Event (relativity)

An event is a physical occurrence specified by a location and a time. In relativity, its spacetime coordinates depend on the reference frame used to describe it.

Spacetime: The four-dimensional structure combining three dimensions of space with one dimension of time. An event is a point in spacetime, rather than a point in space alone.
Lorentz transformation: A linear transformation relating spacetime coordinates measured in inertial frames moving at constant relative velocity. It converts the coordinates assigned to the same event by different inertial observers.
Causal structure: The pattern of which events can influence which other events, constrained by spacetime geometry and signal speeds. Event relations determine whether a physical influence could connect two occurrences.
Invariant: A quantity or property that remains unchanged under a specified class of transformations. The event's coordinates vary by frame, while quantities such as intervals can remain invariant.
Reference frame: A coordinate system and set of clocks used to assign positions and times to physical occurrences. Event coordinates are reported relative to a chosen frame.
Minkowski diagram: A spacetime graph that represents events, worldlines, and light cones, usually with one space axis and one time axis. It makes coordinate changes and causal relations between events visible.
Radar time: A method for assigning times and distances using the travel time of light signals sent and received by an observer. It gives an operational way to assign coordinates to distant events.
Absolute time: A universal time assumed to pass identically for all observers, independent of motion or location. Relativity rejects a single frame-independent time coordinate for all events.
Worldline: A path through spacetime representing the history of an object or particle. A moving object's successive events trace its worldline.
Spacetime interval: A quantity formed from coordinate differences between two events that is invariant under Lorentz transformations. Unlike coordinate separations, the interval is unchanged between inertial frames.

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