5 Bizarre Paradoxes Of Time Travel Explained

5 Bizarre Paradoxes Of Time Travel Explained

There is nothing in Einstein’s theories of relativity to rule out time travel, although the very notion of traveling to the past violates one of the most fundamental premises of physics, that of causality. With the laws of cause and effect out the window, there naturally arises a number of inconsistencies associated with time travel, and listed here are some of those paradoxes which have given both scientist and time travel movie buffs alike more than a few sleepless nights over the years.

The Time travel paradoxes which follow fall into two broad categories, namely 1) Closed Causal Loops, such as the Predestination Paradox and the Bootstrap Paradox, which involve a self-existing time loop in which cause and effect run in a repeating circle, but is also internally consistent with the timeline’s history, and 2) Consistency Paradoxes, such as the Grandfather Paradox and other similar variants such as The Hitler paradox, and Polchinski’s Paradox, which generate a number of timeline inconsistencies related to the possibility of altering the past.

1: Predestination Paradox

A Predestination paradox occurs when the actions of a person traveling back in time ultimately causes the event he is trying to prevent to occur. He then becomes trapped inside a ‘temporal causality loop’ in which Event 1 in the past influences Event 2 (time travel to the past) which then causes Event 1 to occur, with this circular loop of events thus ensuring that history is not altered by the time traveler’s journey to the past. In other words, any attempts to stop something from happening in the past will simply lead to the cause itself, instead of stopping it. This paradox suggests that things are always destined to turn out the same way, and that whatever has happened must happen. Sound complicated? Imagine that your lover dies in a hit-and-run car accident, and you travel back in time to save her from her fate, only to find that on your way to the accident you are the one who accidentally runs her over.

Therefore, you cannot change the past and furthermore, anyone attempting to do so may literally find themselves trapped within a repeating loop of time. One way of dealing with this type of paradox is to assume that the version of events experienced are already built into a self-consistent version of reality, and that a time traveler trying to alter the past will only end up fulfilling his role in creating history, not altering it.

Examples of Predestination paradoxes in the movies include TimeCrimes (2007), The Time Traveler’s Wife (2009) , and Predestination (2014).

2: Bootstrap Paradox

A Bootstrap Paradox is a type of paradox in which an object, person, or piece of information sent back in time results in an infinite loop where the object has no discernible origin, and exists without ever being created. It is also known as an Ontological Paradox, as ontology is a branch of philosophy concerned with the nature of being, or existence. George Lucas traveling back in time and giving himself the scripts for the Star War movies which he then goes on to direct and gain great fame for would create a bootstrap paradox involving information, as the scripts have no true point of creation or origin. A bootstrap paradox involving a person could be, say, a 20 year old male time traveler who goes back 21 years, meets a woman, has an affair, and returns home three months later without knowing the woman was pregnant. Her child grows up to be the 20 year old time traveler, who travels back 21 years through time, meets a woman, and so on. American science fiction writer Robert Heinlein wrote a strange short story involving a sexual paradox in his 1959 classic “All You Zombies”.

These ontological paradoxes imply that the future, present and past are not defined, thus giving scientists an obvious problem on how to then pinpoint the “origin” of anything, a word customarily referring to the past, but now rendered meaningless. Further questions arise as to how the object/data was created, and by whom. Nevertheless, Einstein’s field equations allow for the possibility of closed time loops, with Kip Thorne the first theoretical physicist to recognize traversable wormholes and backwards time travel as being theoretically possible under certain conditions.

Examples of Bootstrap paradoxes in the movies include Somewhere in Time (1980), Bill and Ted’s Excellent Adventure (1989), the Terminator movies, and Time Lapse (2014).

