There are many SF-stories about time-travel. But with time-travel come the possibility for a paradox. As far as I can see, there are different solutions from different fictional works. Maybe some author even uses a paradox as part of the story.

Can someone provide a more or less complete list of these solutions for time-travel?

Best would be each illustrated with an example-work.

  • 1
    Wikipedia has an extremely thorough article on the subject.
    – user1027
    Commented May 2, 2012 at 2:30

7 Answers 7


There are two camps of resolutions to the problem of paradox, and one additional non-solution.


Predestination implies that history, and the future for that matter, are immutable. You can't go back and kill your grandpa, because your childhood would already have reflected that possibility. A fine example of this is 12 Monkeys. Primer is another example, however it is much harder to decode.

Multiple Universes

A multiple universes scenario involves each time jump creating an alternate universe where those changes may affect the future, jumping forward from there gets you a changed future. This is the only way you can change the future without a paradox. The new Star Trek movie illustrates this. By going back and killing loads of people, the entire history of the universe is altered. While it isn't explicitly stated, this created an alternate universe where the new film takes place.

Muddy thought

Back to the Future is a great example. It almost gets there, however the 'fading' aspect suggests some kind of probabilistic nature that doesn't really hold together and isn't addressed in the rest of the films, while their own logic is also defeated by Old Biff being able to RETURN to the future he left from to give Young Biff the Almanac. He should have returned to a Middle Age Biff ravaged future never shown in the films. He should have left Marty and Doc stranded in the happy future. (Where they could just build another time machine) Without these muddy-headed things, it would have been another example of a multiple universes time travel film.

  • +1 I see you have thought about these things in great depth :)
    – user296
    Commented Feb 4, 2011 at 8:45
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    Are you sure Primer belongs in the first category? Commented Jan 15, 2012 at 21:13
  • You are wrong about the Muddy Thought at least for Back to the Future. The amount of time spent altering a time line in the past and the deviation to the current timeline is relative to the time it takes to affect the future. The proof of this is shown by the fading. Old Biff stayed a little while and affected a major change. Marty made a small change and stayed a week. Commented Feb 17, 2012 at 16:30
  • @KevinHowell I don't follow. Are you suggesting that the two different timelines (biff hell and normal) converge somehow to allow for the same future? What in physics can account for this recombination? Old biff changed the future, that means he should go back to the future of the world he changed. A 'time delay' which the fading represents is meaningless, surly it wouldn't have taken old biff 50 years to fade away.
    – DampeS8N
    Commented Feb 17, 2012 at 22:34
  • I think I explained it best in my answer here movies.stackexchange.com/questions/1193/… Commented Feb 20, 2012 at 15:49

Wikipedia's compilation of Time Travel Rules is a good resource to check about this subject.

My two cents:

  • 1) Travelling back in time modifies the future, as in A Sound of Thunder

  • 2) Travelling backwards or forwards in time is part of the current timeline, as in Terminator or La Jetée

  • 3) Travelling in time allows one to visit many different worlds, each one a variation of the current timeline, as in Voyagers! or Sliders

  • 4) "All You Zombies" by Robert Heinlein. It deserves a unique category :)

  • 1
    AYZ is a great example of a predestination based time travel system.
    – DampeS8N
    Commented Jan 13, 2011 at 16:27
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    +1 for All you zombies. A must read for everyone interested in time travel.
    – fabikw
    Commented Jan 13, 2011 at 19:33
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    Hmm, in Terminator later the history is changed, so that's modifies the future. I'm not sure about it though. The movie that inspired Twelve monkeys is written roght 'La Jeteé'.
    – Mnementh
    Commented Jan 13, 2011 at 22:37
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    The other day I forwarded this link to a friend, All You Zombies ftw!
    – juan
    Commented Jan 17, 2011 at 20:15
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    The Terminator movies and TV series fall under point 1 (modifies the future), as the events of each movie did push back Judgment Day a few years.
    – Izkata
    Commented Dec 1, 2011 at 1:45

There is an essay by Larry Niven called "The Theory and Practice of Time Travel" in which he states many rules and different kind of time-traveling possibilities.


In Stephen Baxter's "The Time Ships", the official sequel to HG Wells's "The Time Machine", Baxter uses the concept of finding the causal link connecting Multiple Universes, so the causal link can be brought into the this reality and mapped and through that a circular time travel paradigm to ensure his time machine has enough and of the correct type of fuel, and a new route back to his beloved in the year 800k something.

As Stephen Baxter is a cambridge educated mathematician and engineer, its all based on current understanding of science like string theory, quantum mechanics and various current papers he has studied (which are listed).


