You saw it in the sci-fi comedy Back to the Future, in the cartoon Futurama, and then in the 2009 blockbuster Star Trek. We are talking about the "grandfather paradox" (or variations on this theme), when someone goes back in time and changes something so significant there that it creates an impossible scenario for the development of events. However, what exactly is the grandfather paradox, how does it work, and what happens if someone actually launches it?
Here is the simplest version of the paradox. Jim Bob III builds a time machine and goes back sixty years, when his grandfather Jim Bob I is only 20 years old and not yet married. He then kills his grandfather on the street (Jim Bob III is in some trouble), which means that Jim Bob II will never be born. However, if Jim Bob II is not born, then neither is Jim Bob III. Consequently, he will never build a time machine or come back, so that Jim Bob I can live a long and happy life. However, then after a while he will have a spoiled granddaughter, prone to arrange paradoxes, and everything will be repeated from the beginning.
There is one solution to this paradox that continues to occupy the minds of theorists, but it also has its own problems. It is called Novikov's principle of self-consistency, which, in essence, consists in the fact that any events that occur during travel to the past correspond to and are identical to the events that have already occurred "for the first time." In other words, there is only one past, and if you eventually went there, then you are already there.
Confused? In fact, you've probably already seen how the principle of self-consistency works. In Harry Potter and the Prisoner of Azkaban, Harry escapes when a mysterious figure calls on the Patronus to drive off the attacking Dementors. On his next journey through time, he learns that this mysterious figure was himself. And in Game of Thrones, Bran discovers that his actions in the present are the cause of Hodor's unique speech defect in the past (we won't get into the how, as we hope to finish the story today).
These examples from pop culture do not eliminate paradoxes as such, but they do show how the paradox could be resolved. The Patronus that saved Harry was already the first time Harry experienced it, and Hodor became Hodor because Bran went back in time. In other words, you cannot go back in time and kill your grandfather because your grandfather was not killed. And if you try, you will fail, because your grandfather never died in the past.
This is what is most important in the grandfather's paradox: it is a paradox only in certain versions of the space-time continuum. Until we actually manage to go back, we will never know for sure how our continuum works. But there are several options.
Fixed timeline. This is exactly how the timeline in the above-mentioned episode from "Futurama" is arranged, and this is exactly the version that Novikov's principle most clearly describes. In this version, what is in the past is in the past. The ink is dry. So when Fry thought that he had irreversibly changed the passage of time by bringing his grandfather to death, he was actually just replaying the old passage of time … which, as it turned out, meant that he was his own grandfather. Y-yes. Well, let's continue.
Dynamic timeline is the most popular concept of how time works, even though it doesn't make much sense when you think about it. This is the time Marty McFly had to deal with. In his reality, the changes he made in the past were reflected in the present, and this is how he almost erased himself from life, finding himself more attractive than his young father. But if the present changes, if you change the past, then won't you change yourself? And if you accidentally make it so that you are not born, then you will not have a single chance to fix it. In this time continuum, paradox triumphs over common sense, and science fiction writers simply have to shrug their shoulders and admit that this is more dramatic.
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The multiple timeline is the one described in Star Trek and easily bypasses any potential paradoxes. Each trip to the past generates a new reality in which this trip to the past took place. The past of the reality that you left is still unchanged, like a fixed timeline, but the new reality that you created can develop in different ways. Thus, the villain Nero, played by Eric Bana, did not have to worry about causing any paradoxes, since the universe he was acting on had nothing to do with the one he came from. Something like "what happens in Vegas stays in Vegas", only with more lasers.
Igor Abramov