On the radiating and nonradiating components of scalar, electromagnetic, and weak gravitational sources

We derive a new description of two complementary classes of scalar, vector, or tensor sources, namely, nonradiating (NR) sources and sources that lack a NR part, i.e., “purely radiating” sources. The class of purely radiating sources is shown to be exactly the class of solutions-within the source’s support-of the homogeneous form of the associated partial differential equation relating the sources to their fields; i.e., purely radiating sources are themselves fields. We show that this result is related to the well-known reciprocity principle and establish a new definition of a localized NR source based on that principle. A broad class of physically relevant sources, all of which possess a NR part, is identified. The role of NR sources in absorption of radiation and energy storage is addressed. 1999