relativity

Einstein’s framework unifying spacetime, gravity, energy, and mass.

• special relativity: the speed of light is constant for all observers in spacetime
• time dilation and length contraction follow from invariance of light speed
• mass-energy equivalence: E = mc², linking rest mass to energy
• general relativity: gravity is curvature of spacetime caused by mass and energy
• predicts black holes, gravitational waves, and expansion of the universe — see cosmology
• classical limit recovers mechanics at low velocities and weak gravity
• merging with quantum mechanics remains an open frontier