Einstein announces the general theory of relativity
Albert Einstein had described the special theory of relativity in 1905. The result of Einstein's thinking about light, this theory introduced brand-new ideas to science. It opened up an entire field of physics, but left Einstein with some nagging questions. The problems of gravity and acceleration would not go away.
After thinking about the problems for 10 years, he published the general theory of relativity. In it, he suggested that gravity is not a force, as Newton had believed, but the result of a curvature of the space-time continuum -- the four dimensional world in which we live. He used a thought experiment to compare the force felt from gravity with acceleration. Imagine you are in an elevator and feel what you believe is the force of gravity, holding you to the floor. According to Einstein, since you cannot see outside the elevator, you cannot tell if you are feeling the force of gravity or if the elevator is being pushed toward your feet. Einstein stated that the two forces are actually identical. Furthermore, if you were in the elevator accelerating upward and a beam of light entered the elevator parallel to the floor, the light beam would appear to bend downward. This meant that light, which ordinarily traveled in straight lines, could curve if it traveled across a gravitational field. This curving path of light meant that that "field" was really a curving of space, which Einstein found was inseparable from time. The curvature would be caused by bodies with great mass.
A weak gravitational field indicates nearly flat space-time, and there Newton's theories seem to apply. But a strong gravitational field throws classical predictions off. Einstein postulated three ways this theory could be proved. One was by observing the stars during a total solar eclipse. The sun is our closest strong gravitational field. Light traveling from a star through space and passing the sun's field would be bent, if Einstein's theory were true. If you could see the star during the day, he predicted, it would be in a different place than at night. The only chance to see it during the day would be during an eclipse.
On March 29, 1919, that opportunity came. British Astronomer Sir Arthur Eddington traveled to Príncipe Island off the western coast of Africa. His team photographed starfields during the eclipse and compared the photos with those of the same starfield taken when the sun was not present. Eddington found the apparent location of the stars had shifted, just as Einstein predicted.
Further proofs of Einstein's theory came with advancing technology through the 1960s and continue in the present. But the immediate impact in 1919 was enormous. World War I had just ended. Einstein became a celebrity, and within a year, more than 100 books had been published about his theories. Leopold Infeld, who worked with Einstein on a book on relativity, suggested, "people were weary of hatred, of killing. . . . Here was something which captured the imagination . . . [t]he mystery of the sun's eclipse and of the penetrating power of the human mind."