While chemists are testing out the deadliest types of poison gas for future wars . . . it is well that there should also be some notable attempts made to conquer the hearts of men by kindness and to demonstrate that one person who heads an expedition to heal the wounds and desolation of war is stronger than a battalion of men under arms.
This was the Quaker war, only able to be waged once peace had come.
This emphasis on intellectual connections meant the German educational system was a particular target for the Quaker relief workers. Ludlam’s group undertook to feed thousands of students in Berlin. Connections with Berlin teachers were particularly important for the relief work at schools. To this end, Eddington suggested that his friend contact one particular faculty member for assistance: the very busy Albert Einstein, who was reported as being “closely connected” to the efforts.
CHAPTER 11
The Test
“The most favourable day of the year for weighing light is May 29.”
USUALLY, WHEN SCIENTISTS test a theory, they get everything nicely under control. Laboratories are carefully designed, protected, and isolated. The whole point of an experiment is to create a space where there is no interference, no confusion—a locale where nothing unexpected can interrupt. These delicate places are what allow, for example, the predictions of quantum electrodynamics to be tested to a precision of 1 part in 10 billion. Accuracy and reliability of that sort can only be achieved when you can have everything just how you want it.
Eddington did not have that luxury. He was going to test Einstein’s theory at a solar eclipse thousands of miles from the nearest precision laboratory. This was not easy. The young Eddington once described the difficulty of eclipse expeditions: “In journeying to observe a total eclipse of the Sun, the astronomer quits the usually staid course of his work and indulges in a heavy gamble with fortune.” Weather and war made true control impossible. Instead, he had to perform a test that was persuasive in spite of the chaotic conditions. He had to make an expedition as scientific as an experiment. He was hardly the first scientist to have this problem, and he had a whole arsenal of strategies to draw on.
Instruments and devices were not the only things that needed to be handled carefully. As much as he needed to create a favorable physical environment for his tools, he needed a favorable social and political environment in which his results could be received. Experiments and observations do not speak for themselves. People must be willing to be persuaded; they need to understand how the data connect to the theory; they must be prepared to accept that the results mean what you say they mean. Scientists must create spaces—sometimes tangible, sometimes intangible—in which science can be done. Eddington had to do both.
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EINSTEIN’S SPACES AT the beginning of 1919 were, at best, unstable. Berlin, his scientific space, was increasingly messy. His lectures on relativity were postponed because the university lacked coal to heat the lecture hall. He was frustrated both by the fecklessness of the new government (he called them “excessively dishonest”) and the viciousness of the Allies in victory (compared to the kaiser, they were “only slightly the lesser evil”). Luckily, he was absent from the city during the bloodiest week of the revolution, when the Spartacists (the group that helped Nicolai escape in a biplane) tried to seize power.
He was away from Berlin because he was trying to get his personal spaces into better order. On February 14—Valentine’s Day—a Zurich court ended his marriage to Mileva on the grounds of adultery (he admitted his affair with Elsa) and “character incompatibility.” Custody of the boys was given to their mother, and Einstein was ordered to pay 8,000 francs annually, drawn from the hoped-for Nobel Prize money. Finally, he was ordered not to remarry for another two years.
Nominally he was in Switzerland to deliver some lectures and earn some money in a currency other than the plummeting German mark. He was stunned by the strangeness of being around “well-fed citizens who have nothing to fear.” Back in Berlin for his fortieth birthday, he still couldn’t get much science done. He gave a lecture on relativity whose admission fee went to support the Socialist Students’ Union. Unimpressed with the state of Germany, he returned to Zurich right away to deliver more lectures. There wasn’t much interest—only fifteen students registered to hear Einstein speak about relativity—and the university canceled the event.
In Berlin it would have been hard to know that the war was over (ironically, the clearest sign was large numbers of armed men in the streets). Food and fuel were still short. All this was because technically speaking the war was only on pause. There had been an armistice but there was no peace. The latter would come only after a formal treaty was signed among the warring countries. Almost nothing to this end had been done in the months since the armistice.
The peacemakers began to gather in Paris in January 1919. Delegates on the Champs-Élysées strolled past captured German cannon. The conference officially opened January 18, which (surely by coincidence) happened to be the anniversary of Wilhelm I’s coronation in 1871. The negotiations involved setting up the League of Nations as well as dividing Africa and the Middle East into new colonial possessions. The victorious empires gobbled up ever more of the world.
* * *
THOSE NEW IMPERIAL boundaries were of huge importance to astronomers, particularly ones who, say, were planning solar-eclipse expeditions for May 1919. This was not unusual. There is a long history connecting imperialism to eclipse expeditions—remember the infrastructure that made Eddington’s overseas work possible back when he was chief assistant at the Royal Observatory. Traditionally governments put forward funds and resources for eclipse expeditions because those projects brought tremendous national prestige—who but a great power could perform science at any spot in the world?
