Via the Edge, here is Sean Carroll on the arrow of time:
Our experience of time depends upon the growth of entropy. You can't imagine a person looking around and saying, "Time is flowing in the wrong direction," because your sense of time is due to entropy increasing. . . . This feeling that we're moving through time has to do with the fact that as we live, we feed on entropy. . . . Time exists without entropy, but entropy is what gives time its special character.Entropy is for Carroll what gives time its "appearance of forward motion," its "directionality, the distinction between past and future." Entropy is also the guarantor that time moves in one direction only, and is not (despite all our searches for lost time in imagining time machines and time travel in our speculative fiction) reversible.
The long and the short of it for medievalists: fieldwork in your area of study is not going to be possible. On the positive side, though, Carroll's work also suggests that this universe may be a fragment of an infinitely larger multiverse -- one in which, for example, you are not a graduate student locked in a dank archive for the summer but the actual ruler of the Angevin empire. Think of that as you are coughing at all the dust on that parchment you're scrutinizing.
As for me, I'm in my office right now, because I've agreed to evaluate THREE tenure and promotion dossiers this summer. The only thing that gets me through this chore is knowing that in some other universe within this multiverse some other Jeffrey J. Cohen is worshiped as a living god and resides on a perpetually sunny beach.
Now go read Eileen's very smart post.