Entropic Time Machine (ETM) is a set of technologies that allows one to travel through time, specifically to any point in the past. It is called “entropic” because its method sidesteps the second law of thermodynamics, which makes it highly improbable for entropy to decrease, thus making time travel to the past possible.

Instead of moving back in time through the current universe, the ETM finds an identical copy of the timeline and inserts the time traveler into it, thus creating an Alternate Timeline. When the time traveler wants to go back to their original timeline, the ETM will insert them into exactly the next frame of their timeline. This means that from the point of view of the external observer in the original timeline the time traveler never left.

The ETM finds it very easy to find new identical timelines, but is very poor at finding already visited timelines, which makes it possible to get permanently lost.