Discover: How Would Time Travel Actually Work (or, something for all the Whovians)?

A good, cogent article on the physics of time travel:
http://discovermagazine.com/2010/mar/02-the-real-rules-for-time-travelers/

I first encountered this explanation of time travel in Harry Potter and the Prizoner of Azkaban, where Harry and Hermione use a Time-Turner to save Sirus and Buckbeak. (I trust that I didn't spoil the book for anyone -- and if I did, you're way behind the times!) The detail of the Time Turner chapter that I found most memorable was the thud of the executioner's axe: Harry and Hermione heard it fall in real time and assumed that Buckbeak had been beheaded; however, when they went back in time to rescue the hippogryff, they realized that the sound they'd thought was Buckbeak's decapitation was actually the executioner swinging the axe into a hunk of wood in frustration upon discovering that Buckbeak had escaped. Harry and Hermione, therefore, did not change the timeline using the Time Turner; rather, events incorporated all nonlinear parts and intersections of the timeline.

The concept of free will, as the above article notes, becomes problematic if this framework of time is true. Nature, it would seem, is more like a Greek tragedy than we Americans would like to believe. We are uncomfortable with the idea of external limits on our free will, and time is too nebulous to most of us to think its laws as insurmountable as, say, those of Newtonian physics. If we were told that our free will is limited by gravity (no matter how much we might wish to levitate through the air, we cannot), we do not find it unjust. Told that Oedipus had no choice but to kill his father and marry his mother, despite all of his attempts to escape his fate, we find his situation grossly unfair. The above description of time travel suggests that our universe operates more like Oedipus's than we'd imagined.

I believe in free will. I do. But I'd love to try time travelling -- it sounds fascinating.