716: Time Machine
Title text: We never see any time travelers because they all discover it's a huge mistake. This is also why your friend at the lab suddenly looked a year older recently.
A common theme in time travel fiction is going back into the past to fix some mistake or stop some tragedy before it happens (see for instance The Terminator movies). In this comic, it is implied that Rob's time traveling turned out to cause a tragedy of some kind, so in order to stop it, Future-Rob must go back in time to stop himself from time traveling in the first place. The last panel supports this by suggesting that at least once a month somebody discovers time travel, but inevitably ends up going back in time to prevent themselves from doing so.
The blood on the bat suggest that future Rob actually killed past Rob. This is of course a paradox like the Grandfather paradox - but there are theories about how it would still be possible - see the link. The obvious paradox is that when Rob dies the future Rob never existed. But also the time travel Future Rob undertakes uses a technique that is now never invented. This was the reason for future Rob's travel.
The title text suggests that this is why we never see any time travelers since they would have stopped their own past selves from time traveling. After getting rid of their past selves they would assume their place in the timeline, hence why a friend would suddenly look older: they have aged, just in another timeline before returning to the past.
- [Rob is working at a workbench. Future-Rob appears out of nowhere with a baseball bat.]
- Future-Rob: Hi, Rob.
- Rob: Whoa, you're me!
- Future-Rob: You're about to have an idea for a time machine.
- Rob: I am?
- [Future-Rob hits Rob over the head with the baseball bat.]
- [Megan approaches Future-Rob working at the workbench, with Rob nowhere to be seen. The baseball bat is stashed behind it.]
- Megan: Hey, Rob. What's up?
- Future-Rob: Nothing.
- This happens somewhere roughly once a month.