A COMMENT ON JUNCTION AND ENERGY CONDITIONS IN THIN SHELLS

This paper contains a suggestion for a slight modification of Israel's covariant formulation of junction conditions between two spacetimes, placing both sides on equal footing with normals having uniquely defin ed orientations. The signs of mass-energy densities in thin shells at the junction depend not only on the orientations of the normals and it is useful therefore to discuss the signs separately. Calculations gai n in clarity by not choosing orientations in advance. Simple examples illustrate our point and complete previous classifications of spherica l thin shells in spherically symmetric spacetimes relevant to cosmolog y.