 In a tour de force of physical argument, Wall (2013a, 2013b) showed that assumption of the Generalized Second Law rules out traversable wormholes, other forms of faster-than-light travel between distant regions, negative masses for physical systems, and closed timelike curves. (See the Encyclopedia entries  Time Machines  and  Time Travel,  and Visser 1996.) Furthermore, if the Generalized Second Law is to be satisfied, then it is impossible for “baby universes” that eventually become causally independent of the mother universe to form. Such baby universes and their eventual independence, however, constitute the fundamental mechanism for currently popular “multiverse” scenarios in cosmology. 