Grandfather paradox
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- This article refers to the time travel paradox. In novels based on the television series Doctor Who, "Grandfather Paradox" is the semi-mythical founder of Faction Paradox.
The grandfather paradox is a paradox of time travel, supposedly first conceived by the science fiction writer René Barjavel in his book "Future times three" ("Le voyageur imprudent", 1943). Suppose you travelled back in time and killed your biological grandfather before he met your grandmother. Then you would never have been conceived, so you could not have travelled back in time after all. In that case, your grandfather would still be alive and you would have been conceived, allowing you to travel back in time and kill your grandfather, and so on.
An equivalent paradox is known (in philosophy) as autoinfanticide — that is, going back in time and killing oneself as a baby — though when the word was first coined in a paper by Paul Horwich it was in the malformed version autofanticide.
The grandfather paradox has been used to argue that backwards time travel must be impossible. However, other resolutions have also been advanced.
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Scientific theories
Complementary time travel
Since quantum physics is governed by probabilities, an unmeasured entity (in this case, your historical grandfather) has numerous probable states; but, when that entity is measured, the number of its probable states singularises, resulting in a singular outcome (in this case, ultimately, you). Therefore, since the outcome of your grandfather is known, you killing your grandfather would be incompatible with that outcome. Thus, the outcome of one's trip backwards in time must be complementary with the state from which one left. (see Professor Dan Greenberger, of the City University of New York, quoted by Kettlewell, BBC, 2005, [1])
Novikov self-consistency principle
The Novikov self-consistency principle and recent calculations by Kip S. Thorne indicate that simple masses passing through time travel wormholes could never engender paradoxes—there are no initial conditions that lead to paradox once time travel is introduced. If his results can be generalized they would suggest, curiously, that none of the supposed paradoxes formulated in time travel stories can actually be formulated at a precise physical level: that is, that any situation you can set up in a time travel story turns out to permit many consistent solutions. Things might, however, turn out to be almost unbelievably strange. For related information on unbelievably strange causality, see quantum suicide and quantum immortality.
Theories in science fiction
Perhaps the most famous and typical example of the paradox is the first Back to the Future film: Marty McFly travels to 1955 and accidentally prevents the first meeting of his parents. While trying to find out a way to return to his own year, he observes that his siblings begin to fade out from a picture he was carrying with him since he averted their own birth. Towards the end of the film, he starts to fade from reality as well, until he manages to make his parents fall in love 'again'.
Parallel universes resolution
There could be "an ensemble of parallel universes" and when you travel back in time and kill your grandfather, you do so in a parallel universe in which you will never be conceived as a result. However, your existence is not erased from your original universe. Parallel universes are also used in Michael Crichton's novel Timeline and Alfred Bester's short story "The Men Who Murdered Mohammed". The latter story starts with this premise, but proposes a timeline which is dependent on perspective, making such paradoxical changes self-limiting. Interestingly enough, Crichton's novel seems to imply that universes set in the past can affect the one we live in. The example given is when a professor trapped in the past sends a message to his graduate students at a medieval cathedral.
Relative timelines resolution
It could be that the universe does not have an absolute timeline that is permanently written after events happen (or, in the deterministic view, at the start of time). Instead, each particle has its own timeline and therefore, humans have their own timeline. This might be considered similar to the theory of relativity, except that it deals with a particle's history, rather than its velocity.
Physical forces affect physical particles. If your body's physical particles go back in time, you will be able to kill your grandfather (no physical forces will mystically stop you), and nothing will physically happen to you as a result, because there are no physical forces that can "figure out" what happened and this new timeline develops, because the universe simply has no mechanism for unmaking it. Your younger self does not need to be born in order to fulfill a destiny of going back in time, because there is no written-in-stone absolute timeline that needs to be followed. If you were able to find and observe the younger versions of the particles that make you up, they too would follow physical laws and hence wouldn't form into a younger version of you (because one of your parents wouldn't be there to form you).
The parallel universes theory obviates this problem of cause-and-effect.
This theory is similar to the parallel universes theory, except that it happens within one universe. If parallel universes cannot interact again after time travel occurs, then essentially the parallel universe resolution and the relative timelines resolution are the same as there is no way of proving a parallel universe still exists or ever did exist.
