Time travel paradox

The time travel paradox, or grandfather paradox, is a paradox probably first coined by the French science fiction writer René Barjavel in his novel Le Voyager Imprudent (The Imprudent Traveler), from 1942. The concept had already been mentioned previously by the American writer Mark Twain in his posthumous novel The Mysterious Stranger (published in 1916), which despite not belonging to the science fiction genre, the plot comes to focus at times on the infinity of alternate lives that could be possible if it were not for a simple action or inaction.

It is assumed that a person travels through time and kills the biological father of his biological father/mother (the traveler's grandfather), before he meets the traveler's grandmother and they can conceive. Then, the father / mother of the traveler (and by extension, that traveler) will never have been conceived, in such a way that he will not have been able to travel in time; by not traveling to the past, his grandfather is not killed, so the hypothetical traveler is conceived; so he can travel to the past and kill his grandfather, but he would not be conceived, and so on indefinitely.

The previous paradox has no solution since the passage of time is linear and there are no alternative time lines. It has been speculated that this would cause reality to collapse to the point where the traveler murders his grandfather. Another speculation is that of multiple timelines and universes, but that would be mixing up the paradox with other theories not connected to time travel.

A variant of the grandfather paradox is the Hitler paradox or the Hitler assassination paradox, a fairly common trope in science fiction, in which the protagonist travels back in time to murder Adolf Hitler before he started World War II. This does not necessarily prevent time travel itself, but said assassination, by avoiding World War II, precisely eliminates the motive for said travel and also eliminates the knowledge of the utility of said travel. If there had been no World War II, then there would be no reason to travel back in time to kill Hitler. Furthermore, the consequences of Hitler's existence are so great and global that, for anyone born after the war, it is likely that their birth was influenced in some way by its effects, and thus the grandfather paradox. would apply directly in some way.

Ultimately, what the grandfather paradox tries to represent is the impossibility of traveling to the past, since absolutely any action carried out in the past, even the simple fact of the traveler being in a certain place even without doing anything else that staying there static would alter the state of the world in the future and could potentially imply the impossibility of the traveler having made the trip in question.

But that does not mean that they have stopped proposing theories of all kinds, based on which it would be possible in one way or another to be able to travel to the past or to what could be called a version or copy of it.

Analysis

As much as one tries to solve a paradox, it is impossible by definition. Even speculating with alternative time lines is not a solution, since as the scientific community expresses it, it would not be traveling in time but between different dimensions, which is not the same.

A paradox is something physically inexplicable, like what lies beyond the event horizon of a black hole.

Hypotheses in science fiction

In the science fiction television series Star Trek, the time travel paradox has also been called the "Pogo paradox" after a phrase from the comic book character Pogo (Walt Kelly, 1971): «We have met the enemy and this is... us».

Solution of parallel universes

If the time traveler reaches the past, and kills his grandfather, he will do so in a parallel universe where he will never be conceived. That is, he will continue to exist in his original universe, but he will not exist in the universe that he originated from by killing his grandfather. Therefore, since said statement is true, he will never be able to return to his original universe, since that causes changes in each universe that existed. Although there is a possibility that the paradox is now in that parallel universe.

This premise is used by Alfred Bester's story, The men who murdered Mohammed (The men who murdered Mohammed), and by John Boyd, The last spaceship on Earth. It is also used in James P. Hogan's novel, Thrice Upon a Time, and in Michael Crichton's novel, Rescue in Time (adapted for the big screen with the original title of the work, Timeline, in 2003).

Solution of relative timelines

The universe may not have an absolute timeline, which remains unchanged once events occur, or from a deterministic point of view, since the beginning of time. Instead, each particle would have its own timeline, and therefore humans would too. This can be considered similar to the theory of relativity, except that it affects the history of a particle rather than its speed.

Physical forces affect physical particles. If all the physical particles of a human being traveled back in time, that person could kill his own grandfather (no physical force would stop him). As a result, he would get nothing physical, because there are no physical forces that can understand what has happened, and this new time line would unfold simply because the universe has no mechanism to undo it. That person's future self does not need to be born to fulfill the destiny of going back in time, because there are no "absolute" timelines that must be fulfilled. If that person were able to find and observe the current versions of his future particles, they would also follow physical laws and therefore would not become his future self (because one of his parents will not be there to see him). procreate it).

