Favorite Time Travel Paradoxes

Huntn

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It's not an anomaly, a conundrum, or a fallacy, it's a PARADOX!

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:
  • 13 Aug- Primer and Time Lapse Added.
    13Jul Loki Added.
  • 12Jul21- Thread resurrected from Null Space.
Movie/TV list (not comprohrnsive)
  • 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
Mentioned In This thread
  • Primer (2004)- post 21 page 2.
  • Time Lapse (2014)- post 22 page 2.

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, based on a single timeline, are two outstanding examples of uncomplicated time travel.
  • Back to the Future (single timeline) 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 and there is no intent to return to he furture, it’s easy to over look the time paradox.
==========
Spoilers follow, Time Travel Links follow that.
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Movie Discussion (Spoilers)

The Terminato
r (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).
  • The Earth is dying and must be abandoned.
  • Cooper is a retired astronaut and now farmer watching his crops die.
  • Murph, his daughter talks about a ghost in her bedroom.
  • This “ghost“ sends them a binary message of geographical coordinates.
  • Coop and Murphy drive to it and locate a secret NASA facility underground.
  • …where Dr. Brandt an acquaintance of his, tells him about the mission to save humanity, Plan A relocate the humanity to another planet. Plan B is to take human embryos to another planet to get a fresh start. Brandt is working on technology to harness gravity as a propulsion force to power manned vehicles out of our atmosphere into space and carry humanity to a new home.
  • Cooper the only one available with previous space mission experience volunteers to lead the mission.
The Convenience
  • Convientently a worm hole appeared in the vicinity of Saturn 40 years ago. It is speculated that an advance civilization placed this worn hole for our use to provide a viable option for humans to relocate.
  • Later in the story Coop finds himself in the Tesseract, a device created by 5 dimensional beings, which allows him to send a variety of messages back to his daughter.
  • To solve the gravity engine equation, quantum data is required, that can only be retrieved from a black hole. Coop and his robot end up entering a black hole (not voluntarily), the robot grabs the quantum data needed. Coop uses the Tesseract to send this data via the Tesseract to his daughter through an identical wrist watch he gave her.
The Paradoxes
  • Coop of the future sends himself the coordinates of the secret base which results in him leading the mission.
  • Coop speculates that the advanced civilization who placed both the wormhole and the Tesseract, might be “future” us. The second paradox would be why would we have to save ourselves if we survived into the future Before this.

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 processi.

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.

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.



 
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Huntn

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Favorite Time Travel Paradoxes- Pt2 Theories and Links

Some of these links have contrary ideas regarding time travel.
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 (parallel reality/universe?) 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.)

-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, so you are no longer their to 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 (new parallel universe/reality?) in which you will 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?)
 
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Part 3

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.

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.

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.

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

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.

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.

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 Paradox.
 

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They're all nonsense. Time travel, whilst possible doesn't account for any other dimension such as movement of the earth, or the expansion of the universe. Travel 50 years into the future and you're liable to end up sucking the deep vacuum of space as anything else.

Yeah, pet peeve of mine!

That said, I do love a good TT paradox and as much as the aforesaid is top on my mind, I let it slide when it comes to particularly well done TT movies.
 

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One of my physics professors gave us an explanation one day about time travel and how it would be possible, except for entropy. The basic premise of his explanation was that entropy fucks everything up and makes processes non-reversible. To be fair though, he did also mention one day that n-th order integrals give him sexual pleasure, so....
 

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List needs Star Trek 4. THE HUMPBACKS WILL SAVE US ALL, PEOPLE!

I hold T2 as probably the best action film ever made, practically forgetting time travel is in it.

I have a high opinion of Back to the Future 2, more than others seem to. It’s pretty dark and goes fully nuts. Needs more Crispin Glover but every film does.
 

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Watched about 2/3 of The Tomorrow War and couldn't finish it. Some of the laziest and trope-filled writing I've ever witnessed. As far as time travel issues...the main character goes to the future and meets his adult daughter who informs him that he becomes a top-shelf asshole when he returns to the past. Um, with that information don't you think he would then not become a top-shelf asshole when he returned to the past? Just seriously lazy writing so they can create some family drama that needs to be mended....as if destroying human annihilating aliens isn't compelling enough of a plot.
 

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Watched about 2/3 of The Tomorrow War and couldn't finish it. Some of the laziest and trope-filled writing I've ever witnessed. As far as time travel issues...the main character goes to the future and meets his adult daughter who informs him that he becomes a top-shelf asshole when he returns to the past. Um, with that information don't you think he would then not become a top-shelf asshole when he returned to the past? Just seriously lazy writing so they can create some family drama that needs to be mended....as if destroying human annihilating aliens isn't compelling enough of a plot.
Well, you'll be happy to know that Amazon have commission a sequel!

Yeah, I'm with you on plot holes up the wazoo. Not the least that they do all these 'checks' to make sure the draftees are already dead by the time the war starts, yet ignore the incredibly dumb issues that exist - e.g. if Forester was meant to die a few years alter, and his daughter's entire life experience was changed because of that, then wouldn't her entire life experience be changed by him NOT dying a few years alter, ergo would she be where she is, ergo would she have helped develop the...

My brain hurts - and it's not that big to begin with.

