  We give a conceptual exposition of aspects of gravitational radiation,
especially in relation to energy. Our motive for doing so is that the strong
analogies with electromagnetic radiation seem not to be widely enough
appreciated. In particular, we reply to some recent papers in the philosophy of
physics literature that seem to deny that gravitational waves carry energy.
  Our argument is based on two points: (i) that for both electromagnetism and
gravity, in the presence of material sources, radiation is an effective
concept, unambiguously emerging only in certain regimes or solutions of the
theory; and (ii) similarly, energy conservation is only unambiguous in certain
regimes or solutions of general relativity. Crucially, the domain of (i), in
which radiation is meaningful, has a significant overlap with the domain of
(ii), in which energy conservation is meaningful. Conceptually, the overlap of
regimes is no coincidence: the long-standing question about the existence of
gravitational waves was settled precisely by finding a consistent way to
articulate their energy and momentum.
