Relativity

Relativity is really two theories. Special relativity (1905) starts from one strange fact — everyone measures the same speed of light, however they move — and follows it to the conclusion that space and time themselves are relative. General relativity (1915) then extends this to gravity, recasting it not as a force but as the curving of spacetime by mass and energy. Both have passed every experimental test thrown at them, to extraordinary precision.^[1]