Eddington’s plans for his expedition were quite different. Rather than affirm national pride, he wanted this journey to shatter patriotism and celebrate what could be done across borders. He wanted to use the tools of empire to fight for internationalism; Dyson’s support meant this was just possible. If everything went well.
Preparations for a solar-eclipse expedition were elaborate. The first step was simply to figure out where and when the eclipse would be visible. The zone of totality—the place from which the moon completely blocks the sun—is typically some miles wide, but the eclipse is visible only for minutes (if one is lucky). The shadow of the moon hurtles across the surface of the Earth at more than a thousand miles per hour, and astronomers need to be in the right place at the right time with their telescopes and cameras.
The path of totality was an arc across the Southern Hemisphere from Africa to South America.
The path of the May 1919 eclipse, as illustrated in Andrew Crommelin’s February 1919 article in Nature
COURTESY OF THE AUTHOR
Many factors entered into the choice of where to make the observations: Did the location have a reputation for good weather? How humid was it? How low in the sky would the eclipse be? Were there nearby steamship and railway networks to carry the astronomers and their heavy equipment? Did the schedules of those networks intersect with the eclipse dates? Was there a telegraph station nearby? Could food and water be procured at reasonable prices? Was there a friendly local government or colonial administration to help support the expedition? Answering these questions often relied on travelers’ reports or expats of dubious trustworthiness.
Dyson and Eddington decided that there were two locales—each would have about five minutes of totality—that best answered all these questions, one on each side of the Atlantic. Sobral, eighty miles inland in Brazil, was on the rail lines. It was not quite in the center of the path, so totality would be a few seconds shorter. But the logistical advantages more than made up for that. Word was that the rainy season would be over by May (no one was quite sure). Much of the information was provided by a circular sent by Dr. Henrique Morize, the head of th
e Rio de Janeiro Observatory.
The other observation site was Principe, an island 110 miles off the west coast of Africa just north of the equator. It was a Portuguese imperial possession known for its cocoa exports. The chocolate industry meant both that it was served by a fortnightly steamer from Lisbon and that there was likely European-style infrastructure there. Its isolation in the ocean was a positive feature—being surrounded by water meant more stable temperatures throughout the day and easy sightlines to the horizon.
Dyson had been given £1,000 for travel costs in 1918 (about $75,000 today). During wartime, that was an enormous grant—£1,000 could buy a lot of bullets. He decided he could stretch that money to cover expeditions to both sites. This was important insurance against bad weather or other mishap, and dramatically increased the expeditions’ chances of success. Eddington would go to Principe, accompanied by Edwin T. Cottingham, a clockmaker who had worked for years with both Dyson and Eddington maintaining the timepieces at their observatories. The observations in Brazil would be conducted by Andrew Crommelin, an assistant at the Royal Observatory, and Fr. Aloysius Cortie, a Jesuit astronomer, of the Stonyhurst College Observatory in Lancashire. Cortie was known both for his cheerful nature and for regularly delivering science-themed sermons at meetings of the BA. At the last moment, however, he could not participate and was replaced by Charles Davidson. Davidson had been overseeing the preparation of the eclipse equipment and had a reputation as an absolute wizard with mechanical devices and scientific instruments. Dyson trusted him implicitly to make any mechanism work properly.
The equipment that Davidson had been preparing included three carefully chosen telescopes. Eddington needed crisp images of stars, which is usually not something eclipse observers are looking for. So the teams decided to use astrographic telescopes—specially designed to capture precise, faint images. Dyson tried to secure two telescopes of this sort that had been used at previous eclipses where they accidentally captured good star images. One currently mounted in Greenwich was easily acquired. The other was at the Oxford observatory overseen by H. H. Turner, the most vocal anti-German astronomer in the country. We do not know how Dyson persuaded Turner to contribute this valuable instrument to Einstein-centric expeditions, but somehow he succeeded. Father Cortie also suggested that they take a smaller four-inch telescope to Brazil as a backup. It had captured good fields of stars at other eclipses, and would not add much logistical stress.
No one would be looking through these telescopes—a camera would be used. The telescope would focus the image of the stars on a photographic plate. The plate would be exposed briefly (five to ten seconds), and then the astronomers would swap it for a fresh one without disturbing the delicate equipment. There was a complicating factor to this procedure, thanks to Copernicus. Because the Earth rotates, the eclipsed sun and the stars appear to move across the sky. Even over the course of just a few seconds this apparent motion will blur the images on the photograph. One solution to this problem is to mount the telescope on a pivot and slowly turn it to match the Earth’s movement. This is not a very good solution for an expedition, though—telescopes are heavy and large, very difficult to move smoothly without shaking or bending that would ruin the image.
The traditional answer was to use a coelostat, the same sort of clockwork mirror Eddington had used in his 1912 expedition. The telescope would be laid horizontally, nicely stable. The lens of the telescope would be pointed at the coelostat mirror, which would then be adjusted so the image of the sun would fall in the middle of the camera. Then the mirror could be smoothly turned during the eclipse to keep the image centered without blurring.
