Space · Time · Gravity
Relativity describes a universe in which space and time are joined, motion changes how they are measured, and gravity reshapes their geometry.
Meet the two theoriesSpecial relativity is Einstein's theory of space, time, and motion when gravity can be neglected. The laws of physics have the same form in every inertial frame, and every inertial observer measures the same speed of light in vacuum. Time, distance, and the simultaneity of separated events can therefore depend on the chosen inertial frame.
General relativity is Einstein's theory of gravitation. Matter and energy shape spacetime geometry; that geometry governs free fall and the paths of light. Clock rates can also differ between regions with different gravitational conditions.