While astronomers surveyed the heavens, physicists were learning how the universe works. The greatest of these researchers, whose name would rank with Newton's, was Albert Einstein, who was born in 1879 in Germany.

In 1915, Einstein proposed a general theory of relativity, a new explanation for gravity. The theory described gravity in geometric terms, as a curvature of space. In this view, a planet or other large body warps the space around it, much as a bowling ball placed on a mattress would form a depression in it. The warping of space causes smaller objects to move in curved paths toward a massive object, just as marbles would roll toward the bowling ball because of the depression in the mattress.

Einstein also pondered the nature of the universe as a whole. The theory of relativity predicted that the universe could not exist in an unchanging condition, but rather had to be either expanding or contracting. But at that time, all the evidence pointed to a static universe. Rather than insisting that his theory was accurate, Einstein altered his equations by adding a factor he called the “cosmological constant,” which counteracted gravity to produce a changeless universe.