A solar eclipse proved Einstein's theory of relativity
In 1919, Albert Einstein was still a relatively obscure physicist. His radical theory of general relativity claimed that gravity could bend light, but he needed proof. British astronomer Arthur Eddington traveled to a remote island during a total solar eclipse to photograph stars near the blocked Sun. The stars' positions appeared slightly shifted because the Sun's mass bent their light, proving Einstein right and launching him into global fame.
A Clash of Gravitational Theories
In the early twentieth century, Isaac Newton's law of universal gravitation had reigned supreme for more than two hundred years. Newtonian mechanics treated gravity as an attractive force acting across empty space between masses. Under a Newtonian framework that treated light as corpuscles or particles with mass, a ray of light passing near a massive body like the Sun would experience a slight gravitational pull. Calculations for this Newtonian deflection, first derived in the early nineteenth century by Johann Georg von Soldner, predicted that starlight grazing the edge of the Sun would bend by approximately 0.875 arcseconds—an angle so minuscule that it represented a fraction of the width of a coin viewed from miles away.
Albert Einstein proposed a radically different understanding of the universe with his theory of general relativity, completed in 1915. Einstein discarded the notion of gravity as a conventional force, proposing instead that mass and energy warp the very fabric of four-dimensional spacetime. Objects, as well as rays of light, simply follow the straightest possible paths, known as geodesics, through this curved geometry. Because general relativity accounted for both the temporal and spatial components of spacetime curvature, Einstein calculated that light passing the limb of the Sun would be deflected by roughly 1.75 arcseconds—precisely double the classical Newtonian prediction.
This distinct numerical disagreement created a rare and decisive test for theoretical physics. If astronomers could accurately measure the deflection of starlight passing near the massive gravitational field of the Sun, they could determine whether Newton's longstanding model remained intact or whether Einstein's radical restructuring of space, time, and gravity was correct.