There’s an interesting post on a blog at Discover Magazine about time travel, a subject which I find endlessly fascinating. I love it because it’s a “top-ten” list that starts at zero, so it’s actually 11 items long. I also like it because it doesn’t shy away from a little bit of science in laying out how time travel is possible, but difficult. A sample:
7. There is no meta-time.
The least realistic time-travel movie of all time might be Back to the Future. When Marty McFly changes the past (violating Rule 6), the future “instantaneously” changes. What the hell is that supposed to mean? Time measures the temporal interval between different events in spacetime, and can be quantified by clocks. There is no set of clocks outside the universe, with respect to which you can go muck around in the past and have effects propagate into the future “at the same time.” Likewise, your brain is not going to change to remember things differently, nor will any other record-keeping device such as diaries or photographs or embarrassing sex tapes. Sorry about that.
There’s some interesting assertions in the list about what you can or can’t do, as a theoretical time traveller, and then he throws it all out the window by saying, “well, if you went to the past in an alternate universe, then maybe!”