Entropic Time Machine (ETM) is a set of technologies that allows one to travel through time. It is called “entropic” because its method is in line with the second law of thermodynamics, which states that its highly improbable for entropy to decrease. This makes time travel to the past virtually impossible. ETM sidesteps the problem by using a completely different approach.

Instead of moving back in time through the current universe, 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.

ETM is very efficient at finding new identical timelines, but is very poor at finding already visited universes. This is due to the fact that there is a potentially infinite number of identical copies of a given timeline, but there is no known way to (efficiently) retrieve a timeline which is not a copy of the current one.

Part of the difficulty stems from not being able to sufficiently define a timeline and, therefore, filter the domain of all available timelines by a set of parameters: no matter which parameters are chosen, the result is still a set of an infinite amount of identical copies.

This results in the fact that all recorded timelines are universes which have been spawned from the Original Timeline. Changing parameters arbitrarily should in theory allow access to the already existing ATLs, but the computation required for such filtering is beyond current capabilities. ETM is limited to a single time frame as a way to identify a timeline.

Additionally, ETM is bound by the second law of thermodynamics. This means that insertion into the past of the time travelers' current timeline will always fail, as most processes in the universe are irreversible. In fact, ETM is programmed to use the failure as a signal to spawn a new branch. So, each time the traveler travels back in time, ETM will first insert them into the past time frame of their current timeline, this will fail and the ETM will spawn a new timeline at the next frame. Return back is possible precisely because initial insertion fails and ETM retains the timeline's frame of departure as its identification.

This makes it possible to get permanently disconnected from one's original time frame. Specifically, there is no way to return to one's original time frame if the jump was made into the future on the same original timeline: all subsequent attempts to return to the past will spawn out a new timeline. While the timeline spawned is going to be identical to the original, from the point of view of external observers of said original timeline the person would never return. Instead, they would suddenly vanish and appear later, at the point they traveled to in the future. And if this point in the future is far away, sometimes family and friends never get to see the time traveler again, while the time traveler meets everyone in an alternate timeline.

It would be possible for the time traveler to realize their mistake by traveling to the future of their new timeline and find it to be different, since this new timeline would have no record of their travels into the future.