An eye for an i #296 Children

Eddington’s expedition confirms Einstein's relativity

Sir Arthur Eddington

Sir Arthur Eddington   | Photo Credit: Smithsonian Institution Libraries


The European Space Agency referred to the solar eclipse of 1919 as “probably the most important eclipse in the history of science”. For it was this eclipse that allowed Sir Arthur Eddington to confirm Albert Einstein’s theory of relativity. A.S.Ganesh hands you the details…

There’s no escaping Albert Einstein’s theory of relativity. You might be waiting to encounter it and learn everything, totally in awe of the theory having explored the subject in depth or trying to grasp it better by further reading about it. One thing is for certain though: you most certainly would have heard about it.

Einstein published his general theory of relativity in 1915 and it has now gone on to become one of the cornerstones of modern physics. But it met with great scepticism in the initial years and even took decades to be largely accepted. The first major experimental test was carried out by Sir Arthur Eddington during the solar eclipse of 1919.

Stunning prediction

One of Einstein’s stunning key predictions from his theory was the bending of light by gravity as the masses of celestial bodies would cause space-time to curve. The prevalent Newtonian physics, which this theory was taking head-on, also predicted light would bend due to gravity, but only by half as much as Einstein’s prediction.

While this offered a means to test the theory, measuring such a tiny curvature still seemed to be beyond the experimental capabilities of the time. It was not for want of trying though, as the earliest attempts were made by Erwin Finlay-Freundlich.

False starts

An astronomer from the Berlin Observatory, Freundlich led an expedition to Crimea to observe an eclipse, but was arrested as a spy as World War I had already begun. While a team from the Lick Observatory in California did make it to the Crimean eclipse, their attempt was foiled by rain. Their camera was impounded by the Russians and wasn’t returned in time for the eclipses in Venezuela in 1916 or the one in U.S.A in 1918.

It was under these circumstances that Eddington entered the fray. A brilliant mathematician, Eddington worked as a professor at Cambridge before finding himself a place at the Royal Observatory in Greenwich, where he rediscovered his love for astronomy. As a Quaker (member of a Christian religious group), Eddington refused to be drafted into the army during WWI. He would have been jailed for that if not for the fact that Frank Dyson, his boss, intervened.

Dyson’s experiment

Dyson, the Astronomer Royal of Britain, had conceived an experiment in 1917 that would plot the position of stars that were close to the sun’s limb when an eclipse was taking place. By comparing this with their natural positions in the sky, Dyson believed, the effects of warped space-time could be observed. Dyson prevented Eddington from being jailed by promising that he would be undertaking this important scientific mission to test Einstein’s theory.

An early convert to Einstein’s new theory, Eddington felt privileged to be leading the expedition to view the solar eclipse of 1919. Things weren’t all that easy, however, as the ongoing war meant that military needs trumped astronomical instruments. By the time WWI ended in November 1918, they had just five months left to pull everything together.

Eddington first measured the baseline positions of the stars in the Hyades cluster in January and February 1919. Having learnt from past experiences of others and in order to minimise the chances of failure, two teams were created. Eddington and his team set off to Principe, a remote island off the west coast of Africa, while another group of scientists made their way to Sobral, Brazil.

On May 29, 1919...

Unusually cloudy weather in Sobral on May 29, 1919, the day of eclipse, threatened to spoil their chances, but a clearing in the clouds a minute before totality – the moment when the moon fully covers the sun – meant that they could have their readings. Eddington’s team had to endure an hour and a half of rains on the day of the eclipse, but they too were able to make observations. The fact that there was six minutes of total eclipse, among the longest in the 20th century, helped both touring parties.

Having returned to England, Eddington studied the data and was able to confirm Einstein’s predictions. Once this announcement became public in November 1919, Einstein moved from relative obscurity to being a celebrity at the speed of light. There were those who didn’t accept the results right away, but further tests and findings ever since have all been in Einstein’s favour.

Gravitational lensing, or the bending of light around massive objects, continues to remain an important tool for astronomers. For it is this effect that allows scientists to look deep into the cosmos, searching for answers regarding dark matter and the expansion of universe.

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Printable version | Jan 29, 2020 11:50:23 AM |

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