What is the Grandfather Paradox?

 | Last modified on June 7th, 2020 at 7:36 pm
Grandfather Paradox In Hindi

Grandfather paradox is a paradox of time travel, in which discrepancies emerge through changing the past. The name comes from the general description of the paradox: a person travels to the past in time and dies his grandfather before his father or mother is born, which prevents the existence of the time traveler.

The grandfather paradox is not particularly concerned with the paradox of simply killing one’s own grandfather to prevent someone’s birth. Rather, this contradiction is the relation of any action that changes the past, in which the past will be different from the way it was before, which is a contradiction.

History of Grandfather Paradox

A description of the Grandfather paradox can be seen in books written in early 1929. In 1931 it was told in a letter to the American science fiction magazine Amazing Stories. Early science stories dealing with this paradox are the short story Ancestral Voices, written by Nathaniel Schachner, published in 1933, and the 1944 book Future Times Three. Many other compositions of the 1930s and 1940s appeared on various themes of this subject.

Types of grandfather paradox

The Grandfather paradox describes any change in the past, and can also be presented in many forms. Physicist John Garrison has shown a variation of the paradox of an electronic circuit, which involves sending a signal through a time machine to shut itself down, and receiving the signal before sending it. The equivalent paradox is known in philosophy as the “retro-suicide paradox” or “autoinfanticide”, going back in time and killing a younger version of itself (such as a child).

Hitler Paradox

Another form of the Grandfather’s paradox is the “Hitler Paradox” or “Hitler’s Assassination Paradox”. Its description can be seen in many science fiction, in which the hero goes back in time to die, Adolf Hitler, before he can provoke World War II and the Holocaust. Instead of physically stopping time travel as necessary, the action removes any reason for the journey, as well as any knowledge that ever existed, thus making any point of travel in the first place Is removed. Additionally, the consequences of Hitler’s existence are so memorable and ubiquitous that for anyone born after the war, it is possible that his birth was in some way influenced by his influence, and thus the lineage aspect of the paradox directly Will apply in some way.

Parallel universe

Some scientists say that if the Grandfather paradox would be possible, it would create a parallel universe. This means that when the time traveler goes back in time and kills his grandfather, he is actually killing the parallel universe version of his grandfather, and the time traveler’s original universe is untouched, that is, the time traveler’s present time is the same. Will remain the same. It has been argued that since the traveler delves into the history of a different universe and does not have its own history, it is not “real” time travel. In other types, the traveler’s actions of that time have no influence outside of his own personal experience, as depicted in Alfred Bester’s short story The Men Who Murdered Mohammed.

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Philosophical analysis of grandfather paradox

Without knowing whether it is physically possible to travel in the past or not, it is possible to show by using modern modal logic that past results are transformed into logical contradictions. If it is true that the past happened in a certain way, then it is wrong and impossible for the past to happen in any other way. A time traveler will not change the past as he is. The past will only act in a way that is already consistent with what was necessary.

The idea of grandfather paradox has led some to the idea that time travel is by its nature paradox, and therefore logically impossible.

General relativity

Considering the possibility of past travel in the fictional universe described by the famous fictional Kurt Godel, there may be a sort of confusion of that time. He suggests something along the lines of a block time view that has just one more dimension, like time-space, in which all events are fixed within this 4-dimensional “block”.

Causal loop

If a grandfather paradox does not produce a time travel of yesterday, it creates a causal loop. Novikov self-consistency theory expresses an approach to how time travel would be possible without the generation of contradictions. According to this hypothesis, timed curves (time machines) in or near physics can only conform to the universal laws of physics, and thus only self-consistent phenomena. Whatever a time traveler does in the past must be a part of history all along, and time travel can never do anything to stop the journey, as it would represent a dissonance.

Novikov et al used the example given by physicist Joseph Polchinski for the grandfather paradox of a billiard ball going toward a time machine. He showed that the ball originates from the old self-time machine and collides with its self, so its short self never enters the time machine.

Grandfather Paradox Billiard Ball
Top: Original billiard ball trajectory. Middle: The billiard ball escapes from the future, and deletes its previous self-strike which prevents the previous ball from entering the time machine. Below: The billiard ball never enters the time machine, which leads to contradiction and puts into question how its old self can ever get out of the time machine and bend its course.

Novikov et al showed how this system can be resolved in a self-consistent way that can avoid grandfather paradox, although this creates a causal loop.

Some physicists suggest that in a similar way to the chronology protection conjecture approximation proposed by Stephen Hawking, causal loops exist only in the quantum scale, so history loops do not occur on larger scales. Another hypothesis, the cosmic censorship hypothesis, suggests that every closed timelike curve passes through an event horizon, which prevents such causal loops from being observed.

Lloyd and other researchers at MIT have proposed an extended version of the Novikov theory, according to which probability bends to prevent contradictions from occurring. As the universe favors impossible events to prevent impossible actions, the results will turn out to be strangers.

According to Quantum physics

Some physicists have proposed that quantum theory allows time travel where the past must be self-sufficient. Deutsch argues that quantum computation occurs with a negative delay — traveling backward — produces only self-consistent solutions, and the chronological-violation field imposes constraints that are not apparent through classical logic.

In 2014, researchers published a simulation that validated Deutsch’s model with photons. Deutsch uses the terminology of “many universes” in his paper in an attempt to express quantum phenomena, but note that this terminology is unsatisfactory. Others have interpreted it to mean that “Deutsch” time travel involves a time traveler emerging into a different universe, escaping the grandfather paradox.

The interaction of multiple universes approaches is a variation of the quantum mechanics of Everett’s multiple-world interpretation. This includes the time of arrival in a different universe from where they came from; It has been argued that since the traveler comes from the history of a different universe and not from its own history, it is not “real” time travel. Stephen Hawking has argued that even if the MWI is correct, we should expect the traveler to experience a single self-contained history each time, so that the traveler lives within their world rather than taking a different journey. Ellen Everett argued that the dictionary’s approach involves “modifying the fundamentals of quantum mechanics; it certainly goes beyond simply adopting MWI”, and even if Deutsch’s approach is correct, its This would mean that any macroscopic object composed of many particles would break apart when traveling back in time with different particles emerging in different worlds.

The conclusion

Grandfather paradox is still considered a theory in modern science because in practice it has not been possible yet. We hope that on the day when we will be able to travel in time, we will also understand the Grandfather paradox better.


  1. Nicholas J.J. Smith (2013). “Time Travel”Stanford Encyclopedia of Philosophy. Retrieved November 2, 2015.
  2. Frank Arntzenius; Tim Maudlin (December 23, 2009), “Time Travel and Modern Physics”Stanford Encyclopedia of Philosophy, retrieved May 25, 2019