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Since light is just a changing electromagnetic wave through space, could you create light by changing a charge density at a certain frequency?

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Closely related question: physics.stackexchange.com/questions/5046/… (I think you will find the top answers there helpful.) –  Nathaniel Aug 6 '12 at 15:21
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up vote 1 down vote accepted

Changing the charge density at optical frequencies is hard to do, but suppose we wind the frequency down a bit to FM radio frequencies. The question is then whether changing the charge density at FM frequencies would cause radio waves to be emitted, and of course the answer is yes because that's what a radio aerial does.

Well, EM waves would only be emitted if there was a dipolar component of the charge density that was changing, but in practice unless your aerial is a perfect sphere I think there will always be some oscillating dipole present and hence your bar of metal will glow (at radio frequencies)

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what's wrong with higher multipoles? If you had a quadrupole source, with vanishing dipole moment, you'd get quadrupolar radiation, which is perfectly normal. –  Emilio Pisanty Aug 6 '12 at 16:45
@EmilioPisanty: agreed. What I meant was you need at least a dipole. If you could work out how to make the charge oscillate on a sphere it wouldn't radiate. –  John Rennie Aug 6 '12 at 16:51
Thanks @JohnRennie, is it that a light wave propagates perpendicular to the direction of it's electric field is the reason that a sphere wouldn't radiate? I guess that makes sense, from my understanding it could possibly go to the sides, because there is no clearly described way that light gets "started", but yeah that makes sense. –  Mike Flynn Aug 6 '12 at 18:06
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Subjecting a charge density to a hight frequency EM field produces a wider spectrum EM through Bremsstrahlung (any accelerated charge radiates). This property is used in laser plasma interaction or in free electrons laser. There are many complex phenomena involved in laser plasma interactions ranging from particle acceleration to frequency doubling, you will find more information if you look at this link: http://en.wikipedia.org/wiki/X-ray_laser An other approach is the free electrons laser where once again the charges are accelerated in an EM field to produce light. http://en.wikipedia.org/wiki/Free_electron_laser

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