Relativity

The first thing to note about the theory of relativity is that there are two different theories of relativity, the special theory put forward in Albert Einstein's (1879–1955) famous 1905 paper, "On the Electrodynamics of Moving Bodies," and the general theory, completed in November 1915 and first presented systematically in a review article published in March 1916.

Special relativity extends the principle of relativity for uniform motion, known in mechanics since the days of Galileo Galilei (1564–1642), to all of physics, in particular to electro-dynamics, the field out of which the theory grew. Although the key contribution was Einstein's, several other scientists deserve credit for it as well, most importantly the Dutch physicist H. A. Lorentz (1853–1928), the French mathematician Henri Poincaré (1854–1912), and the German mathematician Hermann Minkowski (1864–1909).

General relativity, by contrast, was essentially the work of one man. It was the crowning achievement of Einstein's scientific career. Its name, however, is something of a misnomer. The theory does not extend the principle of relativity for uniform motion to nonuniform motion. It retains the notion of absolute acceleration—that is, acceleration with respect to space-time rather than with respect to other bodies. In this sense, general relativity is no different from Newtonian theory or special relativity. Absolute acceleration, however, is much more palatable in general relativity than in these earlier theories.

From the point of view of modern physics, the question to what extent general relativity fulfilled Einstein's original hopes of relativizing all motion is of secondary importance. What matters most is that general relativity is a powerful new theory of gravity, based on the insight, called the equivalence principle, that the effects of gravity and those of acceleration ought to be described by one and the same structure: curved space-time.