A Determination of the Deflection of Light by the Sun's Gravitational Field
Starlight grazing the Sun bends 1.75″ — just as Einstein said, twice what Newton allowed.
In 1919, astronomers waited for the Moon to blot out the Sun — and caught starlight bending by exactly the amount Einstein had dared to predict.
The big idea
Einstein's general relativity says that mass bends the shape of space and time around it, and that light, taking the straightest available path, must follow that curve. So a ray of starlight skimming past the Sun should get bent slightly, and a star seen right next to the Sun should appear nudged a little away from it.
Einstein put a number on it: a star at the very edge of the Sun should shift outward by about 1.75 arcseconds — a hair's width of an angle, but twice as much as anyone could explain with Newton's older physics. The whole test came down to measuring that tiny shift, and seeing whether it matched Einstein's number or the smaller Newtonian one.
How it came about
The trouble is that you cannot see stars next to the Sun — its glare drowns them. The one exception is a total solar eclipse, when the Moon covers the Sun for a few minutes and stars pop out in a darkened daytime sky. The eclipse of 29 May 1919 was perfect: the hidden Sun would sit in a thick cluster of bright stars, the Hyades.
Britain sent two expeditions to be safe against clouds — one to Sobral in Brazil, one to the island of Príncipe off West Africa, where Arthur Eddington went himself. They photographed the stars around the eclipsed Sun, then compared with photographs of the same stars taken at night months later. The stars near the Sun had shifted outward. When Frank Dyson announced the numbers in London that November, the result made headlines around the world and turned Einstein, almost overnight, into the most famous scientist alive. Honesty requires a footnote: the data were noisy, and one blurred instrument that disagreed was set aside — a choice that later drew accusations of bias, though re-analysis has since confirmed the result.
Why it mattered
For two centuries Newton's gravity had been the unquestioned law of the cosmos. The 1919 eclipse was the first hard evidence that it was not the final word — that near a heavy enough body, Einstein's stranger picture of curved spacetime described reality better. It turned general relativity from beautiful mathematics into tested physics, and opened the century of black holes, the expanding universe, gravitational waves and GPS — all of which lean on the theory those eclipse plates confirmed.
An everyday picture
Imagine the fabric of space as a taut rubber sheet. Set a heavy ball — the Sun — in the middle, and the sheet sags into a dip around it. Now roll a marble straight across the sheet, well clear of the ball: it veers slightly as it crosses the slope of the dip, even though nothing pushed it sideways. Starlight does the same near the Sun — not pulled by a force in the old sense, but following the valley that the Sun's mass carves into space itself. The closer the marble passes to the ball, the steeper the slope and the sharper the bend — which is exactly why the deflection is largest for light grazing the Sun's edge.
Where it sits
This is the experiment that confirmed the theory laid out in this Library's entry on Einstein's general relativity (1916). It is the hinge between Newton's Principia (1687), whose gravity it dethroned at the edges, and the modern science of gravity that followed — the gravitational waves caught by LIGO in 2016, the physics of black holes, and the lensed galaxies that let astronomers weigh dark matter. Every one of those rests on the curved spacetime that a few photographic plates first caught the Sun bending light around.
The purpose of the expeditions was to determine what effect, if any, is produced by a gravitational field on the path of a ray of light traversing it.
(1) The path is uninfluenced by gravitation. (2) The energy or mass of light is subject to gravitation in the same way as ordinary matter. If the law of gravitation is strictly the Newtonian law, this leads to an apparent displacement of a star close to the sun's limb amounting to 0″·87 outwards. (3) The course of a ray of light is in accordance with Einstein's generalised relativity theory. This leads to an apparent displacement of a star at the limb amounting to 1″·75 outwards.
Thus the results of the expeditions to Sobral and Principe can leave little doubt that a deflection of light takes place in the neighbourhood of the sun and that it is of the amount demanded by Einstein's generalised theory of relativity, as attributable to the sun's gravitational field.