A hundred years ago, a team of British scientists proved the truth of Einstein's theory of relativity by tracing the deflection of starlight during a total solar eclipse in May 1919. The article describes in detail what difficulties the participants in the experiment had to overcome, how the experiment itself went, and what was the result of its success.
Usually, when scientists test a theory, they manage to keep the situation under control. However, in 1919, at the end of the First World War, the British astronomer and physicist Sir Arthur Stanley Eddington (Sir Arthur Stanley Eddington) could not boast of such a luxury. He was going to test Albert Einstein's theory of relativity with a solar eclipse, which could only be observed a few thousand miles from the nearest laboratory providing accurate measurements. It wasn't easy. "When traveling to observe a total solar eclipse, the astronomer interrupts the measured flow of his work and enters into a cruel game with fate," wrote the young Eddington. In his case, it was even more difficult to ensure full control over the situation - because of the treacherous weather and war.
Einstein's position was also extremely unstable. In Berlin, his familiar scientific space, more and more chaos reigned. His lectures on the theory of relativity had to be postponed due to the lack of coal to heat university classrooms. While temporarily lecturing in Zurich, Einstein showed no particular interest in his work there either; only 15 students signed up for his lecture on relativity - and the university canceled the event.
In Berlin, it was difficult to understand that the war was over, besides, true peace was possible only after the belligerent countries agreed to conclude a binding agreement. During the negotiations, the creation of the League of Nations, as well as the division of Africa and the Middle East into new colonial possessions were discussed. While scientists were conducting their research, the victorious empires seized more and more lands.
These new frontiers of empires were of immense importance to astronomers planning expeditions to observe the solar eclipse in May 1919. The first step for Eddington and his colleague, physicist and astronomer Royal Frank Watson Dyson, was simply to figure out where and when an eclipse could be seen. The Zone of Totality - the place from which the Moon can be seen completely obscuring the Sun - is usually several thousand miles wide, but an eclipse can only be seen for a few minutes (if you're lucky). The moon's shadow sweeps across the surface of the Earth at over a thousand miles an hour, and astronomers with their telescopes and cameras must be in the right place at the right time. The path of totality stretched across the Southern Hemisphere from Africa to South America. Many factors influenced the choice of the location for the observation:how favorable is the weather at this time of year? How low in the sky will the eclipse pass? Are there steamship and rail networks in the area for transporting astronomers and their heavy equipment? Is there a telegraph station nearby?
Ultimately, Dyson and Eddington decided that two locations on opposite sides of the Atlantic were best suited to these conditions - each scientist would have about five minutes of totality at their disposal. One of these locations - the Brazilian city of Sobral, 80 miles off the coast - had rail links. The city was not located exactly in the center of the totality zone, so the eclipse period lasted a few seconds less. However, this disadvantage was more than compensated for by the logistics advantages. It was believed that the rainy season was ending in this area by May, although no one could vouch for this.
Príncipe, an island 110 miles off the west coast of Africa north of the equator, was chosen as another location. The island was part of the imperial possessions of Portugal and was famous for the export of cocoa. The booming chocolate industry meant there was a biweekly steamer from Lisbon and that the island was likely to have European-style infrastructure. The remoteness of the island played into the hands of scientists, as the surrounding water masses provided more stable temperatures throughout the day and an easy view of the horizon.
In 1918, Dyson was allocated one thousand pounds (by today's standards, 75 thousand dollars) for travel expenses. Given the wartime, this was a very impressive grant - Dyson decided that with this money he could cover the costs of both expeditions, which was an important insurance against bad weather or other accidents and dramatically increased the chances of success.
It was agreed that Eddington would travel to Principe accompanied by Edwin T Cottingham, a watchmaker who had worked for many years at the Dyson and Eddington observatories, keeping the chronometers there. Meanwhile, observations at Sobral were led by Charles Davidson, who had a reputation for being an absolute wizard with mechanical devices and scientific instruments. Dyson could completely trust him with any mechanism.
