Twin paradox

A thought experiment in which twins take different paths through spacetime and reunite having aged by different amounts. It illustrates how elapsed proper time depends on a traveler’s route, not merely on relative motion.

Proper time: The time measured by a clock following a particular path through spacetime. Each twin’s age at reunion corresponds to proper time accumulated along that twin’s path.
Special relativity: A theory of space, time, and motion that applies to inertial frames without gravity. The standard twin-paradox analysis follows from its treatment of inertial motion and clock readings.
General relativity: A theory describing gravity as the curvature of spacetime. It extends clock comparisons to journeys where gravity or curved spacetime matters.
Langevin's twin paradox: Paul Langevin's early presentation of the twin-aging thought experiment in 1911. It helped establish the paradox as a prominent illustration of relativistic time.
Spacetime diagram: A graph that represents events and worldlines in space and time. The twins’ paths can be compared geometrically between departure and reunion.
Inertial frame: A reference frame in which an unforced object moves at constant velocity. The outbound and inbound portions of the traveling twin’s trip can each be analyzed in an inertial frame.
Bell's spaceship paradox: A thought experiment about the changing distance between accelerating spaceships connected by a string. It also examines acceleration in relativity, but concerns length and stress rather than unequal aging.
Spaceflight twin paradox: A version in which one twin travels on a relativistic spaceflight and returns to Earth. The familiar outbound-and-return journey makes the unequal proper times concrete.
Worldline: A path through spacetime representing an object's history. Each twin follows a distinct worldline, and their elapsed times need not match.
Relativity of simultaneity: The principle that observers in relative motion can disagree about which distant events occur at the same time. It resolves the apparent symmetry by changing the traveling twin’s assignment of distant times at turnaround.

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Ladder paradoxRelated: It is a well-known relativity paradox resolved by distinguishing inertial frames and event histories.