This theory is similar to the theory of parallel universes, except that it occurs in a single universe. It should be noted that it is gaining support among scientists, especially those who affirm that the different possible quantum states exist simultaneously and that by examining them and collapsing the wave function, what is achieved is choosing which universe to stay in. In other words, Schrödinger's cat is alive in one universe and dead in another. This paradox is evidenced in the film "Coherence, where its protagonists, while a comet passes close to Earth, get trapped in a Schrödinger box in real time, where millions of alternate realities that coexist mutually converge. Thus, the characters randomly change from reality to reality, unable to return to their original reality, facing the "dangerous and mysterious visitors" (other versions of themselves).

Albert Einstein's theory offers a simpler explanation: energy will always turn into something else, it will never disappear. If an individual travels in time and avoids his own birth, he does not have to disappear or "fade away"; it will continue to exist, but perhaps with some difference. Perhaps he himself is the only one who is aware of his existence, and all the others would never have found out that he existed.

Quantum Regression Solution

It has been postulated that some particles possess the natural quality of "going back in time," which at a quantum level means that if a certain physical process occurs, it can randomly reverse itself and return to the particle(s) involved to the previous state. Since it is not yet known when it will happen and the reason for doing it, it is a process that cannot be repeated in a controlled situation. This is equivalent to saying that if a piece of paper were burned, then for no apparent reason the smoke and ashes would come together again to form the same piece of paper.

It was even postulated that if this phenomenon could be controlled, a machine could be built that could reverse all the physical processes that occur there at a certain time and, later, build an even more advanced one that would reverse the processes that occurred outside of it, maintaining stable inside. Thus, the occupants who were inside would see time go back around them, but only in the historical sense. The rest of the universe, those objects that were beyond the reach of the machine, would continue their course without being altered. It is worth mentioning that in a CERN experiment it was believed to document a certain number of particles that took a slight time jump in their particle accelerator, only to later discover that it was nothing more than a failure in data collection.

Restricted access solution

Another solution, exemplified by Novikov's self-consistency principle, holds that if a person were to travel backward in time, natural laws would forbid any action that would result in his time travel not happening.. This theory can lead to doubts about the existence of free will (Which, in this model, can be an illusion). He also assumes that causality must be constant. This is, that nothing can happen if there is no cause, while other theories maintain that an event can continue despite the fact that its initial causes disappear. It is also possible that the action intended by the traveler is completed, but never successfully enough to result in a cancellation.

Creating a new future

It is also possible that from the moment you manage to travel to the past, you are actually creating an alternate line, where the traveler does not alter the past, but rather the future of a parallel universe, from where you cannot alter absolutely nothing of the universe from which it came.

This is the main plot of the movie Back to the Future Part II. Marty McFly, the protagonist, travels to the future to the year 2015 specifically, and acquires a sports almanac with the results of the last fifty years. Immediately afterwards, the antagonist, Biff Tannen (from the future), meets him by chance. After learning of the existence of a time machine, he manages to steal it along with the almanac, fleeing to the past to deliver it to himself as a young man. When McFly returns to his time, he discovers that life has changed, the result of an alternate future: Biff had become rich and married Marty's mother.

Something similar can be seen in the movie Déjà vu, where the protagonist, Doug Carlin, manages to travel back in time to save a girl. An alternate future is created and in the end Doug manages to save her, but ends up dying in the explosion of a car that had fallen into the water with the two of them inside it. When the girl is rescued, the police officers tell her that someone will come to talk to her, and at that moment Doug Carlin from the past appears, who has not yet met the girl, but when he asks her if they know each other, she answer yes. In this way, the future has been modified: in the "first" future, Doug Carlin investigated from the death of the girl and an accident on a ferry, ending up traveling to the past to change the facts. In this "second" created future, the girl is alive and the ferry accident had been prevented by the Doug Carlin who died, but the Doug Carlin of the normal timeline is still alive and arriving at the scene to question the girl he himself had saved minutes before.

