The world war that would claim over fifteen million lives was tearing Europe apart in 1915. So scientists outside Germany didn't know that a distinguished physicist had presented a theory that would shake up the way we see the Universe. The physicist was Albert Einstein, but his face was unknown to the world, his name not yet a synonym for genius. How did a solar eclipse in 1919 change all that?

Willem de Sitter
The Netherlands was neutral, and scientists there stayed in touch with others on both sides of the conflict. Dutch physicist Willem de Sitter was engrossed in Einstein's relativity theories. He probed them with Einstein, and discussed them with colleagues. De Sitter also wrote to Arthur Eddington, the secretary of the Royal Astronomical Society in England. He sent Einstein's paper on general relativity and agreed to write a commentary for the society.

Arthur Eddington
Although Arthur Eddington was only in his early thirties, he was the Plumian Professor of Astronomy at Cambridge University, a chair once held by Isaac Newton. A noted astrophysicist who did pioneering work on the internal structure of stars, he was also an exceptional mathematician – just the right person to be trying to make sense of Einstein's demanding theory. However Eddington was also a Quaker, a member of a pacifist religious society. Since he wouldn't fight, it looked as if he might end up in prison or a labor camp instead of pursuing science.

Gravity: Newton and Einstein
Newton's physics describes the world of our own everyday experience. Einstein's theories consider the Universe in an entirely different way, a way that defies common sense. Most of the physicists of the time couldn't follow the mathematics, but that wasn't the only problem that they had with relativity. The worldview involved wasn't any easier for them than for most of us.

For Newton, space is where things happen, and time passes in the background. You can describe space using Euclid's geometry, e.g., a straight line is the shortest distance between two points. Gravity is a force of attraction between masses.

But for Einstein, space and time are united in four-dimensional space-time. Mass causes space-time to curve around it. The greater the mass, the greater the curvature. The curvature around an astronomical body is often known as a gravity well. The stronger the curvature, the deeper the well, and the harder it is to escape from it.

This diagram shows how the Moon and an apple move in Earth's gravity well. The apple falls in. Although the fast-moving Moon can't escape, it doesn't fall in. Gravity isn't a force, objects are simply following the geometry of space-time.

How could you test Einstein's theory?
Most of the theory of general relativity couldn't be tested early in the twentieth century. However Einstein said that even the path of light rays would be affected by the way space-time curved around a massive body. Since the Sun is a massive body, starlight passing near the Sun should be deflected.

Problem: when the Sun is out, we can't see the stars. With one exception. You can see both the Sun and stars during a total solar eclipse. This means comparing star positions for when the Sun isn't in the way and when their light skirts the Sun.

Although the idea is simple, its execution isn't. Measuring the angle of the starlight's deflection is difficult, and it was even harder a century ago. Einstein's theory predicts a deflection of 1.75 arc seconds. Making it even trickier is that Newton's theory also predicts a deflection, but only half that of Einstein's. This means that the measurement needs to be good enough to detect not only a deflection, but also to distinguish between the theories.

If you're wondering how small an angle of 1.75 arc seconds is, try to imagine slicing a pie into 360 equal wedges. Then one of those slices would have to be sliced into 3600 slices. Each piece would represent an angle of one arc second.

Solar eclipse May 1919
Frank Dyson, England's Astronomer Royal, persuaded the British government that Eddington's internationally-noted scientific work should exempt him from conscription. One element of Dyson's submission was the contribution Eddington would make to planning for and observing a total eclipse on May 29, 1919, if the war had ended by then. Besides saving a brilliant scientist, Dyson must have realized that a successful observation would be something of a coup for British astronomy. The eclipse had great potential for good results, because it lasted a long time, and the Sun would be in front of a cluster of bright stars.

There were two expeditions. One went to Sobral in Brazil and the other, led by Eddington, went to Principe, an island off the west coast of Africa. Despite the inevitable problems of such an enterprise, they collected enough data for them to be confident that their results were consistent with Einstein's theory, and not with Newton's.

A celebrity
The paper, A Determination of the Deflection of Light by the Sun's Gravitational Field, was presented to a joint meeting of the Royal Society and the Royal Astronomical Society in London. It wasn't just a sensation among the scientists, it was also big news around the world. The headline from the November 10 New York Times proclaimed: Lights all askew in the heavens”. Albert Einstein became an international celebrity.