ИСТИНА

We discuss the problem of radiation from massless particles in classical and quantum electrodynamics and gravity. Since the Compton length of a charge goes to infinity , one can hardly expect radiation from the (hypothetical) massless charge moving in some external field to have classical limit. We illustrate this considering the synchrotron radiation from massless scalar charge. The total radiation power diverges in the classical limit, while the massive QED result for synchrotron radiation goes smoothly to the result of massless QED. The situation in gravity is different: the total power of the classical gravitational synchrotron radiation form a massless particle moving along the null closed geodesic in the vicinity of a black hole remains finite in the massless limit, in spite of the fact that the frequency cutoff diverges. Invoking the result of Weinberg, we also discuss whether the bremsstrahlung problems in electrodynamics and gravity share similar features.