Beerus: So this is a time machine, huh?
Whis: It certainly amazing but, to manipulate time and go in the past or future is a serious crime. Time is something that should flow in only one direction. If you go back to the past and change something, like, for example, picking a flower, it could greatly affect the established story. It could change entire cities or even an entire civilization. It might even result in an entire planet disappearing from the universe. Therefore, traveling through time so easily is strictly prohibited, even among the gods.Say, wouldn't be a good idea to travel back in the past and fix something? Wait, according to the Time Police, the act of traveling in time is prohibited! You will go to Time jail! This trope exists to provide an In-Universe explanation on why time traveling is forbidden. Maybe is to avoid a Butterfly of Doom scenario, the creation of an Alternate Universe, or to avoid the pain in the ass to question how things worked out and you became your own grandfather. Maybe time itself has a way to punish those who mess with time, by sending the Clock Roaches, or worse, messing with time will unleash them to destroy all of existence! Related trope to Our Time Travel Is Different, Alternate Universe, Butterfly of Doom, Grandfather Paradox, and Time Police. Contrast Chekhov's Time Travel, where there is typically an In-Universe justification for the need to use it.
- Dragon Ball Super
- During both Dragon Ball Z: Revival of F and the arc that adapts it, Jaco explains to Bulma that time manipulation is illegal according to the Galactic Patrol. When Whis travels back in time, he turns a blind eye mostly because is pointless for him to say anything. Likewise, both he and the Galaxy King don't bat an eye when Hit's time abilities are shown in the Champa saga, mostly because it is also pointless.
- After Future Trunks goes to the present to escape from Goku Black, who has been destroying his home in the future, Whis states his surprise that Earthlings managed to create a time machine, but also say that time traveling is a taboo to the gods for the Butterfly of Doom justification. Trunks was lucky Beerus was in a good mood and allowed the heroes to keep traveling in time to deal with the villain, who had a time ring. This trope is explored further with the Time Ring, a magical artifact that the Supreme Kais use to travel into the future to see how a civilization has evolved through time, and every time a new timeline is created by using a time machine, a new Time Ring of green color appears as a way to travel to that Alternate Universe. In fact, one of the reasons Goku Black hypocritically proclaims he invaded Trunks's timeline was due to him sinning, by traveling in the past the first time and saving Goku from his heart disease.
Goku Black: 'You've committed a sin far greater than me. Trunks, you're one of the reasons why I wished for the extinction of humanity, by rewriting the past, a new Time Ring was created. That itself is proof of your sin. How many times have you came back through time?Zamasu: We know. We know that you continued committing a taboo among gods, despite being a mortal.
- After the Future Trunks arc is over and Trunks returns to his own era, Bulma tries to recreate another time machine despite Beerus warning her it's forbidden. Without the presence of a menace like Goku Black to the timelines, he destroys the time machine once and for all, to Bulma's misfortune.
- Star Trek:
- Star Trek: The Original Series: The franchise's Ur-Example of this trope involves the planet Gateway, from "The City on the Edge of Forever". The Federation is nearly erased when Dr. McCoy unwittingly changes history by saving a 1930s peace activist who delays the USA's entry into World War II, enabling Nazi Germany to win the conflict and Take Over the World. After this situation is resolved, the planet is placed under strict quarantine. Some non-canon licensed works up the ante to the same death penalty used for Talos IV.
- Star Trek: Voyager and Star Trek: Deep Space Nine: These two concurrent series simultaneouslynote introduced the timeline's current Department of Temporal Investigations and the future Temporal Prime Directive. The former is relatively flexible and informal, but it introduces a taboo to the series canon. The latter is enforced by 29th century officers who work to preserve the timeline, and will go so far as to eliminate people from history to do so.
- In The Dresden Files, time travel is one of the acts forbidden by the laws of magic, with death as the penalty for a single transgression. The exact reason is not given, but presumably has to do with paradoxes or other problems.
- In the Doctor Who New Adventures, there are technological measures (the Backtime Buffers) preventing attempts to take a TARDIS into Gallifrey's past, which is strictly forbidden.