Spoilers Favorite Time Travel Paradoxes

For time travel stories, my impression is that despite the different names, they all involve inconsistent causal loops, unless the multiverse is introduced. The difficult part of time travel paradoxes is to think out of all of the possible ramifications due to the circular nature of events.
Time travel links that describe types of Time Travel are after the spoiler section.

Updated:

• 15Oct22- Added Mastadonia
• 1Aug21- Added a shielded spoiler/plot section to Interstellar. Added Loki.
• 13 Apr21- Added Avengers: End Game clarification regarding its time travel scenario.
• 18 Sep- Added Unbrella Academy
• 27May- Added Back To The Future Part 2.
• 2 May20-Added The Final Countdown
• 21Mar20- Added Harry Potter And The Prisoner Of Azkaban.
• 4Jan 2020- Update Interstellar description, added more paradox examples.
• 16Sep19- Added Timeline.
• 13Sep19- Added Deja Vu and Cause and Effect
• 10Sep19- Added The Time Machine, The Sound of Thunder, and Avengers Endgame.

Books

• Mastadonia (1978) Clifford Simak.

Movie/TV list (not comprehensive)

• 12 Monkeys (1995)
• Avengers: Endgame
• Back To The Future (1985)
• Back To The Future Part 2 (1989)
• Deja Vu
• The Final Countdown (1980)
• Harry Potter And The Prisoner of Azkaban (2014)
• Interstellar
• Jumangi (1995)
• Loki (2021-)
• Looper
• The Sound of Thunder
• Star Trek Next Generation: All good things.
• Star Trek Next Generation: Cause and Effect
• The Terminator (1984)
• Terminator 2: Judgement Day (1991)
• Timeline
• The Time Machine
• The Umbrella Academy (2019-)

My take on time travel movies (spoilers)

• It’s intriguing, it can make for great stories, as long as they don’t go overboard. It’s best when they don’t make you think about it... too much.
• Traveling to the future has been proven by the theory of relativity, related to time. No paradoxes.
• Traveling back in time is much more problematic if the idea is you can travel both ways, especially if you do anything significant in the past and plan to go back to the same future, you left, because anything you do in the past will either change the future or split off the time line you are in, and you can’t go back.
• The Time Machine and The Sound of Thunder are two outstanding examples of uncomplicated time travel.
• Back to the Future is good because it addresses the altered time line because of Marty McFly first disrupting his parents romance, and then the way he got them back together. When he comes back to the present, his family's circumstances have changed significantly in mostly a good way.
• Back To The Future Part 2 (1989)- Not as good as the first one. However they took the time to explain how time travel works, at least this version of it.
• Loki (2021)- gets my seal of approval for innovation. Who needs to worry about paradoxes when the TVA controls all timeline and can move between at will?
• Looper and Avengers:Endgame are problematic if you are looking for coherence, IMO.
• Interstellar has a significant paradox which I was able to overlook.
• Harry Potter And The Prisoner of Azkaban- Incorporates one of the most seamless time travel sequences into a book/movie. You watch the sequence with the end result, than you flip back and see what specifically happened. Two lives are saved.
• The Terminator and Terminator 2:Judgement Day- These movies are so good, it’s easy to over look the time paradox.
• The Umbrella Academy- Just go with what the story tells you.

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Spoilers follow, Time Travel Links follow that.
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Movie Discussion (Spoilers)
The Terminator (1984) and Terminator 2 (1991)- These stories are so good, it’s easy to overlook the time paradoxes. The whopper in this story is that future John Connor sends back Kyle Reece to save this mother from Terminator assassination and Reece becomes his father!

Timeline (2003)- Exciting Michael Crichton story, the arrogance of a corporation, who plays fast and loose with people’s lives after they accidentally discover time travel trying to fax 3D package. There are elements of an altered timeline, and a bootstrap paradox.

The Time Machine (1960)- is easy to comprehend because the time traveler goes to the future, and when he comes back it’s a week later and he stays long enough to grab some tools and personal items, then returns to the future. As best as I can tell no paradox involved.

