The discussion revolves around whether a uniformly accelerated charged particle radiates electromagnetic waves, referencing the equivalence principle and its implications in different contexts, including gravitational fields and the nature of radiation. The scope includes theoretical considerations, interpretations of the equivalence principle, and references to historical debates in physics.
Participants do not reach a consensus; multiple competing views remain regarding whether uniformly accelerated charges radiate, with significant disagreement on the implications of the equivalence principle and the nature of radiation.
The discussion highlights limitations in definitions of "radiate" and the conditions under which the equivalence principle is considered valid, as well as the unresolved nature of the mathematical and conceptual arguments presented.
This discussion may be of interest to those studying theoretical physics, particularly in the areas of electromagnetism, general relativity, and the philosophical implications of the equivalence principle.
This is thermal radiation. Gravity just helps to compress and heat stuff, it does not generate the radiation itself.K^2 said:Don't we have direct evidence of orbiting charges radiating from black hole X-Ray radiation? The metric is the same, so I don't see why it should be much different on Earth's orbit, other than in magnitude, of course.
Does that mean that a charge in free-fall will always have a non-symmetric electric field - right down to the smallest distances? That seems to be something that is testable. It seems also to violate the principle of equivalence because it shrinks the meaning of 'local' to an arbitrarily small region.atyy said:Yes, there is a real difference. I'm not sure about the electric dipole specifically, but as a rule of thumb one cannot naively apply the equivalence principle to charged particle trajectories. For example, one might be inclined to think about a "free falling" charge - but generally a charge cannot be free falling because it is acted on by the field it produces.
Andrew Mason said:Does that mean that a charge in free-fall will always have a non-symmetric electric field - right down to the smallest distances? That seems to be something that is testable. It seems also to violate the principle of equivalence because it shrinks the meaning of 'local' to an arbitrarily small region.
Andrew Mason said:I think the dipole is important because one could always create an electric dipole by separating + and - charges in a neutral body. Many molecules (eg. water) have an electric dipole moment. If a dipole in free-fall radiates, then a great deal of matter in freefall must radiate. Mind you, there is likely a quantum mechanical threshold for radiation for an electric dipole ie. quantum harmonic oscillator.
I think you would have to reduce the water temperature to close to absolute 0 as well. Otherwise the thermal radiation would predominate. So it would be a pretty difficult thing to test.atyy said:I don't know the quantitative answer, but here's my guess. Let's first ignore gravity and the equivalence principle, and do classical electrodynamics in flat spacetime. If we put water in a car and accelerate it, we generally do not detect radiation, so water can be treated as effectively neutral and classical at our level of experimental accuracy, ie. we don't worry about water violating classical electrodynamics even though we don't detect radiation when we accelerate it. For this reason, at the same level of accuracy for which water in a car is considered neutral, we also don't expect to detect any violation of the equivalence principle when water is accelerated by a gravitational field.