American Scientist 93 (2005): 104.
J. Donald Fernie
This year we celebrate the centenary of Albert Einstein's special theory of relativity. Indeed, 1905 was the year in which Einstein first gave notice of his astonishing abilities. He was but 26 and had just earned his doctorate, but that year he published four papers on separate topics, each of which marked a major advance in physics. The first of these, on the photo-electric effect, would bring him the Nobel Prize, but it was the third, on special relativity, that made him both famous and controversial. A decade after this flurry of papers, in 1915, he unveiled the theory of general relativity, shaking again the foundations of science.
So different was relativity from the prevailing beliefs that most physicists demanded proof that it could explain phenomena that Isaac Newton's canon could not. Satisfying such demands was difficult, because the difference between the two models could only be apparent under extreme conditions. There seemed little hope that any terrestrial experiment could decide between them, but Einstein later identified three astronomical tests. The first was the proper calculation of the orbit of the planet Mercury—a feat that was beyond Newtonian physics (see "In Pursuit of Vulcan" in the September-October 1994 American Scientist). The second test required the comparison of light emitted from atoms in the Sun with light from similar atoms on Earth—relativity predicted that the Sun's light would have a longer wavelength (an example of the so-called redshift). The third test posited that if relativity was true, then rays of starlight that passed near the Sun would be bent compared to the same rays when the Sun was elsewhere in the sky. In each case, the relativistic effects are caused by gravity from the Sun's huge mass.
Early attempts to perform these tests did not silence Einstein's critics, because some observations supported his theory and others did not. Thus, the general theory of relativity yielded a much better solution to the Mercury problem than did Newtonian models, but another prediction of relativity, the redshift of the solar spectrum, could not be verified. (Eventually, astrophysicists learned that several other factors complicated the observation of this phenomenon.) So with one result in favor and another in doubt, the third test became something of a deciding vote for or against relativity.
Einstein first suggested how this light-bending effect could be measured in 1911. He predicted that those rays of starlight that passed closest to the Sun would be deflected by 0.85 arcseconds (0.00023 degree) because of the Sun's gravitational field. However, stars that appear next to the Sun are only visible during a total solar eclipse. To test Einstein's hypothesis, one would have to take photographs during an eclipse that showed background stars near the Sun's disk and compare them with photos taken months earlier or later, when the same stars rose in the night sky. Did stars appearing on opposite sides of the Sun's disk maintain the same spacing when the Sun was gone, or not?
This prediction seemed easy to check. Many pictures of solar eclipses already existed, as did photos of the night sky. Even so, skepticism about Einstein's theory was so prevalent that few astronomers rushed to their archives. And when they did examine previous photographs of solar eclipses, they found that the pictures were unsuited to proving or disproving Einstein's claim: The telescopes had been set to track the Sun's motion across the sky, not the stellar motions, and the slight differences between these perspectives obscured the small, predicted shifts in star positions. However, as time went by and other experiments gave equivocal results, the solar-eclipse experiment represented the best chance to test the truth of relativity.
Hoping for a Dark Noon
As early as 1912 it seemed possible to capture the necessary photographs with little fuss. In October of that year, a total solar eclipse was to run across the northern parts of South America, and the astronomical observatory of Córdoba in central Argentina was near enough to mount an expedition. Unhappily, almost all of South America was under clouds that day.
Another suitable eclipse loomed in August 1914, running northwest to southeast across eastern Europe. Erwin Freundlich, a young German astronomer, was determined to test Einstein's theory but encountered grave difficulty raising money for the trip. The scientific establishment in Germany was uninterested in paying for it, leading Einstein himself to offer his own none-too-abundant finances. With so few options, Freundlich appealed to other countries for collaborators that would help fund the expedition. He had only one taker: William Wallace Campbell and a team from the Lick Observatory in California. Later, the Berlin Academy provided additional support.
