Retarded time

Retarded time is the earlier time at which a source emitted a signal now reaching an observation point, accounting for finite propagation speed.

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Light-cone: The set of spacetime events that can be connected to a given event by signals traveling at light speed. Its past boundary identifies events capable of influencing an observation.
Finite propagation speed: The principle that physical influences travel at a bounded speed rather than acting instantaneously. A nonzero travel time separates emission from observation.
Radar: A system that detects objects by transmitting radio waves and analyzing their returned signals. Radar infers target range from the delayed return of an emitted pulse.
Advanced time: The later source time associated with a signal propagating backward from an observation event. It points toward future source events rather than past emissions.
Liénard–Wiechert potentials: The exact electromagnetic potentials generated by a point charge moving along an arbitrary trajectory. Their source position and velocity are evaluated at retarded time.
Speed of light: The speed at which light travels in vacuum, approximately 299,792,458 meters per second. It sets the delay for electromagnetic signals across vacuum.
Electromagnetic field: A physical field that describes electric and magnetic influences throughout space and time. Retarded source data determine time-dependent electromagnetic fields.
Instantaneous action at a distance: A model in which one body influences another without a propagation delay across space. It omits the finite travel time built into retarded time.
Jefimenko's equations: Time-dependent equations expressing electric and magnetic fields in terms of charge and current distributions. They calculate fields from source distributions at their retarded times.
Spacetime: The four-dimensional framework combining space and time into a single structure. Emission and observation are distinct events within spacetime.