If Einstein were alive this year, I’m pretty sure he would be feted with a huge centennial bash. That is because a hundred years ago, a key prediction of his recently formulated Theory of General Relativity was confirmed in spectacular fashion by a solar eclipse. It was a triumphant moment for science, and since his theory was mathematical, also a glorious affirmation of “the unreasonable effectiveness of mathematics” in the natural sciences, to quote the famous words of Eugene Wigner. But I’m getting ahead of myself. Here is the full story of the empirical confirmation of Einstein’s prediction in 1919 from the beginning.

In 1915, Albert Einstein published his celebrated **Theory of General Relativity** in which he proposed that the space-time coordinate system that we use to mark out the events of our universe isn’t static but a fully dynamic, live “creature” (a “manifold” in mathematical jargon). This space-time manifold can bend, flex, and warp under the influence of mass and its equivalent, energy, like a “fabric of the universe”. It is this rugged geometry that gives rise to the force of gravity. And on this “fabric”, light and physical objects would travel along paths that are determined by the equations of curvature spelt out precisely in Einstein’s paper (see Notes below).

Einstein of knew that his theory was a radically new approach compared to that of Newton and that it was beautiful. But he was also a realist. To him, the ultimate test of a theory is not how one likes the idea, but how well it describes the real world. The key is to find a testable prediction and an experiment to either confirm or refute that prediction. This is where the eclipse story comes in.

Nothing knows more about the difficulty of moving along the space-time terrain better than **light** itself. Forced to follow every hill, valley, bump and wrinkle in the universe, light’s path would constantly jostle back and forth as it tries in vain to follow a straight and narrow path. Einstein’s theory’s predicted that the presence of a nearby massive object like the sun will deflect light from its original path, by exactly1.75 arc seconds. This is a small but measurable amount. An elegant way to test this prediction is to measure the bending of light as it passed through the warped space near the mass of the sun during an **eclipse**.

As World War I raged, Einstein sent a copy of his paper to Willem de Sitter in Leiden, Netherlands, who then passed his copy to Arthur Stanley Eddington in England. In 1916, Eddington, then 34 years old, was already a Professor of Astronomy at Cambridge and a brilliant theoretical scientist. Eddington himself became a powerful champion of Einstein’s ideas, promoting them to scientists and explaining them to a wider public. More importantly, when Word War I ended, he made two “eclipse trips”- one to Sobral in northern Brazil and the other to the island of Principe off the coast of West Africa, to test Einstein’s light deflection prediction. Long story short, the prediction was confirmed during a May 1919 solar eclipse visible throughout most of South America (total eclipse) and Africa (partial eclipse). It was an event which Eddington later called “the greatest moment in my life.”

**Further reading**

Robert P. Kirshner, *The Extravagant Universe: Exploding Stars, Dark Energy, and the Accelerating Cosmos*, Princeton University Press, 2016

**NOTES: THE MATHEMATICS OF GENERAL RELATIVITY IN A NUTSHELL**

From a mathematical perspective, the main goal of general relativity is to understand the global structure of solutions of Einstein equations. These are a system of non-linear partial differential equations (PDEs) expressed in hyperbolic form. They describe our universe by relating its geometric structures to matter and energy content whereby gravitation arises through curvature. Solutions of the Einstein equations are Lorentzian metrics. That is, the universe or its parts are modeled as four-dimensional manifolds equipped with these metrics which we call space-times. With this framework, general relativity has thus solved challenging mathematical problems and given answers to long-standing questions about the nature of three-dimensional space, and the extra dimension of time.