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| {{electromagnetism}}
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| In [[electromagnetism]], [[electromagnetic waves]] in [[vacuum]] travel at the [[speed of light]] ''c'', according to [[Maxwell's Equations]]. The '''retarded time''' is the time when the field began to propagate from a point in a [[charge distribution]] to an observer. The term "retarded" is used in this context (and the literature) in the sense of [[propagation delay]]s.
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| ==Retarded and advanced times==
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| [[File:Universal charge distribution.svg|250px|right|thumb|Position vectors '''r''' and '''r′''' used in the calculation.]]
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| The calculation of the retarded time ''t<sub>r</sub>'' is nothing more than a simple "[[speed|speed-distance-time]]" calculation for EM fields.
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| If the EM field is radiated at [[position vector]] '''r'''' (within the source charge distribution), and an observer at position '''r''' measures the EM field at time ''t'', the time delay for the field to travel from the charge distribution to the observer is |'''r''' − '''r''''|/''c'', so subtracting this delay from the observer's time ''t'' gives the time when the field ''actually began to propagate'' - the retarded time<ref>Electromagnetism (2nd Edition), I.S. Grant, W.R. Phillips, Manchester Physics, John Wiley & Sons, 2008, ISBN 978-0471-927129</ref><ref>Introduction to Electrodynamics (3rd Edition), D.J. Griffiths, Pearson Education, Dorling Kindersley, 2007, ISBN 81-7758-293-3</ref>
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| :<math>t_r = t - \frac{|\mathbf{r}-\mathbf{r}'|}{c}</math>
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| which can be rearranged to
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| :<math>c = \frac{|\mathbf{r}-\mathbf{r}'|}{t - t_r}</math>
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| showing how the positions and times correspond to source and observer.
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| Another related concept is the '''advanced time''' ''t<sub>a</sub>'', which takes the same mathematical form as above:
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| :<math> t_a = t + \frac{|\mathbf r - \mathbf r'|}{ c}</math> | |
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| and is so-called since this is the time the field will advance from the present time ''t''. Corresponding to retarded and advanced times are [[retarded and advanced potential]]s.<ref>McGraw Hill Encyclopaedia of Physics (2nd Edition), C.B. Parker, 1994, ISBN 0-07-051400-3</ref>
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| ==Application==
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| Perhaps surprisingly - electromagnetic fields and forces acting on charges depend on their history, not their mutual separation.<ref>Classical Mechanics, T.W.B. Kibble, European Physics Series, McGraw-Hill (UK), 1973, ISBN 007-084018-0</ref> The calculation of the electromagnetic fields at a present time includes integrals of [[charge density]] ρ('''r'''', ''t<sub>r</sub>'') and [[current density]] '''J'''('''r'''', ''t<sub>r</sub>'') 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.
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| ==See also==
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| *[[Antenna measurement]]
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| *[[Electromagnetic four-potential]]
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| *[[Jefimenko's equations]]
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| *[[Liénard–Wiechert potential]]
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| *[[Light-time correction]]
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| ==References==
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| {{reflist}}
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| {{DEFAULTSORT:Retarded Time}}
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| [[Category:Time]]
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