Newton said gravity is a force reaching across space. Einstein said something stranger and more beautiful: there is no force at all. Mass and energy bend spacetime itself, and everything else simply coasts along the straightest available paths through that curved geometry. A falling apple and an orbiting planet are both just going straight — in a spacetime warped by the Earth and the Sun.
General relativity is that idea made exact. This branch introduces the
equivalence principle, the metric that measures a curved spacetime, the geodesic equation of
motion, the Schwarzschild solution and its black hole, and the classic tests — bending starlight,
Mercury's wandering orbit, and the slowing of time in gravity. It builds on