The equipment that Davidson was preparing included three carefully selected telescopes. Eddington needed clear images of the stars, not what eclipse observers usually want. So the teams decided to use astrographic telescopes - specially designed to get accurate images of subtle objects. Dyson was trying to get hold of two telescopes of the kind used in previous eclipses. One of them, installed in Greenwich, was not difficult to get. The other was at the Oxford Observatory, which was directed by H. Turner, Germany's fiercest foe among domestic astronomers. We do not know how Dyson persuaded Turner to put this valuable tool at the disposal of the expedition, whose main task was to test Einstein's theory, but somehow he succeeded.
Even with the proper equipment, this kind of measurement in 1919 was extremely difficult to carry out. As the Earth rotates, the Sun is in an eclipse phase and the stars also move across the sky. Because of this, even if it is only a matter of seconds, photographic images are blurry. One solution to this problem is to mount the telescope on an axis and rotate it slowly in accordance with the movement of the Earth. However, this is not the most suitable option for an expedition: telescopes are heavy and cumbersome, and very difficult to move - inadvertently you can shake the lens or change the tilt and thereby spoil the final image. The traditional solution was a coelostat, a kind of "swing mirror" that Eddington had used in the past.
The telescope is placed horizontally and is stabilized. The telescope lens is directed at the coelostat mirror, which is adjusted so that the image of the Sun falls into the center of the camera. And then during an eclipse, the mirror can be smoothly rotated and thus maintain a clear image in the center.
In Greenwich, there was a whole set of such coelostats - they were already used more than once on expeditions. Unfortunately, these devices were in use for a very long time and could not be relied on. As a rule, the modernization of these devices was an unpretentious, but rather tedious process, but the first preparations for the expedition took place in wartime, and the appropriate permission from the Ministry of Military Supply was required to carry out precision processing. So, as a reserve, the researchers took with them several small four-inch telescopes - just in case.
The members of the expeditions were by no means passive observers who, during an eclipse, try to detect any curious phenomena. Their goal was to test the specific prediction of Einstein's theory of relativity. Einstein suggested looking at a star that appears to be at the very edge of the solar disk (in fact, this star may be trillions of miles away from the Sun - it's just that it is currently in line with the edge of the disk). The image of this star is transmitted by a beam of light. When a stream of light passes near the Sun, the curvature of space-time (created by solar gravity) will bend this light beam as well. Anyone who follows the image of a star from Earth will notice its slight displacement from its original position, which is a consequence of the bending. General relativity predicted the exact angle between the point where a star should be in the absence of solar gravity in its path, and where it would be under its influence. This angle was measured in arc seconds (one-60th of one -60th degree). According to Einstein, this change should be 1.75 arc seconds. On the photographic plates that Eddington was going to use, this figure was approximately one sixtieth of a millimeter.this figure was equal to about one sixtieth of a millimeter.this figure was equal to about one sixtieth of a millimeter.
Astronomers were able to make these accurate measurements because they tried to take all factors into account. The photographs taken during the eclipse were subject to comparison with photographs of the same field of stars, where the Sun was no longer in front of them during the eclipse phase. Scientists were primarily interested in changing the position of the star - for this they needed a reliable starting point. It could take months for the Sun to move far enough across the sky that images are not distorted by its gravity.
This means that the second series of photographs must be taken several months before or after the eclipse itself. In addition, the same lenses and photographic setup should be used when creating these images - all lenses are slightly different from each other, and it is imperative to make sure that the apparent change in star position is not due to inaccuracies in the other lens. Thus, the photographs of the stars that the researchers were going to measure were taken in England with the lenses they planned to use on the expedition.
Wanting to get the preliminary findings home as soon as possible, Eddington and Dyson even came up with a special telegraph code. Before leaving, Eddington wrote an article in which he provided his colleagues with all the information they needed to know how to interpret the results until the expedition returned. Eddington announced three options: no rejection; the deviation is 1.75 arc seconds, as predicted by Einstein; or it is 0.87 arc seconds - an indicator that testifies in favor of Newtonian gravity and challenges Einstein's ideas. In proposing this kind of formulation, Eddington was rather clever. Suddenly, the experiment turned into an open struggle between Einstein and Newton - a unique case when this upstart German could throw off the pedestal of the greatest thinker in history. Eddington created a narrative and a compelling context within which the results of the expeditions could be presented.
