The Eclipse of 1919 - Putting Relativity to the Test

Eddington and Einstein

Einstein first formulated the Theory of Relativity and presented it along with his field equations in Berlin to the Prussian Academy of Sciences on November 25, 1915. They state a radical new view of gravity in which gravitationally large objects curve space and time. After years of work, Einstein had finally arrived at a truly general theory of relativity. He described his new equations as "the most valuable discovery of my life."

In England, Sir Arthur Eddington had been receiving whisperings of the new equations from his friends in the field of astronomy and was quite excited about them. He wanted to prove the Theory of Relativity somehow. The task would be reasonably done since Einstein had clear predictions that could be used to test his theory.

Light from the Hyades bending around the sun on its way to Earth. Light from the Hyades star constellation is bent as it passes by the sun on its way to Earth.

One prediction from the new theory was that light would bend as it passed by a large source of gravity, such as the sun. Eddington imagined that it would be possible to measure the effect of the sun's gravity on the path of light emitted from stars passing near it. But the sun's intense rays made observing stars extremely inaccurate while they were so close. Luckily a solar eclipse was predicted to occur on May 9th of 1919, an ideal opportunity to prove Einstein's new theory. Even more lucky was that a particular cluster of stars, the Hyades, was predicted to pass just behind the sun.

Eddington's experiment proposed observing the Hyades at two different times of the year. He would first measure the location of the stars when the sky was completely clear of anything that might bend their light waves. He would then measure their location again, this time with the sun in front of them. It would have to be done during an eclipse, when almost all of the light from the sun would be blocked by the moon, to ensure the accuracy of the data. Comparing the two measurements would show if the sun caused any deflection of the light from the stars. Einstein's equations predicted a deflection of about four-thousandths of a degree, or 1.7 arc seconds. If this deflection could be verified, Einstein's new theory would be verified as well. The goal was quite clear and simple.

There was only one small problem. Europe was in the middle of World War I, one of the largest wars it had ever seen. The war had been raging since 1914. In 1916 England suffered major losses during the Somme Offensive, after which the draft was instituted. By 1918 the risk of the German army completely overrunning the British and French had grown, leading to a new wave of conscriptions. Eddington was called to service in the British army but refused. While he had the support of the Astronomer Royal, Frank Dyson, he also faced being thrown in jail for refusing to fight in the war.

A tribunal was held in Cambridge to look into Eddington's stance. He was likely going to be refused exemption from the war effort until Frank Dyson stepped in, stating that "under present conditions the eclipse will be observed by very few people. Professor Eddington is peculiarly qualified to make these observations, and I hope the Tribunal will give him permission to undertake this task." The eclipse intrigued the tribunal and Eddington was given an exemption for "national importance."

The Experiment

The Great War does end in November of 1918, thankfully, though still leaving behind a devastated world. Although the goal of Eddington's experiment is clear, achieving the results will not be easy. To observe a total eclipse of the sun the team travels to the southern hemisphere. Eddington, along with Edward Cottingham from the Greenwich Observatory, set up off the west coast of Africa on the island of Principe. A backup team of two astronomers, Andrew Crommelin and Charles Davidson, are dispatched to the village of Sobral in the interior of Brazil.

1919 solar eclipse path The path of the 1919 solar eclipse.
photo credit: NASA

Eddington and Cottingham primed their telescope for the heat of the day. On the morning of the eclipse, it rained heavily on the island of Principe. Less than an hour before totality, the clouds began to clear. By 2:15 in the afternoon the sky was clear and 16 photographic plates were taken of the sun with the Hyades lurking in the background. Eddington telegraphed a message to Frank Dyson: "Through cloud. Hopeful."

The cloudy start on Principe may have saved their experiment. In Sobral it was a perfectly clear and hot day. So much so that the heat warped their main telescope and rendered its photographic plates useless. It was only with the use of a smaller backup telescope that the Sobral team was able to contribute data to the experiment. Crommelin and Davidson took their measurements in the company of excited locals who came to witness the historic event.

The Results

The journey home for the team was long and they did not return to England until late July to analyze their data. Of the sixteen plates that Eddington had recorded, only two had enough stars to properly measure the deflection. The value they calculated was 1.61 arcseconds with an error of 0.3 arcseconds, consistent with Einstein's prediction of 1.7 arcseconds. When the plates from Sobral were analyzed, the results were, at first, worrying. The value measured was 0.93 arcseconds, far from the relativistic prediction and close to the Newtonian prediction. But these were the plates taken from the telescope that had been deformed in the heat. When the backup observations from Sobral were analyzed, the deflection was calculated to be 1.98 arcseconds, with a very small error of 0.12 arcseconds. Again what Einstein's equations predicted.

On November 6, 1919, the team presented their results to a joint meeting of the Royal Academy and the Royal Astronomical Society. In a series of presentations led by Frank Dyson, the team showed that the eclipse measurements taken spectacularly confirmed Einstein's equations. J. J. Thompson, the president of the Royal Society, described the experiment as "the most important result obtained in connection with the theory of gravitation since Newton's day." He added, "If it is sustained that Einstein's reasoning holds good - and it has survived two very severe tests in connection with the perihelion of Mercury and the present eclipse - then it is the result of one of the highest achievements in human thought."

Time Magazine Sir Walter Eddington 1934 Cover of Time Magazine with Sir Walter Eddington
photo credit: Time Magazine

The experiment had proven that it was in fact possible to see an object that was physically behind another object. The achievement of Einstein's new theory and Eddington's experiment to prove that theory were enormous accomplishments in a continued golden age of astronomy which we are still enjoying today.

The day after the Burlington House meeting, Thompson's words appeared in the London Times. The New York Times published its own headlines: "All Lights Askew in the Heavens," and "Stars Not Where They Seemed or Were Calculated to Be but Nobody Need Worry." And of course Einstein instantly becomes a household name as the news spread around the world.

Greater Affects

The confirmation of the new scientific theory came just after World War I had ended. It was to many people an encouraging development that gave the world hope that humanity still showed promise even in the immediate aftermath of the apocalypse that was World War I.

"Not only is the universe stranger than we imagine, it is stranger than we can imagine."
~ Sir Arthur Eddington

  The Perfect Theory by Pedro G. Ferreira

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