A dumb movie - agreed.
 
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I thought most time travel paradoxes are “solved” by the multiverse theory. Every possible difference is a branched parallel universe. So your reason to kill hitler or your being born isn’t affected, because by killing your great grandpa Adolf you’ve created a new universe… which you’re now stuck in, and thus you’ll never return to the “present” you left.

ie if you “filmed” it from the point of view of the place you left from - you’d just never come back, and nothing would change for those people.

of course agents of shield had to have a solution for that too.
 
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Coincidentally I started watching a recentish (2016 maybe?) “chase bad guy into the past” series recently. Timeless? Or something. Kinda predictable so far - it tries to explore the cause/effect ripples a bit, but apparently only for one character. For the others literally nothing changes it seems.
 
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I thought most time travel paradoxes are “solved” by the multiverse theory. Every possible difference is a branched parallel universe. So your reason to kill hitler or your being born isn’t affected, because by killing your great grandpa Adolf you’ve created a new universe… which you’re now stuck in, and thus you’ll never return to the “present” you left.

ie if you “filmed” it from the point of view of the place you left from - you’d just never come back, and nothing would change for those people.

of course agents of shield had to have a solution for that too.
I believe it is an accepted scientific fact that in a single time line you can travel forward in time via the theory of relativity by the virtue of relative speed.

If traveling back in time was regarded as possible, the theory of the multiverse could eliminate paradoxes based on some caveats.
  • If it is believed that going back in time, just your presence there would alter the past and therefore create a branching timeline which would represent a new parallel universe/reality.
  • The original timeline remains unchanged so no one would be able to fix anything in their timeline.
  • If you are relying on some kind of technology to travel in time and return, would that technology still exist in the future of the timeline you are in, to bring you back? That’s an issue whether it’s a single or branching timelines.
IMO, this is the issue with Avengers: Endgame, a desire to reverse the snap (where half of life has been wiped by Thanos) 3 teams go back and change history by stealing the Infinity Stones at 4 different times in the past.

If a single timeline, what a mess! For each change each team made, they would have immediately altered the timeline from that point forward. It is easy to imagine that they’d end up with a present (from where they started) that has become unrecognizable.

Using the muliverse as a story element to avoid paradoxes, each team would be creating separate branching timelines, and there should be no way to return and rendezvous with each other as they are all ended up in different, seperste timelines.

Another issue is that by relying on technology to time travel there could be a scenario where the type of technology you used to time travel, does not exist in your (branched timeline’s) new future to bring you back to the future. Example:
  • Thanos destroys half of all sentient life in the universe.
  • We need a time machine to go back and reverse this.
  • We travel back in time but by changing time the entire timeline is altered immediately: no Infinity War, No snap, no reason to build a time machine.
  • Option 2 multiverse, changing time branches your timeline off. Would a time machine have been built in the new future your created?
  • So could you get back? As I said if coincidentally a time machine had been built that uses the same tech, then it might be possible.

So it’s easy to say I’ll travel back in time and change something to fix my present:
  • I can easily accept going back and by changing my past will change my (former) present as long as I have no plans to return to my own future.
  • Whether a single timeline or branching timeline, you would not be able to return unless coincidentally despite the change you made in the past, the technology was still created to allow you to return to a future via a time machine.
The story telling solution:
In real science, the quantum world is mind boggling. I believe (in science) there is evidence or at least theories that the quantum world spans and connects something like the multiverse.

For Endgame imo, it would of have been easier to swallow if instead of have a time machine fixed in a particular timeline, they used Ant Man technology, and simply said Pim’s suit was built to travel in the quantum world, a world that can access the Multiverse and added a quantum (GPS style) navigator to take you to any timeline at any point in that timeline. No time machine anchored in the future required. However the changes you make in a particular timeline would still carry forward in that specific timeline.

For the Loki series, Disney introduced the TVA (Time Variance Authority) which is brilliant, they created an entity that not only manages multiple parallel timelines/realities/the multiverse, but includes the technology (magic ;)) to move around and cross over to any timeline they need to.

As I write this, we won’t actually, possibly know what the TVA really is until tomorrow when the last episode of Loki drops. However, I think we can say that the TVA does have the ability to interact with multiple timelines.
 
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In real science, the quantum world is mind boggling. I believe (in science) there is evidence or at least theories that the quantum world spans and connects something like the multiverse.

they used Ant Man technology, and simply said Pim’s suit was built to travel in the quantum world, a world that can access the Multiverse and added a quantum (GPS style) navigator to take you to any timeline at any point in that timeline. No time machine anchored in the future required.
This is basically what Agents of SHIELD does in the final season, but it's not revealed to the audience until the very last episode. But even then they still have an "anchor" of sorts IIRC. It does get very weird in that series though, as soon as time travel is introduced.

This is part of why I like Interstellar - it doesn't try to explain how its all possible, even the character acknowledges that he doesn't actually know how he's doing what he's doing - but it alludes to a theory that is easy enough to understand for most of us to just be able to enjoy it: gravity transcends time and space.

Also, it's got sarcastic as fuck robots, so I'd have accepted almost any storyline. Cooper could have physically gone back in time via a black hole, married and then killed his own grandmother and I would give all that a pass, so long as the sarcastic robots went with him.
 
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