The Built on Facts blog asks a question in Physics which anyone can understand, and could have been asked at any time after Maxwell's work 150 odd years ago.
Short version: an accelerating charge radiates. So if you let an electron fall in a gravitational field, it should radiate. But a person (or detector) falling along beside it does not perceive the electron as accelerating and so the electron shouldn't radiate in their frame. How to reconcile the perspectives?Commenter Alex Besogonov says that the answer may be the Unruh effect.