Greenwich had a set of these coelostats that had been used for many previous expeditions. Unfortunately they had been used for many previous expeditions and were old and unreliable. Normally, overhauling them would be a straightforward, if tedious, process, but the early preparations for the expeditions were happening during wartime, and a “priority certificate” from the Ministry of Munitions was required to get any precision work done. That was impossible while the war was on. Once the armistice arrived, Cottingham did the best he could to get the coelostats running smoothly. Mountains of fine, delicate work had to be compressed if the expeditions were to leave on time in the early spring.
Science is more than good equipment. Eddington and Dyson needed to make sure other scientists were ready to think about—to understand—the results captured by that equipment. The expeditions were not passive attempts to just look for something interesting during the eclipse. Their goal was to test a specific prediction of Einstein’s theory of relativity. And Eddington had his own pacifist goals for the expeditions. Both the scientific and the political goals required groundwork to be laid ahead of time.
The first part was understanding Einstein’s prediction. Einstein said, let’s look at a star that appeared to be just at the edge of the sun’s disk (the star was actually trillions of miles away, it just happened to line up with the edge). The image of that star is being carried to us by a ray of light. As that light passes by the sun, the curvature of space-time there (in other words, the gravity) will bend that ray of light. To an observer on Earth looking at the star’s image, the bending means the image will be shifted slightly from its original location. General relativity predicted the exact angle between where the star should be when the sun’s gravity was not in the way and where it appeared to be when the sun’s gravity was. That angle was measured in arc-seconds (one-sixtieth of one-sixtieth of a degree). Einstein said the change should be 1.75 arc-seconds. On the photographic plates Eddington would be using, that would translate to about one-sixtieth of a millimeter. Some scientists at the time objected that this was a very small size to measure precisely. To astronomers, though, this was no great challenge—they measured effects that small every day. Eddington reassured everyone that “this in itself calls for no extravagant precautions of accuracy.”
Astronomers were able to make these precise measurements because they took everything into account. The photographs taken during the eclipse needed to be compared to check plates—photographs taken of the same field of stars when the eclipsed sun is not in front of them. It is the change of position of the star that matters—they had to have an exact reference for that change. It can take months for the sun to move far enough across the sky that the images would be undistorted by its gravity. That means the check plates needed to be taken either months before or after the eclipse itself. Further, they had to be taken with exactly the same lens and photographic setup—every lens is a little bit different, and it was essential to make sure that an apparent change in the star’s location was not really due to an imperfection introduced by a different lens. So photographs of the stars they would measure were taken from England with the lenses they planned to use in the field. Ideally additional check plates would be taken in the field to account for any atmospheric strangeness unique to that location. Calculation of the full results would take months of work back in Britain, but Eddington hoped that they could make preliminary measurements in the field. That required special tools, as well as research into how to develop the photographic plates in tropical conditions—every photographic manufacturer (and even individual production lines) required slightly different techniques. Hoping to get those preliminary results home as soon as possible, he and Dyson even arranged a special telegraphic code.
Before his departure, Eddington wrote an article presenting all this information to his colleagues so they would know how to interpret the results as they came back. Eddington declared that there were three possibilities: no deflection; 1.75 arc-seconds, the Einstein prediction; or 0.87 arc-seconds, sometimes referred to as the half-deflection.* The half-deflection probably came from Lodge’s alternative theory based on Newtonian gravity. Thus Eddington was able to present only three possibilities: null, Einstein, or Newton.
Modern scholars have pointed out that Eddington made a shrewd choice in framing the
possible results this way. This was a “false trichotomy”—there were certainly more than three prospects. There were other alternative theories of gravity (remember Nordström, Einstein’s early rival). There were other possible effects that could mimic gravitational light deflection, such as ether condensation or refraction in the solar atmosphere. Deciding to leave those possibilities out of the “official” predictions created a scenario known as a crucial experiment, where a single measurement could immediately decide between two rival theories. The null result not being very interesting, the test suddenly became a direct struggle between Einstein and Newton—a single moment in which this upstart German could dethrone the greatest thinker in history. There are always many possible explanations for any experimental result, and the crucial experiment (ironically, Newton’s favorite setup) narrows the options. This is helpful epistemologically and practically—one can never account for every possibility—but Eddington was probably more interested in the narrative value. It created a thrilling background against which to present the expeditions’ results. One could hardly hope for a better title card—two geniuses enter the ring, one leaves—to gain attention in the world of science.
During preparations, Dyson found himself explaining these possibilities to Cottingham, the clockmaker going to Africa with Eddington. He was a hands-on worker with little interest in the mathematics of relativity; he just needed to know the measurement they were looking for. As Eddington described Dyson’s lesson, Cottingham “gathered the main idea that the bigger the result, the more exciting it would be.” Cottingham asked, “What will it mean if we get double the deflection?” (3.5, twice the Einstein prediction). “Then,” said Dyson, “Eddington will go mad, and you will have to come home alone.”
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