May 29, 1919: A Major Eclipse, Relatively Speaking
1919: During a total solar eclipse, Sir Arthur Eddington performs the first experimental test of Albert Einstein’s general theory of relativity.
The findings made Einstein a celebrity overnight, and precipitated the eventual triumph of general relativity over classical Newtonian physics.
In 1919, Newton’s law of universal gravity still dominated scientific discourse, as it provided extremely accurate explanations of physical observations. But Einstein had a major issue with Newton’s theory: It wasn’t consistent with his own special theory of relativity, which predicted that space and time were relative, forming a four-dimensional continuum called spacetime. He conceived a general theory of relativity, in which gravitational fields would cause warps in spacetime, thus weaving gravity into the continuum.
One prediction of general relativity was that light should not travel in a perfectly straight line. While traveling through spacetime and nearing the warp induced by an object’s gravitational field, light should curve — but not by much. A ray of light nicking the edge of the sun, for example, would bend a minuscule 1.75 arcseconds — the angle made by a right triangle 1 inch high and 1.9 miles long. Newtonian physics also predicted light would bend due to gravity, but only by half as much as Einstein’s theory predicted.
Such a tiny difference seemed impossible to measure by earthly experiments. In fact, the two theories, though fundamentally opposed, made highly similar predictions for almost all tests of gravity and light. As a result, it was futile to try to understand which one provided a more accurate description of the fundamental laws of the universe.
Sir Frank Watson Dyson, Astronomer Royal of Britain, conceived in 1917 the perfect experiment to resolve the issue. A total solar eclipse on May 29, 1919, would occur just as the sun was crossing the bright Hyades star cluster. Dyson realized that the light from the stars would have to pass through the sun’s gravitational field on its way to Earth, yet would be visible due to the darkness of the eclipse. This would allow accurate measurements of the stars’ gravity-shifted positions in the sky.
Eddington, who led the experiment, first measured the “true” positions of the stars during January and February 1919. Then in May he went to the remote island of Príncipe (in the Gulf of Guinea off the west coast of Africa) to measure the stars’ positions during the eclipse, as viewed through the sun’s gravitational lens.
Eddington also sent a group of astronomers to take measurements from Sobral, Brazil, in case the eclipse was blocked by clouds over Príncipe. Outfitting and transporting the dual expeditions were no small feats in the days before transoceanic airplanes and instantaneous global communication.
Both locations had clear skies, and the astronomers took several pictures during the six minutes of total eclipse. When Eddington returned to England, his data from Príncipe confirmed Einstein’s predictions. Eddington announced his findings on Nov. 6, 1919. The next morning, Einstein, until then a relatively obscure newcomer in theoretical physics, was on the front page of major newspapers around the world.
The bending of light around massive objects is now known as gravitational lensing, and has become an important tool in astrophysics. Physicists now use gravitational lensing to try to understand dark matter and the expansion of the universe.
Sources: International Astronomical Union, Wikipedia, NASA
Photo: Observations of this 1919 solar eclipse confirmed Einstein’s general theory of relativity.