Categories: Astronomy ListsTime & Time Travel

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 becomes part of past events, and may ultimately causes the event he is trying to prevent to take place. This results in a ‘temporal causality loop’ in which Event 1 in the past influences Event 2 in the future (time travel to the past) which then causes Event 1 to occur, with this circular loop of events ensuring that history is not altered by the time traveler, and that 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. Your attempt to change the past has therefore resulted in a predestination paradox. One way of dealing with this type of paradox is to assume that the version of events you have experienced are already built into a self-consistent version of reality, and that by trying to alter the past you will only end up fulfilling your role in creating an event in history, not altering it. In The Time Machine (2002) movie, for instance, Dr. Alexander Hartdegen witnesss his fiancee being killed by a mugger, leading him to build a time machine to travel back in time to save her from her fate. His subsequent attempts to save her fail, though, leading him to conclude that “I could come back a thousand times… and see her die a thousand ways.” After then traveling centuries into the future to see if a solution has been found to the temporal problem, Hartdegen is told by the Über-Morlock:

“You built your time machine because of Emma’s death. If she had lived, it would never have existed, so how could you use your machine to go back and save her? You are the inescapable result of your tragedy, just as I am the inescapable result of you.”

Other examples of predestination paradoxes in the movies include 12 Monkeys (1995), 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

This 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.

Peter Christoforou :

View Comments (46)

  • 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!

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

  • 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.

  • I believe it would be close to The Multiverse hypothesis in that by traveling back in time, you end up in a parallell universe.
    However, this parallell universe is not created neither when you travel or cause a time altering event. Imagine the amount of energy it would take to create it? It's more likely that paralell universe already existed and whatever action you take in it will follow all natural laws of nature.
    To return to your own time line, I can only see two possibilities:
    a) Time is not linear and everything that ever has, is, and will, all exist at same time. To then go into the future, you could end up in an infinite number of alternate realities.
    b) You can't go to the future other than the normal flow of time. While you might find a way to get back 10 years in the past, you would have to live in that timeline for 10 years until you back to your "own" time. And that timeline would most likely not be the same you came from.

    Finally, would you be able to interact with yourself in a past timeline? Perhaps some weird effects of quantum mechanics would just send you into a timeline where you didn't exist. Either you died, or you were never born. Your knowledge of the future events would most likely also become false. Perhaps you know the winning lottery number and place all your money on it, only to lose it all. Trying to convince others and people would also be futile if you couldn't even "predict" future events you remembered.

  • This is why many of us think science is full of shit (besides the fact it keeps changing). Why all this discussion about something that is obviously impossible? Only scientists ....

    • If you're only going to talk about things that are actually definitely possible, how are you going to find out what's possible without science?

  • How about the observers? Beings from the future that has traveled to our past and present. In their timeline they have no record of our life so they travel to the past to create a record for their own timeline. Apparently there is some form of catastrophic event that happens and completely destroys all records prior. Are these observers changing our timeline by coming here, or does it not matter due to the fact the upcoming catastrophic event destroys all evidence of their presence in our timeline anyway.

  • This all paradox's thus explain us that everything is planned for everyone & nothing can change it until something much powerfull than that planner interrupts in between all the plans.

  • Please consider these.
    You mentioned that the things in the bootstrap and predestination paradoxes occur in a cyclic and never ending fashion and you pointed it as the main problem.
    But in the Grandfather paradox you raised a question"Why should I go and kill Hitler as there is no reason and trigger for me to go back if I have already killed him".If you look up the above sentence carefully you will notice that you(the creator) have tried to connect all these events in a cyclic fashion that you never wanted in the bootstrap paradoxes.

    First you didn't accept the cyclic, origin less events.
    Next you mentioned grandfather paradox as a paradox because the cycle is not completed.
    Both paradoxes are contradictory ..........

    Whats this........................

  • A man walks down a street in NYC, enters a telephone booth and calls a cab. The cabby tells him they will be there in ten min, cabby arrives at destination a woman walks up to the cab and complains about how late the service is. Confused and baffled the cabby gives the woman a discount, the cabby drives around the block for twenty minutes never realizing once there was no one in the car. But his pay check just doubled and the his meter reads as never having left the spot.