Choosing the Lorenz gauge implies that sources of the EM potentials at a given point are the charge density (for scalar potential) and current density (for vector potential) that cross a collecting sphere converging(adsbygoogle = window.adsbygoogle || []).push({}); at the speed of lighttoward that point. It is often said that the retarded potentials thus obtained satisfy causality.

My question is the following: Why is the collecting sphere converging at exactly the speed of light? Couldn't it converge at a lower speed?

What is the justification for assuming that an informationthat is not lightmust travel at the speed of light? Does some equation require it, is it empirical knowledge or is it just a postulate?

Thank you for your help,

Bakshi

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# Causality and EM potentials

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