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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 delays. ==Retarded and advanced times== The calculation of the retarded time ''tr'' or ''t''′ is nothing more than a simple "speed-distance-time" calculation for EM fields. 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, ''t''′.〔Electromagnetism (2nd Edition), I.S. Grant, W.R. Phillips, Manchester Physics, John Wiley & Sons, 2008, ISBN 978-0471-927129〕〔Introduction to Electrodynamics (3rd Edition), D.J. Griffiths, Pearson Education, Dorling Kindersley, 2007, ISBN 81-7758-293-3〕 : which can be rearranged to : showing how the positions and times correspond to source and observer. Another related concept is the advanced time ''ta'', which takes the same mathematical form as above, but with a “+” instead of a “−”: : 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 potentials.〔McGraw Hill Encyclopaedia of Physics (2nd Edition), C.B. Parker, 1994, ISBN 0-07-051400-3〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Retarded time」の詳細全文を読む スポンサード リンク
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