- noun a theoretical explanation of the nature of gravitation and of motion
- A pair of theories about the nature of gravitation and of motion, respectively called General and Special Relativity, and developed mainly by Einstein. The General Theory was confirmed by astronomical observations of the effect of the mass of the Sun on passing starlight, while the Special Theory remains controversial, although evidence for some of its predictions, like the increase in mass of particles approaching the velocity of light, has been obtained in particle accelerators. The effect of relativity on astronomy cannot be underestimated. It starts by showing how stars shine and continues with the whole field of relativistic astrophysics, the content of which covers black holes, the big bang, and almost all other major issues surrounding the origin, large-scale structure and evolution of the universe. Gravity Probe B, also called the Relativity Mission, was a NASA space probe launched in 2004 to test the general theory of relativity by looking for the twisting of spacetime caused by the mass of the Earth.