Einstein's new general theory of relativity predicted a remarkable effect: when a ray of light passes near a massive body, the ray should be bent. For example, starlight passing near the sun should be slightly deflected by gravity. This deflection could be measured when the sun's own light was blocked during an eclipse. Einstein predicted a specific amount of deflection, and the prediction spurred British astronomers to try to observe a total eclipse in May 1919. Feverish preparations began as the war ended. Two expeditions, one to an island off West Africa and the other to Brazil, succeeded in photographing stars near the eclipsed sun. The starlight had been deflected just as Einstein had predicted.

In a letter to an astronomer in 1913, Einstein included a sketch (right) that showed how gravity should deflect light near the sun, making stars appear to shift their positions. A photograph (below) from one of the expeditions shows the eclipsed sun. Some stars are circled and artificially enhanced in this reproduction. These apparent positions deviated from the positions of the stars photographed when the sun was elsewhere in the sky. As a ripple a pane of in glass is detected when objects seen through the glass are distorted, so we detect here a warping of space itself.

Click here to see a "gravitational lens"