Albert Einstein. Caricature of the Swiss-German physicist Albert Einstein (1879-1955) displaying his watch, representing his theories of time. Einstein is best known for his theories of special and general relativity, which together laid the basis for much of modern physical and cosmological thought. The theories explain gravity as a curvature of spacetime, say that time moves more slowly in fast-moving objects, state the speed of light is the same in any frame of reference and make it possible to derive the famous equation E=mc2. This shows the equivalence of mass and energy, and allows the power of nuclear weapons to be calculated, amongst other things.