3: Grandfather Paradox

5 Paradoxes Of Time Travel ExplainedThis time paradox gives rise to a ‘self-inconsistent solution’, because if you traveled to the past and killed your grandfather, you would never have been born and would not have been able to travel to the past- a paradox.  Let’s say you did decide to kill your grandfather because he created a dynasty that ruined the world. You figure if you knock him off before he meets your grandmother the whole family line (including you) will vanish and the world will be a better place. According to theoretical physicists, the situation could play out as follows:

Time line protection hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger but the gun fails to fire. Click! Click! Click! The bullets in the chamber have dents in the firing caps. You point the gun elsewhere and pull the trigger. Bang! Point it at your grandfather.. Click! Click! Click! So you try another method to kill him, but that only leads to scars that in later life he attributed to the world’s worst mugger. You can do many things as long as they’re not fatal until you are chased off by a policeman.

Multiple universes hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger and Boom! The deed is done. You return to the “present” but you never existed here. Everything about you has been erased, including your family, friends, home, possessions, bank account, and history. You’ve entered a timeline where you never existed. Scientists entertain the possibility that you have now created an alternate timeline or entered a parallel universe.

4: Let’s Kill Hitler Paradox

Similar to the Grandfather Paradox which paradoxically prevents your own birth, the Killing Hitler paradox erases your own reason for going back in time to kill him. Furthermore, while killing Grandpa might have a limited “butterfly effect”, killing Hitler would have far-reaching consequences for everyone in the world, even if only for the fact you studied him in school. The paradox itself arises from the idea that if you were successful, then there would be no reason to time travel in the first place. If you killed Hitler then none of his actions would trickle down through history and cause you to want to make the attempt.

By far the best treatment for this notion occurred in an a Twilight Zone episode called Cradle of Darkness that sums up the difficulties involved in trying to change history, with another being an episode of Dr Who called ‘Let’s Kill Hitler’.

5: Polchinski’s Paradox

American theoretical physicist Joseph Polchinski proposed a time paradox scenario in which a billiard ball enters a wormhole, and emerges out the other end in the past just in time to collide with its younger version and stop it going into the wormhole in the first place. Polchinski’s paradox is taken seriously by physicists, as there is nothing in Einstein’s General Relativity to rule out the possibility of time travel, closed time-like curves (CTCs), or tunnels through space-time. Furthermore, it has the advantage of being based upon the laws of motion, without having to refer to the indeterministic concept of free will, and so presents a better research method for scientists to think about the paradox.

When Joseph Polchinski proposed the paradox, he had Novikov’s Self-Consistency Principle in mind, which basically states that while time travel is possible, time paradoxes are forbidden. However, a number of solutions have been formulated to avoid the inconsistencies Polchinski suggested, which essentially involves the billiard ball delivering a blow which changes its younger version’s course, but not enough to stop it entering the wormhole. This solution is related to the ‘timeline-protection hypothesis’ which states that a probability distortion would occur in order to prevent a paradox from happening. This also helps explain why if you tried to time travel and murder your grandfather, something will always happen to make that impossible, thus preserving a consistent version of history.

Are Self-fulfilling Prophecies Paradoxes?

A self-fulfilling prophecy is only a causality loop when the prophecy is truly known to happen and events in the future cause effects in the past, otherwise the phenomenon is not so much a paradox as a case of cause and effect.  Say,  for instance, an authority figure states that something is inevitable, proper, and true, convincing everyone through persuasive style. People, completely convinced through rhetoric, begin to behave as if the prediction were already true, and consequently bring it about through their actions. This might be seen best by an example where someone convincingly states:

“High-speed Magnetic Levitation Trains will dominate as the best form of transportation from the 21st Century onward.”

Jet travel, relying on diminishing fuel supplies, will be reserved for ocean crossing, and local flights will be a thing of the past. People now start planning on building networks of high-speed trains that run on electricity. Infrastructure gears up to supply the needed parts and the prediction becomes true not because it was truly inevitable (though it is a smart idea), but because people behaved as if it were true.

It even works on a smaller scale – the scale of individuals. The basic methodology for all those “self-help” books out in the world is that if you modify your thinking that you are successful (money, career, dating, etc.), then with the strengthening of that belief you start to behave like a successful person. People begin to notice and start to treat you like a successful person; it is a reinforcement/feedback loop and you actually become successful by behaving as if you were.

