From Eternity to Here

by Sean Carroll

The weirdest thing about the idea that the space of states changes with time is that it requires an external time parameter—a concept of “time” that lives outside the actual universe, and through which the universe evolves. Ordinarily, we think of time as part of the universe—a coordinate on spacetime, measured by various sorts of predictably repetitive clocks. The question “What time is it?” is answered by reference to things going on within the universe—that is to say, to features of the state the universe is currently in. (“The little hand is on the three, and the big hand is on the twelve.”) But if the space of states truly changes with time, that conception becomes insufficient. At any one moment, the universe is actually in one specific state. It makes no sense to say, “The space of states is smaller when the universe is in state X than when it is in state Y.” The space of states, by definition, includes all of the states the universe could hypothetically be in.