Worldline

A worldline is a curve in spacetime that traces an object's position through time. Its shape records the object's motion and, in relativity, the elapsed proper time along its path.

Spacetime diagram: A plot that represents events using spatial and time coordinates. It depicts worldlines as paths through spacetime coordinates.
Spacetime: The four-dimensional structure combining three dimensions of space with one of time. A worldline is a path within this unified structure.
Twin paradox: A relativity thought experiment in which twins following different spacetime paths reunite at different ages. The twins' distinct worldlines explain their different elapsed proper times.
Worldsheet: A surface traced through spacetime by an extended object as it evolves. A string sweeps out a two-dimensional worldsheet rather than a one-dimensional worldline.
Proper time: The time measured by a clock traveling along a particular path through spacetime. It measures elapsed time along a timelike worldline.
Event (relativity): A physical occurrence specified by a location and a time in spacetime. A worldline is a continuous sequence of events associated with an object's history.
Muon: An unstable elementary particle similar to an electron but much heavier. Its worldline helps explain why fast-moving muons reach Earth's surface before decaying.
Spacelike curve: A spacetime curve whose tangent lies outside the light cone. It cannot represent the path of a physical object moving at or below light speed.
Four-velocity: The rate of change of spacetime position with respect to proper time. It describes the tangent direction and magnitude of a massive object's worldline.
Reference frame: A coordinate system and set of clocks used to assign measurements to events. Coordinates draw a worldline, while different frames assign different coordinates to it.

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