 That black holes, purely gravitational objects, possess a physical entropy strongly suggests that the gravitational field itself in general may possess entropy, as Penrose (1979) hypothesized. Indeed, there are a number of reasons to suspect that the thermodynamical character of gravity should extend to gravitational systems and structures beyond just those provided by black holes. Because gravitational “charge” (i.e., mass-energy) comes with only one sign (as opposed, e.g., to electromagnetic charge, which can be of either positive or negative sign), bits of matter tend to accelerate towards each other, other things being equal. This already suggests that gravity has a built-in thermodynamical character, since it provides an objective, invariant measure of a direction for time: it is characteristic of future time-flow that bits of matter tend to accelerate towards each other, and so become more inhomogeneous in the aggregate. (See  section 7  for discussion of the possible relation of gravitational entropy to an arrow of time.) 