The grandfather paradox is essentially about a time traveler going back in time to kill his grandfather. If this happens before he meets the time traveler's grandmother, logical contradictions arise, since the time traveler extinguishes a compelling cause of his own existence, which in turn means that neither time travel nor murder can happen. The grandfather paradox is the most common example used to illustrate causality problems in time travel .

## Problem

With the invention of a time machine or some other way of traveling backwards in time, logical problems arise: With time travel, a known past can be changed.

The grandfather paradox is usually presented as follows: someone who has the option of time travel travels into the past at a point in time that is before his father was conceived and kills his grandfather there. This situation is paradoxical because the time traveler cannot be born without the existence of his father, who is not born because of the death of his grandfather, and consequently could not have traveled back in time to kill his own grandfather. The grandfather paradox thus shows that the assumption of being able to undertake a journey through time that changes the past leads to a logical contradiction.

## Dissolution through a self-consistent universe

A possible resolution offers a self-consistent universe : it is possible to travel in time, but not to produce causality violations. Everything that the time traveler does in the past is already part of that past. Applied to the grandfather paradox, the following picture could emerge: the time traveler tries to kill his grandfather, but fails or - on the contrary - even causes grandfather and grandmother to get to know each other through his excursion into the past.

Strictly speaking, exactly the same past would have to be traversed that was traversed up to the time of the time travel (the appearance of the time traveler is already part of the past). If one takes the interpretation a little further, the only requirement is that all changes triggered must ultimately lead to identical starting conditions for the journey through time. The time traveler can change something in the past, but only within the framework of narrow restrictions (namely the requirement for a consistent restoration of the initial situation of his time travel).

### Cases from film and literature

An example of such a journey through time in a self-consistent universe is the finale of the book Harry Potter and the Prisoner of Azkaban and that of the film adaptation of the same name. The German science fiction series Dark also takes place in a self-consistent universe and expresses this directly in the third episode of the second season: This is the bootstrap paradox for causal cycles. The time traveler was and is also part of the past, although the past is before his date of birth. Conversely, there are objects (e.g. the book A Journey through Time and the Time Machine in a Suitcase) that are taken into the past and serve as an example for the author or builder and are thus causally responsible for their own existence. The objects have no causal origin, their existence is based on their existence.

Even Greek myths make use of this possibility: Due to an oracle which prophesied that he would by his own son Oedipus die sets Laius those of a newborn. This act becomes, so to speak, the cause or the impetus for the prophecy to be fulfilled at all ( self-fulfilling prophecy ). Oedipus now leaves his foster parents in order not to become a patricide. As a result, however, he meets his biological father and the oracle's saying fulfills itself, so to speak. Something similar happens at Matrix when Neo breaks a vase while visiting the oracle because the oracle says he shouldn't take care of the vase. Then the oracle adds: "Much more agonizing for you will later be the question: Would you have knocked the vase over if I hadn't said anything?"

## Dissolution through parallel worlds

Another possible resolution of the paradox is based on the assumption of parallel worlds . The time traveler does not actually travel into his own past, but rather travels into an independent timeline in a parallel world that no longer corresponds to the time traveler's original past once the time traveler arrives. By entering a new world, you no longer extinguish yourself if you kill your supposed grandfather, since he is the grandfather of the parallel world. Your own grandfather, on the other hand, is unaffected and ensures that you are born in the present because he does not interact with the other grandfather of the parallel universe. With this assumption, each time travel through time creates its own universe, otherwise the paradox would arise again.

### Cases from film and literature

In the Doctor Who episode Before the Flood ( episode 9.04 ) this paradox is pointed out with the help of Beethoven's 5th Symphony . The doctor explains that a time traveler is returning to the past to have Beethoven sign the sheet music for the symphony. However, he states that Beethoven does not exist. He then copied the scores and published them himself under the name Beethoven. This raises the question of who was the inventor of the 5th symphony.

In the Futurama episode Roswell Good - All Good , the grandfather paradox is directly referred to as Fry travels back in time and inadvertently kills his grandfather in the process. Here the paradox is solved very curiously by the fact that Fry fathered his own father and thus became his own grandfather.

In addition, several Star Trek films and series episodes experiment with the idea of ​​changing timelines: cracks in space and time usually temporarily change the past and must be corrected over the course of the story.

In the film Back to the Future , too, the manipulation of the past is often played with, where the phenomenon is graphically illustrated by the doc.

However, while the classic parallel world resolution assumes that both versions of the present exist side by side, in most science fiction stories the original timeline is obliterated and replaced by an alternative timeline.

There are also parallel worlds like this in the television series The Flash .