Author Orson Scott Card used this theory to allow his characters to travel back in time and prevent the European colonization of the New World in his novel, Pastwatch: The Redemption of Christopher Columbus. This model also appears in James P. Hogan's novel "Thrice Upon a Time", although Hogan confusingly uses the term "universes" to describe different moments on the same timeline rather than separate timelines.
In Anderson's Time Patrol, the Patrol's purpose is to prevent such changes in time, and when they have occurred, undo the changes as neatly as possible, to revert to the "normal" timeline. Such a Time Patrol, under one name or another, is a common feature in stories using this resolution.
In the television series Seven Days, NSA Agent Frank Parker uses a device called the chronosphere to go back in time, usually one week, to "undo" catastrophic events. This would only be possible if the relative timelines resolution holds, because if Parker succeeds, there would never have been any reason to send him back in time.
In the essay The Theory and Practice of Time Travel, Larry Niven proposes that, after some unknown number of revisions of history, the effect of some episode of time travel will be to create a universe where time travel, although possible, is simply never discovered. Such a timeline is stable, and in it no paradoxes occur, and so need no resolution.
Restricted action resolution
Another resolution, of which the Novikov self-consistency principle can be taken as an example, holds that, if one were to travel back in time, the laws of nature or other intervening cause, would simply forbid the traveller from doing anything that could later result in their time travel not occurring. For example, a shot fired at the traveler's grandfather will miss, or the gun will jam, or misfire, or some other event will occur to prevent the attempt from succeeding. In effect, the traveller will be unable to change history, because for him it has already occurred.
Normally, in fiction, the time traveller does not merely fail to prevent the actions he seeks to prevent; he accidentally precipiates them. The oldest form of this may be the numerous folk tales involving prophecies -- where in the time travel is of information -- such as Oedipus Rex, wherein the very actions undertaken to thwart the decree of the prophecy bring it about: Cronus' swallowing of his children to prevent their usurping his power is what encouraged Zeus to overthrow him, and Oedipus's being abandoned led him to meet his mother without being aware of her presence.
This theory might lead to concerns about the existence of free will (in this model, free will may be an illusion). This theory also assumes that causality must be constant: i.e. that nothing can occur in the absence of cause, whereas some theories hold that an event may remain constant even if its initial cause was subsequently eliminated. It is also possible that the time travelers' intended action might be completed, but never successfully enough to result in cancellation—see Novikov self-consistency principle.
This premise was shown in the beginning of the 2002 movie version of The Time Machine, in which the main character cannot save his girlfriend by going back in time, as he only started building the time machine out of frustration of her death. This loop is not present in the original book. Likewise, in the film 12 Monkeys, the main character not only is unable to prevent a tragic past event from occurring, but even realizes that, as a child, he witnessed his adult self failing in the attempt.
Douglas Adams's Hitchhiker's Series and Harry Harrison's Technicolor Time Machine make use of this resolution for light-hearted and comic plots.
Closely related but distinct is the notion of the time line as self-healing. The time-traveler's actions are like throwing a stone in a large lake; the ripples spread, but are soon swamped by the effect of the existing waves. For instance, a time traveler could assassinate a politican who led his country into a disastrous war, but the politician's followers would then use his murder as a pretext for the war, and the emotional effect of that would cancel out the loss of the politican's charisma.
It also may not be clear whether the time traveler altered the past or precipitated the future he remembers, such as a time traveler who goes back in time to persuade an artist -- whose single surviving work is famous -- to hide the rest of the works to protect them. If, on returning to his time, he finds that these works are now well-known, he knows he has changed the past. On the other hand, he may return to a future exactly as he remembers, except that a week after his return, the works are found. Were they actually destroyed, as he believed when he traveled in time, and he has preserved them? Or was their disappearance occasioned by the artist's hiding them at his urging, and the skill with which they were hidden, and so the long time to find them, stemmed from his urgency?
Destruction resolution
Some science fiction stories suggest that causing any paradox will cause immense damage to, or the destruction of, everything the time traveller has ever affected in any way, which may be as wide-ranging as the entire universe. The plots of such stories tend to revolve around preventing paradoxes.
In Back to the Future Part II, it was speculated by Doc Brown that "the encounter could create a time paradox, the results of which could cause a chain reaction that would unravel the very fabric of the space-time continuum and destroy the entire universe! Granted, that's a worst-case scenario. The destruction might, in fact, be very localized, limited merely to our own galaxy". However, it must be noted that this is not the method of resolution in the Back to the Future trilogy for all time-travel paradoxes.
