A note on spacelike and timelike compactness

Understand

When studying the causal propagation of a field in a globally hyperbolic spacetime M, one often wants to express the physical intuition that it has compact support in spacelike directions, or that its support is a spacelike compact set.

• We compare a number of logically distinct formulations of this idea, and of the complementary idea of timelike compactness, and we clarify their interrelations.
• E.g., a closed subset A of M has a compact intersection with all Cauchy surfaces if and only if A is contained in J(K) for some compact set K.
• (However, it does not suffice to consider only those Cauchy surfaces that partake in a given foliation of M.) Similarly, a closed subset A of M is contained in a region between two Cauchy surfaces if and only if the intersection of A with J(K) is compact for all compact K.