Are Time Paradoxes Inevitable?

The Butterfly Effect is a reference to Chaos Theory where seemingly trivial changes can have huge cascade reactions over long periods of time. Consequently, the Timeline corruption hypothesis states that time paradoxes are an unavoidable consequence of time travel, and even insignificant changes may be enough to alter history completely.

In one story, a paleontologist, with the help of a time travel device, travels back to the Jurassic Period to get photographs of Stegosaurus, Brachiosaurus, Ceratosaurus, and Allosaurus amongst other dinosaurs. He knows he can’t take samples so he just takes magnificent pictures from the fixed platform that is positioned precisely to not change anything about the environment. His assistant is about to pick a long blade of grass, but he stops him and explains how nothing must change because of their presence. They finish what they are doing and return to the present, but everything is gone. They reappear in a wild world with no humans, and no signs that they ever existed.. They fall to the floor of their platform, the only man-made thing in the whole world, and lament “Why? We didn’t change anything!” And there on the heel of the scientist’s shoe is a crushed butterfly.

The Butterfly Effect is also a movie, starring Ashton Kutcher as Evan Treborn and Amy Smart as Kayleigh Miller, where a troubled man has had blackouts during his youth, later explained by him traveling back into his own past and taking charge of his younger body briefly. The movie explores the issue of changing the timeline and how unintended consequences can propagate.


Scientists eager to avoid the paradoxes presented by time travel have come up with a number of ingenious ways in which to present a more consistent version of reality, some of which have been touched upon here,  including:

The Solution: time travel is impossible because of the very paradox it creates.

Self-healing hypothesis: successfully altering events in the past will set off another set of events which will cause the present to remain the same.

The Multiverse or “many-worlds” hypothesis: an alternate parallel universe or timeline is created each time an event is altered in the past.

Erased timeline hypothesis: a person traveling to the past would exist in the new timeline, but have their own timeline erased.

  • Anonymous

    Stiens;gate(anime) is also an example of predestination example. Add it to the list if possible.

    • Anonymous

      I meant steins;gate is also an example for predestination paradox(most episodes until Okabe realizes).

    • TheBiggestMoist

      That show is exactly why I’m here learning about paradoxes! Can’t recommend Steins;Gate enough. Best show I’ve ever seen.

    • Alexis Zarco

      More like the grandfather paradox, multiverses hypothesis. Isn’t it?

  • Etienne Bideau

    The self-healing hypothesis is just predestination… Or did I miss something ?

    • Jeffrey Montgomery

      Self healing example – If you take the Polchinski’s Paradox as an example, the billiard ball would knock itself off course, still enter the wormhole, but at the other end of that one, it misses. This “self healing” would then have one interaction whereby the interaction causes the next loop of the paradox to fail.

    • Warrick Lehmann

      its not predestination. example – if someone goes back in time to save their loved one from dying in a car accident, that person may be successful but shortly after she will die some other way. It has been in many movies and tv series where the main character will go crazy trying many times and different ways to save their loved one, and keep them alive. With each rescue the “universe” makes little alterations to restore the balance and they loved one will die shortly after the save.

      • NamelessUser

        Just like „The Time Machine”. After the death of his wife, the protagonist works on a time machine and safes her multiple times, but she dies over and over again of new causes.

    • http://www.astronomytrek.com/ Pete

      Basically, Novikov self-consistency principle helps resolve the Predestination Paradox

    • Rey

      In Self-healing hypothesis the alteration made in past ends up creating a new timeline, but to a certain extent later it will follow the actual timeline…Predestination hypothesis is juz for a certain event which will occur no matter what…..its juz that some of the other events in the altered timeline will be different than the real timeline….so yea, for that certain event that will occur for sure you can say both are same, but overall its different..kind of complicated:p

    • paul cuzz

      The way I understand it is self healing like this. You go back in time and kill Hitler, but then Himmler or Goring or Goebbels would have stepped in and continued on.

