Princeton University Press), will be released in spring 2006. jcrelinsten@impactg.com