# Causality and EM potentials

1. Nov 13, 2005

### bakshi

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 at the speed of light toward 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 information that is not light must 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

2. Nov 13, 2005

### bakshi

I found the answer to my own question. You can choose any gauge you want for the potentials: no matter what the speed of the collecting sphere (which is even infinite for the scalar potential in the Coulomb gauge), they will all lead to the same equations for the fields, and these equations are causal at speed c. Therefore if potentials are not considered real in classical physics it should not be mentioned that the Lorenz gauge has to be chosen for causality reasons.

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