Eddington was in a hurry to launch his show. In early March, he hit the road, covered five thousand miles across the ocean, and on April 26 arrived with Cottingham to the shores of Africa. The men spent about a week in the port of St. Anthony on Principe Island, looking for suitable observation points. Finally, they chose the Roça Sundy Plantation in the northwestern part of the island, away from the mountains above which clouds usually gathered - it was a plateau overlooking the bay, located 500 feet above sea level.
The place and date - May 29 - turned out to be extremely favorable. As it turns out, this particular eclipse must have happened right in front of Hyades, a rather bright constellation that is ideal for measuring Einstein's deflection. Eddington needed just such bright stars so that they could be easily seen in the photograph. In addition, several stars, as opposed to one, could demonstrate different degrees of deflection as they were removed from the Sun: a star right at the edge of the solar disk should show a deflection of 1.75 seconds; another star located a little further is a slightly lower indicator; and the most distant star of the constellation should have shown almost no deviations. Einstein predicted not only deflection, but how it would change according to distance from the edge of the Sun. The presence of the constellation made it possible to check this aspect of his predictions.
Astronomers of past or future eras may have to wait for such favorable conditions for centuries or millennia. The Hyades are located in the constellation Taurus. They form the head of a bull and are located right next to the sparkling red star Aldebaran. The stars were named after the five nymphs, daughters of Atlas. Mourning the death of their brother, they were in heaven in the immediate vicinity of the voluptuous Orion. One of the brightest star clusters, the Hyades are visible to the naked eye and have attracted the attention of astronomers since ancient times. They belong to the constellations placed on the shield of Achilles, along with Orion and Ursa Major. In the view of the ancients, these stars acted as messengers of the heavenly kingdom.
Eddington, unlike Achilles, did not have a shield on which he could catch these stars - he could only catch their meaning through a telescope. To test the deflection of light from these stars, he had to point the telescope into the darkness of a total eclipse, when the ambient temperature drops, the birds stop singing and (most importantly for Einstein) the stars become visible.
On Thursday, May 29, 1919, it was cloudy in Sobral. The local community intended to turn the eclipse into a public event, and preparations for it were in full swing. A small observatory, located at the edge of the eclipse, sold tickets to those wishing to look through a telescope. By the beginning of the eclipse, the sky was covered with dense clouds. When the front edge of the moon touched the solar disk (called the "first touch"), astronomer Andrew Crommelin, who accompanied Dyson, assumed 90 percent cloud cover. But it quickly began to fade, and during the period of totality, the Sun was in a rather large gap between the clouds.
Everything plunged into surreal darkness, and the astronomers set to work. One of the Brazilians was watching the clock and counting the seconds out loud so that he could have time to take pictures. With the help of a large telescope, nineteen photographs were taken for exposure, and with the help of small four-inch lenses, eight. The sky was clear throughout the eclipse; the experiment went smoothly. The scientists immediately sent home a telegram: "Magnificent Eclipse."
Across the Atlantic, guests of honor of Principe Island came to Rosa Sandy on the morning of the eclipse. And they were greeted with a heavy downpour - which British subjects had never experienced before and which was not typical for that time of year. It ended around noon, just a few hours before the eclipse. The clouds, in the words of Eddington, "almost deprived us of our last hope."
At the first touch, the sun was not visible behind the clouds. It was only at 13:55 that astronomers began to discern its disk in the sky, transformed into a crescent by the inexorably creeping moon. He then appeared from the clouds, then plunged into them again. Even in good conditions, the last few seconds before totality were described as "almost painful." We can only guess what the scientists were experiencing at that moment. It was calculated that the totality should have come five seconds after 14:13. At that moment, astronomers were turning into machines that strictly followed the sequence of planned procedures regardless of what they could see with the naked eye - they were machines driven by hope and anticipation. Eddington put it this way: "We had to faithfully carry out our program of planned images."All their attention was absorbed by the telescope. Cottingham oversaw the coelostat mechanism and gave Eddington fresh plates; Eddington removed the finished photographs and inserted new plates. After each shift, he had to pause for a second, otherwise the movement could cause a tiny shiver that would ruin the image.