The 2005 Doctor Who series episode Father's Day provided a unique version of the destruction resolution. A paradox causes a wound in space-time, which attracts flying carnivorous monsters, Reapers. The Reapers act like bacteria around a real wound, devouring everything, starting with the youngest people and objects, until the wound is "sterilized" and the paradox resolved by its destruction.
In Stephen King's short story The Langoliers, a group of travelers pass through a strange time portal and end up in the past. Just when they realize what has happened to them, they see that the entire world around them is being eaten up by the Langoliers, strange creatures with razor sharp teeth devouring everything in their path, including the land and sky, as if somehow the past needs to be cleaned up to allow the future to continue properly.
In the 2001 film Donnie Darko, a rift in spacetime is created when a jet engine lands on the title character's house approximately 28 days before the plane carrying it flies over. This creates an unstable parallel (or tangent) universe which will cease to exist at the end of those 28 days if the engine is not returned to the primary universe. The laws of nature in the parallel universe are roughly the same, except characters and events close to the time portal are "manipulated" to return the engine to the primary universe before the parallel universe collapses on itself and becomes a blackhole of spacetime. If the portal were still open when this happened, it would destroy the primary universe and all of spacetime. How this works is never explained, but implies the idea of some universal defense mechanism.
Other considerations
One of the main assumptions of the grandfather paradox is that causality is an active physical force which can "follow" a time-traveller back through time. But it may very well be that this is not the case. In other words, if you traveled back to the year 1970 to kill your parents and prevent your birth in 1971, you would not disappear from history, because simply by travelling back to the year 1970, you would have established that you already existed in your adult form, complete with memories, in the year 1970. In effect, you would have prevented nothing but a redundant, younger copy of yourself from co-existing with you. Going back in time to prevent the premature existence of yourself would likewise only push back the time at which the existence of yourself was historically established.
Of course, while this would negate the grandfather paradox itself, it says nothing about other changes made to the timeline (for example: what if you killed someone else's parents?), and may assume a separation between the individual's timeline and the "main" timeline of the rest of the universe, essentially resulting in its own paradox -- that the lack of causality as an active force that affects the individual separately from the rest of reality in itself requires that causality be an active force that affects the individual separately from the rest of reality.
Questions like these lead some back to the simpler idea that time travel is by its very nature paradoxical and therefore logically impossible, on the same order as round squares and portable holes -- or at least, impossible under any of the conditions in which human life is thought to be able to exist.
These same questions also led Kurt Gödel to assert that the possibility of time travel would imply that time itself doesn't exist. [2]
References
Newspaper/magazine articles (or online periodicals)
- David Lewis, "The Paradoxes of Time Travel," American Philosophical Quarterly 13 (1976), 145-152
See also
- The Futurama episode, Roswell That Ends Well, episode 3ACV19, in which a main protagonist, Philip Fry, travels back in time and humorously solves the paradox by (accidentally) becoming his own grandfather.
- Robert A. Heinlein's short story "'—All You Zombies—'"
- Spider Robinson's short story "Father Paradox"
- Connie Willis's novel To Say Nothing of the Dog
- Predestination paradox
- The Terminator
- Back To The Future
- Pogo Paradox at Memory Alpha
- The popular webtoon Bonus Stage episode 87, Bonus Stages, where main charecter, Phil Argus, goes back in time to the first episode of the series, and kills his old self, putting a slightly different twist on the paradox, and destroying the rest of the Bonus Stage universe.
- The online machinima comedy series Red vs Blue - The character Church travels back in time to prevent his own death, and to avert the events and disasters that put the characters into their current situation, and ends up causing all of the problems and situations of the first 48 episodes — including his death — entirely by accident.
- The Red Dwarf epsiode, Tikka To Ride, episode 7.1, in which The future version of the crew destroyed the present crew; therefore, the future crew no longer existed and therefore were unable to go back in time and kill themselves, hence they survived.
- The Red Dwarf epsiode, Ouroboros, episode 7.3, in which Lister discovers he is his own father and travels back in time to place his son, aka himself, under the pool table where his adoptive parents first found him.de:Großvaterparadoxon
es:Paradoja del viaje en el tiempo fr:Paradoxe du grand-père he:פרדוקס הסבא pl:Paradoks dziadka fi:Isoisäparadoksi zh:祖父悖論