A time machine is any means of conveyance that allows the user to travel through time to points in the past or the future
. A popular trope of science fiction, there have been many different types of time machines with different capabilities and restrictions. One main difference is whether a time machine is mobile or immobile. Immobile time machines, such as Mr. Peabody's WABAC Machine or the time travel machine from the Terminator
series, allow a traveler to begin their voyage from within a home base, but may require a pre-set means of return to the present, if they allow a return trip at all. Mobile time machines are handy because they let you take the machine along for the ride, however it is still possible to become stranded if the time machine is broken or requires special fuel that is not readily available. This would include the time-traveling DeLorean from Back To The Future
, whose flux capacitor required both a 1.21 gigawatt power supply and a forward velocity of 88 miles per hour to activate.