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.

Anomalies are a complex phenomena which are at the core of entropic time travel. While it is believed that spawned timelines are identical copies of the original timeline, from time to time a new timeline will exhibit an anomalous event that will be unique to it alone, and never occur in other timelines.

Anomalies were discovered by a group of NOTTs in the 1980s of the Original Timeline. This has overturned decades of theoretical work in temporal physics and has deeply affected our understanding of entropic time travel.

This has also partially explained the previously completely mysterious fact that all copies of the Original Timeline, while usually identical to each other, are always different to the OTL in one important way - no known ATL contains the Entropic Time Machine. It is now widely believed that the ETM itself is an anomaly.

The discovery of anomalies has spawned a whole new area of research focused on understanding why anomalies happen, what they are and if they can be replicated. The latter had already been an area of some interest due to there being no functioning ETMs in other timelines. Decades of investigations have revealed little insight, specifically because it is not possible to examine the past of the OTL in detail: any trip to the OTL's past simply spawns a separate timeline that has no record of an ETM ever existing.

Finding anomalies in other dimensions has allowed to get closer to them occurring and, thus, try to understand what is different in these cases.

The leading hypothesis is that it has something to do with insertions, since all anomalies have always happened in time frames following the arrival of a time traveler or a group of time travelers, although the time between arrivals and anomalies varies greatly from days to years. This begs the question if ETM is an anomaly that happened due to a traveler from another universe arriving to the OTL. No such individual has ever been found.

Another important hypothesis is that every ATL has an anomaly, it's just that in most ATLs it is undiscovered due to not happening to prominent people/places/events or due to it yet to happen in the far future.