In the Back to the Future movie, the concept is easy to understand that if McFly does not get his parents together, he will cease to exist, by virtue of a fading photograph, which as I recall, a photo was used in both the 1st and 3rd movies. However what the movie does not address is all of the peoples' lives he has effected by interacting with them, other than the benefit to his own family. Maybe he inspired the "soda jerk" to become mayor. This is an altered timeline. Best not to think about all the changes that occured to Marty by virtue of the altered time line, and how he would mesh returning back to his now changed former life.

Back To The Future Part 2 (1989)- Not as good as the first one. However they explained how when old Biff gave the sports almanac to young Biff, it caused a tangent in the time line so things in the future where they were (2015) did not change, because a new timeline was created in 1955 with the handing over of the book to young Biff. The solution was to go back before the divergence and take the book back.

The fallacy with this time travel example is that they use this one event, knowledge of the future outcome of sporting events, as the only thing that would drastically alter the future, although cumulatively all of the changes caused by Marty going back, then the Professor and Marty going back and interacting with people, any number of minute things could drastically altered the furture, but hey, that gets too complicated fast and... it’s a time paradox.

The Sound of Thunder- One of the earliest time travel stories I am aware of is The Sound of Thunder, a 1952 Ray Bradbury story which is the origin for the term "butterfly effect", where a time traveler steps on a butterfly and the future is changed. Also a mediocre 2005 movie, about traveling back in time, to hunt dinosaurs. If I recall properly, dinos that we’re going to die anyway in a particular time frame, but you can’t stray off a designated elevated path. Things go wrong, a character strays off the path, and the future is dramatically changed. This is an outstanding take on time travel, and an altered time line because it illustrates that a small change in the past could produce an unimaginable change in the future you left.

In 12 Monkeys (1995), a character Cole is sent back in time to look for an organization called the The Army of The 12 Monkeys, an organization believed to be responsible for the outbreak of a deadly disease that wipes out most of humanity. However he arrives earlier than the target date, inquires, and inspires an inmate at a mental institution, which starts a chain of events. This would be a bootstrap paradox.

In Interstellar,
I decided I really like this science fiction story because it‘s equal parts SciFi and an examination of human emotion in extreme circumstances, and what could be described as a perfect time paradox, a visual depiction of time as non-lineal and under certain vague hypothetical circumstances (a Tesseract) provided by advanced beings, (maybe advanced us), accessible at different points.

It also includes other vague plot points such as quantum data being transmitted from inside a black hole to help scientists on a dying Earth figure out how to create a gravity engine. But the important thing is that I was comfortable not focusing on hard technical, scientific details which are sparse and going with the story and accepting the narrative as presented. (Description updated, spoiler added (Aug 2021).

My favorite paradox occurred in Star Trek Next Generation series final All Good Things where Capt Piccard finds himself traveling through time, jumping back and forth in his life. This is another test perpetrated by Q. In the past, he becomes aware of a temporal anomaly close to the Devron System in the Neutral Zone and discovers that by virtue of being a temporal anomaly, it is moving backwards in time, growing as it moves backwards. His retired self vowes to cajole his friends into transporting him to the Devron System to look for it at an earlier state, searching by means of deploying a reverse tachyon beam. The cause of the paradox is that he is the one who causes its creation by looking for it!

Star Trek Next Generation: Cause and Effect- A brilliant episode.
The destruction of the Enterprise near a distortion in the space-time continuum causes a temporal causality loop to form, trapping the ship and crew in time and forcing them to relive the events that led to their deaths.

Jumangi (1995)- A favorite movie, no time travel, but creates a time line that is later erased when the game concludes. No paradox, simple to understand.

Looper- the worst time travel movie I've seen if the goal is to all most understand why things happened the way they did. Nine Problems With Looper.

Avengers: Endgame (2019)- This is a top rated fan film that not only features time travel, but shoves time travel in your face to undo a huge event, in a very specific way, as if you can go back and forth in time, but somehow not scrambling everything in the process.

Six immensely powerful relics, are removed from the past, but somehow does not turn the future completely upside down, but manages to restore half of all life that was destroyed (which makes sense in itself, because Thanos used those stones to destroy that life, and he could not find the stones, because they were taken), have a big fight, then go back to the past, put the stones back and somehow that does not undo everything that the time travel changed. Bottom line: Just Don’t Think About It.

