I read about an interesting counter-point to this a while ago. Imaging a ring of matter facing us, with a small source of radiation in the middle, which periodi

I read about an interesting counter-point to this a while ago. Imaging a ring of matter facing us, with a small source of radiation in the middle, which periodically emits a bunch of radiation. This radiation will strike the ring roughly at the same time, which is then re-rediated at us, in synchrony, even though the ring may very large (e.g. a light year across).