Relativity

Relativity is the framework in which measurements of space and time depend on an observer’s motion, while gravity is described as the curvature of spacetime.

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Principle of relativity: The principle that physical laws take the same form in all inertial reference frames. It underlies the claim that no inertial observer has a privileged status.
Special relativity: A theory of space and time for inertial observers, built on the constancy of light speed. It explains how motion changes measured durations, lengths, and simultaneity.
Michelson–Morley experiment: An 1887 experiment that found no detectable difference in light speed caused by Earth's motion through a hypothesized ether. Its null result challenged the ether hypothesis preceding special relativity.
Newtonian mechanics: A theory of motion based on absolute time and forces, highly accurate at everyday speeds and weak gravity. Relativity replaces its absolute time while recovering its predictions in ordinary conditions.
Mass–energy equivalence: The relation between mass and energy expressed by the equation E = mc². Special relativity makes mass a form of energy with measurable consequences.
Inertial frame of reference: A reference frame in which objects with no net force move at constant velocity. Special relativity states its laws in frames moving uniformly relative to one another.
General relativity: A theory in which gravitation arises from the curvature of spacetime caused by mass and energy. It extends relativity to gravity and accelerated motion.
Perihelion precession of Mercury: The gradual rotation of Mercury’s orbital ellipse, including an anomalous component explained by general relativity. General relativity accounts for the excess precession unexplained by Newtonian gravity.
Newtonian gravity: A theory describing gravity as an instantaneous attractive force between masses. It contrasts with general relativity’s account of gravity as spacetime curvature.
Relativistic momentum: Momentum defined to remain consistent with special relativity at all speeds below light speed. It replaces classical momentum when objects move near light speed.