When Rhodey talks about going back and killing baby Thanos to take care of the situation, and all the time travel movies he has seen, Banner even says: Time does not work that way, your (old) future would become your past. And then the writers proceed to do exactly what Banner said they could not do! Yes, it’s true.

The Problem With End Game: As said before, the easiest solution is to accept what the story tell you. However, if you think about it, The Avengers under the guidance of Banner send 3 teams back in time to retrieve 6 Infinity Stones. As soon as the time line was disrupted by these stones being intercepted, there are three possibilities.

1. They split off time into a new or several new timelines, indeed their future had become their past, and could not return to their original time line.
2. If the premise is altering a single time line, there never would have been an Infinity War, and there would have been no reason to build a quantum tunnel (time machine) to go back in time to fetch stones. Therefore the teams who went back in time, their future would have become their past, and they would have cut themselves off from returning to their former future, as the new future they created unfolds.
3. Now if the writers were clever enough, they might have concocted a story with a new future, where a quantum tunnel had been built giving the Avengers back in time, a route to return, but it would still not be the original future they left.

Deja Vu (2006) is an enjoyable Denzil Washington movie where a police officer travels back in time (about a week) to prevent a terrorist attack on a ferry carrying passengers and automobiles. Yes, there are now 2 of him, but the story is surpringly coherent.

Loki (2021) A Marvel/Disney time travel story that avoids the usual complications by having multiple time lines (if you go back in time and change something, it splits off into a new timeline), and the original timelines remains unaltered and includes the mysterious TVA.

Of note:

• In the original Avengers at the end of movie, Loki is taken to Asguard by Thor and imprisoned and the Tesseract is stored away there.
• In Endgame, in an attempt to grab the Tesseract, Loki ends up escaping with it and ends up in the desert. This is Loki in a branching timeline, where the Loki series takes place.
• The solution for the Avengers in Endgame (the original timeline) is to go farther back in time to 1970 and grab the Tesseract there.
• Since the last Loki episode has yet to drop (when this was added) we don’t know if he stays in a seperate timeline or is somehow put back in our timeline because here, he is dead,

Disney is the Master trickster going with multiple timelines. And for story telling purposes you need either magic or technology that can not only send you through time, but to maintain a link to our timeline, technology that can jump from timeline to timeline. Disney presents the Time Variance Authority.

This will be the way to bring the Loki who escaped with the Tesseract in End Game back into our timeline resurrecting him by means of the “Time Keepers“ who manage multiple, parallel time lines, and can jump to any time line they desire with one of their time gadgets. That is the answer and my guess is this is how we will see Loki we know will rise once again in our Universe.




Some of these links have contrary ideas regarding time travel.

• A TimeTravel Website
• Rules of Time Travel@ Wikipedia.

1. Single fixed history, which is self-consistent and unchangeable. In this version, everything happens on a single timeline which does not contradict itself and cannot interact with anything potentially existing outside of it.
2. History is flexible and is subject to change (Plastic Time). Events can be altered, but there are different variations of this branch from time is easy to change (Back To The Future) to major changes are hard to achieve.
3. Alternate timelines. In this version of time travel, there are multiple coexisting alternate histories, so that when the traveler goes back in time, he/she ends up in a new timeline where historical events can differ from the timeline he/she came from, but his/her original timeline does not cease to exist (this means the grandfather paradox could be avoided.)

• Three types of Time Travel- Fixed Timeline, Dynamic Timeline, and Multiverse.

-Fixed Timeline- Even when parties travel back in time... the future they left cannot be changed. All events remain as fixed poinst in time. The actions of the traveler in the past have already become part of hisotry. This is known as the Novikov Self-Consistency Principle. For Example, say you travel back in time in order to kill Adolf Hitler as a baby in order to prvent WWII. You replace him with a orphaned baby, so that the family will not notice. You trave back to the Future, and the replaced baby grows up to become Adolf Hitler Himself (The Terminator, Harry Potter 3, 12 Monkeys).
-Dynamic Timeline- Altered events in the past have definite impact on the present. For example: If you travel back in time and kill your Grandfather... you also prevent your own birth, and your eventual trip back in time, in turn, your Grandfather is never killed, and you are born again, only to go back in time and kill Your Grandfather anyway. A Paradox as seen in Back to the Future. Confusing? YES.
-Alternate Timelines- With an infinite number of parallel universes, traveling into the past causes a new divergent timeline from the first. Because of this, the traveler can do anything with impunity, and only the new timeline will be effected. For example, if you kill all your grandparents, nothing happens. There is no paradox, you have simply created a new timeline in which you will not exit, but the original timeline is unaffected. However, you cannot return to the original timeline. (But I assume you are still existing in the new reality you created, you just magically appeared there?)

