May 29, 1919, is the date of a solar eclipse that caused a revolution in science. The eclipse is famous for testing Albert Einstein’s theory of general relativity. Einstein was relatively unknown at the time. He had proposed general relativity in 1915, and scientists had been intrigued by the entirely new way of thinking about gravity – for example, the idea that mass causes space to curve – but no one had experimentally proven the theory to be correct. Then, on May 29, 1919, an expedition of English scientists – led by Sir Arthur Eddington – traveled to the island of Príncipe off the west coast of Africa to observe a total solar eclipse. If the theory were right, the light from stars should be bent by the gravity of the sun and appear displaced. An eclipse, where the moon blocks the sunlight enough for stars to be seen near the sun, was the perfect opportunity to test this.
The scientists’ measurements during the eclipse showed that, astoundingly, Einstein’s predictions were indeed correct. The locations of the now visible stars appeared displaced, due to the fact that their light had to travel to us on the curved space around the sun caused by its gravity, as described by Einstein.
From anonymity to stardom via a solar eclipse
Later that year – on November 6, 1919, in London – England’s Astronomer Royal, Frank Dyson, who had organized the expedition, presented the results at a joint meeting of the Royal Astronomical Society and the Royal Society. Dyson said “there can be no doubt” that measurements made during the May 29, 1919, solar eclipse “confirm Einstein’s prediction.”
As part of the celebration of the 100th anniversary of this legendary solar eclipse, Caltech physicist Sean Carroll explained to NBCNews in 2019:
General relativity was the poster child for being a crazy, new, hard-to-understand theory, with dramatic implications for the nature of reality. And yet you could see [the results]; you could photograph it. So people got caught up in that excitement.
And so Albert Einstein was catapulted to rock star fame, a status in popular culture he has retained ever since.
A new perspective on gravity and the universe
Einstein’s general theory of relativity underlies our most basic modern cosmology, our way of looking at the universe as a whole. Before Einstein, scientists relied on Isaac Newton’s theory of gravity. Newton’s way of looking at gravity is still valid and is still taught to physics students. But while Newton’s formulation of gravity is more of a special case under specific conditions, Einstein’s theory is a refinement of scientists’ understanding of gravity that covers the big picture … and what a mind-blowing big picture! Einstein proposed that mass causes space to curve. So, for example, although there appears to be a “force” (as described by Newton) that causes our Earth to be pulled towards the sun by gravity, that force can “simply” be described as Earth traveling in curved space around the sun, according to Einstein.
Einstein’s general theory of relativity not only explains the motion of Earth and the other planets in our solar system. In our modern cosmology, it also describes extreme examples of curved space, such as around black holes. And it helps to describe the history and expansion of the universe as a whole.
The solar eclipse was the first proof of many
In the century and a bit since the 1919 total solar eclipse, Einstein’s relativity theory has been proven again and again, in many different ways. You might have seen the recent first-ever photo of a black hole?. It also proved, once again, that Einstein was right.
Now and then
The Royal Astronomical Society (RAS) described modern-day practical applications of Einstein’s theory:
The theory fundamentally changed our understanding of physics and astronomy, and underpins critical modern technologies such as the satellite-based Global Positioning System (GPS).
The theory of relativity is essential for the correct operation of GPS systems, which in turn are relied on in many common applications including vehicle satellite navigation (SatNav) systems, weather forecasting, and disaster relief and emergency services. However, the world had to wait decades before the applications of such a blue skies result could be realized.
Back in the day of the 1919 eclipse, Sir Arthur Eddington attended a dinner of the same organization – RAS – shortly after the successful expedition. He then showed his humorous side by reciting a verse he had written on the feat:
Oh leave the wise our measures to collate
One thing at least is certain, light has weight
One thing is certain and the rest debate
Light rays, when near the sun, do not go straight.
Bottom line: The solar eclipse of May 29, 1919, was the day astronomer Sir Arthur Eddington verified Einstein’s general theory of relativity, by observing how stars near the sun were displaced from their normal positions. This apparent change in position happens because, according to Einstein’s theory, the path of light is bent by gravity when it travels close to a massive object like our sun.