Application
Perhaps surprisingly - electromagnetic fields and forces acting on charges depend on their history, not their mutual separation. The calculation of the electromagnetic fields at a present time includes integrals of charge density ρ(r', tr) and current density J(r', tr) using the retarded times and source positions. The quantity is prominent in electrodynamics, electromagnetic radiation theory, and in Wheeler-Feynman absorber theory, since the history of the charge distribution affects the fields at later times.
Read more about this topic: Retarded Time
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