• Top Five Philosophical and Science Fiction Paradoxes

1. Grandfather
2. Bootstrap
3. Paradox of Value
4. Dream Argument
5. Paradox of Hedonism?

Bootstrap Paradox:

The bootstrap paradox, or ontological paradox, is a paradox of time travel that refers to scenarios whereby items or information are passed from the future to the past, which in turn become the same items or information that are subsequently passed from the past to the future - this creates a circularity of cause-effect such that the items or information have no discernible origin. Thus, the paradox raises the ontological questions of where, when and by whom the items were created or the information derived.

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Grandfather Paradox- Inconsistent Causal Loop.

You're a time-traveling assassin, and your target just happens to be your own grandfather. So you pop through the nearest wormhole and walk up to a spry 18-year-old version of your father's father. You raise your laser blaster, but just what happens when you pull the trigger?
Think about it. You haven't been born yet. Neither has your father. If you kill your own grandfather in the past, he'll never have a son. That son will never have you, and you'll never happen to take that job as a time-traveling assassin. You wouldn't exist to pull the trigger, thus negating the entire string of events. We call this an inconsistent causal loop.

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5 Bizarre Paradoxes Of Time Travel Explained

1. 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.
Movies: Examples of predestination paradoxes in the movies include 12 Monkeys (1995), TimeCrimes (2007), The Time Traveler’s Wife (2009), and Predestination (2014).
Books: An example of a predestination paradox in a book is Phoebe Fortune and the Pre-destination Paradox by M.S. Crook.

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2. 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.

– Information: 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.

– Person: 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“.

Movies: 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). The Netflix series Dark (2017-19) also features a book called ‘A Journey Through Time’ which presents another classic example of a bootstrap paradox.
Books: Examples of bootstrap paradoxes in books include Michael Moorcock’s ‘Behold The Man’, Tim Powers’ The Anubis Gates, and Heinlein’s “By His Bootstraps”

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3: Grandfather Paradox

The Grandfather Paradox concerns ‘self-inconsistent solutions’ to a timeline’s history caused by traveling back in time. For example, 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 then 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.

Movies: Example of the Grandfather Paradox in movies include Back to the Future (1985), Back to the Future Part II (1989), and Back to the Future Part III (1990).
Books: Example of the Grandfather Paradox in books include Dr. Quantum in the Grandfather Paradox by Fred Alan Wolf, The Grandfather Paradox by Steven Burgauer, and Future Times Three (1944) by René Barjavel, the very first treatment of a grandfather paradox in a novel.

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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.

Movies/Shows: By far the best treatment for this notion occurred in 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’.
Books: Examples of the Let’s Kill Hitler Paradox in books include How to Kill Hitler: A Guide For Time Travelers by Andrew Stanek, and the graphic novel I Killed Adolf Hitler by Jason.

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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.

Books: Paradoxes of Time Travel by Ryan Wasserman is a wide-ranging exploration on the topic of time travel, including Polchinski’s Parade.

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velocityg4 said:

What I like about Back to the Future is the inconsistency. How come in one his changing the past threatens his existence? When Biff changes the past it creates a divergent timeline.

Going by the BtF 2 rules. When Marty prevented his parents relationship. He would have just created a divergent timeline. While he would still exist in his universe. When he went to the future it would have been in a universe which he did not exist.

Which is also the only possible outcome. Preventing his own conception would create a impossible paradox without a divergent timeline and universe. By preventing his conception he would not exist to have prevented it. Nor would the time machine have made it back.

Of course there is also the problem of the Time Machine only being able to go to the same position in the past or the future that it departed from. This would mean that any time travel would most likely result it appearing in outer space. As the universe is mostly empty space.

