xkcd.WTF!?

Explanation ➲

Cueball activates a time machine to go back into the past. The time machine rewinds time, but in the process rewinds the event where the time machine itself was turned on, turning the time machine off in the process. He is now a few seconds in the past, prior to having activated the time machine, but he is baffled that he does not seem to have accomplished anything and turned off the time machine unintentionally. It would seem that the time machine is the world's most technologically-advanced^{[citation needed]} "useless machine" (a device whose only purpose is to switch itself off when it is switched on).

The title text mimics a countdown to an event. "T minus 10," for example, means 10 seconds until the event. When the event is the activation of a time machine traveling back in time, after 10 seconds it will once again be "T minus 10," and a second later it will be "T minus 11," counting up rather than down. This casts doubt on the value of the countdown because, from the perspective of the time traveler, the event has already taken place.

Cueball is only able to travel back in time a few seconds because in this comic time is seen as continuous and linear from Cueball's point of view, so he can only travel back in time to the moment he activated the machine (the first series of "E"s is the machine warming up and the second series of "E"s is that in reverse) the logic behind this is that because time appears to be continuous, Cueball's input was required for the machine to work. Since it does not appear to be a traveling vessel, it is also possible that Cueball could trap himself in the past by traveling to a time before the machine was created, and it would remain in the present.

It is very fortunate that Cueball retains knowledge of the time travel process, brief as it was; if he didn't, then the universe would become trapped in a time loop, with Cueball flipping the machine on, which then reverts time back to before it was turned on, leading Cueball to (believing that he has not yet turned on the machine) immediately reactivate it, dooming the universe to repeat the same few seconds indefinitely.