I had this on the list of rhetorical questions to ask about time travel into the future, to try and get the students to think about what we do and don’t want to count as time travel. The earlier questions were Is waiting for time to pass time travel? and Is sleep time travel?. (If yes to the latter, the follow-up might be Is a bottle of gin a time machine?, but I don’t think I’ll take that path in my first year class!)

At first I thought the answer to each question was obviously no, and I still think that for the first two. But I’m not so sure about hibernation any more. The worry is that time travel is, as Lewis says, where personal time and external time don’t match up. Now there’s a few ways to measure personal time. If we just do it psychologically, then dreamless sleep will count as time travel, because no psychological time is passing. That’s clearly wrong, so presumably we should use some physiological measures of personal time as well. (Lewis mentions hair growing as one criterion of the passage of personal time.) But hibernation, at least as I understand it, also involves the slowing down of physiological processes.

If a bear wore a wristwatch, that wouldn’t slow down during hibernation, but we don’t want to make watches that important do we?

At some level, maybe cellular maybe lower, processes don’t slow down during hibernation. Maybe that’s what is relevant to time travel. But now I worry we’ll rule out some kinds of things we do want to count as time travel. Is H.G.Wells’s machine not really a time machine (when moving forward) unless the electrons in the time traveller’s body slow down? Perhaps, but that seems a much stronger condition on forward time travel than I’d have thought necessary.