When the totality ended, the world returned to its previous state, as if there was no violation of the natural order at all. Eddington could take a breather. His short telegram to Dyson looked like this: “Through the clouds. We do not lose hope."
The decision was made to develop the photographs on the ground: in Brazil and on the island of Principe - but this was explained not only by "impatience". The glass plates were too fragile and could easily get damaged on a long journey. Developing them in the field and carrying out preliminary measurements at least guaranteed some results, albeit not obtained in the most perfect conditions. The following night at Sobrala, Davidson and Crommelin printed four astrographic photographs. They were shocked to see that the images of the stars were slightly distorted, as if the focus of the telescope itself was changing.
This change in focus can only be explained by the uneven expansion of the mirror due to solar heat. The focus scale readings were checked the next day: during this time they remained unchanged at the 11 mm mark. The quality of the plates left much to be desired. In the course of ordinary observations of a solar eclipse, this effect would not be taken into account. However, the deviation indicated by Einstein was so small that such a phenomenon could easily absorb it.
Images from the four-inch telescope, which they captured just in case, turned out to be much better. So there was hope. In any event, astronomers had a long wait. They had to stay in Brazil until July to photograph the Hyades at a time when the Sun was no longer in their way. Eddington was not in the mood to sit and wait. While there were good technical reasons for studying the photographs immediately, it seems that his incentive was more personal. For six nights after the eclipse, he and Cottingham developed two plates each night. The results were not entirely satisfactory: “The first 10 photos show almost no stars. The images on the last six, I hope, will give us what we are looking for; but all this is very annoying."
Eddington spent all of the ensuing days on photographs, trying to make accurate measurements using a complex device called a micrometer. Even with Eddington's legendary mathematical speed, it still took him three days of feverish work. This task turned out to be more difficult than he expected, because the images of the cloudy sky forced him to use methods that were different from those previously planned. But one day in the first week of June 1919, Eddington put aside the pen with which he was doing his calculations. The answer was received: "I realized that Einstein's theory has stood the test, and from now on, a new direction of scientific thought should prevail."
True, this statement of Eddington was more like self-hypnosis. His preliminary calculations were by no means sufficient to convince his British colleagues of the results obtained. This still required a lot of work. Eddington hoped to stay on Principe to complete some of this work, but his plans were thwarted by problems with the local shipping company. He was informed that if the scientist does not hit the road immediately, he risks being stuck on the island indefinitely. The Governor of Principe arranged for him and Cottingham to sit on the last ship to leave the island that summer (SS Zaire). Returning home, Eddington found himself in a new world of "international" science, which officially included "everyone except Germany and Austria." Meanwhile, he brought with him a suitcase full of photographs,closely related to the theory developed in Berlin.
Scientific observations do not speak for themselves and are in no hurry to reveal their secrets. It took Eddington months of tedious measurements and calculations to convince the world that Einstein was right on the basis of his conclusions.
Dyson and Eddington continued to work separately even while analyzing the data. They probably thought that independent measurements would be more reliable. Photos from Principe Island were analyzed in Cambridge and from Sobral in Greenwich. In all likelihood, Eddington did the measurements and calculations for the former himself, while Davidson worked with the staff of the Royal Observatory; the members of the Sobral expedition faced a less difficult task. Since they were able to take test pictures in situ, they were able to directly compare them to photographs of the eclipse. In addition, in both cases, the photographs were taken at the same location using the same telescope. Scientists had to simply measure the distance by which the image of a particular star moved in the presence of solar gravity.
True, for this it was not enough to attach a ruler and draw a line by eye. The measurements were made using a micrometer, which allowed us to estimate much tinier distances beyond the reach of the human hand. These measurements required a lot of preparation and patience, but were part of the astronomer's standard practice.