The eclipse was due August 21, but the team of Germans and Americans established a camp near Kiev well before that date to prepare for the event. Unfortunately, history intervened: On August 1, 1914, Germany declared war on Russia, and the German astronomers were taken prisoner. Russian forces expelled the older scientists and held the younger ones as prisoners of war. The Russians did allow the Americans to stay for the eclipse, but again the sky was totally clouded out. Campbell later wrote "I never knew before how keenly an eclipse astronomer feels his disappointment through clouds. One wishes that he could come home by the back door and see nobody."
The next year, at the height of the First World War, Einstein published his general theory of relativity. This timing greatly complicated the theory's dissemination because German scientific journals were then unavailable to the English-speaking world. It was an astronomer from neutral Holland who brought word of the new theory to Britain. Moreover, Britain was going through a period of almost hysterical opposition to all things German. Ardently opposed to this mindless, pervasive hatred, a young British astrophysicist named Arthur Stanley Eddington stood almost alone. Eddington was not only a rising star in astronomy but a Quaker—a religious pacifist. As such, he refused to fight in the war, although he was willing to risk his life providing aid to civilians caught in the violence. Because of his beliefs, Eddington lived on the verge of imprisonment during much of the war and suffered vicious attacks for his pacifism and efforts to counter his peers' nationalistic hostility toward German science.
Eddington learned of Einstein's general theory from the Dutch astronomer Willem de Sitter and was immediately taken with it. He was almost certainly the first (and, for a while, the only) English-speaker to understand the theory and appreciate its significance. Eddington grasped the fact that Einstein's new work meant that the eclipse experiment was an even more significant test of relativity—the general theory predicted twice as much deflection of light rays passing the Sun as did the special theory. Another suitable eclipse would occur in 1919, and although in 1915 there was no immediate hope for peace, the British Astronomer Royal, Frank Dyson, began to lay plans (no doubt at Eddington's prompting) for an expedition to photograph the event. Eddington, of course, was eager to lead such an expedition but worried that his uncertain standing with the authorities might cause difficulties for the project. Then, in a stroke of genius, Dyson wrote a carefully worded letter to officialdom. In response, the government notified Eddington that he was lucky so far in having avoided prison, and that his only hope of remaining that way was to lead Dyson's expedition, whether Eddington liked it or not! Eddington dutifully bowed to the hoped-for ultimatum.
Around the same time, an eclipse in the United States in June 1918 was almost entirely obscured by clouds, but Campbell's team did get some photographs. These poorly exposed plates seemed to indicate no relativistic effects, much to the delight of Einstein's skeptics, including Campbell.
The eclipse of May 29, 1919, was to start near the border between Chile and Peru, then traverse South America, cross the Atlantic Ocean and arc down through central Africa. No part of the path was far from the equator, and the desirable, longest-lasting portion was in the Atlantic, a few hundred miles from the coast of Liberia. The British planners decided that the tiny island of Principe, nestled in the crook of Africa's Gulf of Guinea, would be best despite the poor astronomical viewing from low-lying tropical regions. The choice of Principe introduced other challenges. One modern travel agency advises prospective visitors to the island that "It's best to go between June and September. The rest of the year is muggy and hot—you'll be swimming in rain and your own sweat." Just in case Principe was cloudy at the crucial time, the British sent a second expedition to observe the eclipse from Sobral, in eastern Brazil.
The main instruments at both sites were existing astrographic telescopes of 33-centimeter aperture designed specifically for photographing star positions with high precision. Although these telescopes were designed to automatically follow the stars, their temporary emplacement in the field required each telescope to be immobilized as a clockwork-driven flat mirror tracked across the sky and fed light to the main lens. As an afterthought, the Brazil contingent added a small 10-centimeter telescope to its roster. In the end, it saved the day.
The expeditionaries set out months ahead of the eclipse to allow for travel difficulties. Although the war officially ended in November 1918, chaos continued for months thereafter. Upon arrival, they had to evaluate the terrain, choose a site, and set up and test their equipment. Eddington's group arrived at Principe in late April and, amid the heat and rain, found themselves under such constant attack by biting insects that they needed to work under mosquito netting most of the time. The rain grew worse as May advanced, and the day of the eclipse began with a tremendous storm. The rain stopped as the day wore on, but the totality phase of the eclipse would start at 2:15 p.m. and last only five minutes. Eddington wrote:About 1.30 when the partial phase was well advanced, we began to get glimpses of the Sun, at 1.55 we could see the crescent (through the cloud) almost continuously and large patches of clear sky appearing. We had to carry out our programme of photographs in faith. I did not see the eclipse, being too busy changing plates, except for one glance to make sure it had begun.... We took 16 photographs ... but the cloud has interfered very much with the star images.
