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. Now did you travel back or not? 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. (Kettlewell, 2005, [1] (http://news.bbc.co.uk/2/hi/science/nature/4097258.stm))
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
Parallel universes resolution
There could be "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 James P. Hogan's novel "Thrice Upon a Time", as well as Michael Crichton's novel "Timeline". Alfred Bester's short story "The Men Who Murdered Mohammed" starts with this premise, but proposes a timeline which is dependent on perspective, making such paradoxical changes self-limiting.
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 only problem is that the particles that you are made of now have no history and from the universe perspective extra energy and matter have been added from essentially nowhere, this problem can be negated if the universe has infinite energy, or if the act of traveling back in time removes equal amount of energy to you when you enter the past. The parallel universes theory also negates this problem.
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.
In the TV 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.
Restricted action resolution
Another resolution, of which the Novikov self-consistency principle can be taken as an example, holds that, if one was 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 traveller'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.
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 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.
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.
References
Newspaper/magazine articles (or online periodicals)
See also
- Predestination paradox
- The Terminator
- Pogo Paradox (http://memory-alpha.org/en/wiki/Pogo_Paradox) at Memory Alphade:Großvater-Paradoxon
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