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- Thread starter Loren Booda
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Hans de Vries

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It's the second one:

The reason is ultimately the conservation of charge. If you could "switch"

a charge on and off then you would have perfectly spherical radiation.

This would however violate the conservation of charge given by [itex]\partial_\mu J^\mu=0[/itex],

where J is the 4-vector current. The radiation would be longitudinally

polarized, with E in the direction of the motion and without a B component.

The fact that we always can describe radiation by orthogonal E and B

fields transversal to the direction of the motion is ultimately also the

result of the conservation of charge: [itex]\partial_\mu J^\mu=0[/itex] leads to [itex]\partial_\mu A^\mu=0[/itex],

which is the conservation of the potential field. This restriction then leads

to a reduction in the degrees of freedom of the electric radiation field.

Regards, Hans

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