100 years ago today, the sun made Einstein a star


Observations made during a solar eclipse provided the evidence to confirm the Theory of General Relativity. Robyn Arianrhod reports.


Careful observation of a solar eclipse proved that Einstein's theory of general relativity was correct.

VCG/VCG via Getty Images

Today – May 29, 2019 – is the first centenary of a legendary eclipse, and of the special scientific expedition designed to carefully observe it. It was the first successful deliberate attempt to test Albert Einstein’s new theory of gravity (or General Relativity) – and its results turned Einstein into a celebrity of superstar magnitude.

A hundred years ago, the excitement surrounding the expedition was palpable. Four long years of war had made Britain and Germany enemies, but in early 1919 – in an act of international reconciliation as well as audacious scientific planning – the British expedition set sail for the tropics to catch the best view of the May 29 total eclipse of the sun.

Their goal was to test which theory of gravity was correct: the German-born Einstein’s, or that of British icon Isaac Newton.

What did the eclipse have to do with gravity?

Einstein had predicted that starlight passing by the sun as it travels to Earth would be slightly deflected by the sun’s gravity.

To test this, he suggested taking photographs during a total solar eclipse, when the stars surrounding the sun became visible for a few minutes. He hoped these photos would reveal a displacement in the stars’ position compared with images of the same stars when they are no longer close to the sun.

According to the standard optical and gravitational theory of the time, however, light should travel in straight lines. It has no mass, so according to Newton’s theory it couldn’t be deflected by gravity the way a tennis ball slammed across the net arcs down towards the ground.

But Einstein didn’t link the force of gravity directly to the masses of two bodies the way Newton did; instead, he predicted that the sun’s large gravitational field actually curves the space-time around it. And this meant that light rays would have to travel on curved paths as they passed by the sun.

This curvature is mind-bendingly small: on their photographic plates of the stars, the British astronomers had to be able to be able to discern position changes of around one-fiftieth of a millimetre!

The technical expertise needed for such a delicate task was enormous, and it’s certainly worthy of celebrating today. The results showed conclusively that the sun does deflect passing starlight, and a new scientific star was born.

In the next edition of the Cosmos quarterly print magazine, out on June 6, Robyn Arianrhod delivers a fascinating and detailed history of the eclipse that cemented Einstein’s reputation. Order your copy here.

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Robyn Arianrhod is a senior adjunct research fellow at the School of Mathematical Sciences at Monash University. Her research fields are general relativity and the history of mathematical science.
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