ONE hundred years ago, on November 25th 1915, Albert Einstein presented his freshly finished general theory of relativity to the Prussian Academy of Sciences. It was the outcome of nearly a decade's dedicated work. He showed that the theory solved a 150-year-old problem: each year, Mercury's closest point of approach to the Sun was moving forward more than it was expected to. All manner of explanations had been put forth, including an unseen planet called Vulcan, but relativity did the job perfectly. In 1916, Einstein predicted that relativistic effects would cause the apparent positions of stars to change during an eclipse, as the sun bent the distant stars' rays. That prediction was proved right in 1919, in a widely publicised expedition that propelled Einstein to global fame. But what exactly is his general theory of relativity all about?
Generally, it is about gravity. In 1905, Einstein had put forth his special theory of relativity, which concerned itself with objects and experimenters travelling at speeds near that of light. Einstein took this to be an absolute speed limit. His flash of insight was to fuse the three dimensions of space with that of time and create a single, mutable whole: spacetime. To make sure that light was always seen to move at light speed, the theory predicted weird effects like the shrinking of physical extent or stretching of time as objects got faster. Counterintuitive though it was, the theory worked. But as its name suggests, it was a special case—for movement at constant speeds in straight lines. Einstein knew, for example, that his ideas didn't match up with Isaac Newton's theory of gravity, which presumed unchanging dimensions of space and made no mention of time.
His quest to crack this problem began as he sat working at the Swiss federal patent office, having what he later described as "the happiest thought of my life": someone falling off a roof does not feel his own weight. That objects in free-fall do not experience gravity was a hint that gravitation and acceleration were identical. Einstein imagined a person in a cabin in outer space (this being long before manned spaceflight, he conjured a "spacious chest resembling a room") being pulled along in such a way that the person inside would find the situation indistinguishable from that on earth. Gravity not only looked like acceleration, he concluded; it was acceleration. But what provides that acceleration? Einstein's great insight this time was that it is an effect of mass actually stretching spacetime, creating a kind of dip into which objects fell, or circled, as around a plughole. Matter is not pulled by gravity, it falls along the path of least resistance, tracing out the shape of spacetime itself.
After the 1919 eclipse, general relativity lapsed into the shadows. Physicists were distracted by another flashy new theory, quantum mechanics, or working on the physics of atomic nuclei, which was also booming. It was not until odd sources of radio waves, eventually called quasars, were discovered in the late 1950s that general relativity started to stage a comeback. Quasars, it emerged, represented black holes, a theoretical outcome of relativity that even Einstein thought too weird to be true. By now they are taken to be fundamental constituents of the cosmos and to lie at the centres of most galaxies. General relativity has become an essential tool for all matters astronomical and cosmological, right back to the beginning of the universe itself. It is used to correct satellite-navigation data (for satellites experience a slightly different spacetime-stretching in orbit than you and your smartphone do) and to plan the kinds of space missions that can steer a space probe with 150-km precision past Pluto, nearly 5 billion km away—as the New Horizons mission did in July. Relatively speaking, Einstein's has become a fabulously successful theory.