  • Erik Stengler

    I Just watched Predestination, and I would say it’s a Bootstrap paradox, not a Predestination paradox, or at least maybe a mix of both (Predestination in that young Hawke becomes old Hawke despite having sworn not to do so, but this is just in order to close the loop)

  • Joel Stoner

    1, 3 and 4 are the same paradox. The Causality Paradox.
    Looper is a bad example, as it never addresses the Paradox. If the man kills himself while he is his young version, then he never goes into the past, to cause the young man to want to kill himself to save the kid. Causality Paradox at work.

    • http://www.astronomytrek.com/ Pete

      Time travel paradoxes fall into two broad categories, namely consistency paradoxes, and casual loops. The former includes the Grandfather Paradox (3) of which The Hitler paradox (4), and Polchinski’s Paradox (5) are well-known variation on the theme. The consistency aspect of the paradox relates to the possibility of past events being changed.

      On the other hand, the Predestination Paradox (1) and Bootstrap Paradox (2) are variants of ‘causal loops’, which relate to a self-existing object, person, or information trapped in time. These paradoxes, however, are consistent and indicate an absence of causality violation.

  • Yonathan Arief Kurniawan

    I think that the solution to any paradox involving time travel is that to realize that something that exists doesn’t necessarily have any influence upon reality and something that influences reality doesn’t necessarily exist. It’s like a mute person and a disembodied voice. The former is like the mute person while the latter is like a disembodied voice. If you killed your grandfather you would still exist but have no influence upon reality, no one would be able to see, hear, or even remember you.

  • John Hamilton

    Maybe the Universe is the result of a Bootstrap paradox.

    • Zac C.

      Hmm, I like this notion. Perhaps years from now, we discover real time travel. Some time later, we also develop a way to spark the creation of an infant universe. Someone decides to experiment with both and, intentionally or non, becomes the initiator of our own eventual creation.

    • Aaron Stephens

      God created the universe so this is impossible.

      • Eddie Hines

        God is also the author of any paradox we talk about since he exist in all times, places, and events. in our future bay be the time he creates this universe. since time has no meaning to him, we are siing his wonders from a very primitive view.

        • willcommentforfood

          Exactly. Such as limited minds so obsessed with their religious dogma that they can’t comprehend a time theory with any complexity, and think it’s an attack on their theology.

          You replied well, because any understanding of the incredible immensity of the infinite God already shows that the complexity of the universe, and any of its potential time paradoxes, if they exist, would be “created” by God, or “allowed” as “bad things” like Hurricanes are allowed”). This gets into dicey theological questions that don’t interest me much, because my view is that God is a higher power force for good, kind of like the AA vision of diety, more concerned with helping us make better decisions, a counselor if you will, rather than a manipulator of events, nations, politicians or natural phenomenon. Safer ground for me, as a human, to comprehend the usefulness of a deity that operates strictly in the spiritual realm, as the material realm operation is far above my comprehension or pay grade.

          So, I find your response excellent, intelligent, and smart.

  • Kev Charlton

    Does this mean if enough of us believe that Time Travel is possible then it will eventually become a reality?

    • willcommentforfood

      Seems like it. Certainly, enough people & governments & scientists seem to think it’s possible. Then, that can create it, because resources are put into creating it. Not by magic, but just like air flight, or submarines, people wanted to make these things a reality, and worked hard to do it. Some call that a self fulfilling prophecy, and it is, but it’s also, sort of, history in the making.

    • Sp♠de

      Did you ever stop to actually contemplate time? There is no “if”. We are all travelling in time right now. Growing older, days going by, etc. The real question, then, is .. is time linear? Can we go backwards as well?
      Just to remain on topic, one of the best time travel movies ever made (in my opinion) is Primer (2004)

  • mark1983

    I really believe that if Time Travel existed, The Bootstrap Paradox theory and Multiple universes hypothesis could be a better explanation, One would go back in time, Kill their grandfather, come back and find that they do not exist in that time line. Or, if he/she goes back in time to give an item that will make him/her richer in the future, then get back in the time machine and find that everything has changed, he/she is rich.

