Benchmarks: May 29, 1919: Solar eclipse "proves" relativity

by Nate Burgess
Thursday, January 5, 2012

On May 29, 1919, the moon’s silhouette crept slowly over the sun, bringing premature night to observers in a broad swath of the Southern Hemisphere between South America and Africa. Few onlookers realized that this event would provide the first successful test of Albert Einstein’s theory of relativity.

A few pinpoints of light — a group of stars called the Hyades — twinkled around the shimmering halo remaining in the sky. These stars were the focus of a team of British scientists who watched the eclipse from both sides of the Atlantic. Astronomer Andrew Crommelin directed efforts in Sobral, Brazil, while astronomer Arthur Eddington led the joint expedition from his camp on the island of Princípe, part of the São Tomé and Princípe archipelago off the west coast of Africa. Both teams had an assortment of steel telescopes, astronomical measuring devices and motorized reflective mirrors — all the equipment necessary to perform a historic experiment that would have profound consequences in all areas of science.

The seed for the 1919 eclipse expedition was planted four years earlier, when Einstein published his general theory of relativity, a theory built upon the foundation of his earlier special theory of relativity. Together, these works proposed a paradigm shift in physics. In this new view, length and time were not consistent everywhere. These measures, instead, varied between frames of reference, while the speed of light remained constant from every perspective. To accommodate these changes, the universe was now described not in three dimensions, but in four-dimensional coordinates of space-time. And, most important for the Eddington experiment, the theory predicted an unusual astronomical phenomenon: starlight would bend around massive objects like the sun.

This idea was not entirely new. Early Newtonian physics had suggested that the gravitational force between Earth and light “particles” would produce curved light beams. This effect was predicted to be extremely tiny and had therefore been nearly impossible to test. Later, contemporary physics had given up on the Newtonian idea that light particles have mass, rendering the prediction meaningless to physicists.

Einstein’s theory of general relativity, however, brought back the Newtonian idea in a new form. This theory, with its new coordinate system, redefined gravity not as an independent force, but rather as a byproduct of curved space-time near massive objects. In this view, light waves would indeed bend near stars — in fact, twice as much as earlier Newtonian predictions. That meant the curvature would be more measurable from Earth.

Scientists seeking to confirm Einstein’s prediction focused their attention on solar eclipses. This is because with the sun in shadow, scientists could observe the position of stars very close to the sun. The position of these stars during the eclipse could later be compared to the position of the same stars without the sun’s light-bending influence. As Peter Cole points out in “Einstein, Eddington, and the 1919 Eclipse,” Sir Frank Watson Dyson, the Astronomer Royal of Britain, first realized that the Hyades stars and the May eclipse would provide an ideal subject for this type of experiment.

Unfortunately, until the end of World War I in November 1918, the factories capable of building the necessary astronomical equipment to make these measurements were engaged in military production. After the war ended, the team had only five months to assemble equipment and embark on the journey to the two observation points.

More challenges awaited them. On Princípe, Eddington experienced heavy rains and clouds the day of the eclipse. Though the sky cleared some as the eclipse occurred, viewing conditions were certainly less than ideal. Across the Atlantic, the Brazil team had a great view, but they discovered that tropical heat had warped the metal in their large telescopes. Fortunately, they had also brought a small 10-centimeter telescope backup. This telescope ultimately provided the best data of the expedition.

After the results were tabulated, both teams' numbers were within two standard errors of Einstein’s predicted deflection value and more than two standard errors away from the Newtonian value. These experimental results indicated that Einstein’s predictions were likely right. Initial scientific opinions on the experiment, however, were ambivalent. For example, some accused Eddington of manipulating data when he threw out values that he assumed were affected by instrumental error.

Despite the mixed opinion, news media celebrated the event — making Eddington and Einstein instantly famous across the globe. Experiments throughout the 20th century would continue testing the ideas of relativity, confirming with ever-increasing confidence that Einstein’s approach, and the science inspired by it, adequately described many facets of nature on the broadest scale.

The 1919 confirmation of general relativity provided science with a new set of theoretical tools used today in many new technologies, including GPS. Global Positioning System technology uses a network of satellites that orbit Earth to provide accurate 3-D locations to users on the ground. To achieve the level of accuracy seen today, GPS satellites are equipped with advanced atomic clocks that are carefully set using a complex system of inter-satellite and ground-to-satellite communication. This system is necessary, in part, because of two observed orbital effects predicted by Einstein in his theories of relativity.

The first of these phenomena, time dilation, comes from Einstein’s special theory of relativity, a work that only considers objects in inertial reference frames (and therefore constant velocities). This law predicts that objects moving very quickly, such as a satellite in orbit around Earth, will experience time more slowly. Consequently, the typical atomic clock on board a GPS satellite in orbit loses about seven microseconds a day relative to the same atomic clock at Earth’s surface.

To more fully understand Einstein's special and general theories of relativity, try "The Einstein Theory of Relativity" by Lillian R. Lieber.

For more information on GPS, check out

An even more significant effect predicted by Einstein’s general theory of relativity acts counter to this seven-microsecond slowdown. According to Einstein, due to space-time curvature near Earth, objects in orbit, which are therefore experiencing less gravitation, will experience time more quickly. GPS satellite clocks gain about 45 microseconds a day relative to terrestrial clocks from this effect.

Together, these two forces have the potential to throw off satellite atomic clocks by up to 38 microseconds every day. Though this certainly seems like a small amount, GPS requires accuracy thousands of times higher to function as a useful navigation tool. In short, if you have ever used a GPS unit, you have benefited from relativity — and from the results of the 1919 eclipse.

Edited: 3 Sept 2009

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