In my last “Ask a Physicist” column, I all but dared the io9 readers to send me questions about time travel. And they did. In droves. It seems that I’m morally compelled to do next week’s column on time machines.
I’m not exactly shy about writing about time travel (and see the links within the links), but I’ve gotten a little disenchanted with it. I have always taken the position that the only time travel that possibly makes any sense to discuss are those permitted by general relativity. Otherwise we’re in the realm of pure science fiction. The problem is that every GR solution for a closed timelike curve (aka time machine) seems to have a possibly insurmountable physical requirement: exotic matter may not exist, cosmic strings may not exist, our universe has no net sense of rotation, etc. (If you don’t know about these time machines, don’t worry, I’ll write about them next week).
But here’s the rub: since general relativistic time machine may not be physically possible, discussing them is only incrementally better than talking about TARDISes and DeLoreans and the like. I’ll put in all the caveats about how most of these things may not be physically possible, of course, but history (and the comments section) teaches me that people will use that as an opportunity not to question the possibility of time travel, but to upgrade their favorite crazy version of time travel to “just as physically viable as wormholes.”
I’ll tell you one thing I won’t do in this column. Except for the briefest of mentions, I’m not going to talk about particular paradoxes. Here’s my plan: I’ll mention their existence, and then the comments will be all about debating them. That, and Primer.
p.s. Since we’re doing time machines one way or another, we should make it a good one. Which time machine models do I need to discuss under pain of your unending disappointment?