The absence of this quantity in the Wheeler-Dewitt equations necessitated approaches that generate this quantity, such as the emergent definition of this quantity in the Page-Wootters formalism. A foliation produces a series of surfaces indexed by this quantity in the ADM formalism. The integral of “ds over c” along a worldline defines one form of this quantity. The “problem” of this quantity tries to reconcile its dynamical treatment in general (*) relativity with its treatment as an absolute background parameter in quantum mechanics. Intervals that are negative, and thus fall inside the light cone, are described as “like” this (10[1])quantity. This quantity changes more slowly for a sibling traveling in a rocket in the twin paradox. For 10 points, relativity unifies space with what quantity? ■END■