Since at any time the total speed of the Earth moving through the universe is between ~750,000 and ~1,900,000 MPH. If you could only go to the same place you departed from. Earth would be around 200 to 500 trillion miles away from you when you arrived about 30 years in the past or the future.

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I think most paradoxes require a suspension of disbelief. That's a great point about location, that I've not considered before. You'd just use your time/space movement calculator to figure out the new location along with the time. I'll need to watch the second one again as I don't remember the "alternate time line" aspect of the story. I'm most clear on the 1st and 3rd movies.

wgnoyes said:

There's a quite decent "Sound of Thunder" movie, as well.

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I remember seeing it where repeated attempts are made to fix the future?

Scepticalscribe said:

Great thread, and I must say that I'm thoroughly enjoying reading it.

Actually, I used to love those 'time travel' episodes in series' such as Star Trek Next Generation (and indeed, the original 'Star Trek' - remember the episode 'City On The Edge Of Forever'?)

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I did not remember this title but resherehed my memory with this link: http://en.memory-alpha.org/wiki/The_City_on_the_Edge_of_Forever_(episode)

I was in Jr. High when this show came out and it was cutting edge for the time. My Mom did not want me watching it. My only complaint with this serie's visits to Earth, is that most times it looked like the studio back lot. The episode that left the biggest impression on my young impressionable mind was the episode where the guy's wife was really a morphing salt sucker who was murdering crew members.

I added a Rules Of Time Travel from Wikipedia to the first post because I like how they are broken down better than the Three Types of Time Travel.

maflynn said:

Evidently Steven King has a book our 11/23/63 that is being made into a movie (mini series I believe). This looks interesting where a man goes back in time to stop the Kennedy assassination. The book is highly rated and I may read it once I'm done with my current book.

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I'll be curious to see King's take on time. Of interest, under the Rules of Time in the OP, History Is Flexible (plastic time) section, there are variations where on one end of the spectrum time is easy to change (Back to the Future) to the other end of the spectrum where insignificant events are easily altered, but significant events like an assination would be much more difficult to succeed.

The thing about Marty McFly was that if he had caused his parents not to get together, he would have never been born, but because he had split from the original time line, he did exist regardless. I think if he had failed, he would have returned to a world where he had never been born, similiar to It's A Wonderful Life when George Bailey is granted his wish, never to have been born... but he still existed.

Its interesting that the new link mentions a Young Adult series called Timeriders where one of the plots is to go back and prevent Kennedy's assassination. To me that would be a huge gamble playing with the Law of Unintended Consequences.

mscriv said:

I'm surprised no one has mentioned The Terminator series yet. It always bothered me that the creators/writers made such an obvious time travel mistake in that series. If my memory holds true the story says that the amazing technology of Skynet was created because of the exposure that the scientist/engineer had to parts remaining from the destroyed Terminator who was sent back to kill Sara Conner in the first place. What?!? That's not possible. Skynet can't be created in the past from it's own technology in the future. In the original timeline Skyline had to be created somehow (without this exposure) so that it could even exist in the future to begin with.

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Lol, I did over look that and T1 and T2 are favorite movies. That -is- the time travel paradox, on one hand it's chicken or egg syndrome, on the other, it's self fulfilling prophecy. Maybe the idea was all ready floating around in the engineer's head, and the fragment from the first Terminator just helped him along. But how about Kyle Reese becoming John Connor's father?! As I recall somewhere in the there Sara Connor says somthing like "it's hard to wrap your head around". Personally, from a practical standpoint, I view time travel stories as cleaver science fiction, but still can enjoy them.

Not favorite- Terminator Genisys (2015). What results when you have too many time lines, resulting in a convoluted mess, and/or are just trying to hard to milk it, resulting in a sore teat and brain trying to keep all the variances connected and coherent.

ucfgrad93 said:

Star Trek: The Next Generation: Yesterday's Enterprise. One of my all time favorite episodes of any tv series.

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I remember thinking how mean to kill Tasha Yar off until I realized she decided to leave. For anyone not familiar, this was a reprisal of that role. Of interest she had been originally cast as Deanna Troy, but got switched and was frustrated with the limited nature of her role as compared to other characters. All Goood Things stands as my favorite time paradox STNG episodes.