The resolutions to the Grandfather paradox (what would happen if you got into a time machine, went back in time and ended up killing your grandfather before your father was born?), as posed by both theoretical physicists and sci-fi authors alike are:

  • Closed loop: the time machine cannot take you back in time further than the point at which the time machine first existed (or was first used). The reasoning is that the theoretical physics behind time travel result in the time machine basically being a conduit back to when it was made. Thus, because you can't even visit a time before you were born (or even right now, an hour before you flip the power switch on the time machine in your living room), you cannot kill your grandfather in a past time. This is based on real-world physics, but to my knowledge it's not suitable for sci-fi (because what's the fun of a time machine that can't take you back in time?).

  • Observer Only: You cannot change ANYTHING in the prior time. You may not even "exist" in corporeal form in the past time. A lot of stories and movies (A Christmas Carol and Its A Wonderful Life are two examples that come to mind given the season) have used this form of time travel; you can see what happened but cannot affect it at all. Usually, this is not seen as "time travel" per se, but instead just looking back on memories of things past, or stepping forward into a probable future based on the current state of affairs. As such, your grandfather cannot die by your going back in time because your doing so cannot affect anything.

  • Prevention: While you can change certain things in the past (or at least think you are), you would somehow be prevented by actions occurring in that time from killing your grandfather. The reasoning here is based on a single-reality universe (a single-verse). In a single-verse, everything that has happened in the past definitely did happen, otherwise you wouldn't be doing what you are doing now. This means that everything you're GOING to do during your time travel, you've already DONE and seen the effects of before you ever left. Because your grandfather exists, and so you do, you simply can't have done anything you're going to do that would result in your grandfather never siring your father. This is generally the form of time-travel in the Harry Potter universe; what the characters, and thus the readers, really "know" has happened (as in they've experienced it and not just inferred or assumed it) does not change; what does change is what they don't know happened, and have simply assumed. This lends itself to its own paradoxes (which authors ignore) and implied a fatalistic "moist robot" predestination; you cannot do anything but build a time machine and go back and change whatever you wanted to change, because your reality is based on you already having built a time machine, gone back in time and changed what you wanted.

  • Alternate Universe: You CAN kill your grandfather, but when you returned to your present time, it would be along an alternate timeline. This is based on a mutli-reality universe or multiverse, where at every point in time at which things could have happened differently, they did, such that an infinite number of stable "alternate realities" exist. You still exist, because you were born in a different universe of the Multiverse, and your grandfather is dead because you killed him in this new universe. However, it's as if you cut Mickey Mouse out of a cartoon's celluloid and pasted him into Avatar; you have no past that would coincide with anything in this reality, which would undoubtedly raise questions sooner or later when the denizens of this reality tried to figure out who you are and where you came from. This is very common in sci-fi, with or without the potential to undo what you have done. The Star Trek reboot movie from 2009 uses this solution; everything in all the previous Star Trek shows and movies really did happen, and in fact those events led up to the Narada going back in time. But now that the Narada HAS gone back in time, the timeline has shifted and we're now in an alternate universe where certain things have happened very differently (such as Kirk never knowing his father, the destruction of Vulcan and the death of Spock's human mother).


There's one resolution to time travelling paradoxes that hasn't been covered so far: the kind where time travel is theoretically possible, but the universe is constructed such that whenever anyone attempts to put theory into practice, disasters always seem to happen. Paradoxes are avoided entirely, because anyone who is on track towards constructing working time machine essentially becomes incredibly unlucky until they give up.

In the short story (and not the paper) Rotating Cylinders and the Possibility of Global Causality Violation Larry Niven explores that formulation; in this particular case, when trying to complete a half-built time machine, a civilization experiences a series of inexplicable setbacks that are initially believed to be the work of enemy agents.

It has also been proposed as a rather tongue-in-cheek explanation (original paper) for the various problems the real-world Large Hadron Collider has been having, as it's possible that some of the particles it creates could travel back in time.


AYZ - seconded (fifthed?) a must read, a gem of the genre. Heinlein do others, notably the 1941 precursor "By His Bootstraps", and some aspects of "The Cat Who Walked Through Walls" later.

Some others that I thought were reasonably good ideas that deal with the consequences of the underlying what-if:

1: Only information can be passed back through time, in such a way that it can be used to change the future. The great James P. Hogan story "Thrice upon a time" is a good illustrations of this trope.

2: Stuck in a time loop until you get it right. "Groundhog Day" for example (also a Xena rip-off that was not too bad).

3: Go back to occupy somebody's body in the past, e.g. "Peggy Sue Got Married", or a recent Warehouse 13 episode. I dimly recall only one instance where the impact of this on the original conscousness was dealt with in depthm but forget the title - ??

I do not really consider parallel worlds to be time travel themes.

There's a list of 213 time travel related movies at Wikipedia.

Most of Hollywood's ideas about time travel are rather silly.

(2012) Here is a 700+ list of movies containing a Time All Time Travel element, which considerably expands on the tropes of Time Travel: All Time Travel movies, 1896-2015.

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