    However, the individual would have to share their fortune with their duplicated paradox (Ref Continuum) unless that individual kills their paradox self, they can live life to the full, unless the future paradox double goes back in time and creates a whole new time line while the previous paradox stays in that time line living their life, the new time line that person created which is a whole parallel universe idea, that is if the parallel universe infinite. It’s like being on a motorway (highway) and changing lanes every time the person goes back in time.

    I also think may be the The Double Occupancy Paradox might have some affect where as the same matter can not occupy the same space (Ref: Timecop)

    All this might sound screwed up, but it’s a better explanation than Back To The Future, 12 Monkeys and even Frequency.

  • Just Me

    the Polchinski’s Paradox could just as easily become a Bootstrap paradox.
    The ball making the original deviate and not go through the portal, but itself going through the portal again. Making the 2nd ball of unsure origin.

  • Sudipta Pradhan

    What is that “something” which prevents me from knocking out my grandpa and doesn’t allow to alter history as in case of predestination paradox??

  • http://tvtropes.org/pmwiki/pmwiki.php/Tropers/StarSword StarSword

    Best example I ever saw of a bootstrap paradox was in The Legend of Zelda: Oracle of Ages. During a chain-of-deals questline, you get a vase from a guy in the present only to then almost immediately trade it to his ancestor in the past, who says it will become his family’s heirloom.

    Of course the same game also has changing things in the past changing the present, and a process in the past visible at the same speed in the present. At which point your roommate has to get a mop to clean up what’s left of your head.

  • Christopher Barber

    If the person meets someone from the future say a relative or someone that is relative to the time travelers timeline will they be forced to see the endless time loop? If so what if someone use powder to erase the effect will that work?

  • Ashish Jog

    I have one question about grandfather paradox; who confirms that you killed the man is Your Grandfather; and as we know the “NATURE” is self adjustable; it can happen that when you return you’ll know your Grandmother had Affair and you result from it! and this loop continues as you go on; killing grandfather or more clearly as you put NATURE in TROUBLE!

  • Stephen Pegg

    Steins gate is somewhat true from what I have an understanding on but its not with tech. I think our brains can do it without any help, I have not really been able to figure it out completely I don’t know if I ever will. But viewing the what if situations that is possible our minds can dream up different paths we can take. You might be able to affectedly take over an alternate version of yourself with enough will to do it. But that version of you must want it as well. When it comes to physically traveling to the past or future there will be A allot of power needed. B a map drawn up of all the possible situations that could happen to prevent the changes from reverting back and also the prevention of your own destruction of you meddling with things you should not be messing with. Time travel would be super messy only in situations that its life or death should it be attempted. Knowing the future is just fooling ones self as if you trust what you see then it will happen but if you did not know what would happen anything is possible.

  • Silverslyph

    So try this, add quantum mechanics: by simply creating these concepts we are changing the universe, because it becomes what we want it to be. So we create time travel and paradoxes because we all participate in stories that makes them real in our heads via reading, TV, etc. Like Star Trek, we all dreamed of communicators and tricorders, which in reality we now have. How long before they build a stargate? What do you call that paradox? Lol!

  • http://www.dinaraeswritestuff.blogspot.com Dina Rae

    awesome article. Really liked how you broke down the theories for the regular person. Dina Rae, author

  • C HAX

    Reading about this reminds me of Primer, such a good film about time travel.

  • Santosh

    Is it possible that you travel back in time and you will not see the same events which happened in your life. basically you end up in another time line. Here the question arise how you will confirm you are in same time line. My other concern is can we invent a device which can communicate across time dimensions just cellphone which connects across spaces. My other thought is quantum entanglement can happen across time.