Time paradox (temporal paradox) - a paradoxical situation that arises as a result of time travel in the opposite direction, when, as a result of some actions in the past, such a journey cannot be undertaken.
A typical example is the grandfather's paradox, when the hero, traveling in the past, kills his grandfather before he can conceive his father. This is a paradox, because by killing the grandfather, the hero prevents his own birth, and thus prevents the time travel that prevents his birth.
Different schools of thought consider the question of how grandfather's paradox will be resolved if the time machine is invented.
Time sequence protection hypothesis
The essence of this hypothesis is that a time traveler simply cannot create a paradox, because the natural passage of time will not allow him to. For example, he will appear in the wrong place and will not be able to do anything at all, or various turns of fate will interfere with him, or even he, without noticing it, will restore the course of events in the form in which he remembers them. This, by the way, creates another paradox - the paradox of predestination, it is also a time loop, when a time traveler is predetermined to go on a journey in time and there to perform some actions that have already occurred. Usually, these actions predetermine the need for travel in the future, an example would be a phone call to yourself.
A similar theory holds that time travel is responsible for the way the world is today, that is, the actions of travelers in the past affect the present. On the other hand, in this case, travelers will try to interact with the past as little as possible out of fear of the consequences.
In Fallout 2, the Chosen One can enter Vault 13 using a temporary portal and accidentally break the water chip, the breakage of which will force his ancestor, the Vault Dweller, to leave the Vault, in order to be later exiled and found a village in which the Chosen One will be born. By the way, the hero may not go to this portal and even never see it during the whole game, but events have already taken place, which makes fans wonder about the true cause of the breakdown.
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Such a course of history can be easily imagined. Imagine that time is a book. The time machine allows you to turn a few pages, but the text in them will not change because of this, neither written about others, nor written about you.
Time sequence damage hypothesis
This hypothesis assumes that any actions of the time traveler have a strong influence on the future and cause the so-called "butterfly effect", that is, even a slight change in the past leads to a complete change in the whole history in the future, as, for example, in the movie "The Butterfly Effect" or in Ray Bradbury's story "And Thunder Came," when during a Jurassic safari one of the participants steps on a butterfly and the wrong president is elected in the United States.
A very popular illustration of this example is Back to the Future, where schoolboy Marty McFly travels in a Doc Brown time machine and tries to first save Doc from death, and then restore the "natural" course of history to return to the familiar world.
In addition, this hypothesis underlies many works with the so-called "alternative history". For example, the Command & Conquer: Red Alert series of games takes place in a world in which, after World War II, Einstein created a time machine, returned to the past and killed Adolf Hitler. As a result, in the 50s the USSR under the leadership of Stalin invades Europe and unleashes a world war.
In World of Warcraft, the Bronze Dragons, the lords of time, sometimes ask the player to restore the natural course of history, arguing that if this is not done, the consequences will be much worse. Thus, players manage to take part in the events that took place in the past of Azeroth, which they saw in previous games from Blizzard: the opening of the Dark Portal between the worlds by the magician Medivh, the battle for Mount Hyjal, Thrall's escape, the Purification of Stratholme by Prince Arthas Menethil.
The multiple universes hypothesis
The essence of this hypothesis is that there are an infinite number of universes, one for each choice. Thus, if the time traveler killed the grandfather, there would be a universe with a living grandfather and a dead grandfather.
Another hypothesis is that the murder of the grandfather creates a new universe in which the murder took place, but this does not affect the killer or his original universe.
For example, in one of the comics, Superman tries to prevent many historical events, such as the murder of Lincoln. However, returning to his time, he does not find any differences, but later discovers a neighboring universe with an alternate history that appeared as a result of his actions.
The merging universes hypothesis
Contrary to the multi-universe hypothesis, this hypothesis assumes that any action taken while traveling in time rewrites the past. Thus, if a time traveler meets his take from the past, he will simply merge with him, becoming part of the time in which the action takes place. The same is true for events: two events will merge into one, which does not generate a paradox. That is, the death of a grandfather in one universe and his life in another will merge so that the hero's past allows him to continue to exist, or, if this is impossible, to destroy all traces of him from the future.
Time Sequence Hypothesis
The idea is that the story changes as soon as the time traveler thinks about time travel. Thus, he can send messages from the past to the future and receive the tools or materials he needs at the moment through another trip to the earlier past. This was described by Harry Harrison in the Steel Rat books and in the movie Bill & Ted's The Bizarre Adventure.
This cannot be, because it can never be
This hypothesis states that the time traveler is the past in relation to the future self. If a time machine is once created in the future, then it will probably be ever replicated and someday a potential time traveler in the future will get to it and send it back into the past. But since this does not happen, time machines were not invented and cannot be invented.
There is also a point of view that the time machine might self-destruct if sent back in time before the date of its invention.
Loss of motive hypothesis
The bottom line is that if the traveler built a time machine with the goal of fixing something specific and achieved this goal by time travel, then the reason for creating a time machine will be removed and thus the machine will not be created, but another paradox will be created. essentially similar to the grandfather's paradox.
On the other hand, this paradox can be bypassed if no amount of time manipulation can change the event that the time traveler needs to change. Then the time machine will still be built, no matter what the traveler does while traveling in the past.
Harmonious universe
Everything is quite simple here: the universe strives for harmony and therefore corrects itself back, even if a certain person, event or idea threatens to destroy this harmony. An example of this in Soviet literature is the work of the Strugatsky Brothers "A Billion Years Before the End of the World" with the concept of a "homeostatic universe."
Empty time
The theory of time travel, roughly described by Stephen King in The Langoliers and in more detail by Clifford Simack in What Could Be Easier than Time. Once in the future or the past, the traveler will find that it is empty. There are no people, matches and gasoline do not burn, food and drinks are tasteless, there is no electricity, the weather and time of day are either static, or change anyhow (in other words, there are only those objects that are deeply rooted in time and are almost eternal - for example, our the planet underfoot; all other objects exist only in the form of their shadows or ghosts).
The past disappears after some time, there is no distant future, and when he hits the nearest, the hero can only wait for the present to catch up (or, as an option, there really is no future, and anyone who gets there will face a complete emptiness, in which there is no even air). At this moment, people will suddenly appear for the hero (and he will suddenly appear for those around him, which can be fraught at best with nothing, at a slightly less best collision, and at worst depending on the author's imagination), matches will burn when striking the boxes, etc. etc.
Memory replacement hypothesis
The essence of this hypothesis is that a person, by changing the past, changes himself, which means he will not remember anything. For example, if a certain person goes back in time and kills Stalin, then Hitler will win the war and conquer the world. But in this case, it turns out that this certain person was born in a fascist society, lived, speaking in German and observing wherever possible and where not a swastika.
This means that when he returns to the future, it will seem to him that nothing has changed (after all, he changed the past and never lived in our society with you), although in reality everything has changed, because Hitler did win the war. A similar example can be seen in the movie "The Butterfly Effect", with the difference that the hero of the film realizes and remembers all the "alternative branches of development" of time.