Eddington had to take an extra step. He was unable to obtain verification images from the island, so direct measurements were excluded. The scientist had to compare the image of the Hyades, obtained by him during the eclipse, with the image of these stars, made by the same telescope at Oxford. But he had to consider the possibility that there was some subtle difference between the two groups of images. Therefore, in both places (Prinisipe and Oxford), he took pictures of another star field and, comparing these photographs, could understand what the difference was.
Armed with this information, the scientist could use it in his final measurements. It is extremely difficult to avoid distortion or error in scientific measurements. Rather, the trick is to understand and fix these problems. The expedition to Principe Island resulted in 16 photographs, although due to cloudiness, only seven of them were useful. Fortunately, all seven have the stars with the highest predicted deflection. However, for reliable measurement, at least five stars were required as matches, and only two plates provided such information. At the very least, this information was consistent and the mean deviation was 1.61 arc seconds, ± 0.30. This degree of uncertainty was quite adequate, albeit high. Einstein's predicted deviation was 1.75. Not a bad result for the first measurement of a completely unknown physical phenomenon, Eddington thought.
As for the results of the expedition to Sobral, here the situation was saved by a four-inch reserve telescope taken at the last moment. Seven of the eight plates he shot, gave excellent images of all seven stars scientists needed. Measurements based on them yielded much better results: 1.98 arc seconds, ± 0.12.
Busy with endless measurements and calculations, Eddington and Dyson somehow took the time to set the stage for the presentation of the results. Dyson has asked the Royal Society Council to schedule a special meeting for November 6 to formally present the results. The way back was closed. Nevertheless, it was still not possible to report this directly to Berlin, so the researchers did differently. The Dutch physicist Hendrik Lorentz sent Einstein an urgent and brief telegram that read: "Eddington found the deflection of stars on the solar disk beforehand between nine tenths of a second [degree] and twice the magnitude."
Unfortunately, we have no eyewitness testimonies who were near Einstein at the time of receiving the telegram. But then he showed a telegram to everyone who came to his apartment, which allows us to trace the scientist's reaction through the eyes of those around him. Ilse Rosenthal-Schneider, a young physics student, sat with Einstein at his desk, perusing a book full of criticisms of his theory of relativity. Einstein suddenly interrupted his reading to take a document from the windowsill. He coldly remarked, "This might interest you," and handed her Lorentz's telegram. Einstein could not think of anything else and was clearly not disposed to hide this news from others.
This was the attitude Eddington hoped to instill in his British colleagues in the halls of the Royal Society at Burlington House on Piccadilly. The listeners sat on benches, and those who did not have enough space were crowded between the columns along the walls. Alfred North Whitehead, a distinguished philosopher and mathematician, was also present in this room. He described the excitement in the audience this way: "The atmosphere of intense interest was exactly like the atmosphere of the Greek drama."
The next day, the London newspaper The Times published the biggest scientific headline in history: "A Revolution in Science." The discovery was attributed to "the famous physician Einstein" (he was neither one nor the other). On Saturday came the next article with the same title and addition "Einstein versus Newton". This was the first public exposure to Einstein, and the scientist appeared to the world exactly as Eddington wanted: in the role of a peaceful genius who rejected the stereotypes of German militarism characteristic of wartime.
A wave of excitement swept across the Atlantic, and on November 10, 1919, The New York Times shouted from the front pages: "Scientists are looking forward to eclipse observations." It is important to look back and remember that this was in fact the Times' first mention of Einstein.
This burst of interest finally allowed Eddington and Einstein to write to each other directly. "All England is talking about your theory … this is the best thing that can happen in the scientific relationship between England and Germany," Eddington wrote to Einstein that same year. Thanks to Eddington, the expedition became a symbol of German-British solidarity. Einstein, for his part, decided to fight militarism in German science by raising the stakes. It was a great moment for science, divided by war, because some scientists have managed to turn it into a single whole.
This article is an edited excerpt from Matthew Stanley's book Einstein's War: How Relativity Conquered Nationalism and Shook the World 2019, published by Penguin Books