The weather in Brazil was much better—beautifully clear, in fact. The observers took 19 photos with the astrograph and eight with the small telescope. But when the photographs were developed, they found that despite their precautions, the astrograph's pictures showed, according to Dyson, "a serious change of focus, so that, while the stars were shown, the definition was spoiled." Even under ideal conditions, the predicted relativistic displacement on the photographs was only 1/60 of a millimeter—about a quarter of the diameter of a star on a sharply exposed image. Although they could measure such a minute shift, the poor focus made this task nearly impossible. By contrast, the small telescope's photographs were clear and sharp, but on a reduced scale.
The original caption for the graphical explanation of the experiment read as follows: The results obtained by the British expeditions to observe the total eclipse of the sun last May verified Professor Einstein's theory that light is subject to gravitation. Writing in our issue of November 15 , Dr. A.C. Crommelin, one of the British observers, said: "The eclipse was specially favourable for the purpose, there being no fewer than twelve fairly bright stars near the limb of the sun. The process of observation consisted in taking photographs of these stars during totality, and comparing them with other plates of the same region taken when the sun was not in the neighbourhood. Then if the starlight is bent by the sun's attraction, the stars on the eclipse plates would seem to be pushed outward compared with those on the other plates…. The second Sobral camera and the one used at Principe agree in supporting Einstein's theory…. It is of profound philosophical interest. Straight lines in Einstein’s space cannot exist; they are parts of gigantic curves." From the Illustrated London News of November 22, 1919.
Weighing the Data
Many months later, back in England, Eddington pondered the inconsistent results. Einstein's theory predicted a displacement of 1.75 arcseconds, but none of the experiments was in perfect agreement with the theory. The usable photos from Principe showed an average difference of 1.61±0.30 arcseconds, the astrograph in Brazil indicated a deflection of about 0.93 arcseconds (depending on how one weighted the individual spoiled photos), and the little 10-centimeter telescope gave a result of 1.98±0.12 arcseconds. The smaller device, in addition to yielding the most precise data, afforded a wider field of view and supported Einstein's theory of how the displacement should vary with angular distance from the edge of the Sun. But the validation of relativity required exact measurements, particularly because physicists had realized that Newtonian theory alone could predict a stellar displacement that was half that of Einstein's, or about 0.83 arcseconds.
Eventually, Eddington, after much discussion with Dyson, suggested an overall measurement of 1.64 arcseconds, which he took to be in pretty good agreement with Einstein, but he also gave the separate results from each telescope so others might weight them as they saw fit. Moreover, Dyson offered to send exact contact copies of the original photographic glass plates to anyone who wished to make their own measurements, which should have gone far to refute the occasional allegation that Eddington had cooked the results.
Ironically, confirmation of Eddington's conclusion (and the theory of relativity) came from Campbell's team at an eclipse in Australia in 1922, for which they determined a stellar displacement of 1.72±0.11 arcseconds. Campbell had been open in his belief that Einstein was wrong, but when his experiment proved exactly the opposite, good scientist that he was, Campbell immediately admitted his error and never opposed relativity again.
I am indebted to Dr. Jeffrey Crelinsten for granting access to his unpublished work on this topic and for providing comments on an earlier version of this article.
- Clark, R. W. 1971. Einstein: The Life and Times. New York: World Publishing Company.
- Crelinsten, J. In press. Einstein's Jury: The Race to Test Relativity. Princeton, New Jersey: Princeton University Press.
- Stanley, M. 2003. An expedition to heal the wounds of war: The 1919 eclipse and Eddington as Quaker adventurer. Isis 94:57-89.
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