In the 2002 film The Time Machine, based on the novel of the same name by H. G. Wells, it is invented that the events that occur in the universe are inevitable and occur in all its timelines. Thus, the protagonist's wife dies in many different ways in each of his trips to the past. The explanation is that, if he had prevented her death, he would never have had a reason to build a time machine, so he could not save her, and so he would build the machine to save her, but by saving it he would not have built it, etc., for what his wife must die in order not to create a paradox similar to that of the grandfather.

Something similar occurs in the first Terminator film, where Kyle Reese, a member of the resistance against robots, travels back in time to protect Sarah Connor, the future mother of John Connor, leader of the resistance, and ends up generating this future leader with it. This is how the paradox arises: if Reese traveled to defend the mother of John Connor, it cannot be himself on the same trip that produced his existence, since if he did not engender him on the trip, he would never have existed, therefore there would be no reason to travel to protect him or his mother-to-be.

An example closer to the original situation can be found in an episode of the animated series Futurama. In it the protagonist, Philip J. Fry, accidentally travels back in time where he meets his grandfather and his then girlfriend. He tries as hard as possible to make sure nothing happens to his grandfather, but he ends up dying in a nuclear test accident. Surprisingly, Fry did not disappear. Thinking that her grandfather's girlfriend was not really her grandmother, he sleeps with her. In the end he discovers that he was, and that the only reason he continued to exist was that by sleeping with her he became her own grandfather.

Solution of the other personality

In science fiction stories it has been suggested that it is possible for a subject to travel through time and murder his father if that subject has taken on another identity (obviously in the sense of really being another person). An example is cited in the game Prince of Persia: Warrior Within, where the protagonist travels back in time to prevent himself from making a mistake in his time travels. For this, he manages to get hold of a mask that transforms him into another identity, with which it is possible for him to alter the facts without creating any paradox.

Spectral Contemplation

In science fiction books and novels, the idea has also been raised that if travelers go to the past, they are not physically part of it, but appear as specters (ghosts). This theory can be observed in the game The Dig, by Lucas Arts, as well as in the series Quantum Leap, when the character Al Calavicci appears in the past in holographic form.. You can "see" the past, but he cannot change or participate in the events that happen there. In short, the travelers would be invisible to the residents of the past.

This theory can be raised from the point of view of quantum physics, you can travel in time to the past through a black hole to another universe but this can never interfere with the other, because although they share the same event of events, these are in different dimensions and can only be joined by a black hole. An example of this theory is seen in the movie Interstellar, where Joseph Cooper travels through a black hole and returns through it and enters a fifth-dimensional area, where he can create events in the original universe from which he came. but cannot be seen by his daughter Murphy. The example given in the film is of Joseph seeing events in his daughter's room that had already happened when she was little, he finds a time line in which she is next to the library where her father is going to move some books. to send you a message.

Messages in time

Going back to the movie Déjà vu, when Doug Carlin begins to investigate the case of the murdered girl, he begins to see puzzles that make no sense: A message found in the refrigerator that says "you can save her", a bloody towel in the sink, a phone number, a call, etc. They are messages made by himself that, if the condition of traveling to the past is true, act as signals that he left in a previous moment, so they will remain constant in "his" of her; universe. When he travels to the past and saves the girl from being killed, these messages become true, since he himself devised and carried them out. This is a very peculiar paradox, where an individual is sending messages to his & # 34; another me & # 34; of a past that, supposedly, is not certain that it exists or ever existed. If he thought of traveling to the past in the near future, the desire would be on his subconscious, but the trip would never happen, unless some important event happened that forced him to do so within his universe.

Origin and destination

Another statement in the public domain that is discussed in science fiction forums is the theory of origin and destiny, which stipulates that a machine and/or condition is necessary to open a portal "entrance" in space-time, and another "output". Because of this, it is impossible for a human to travel until before the creation of the first time travel mechanism that he could recreate the conditions to open another portal. This theory is under debate and in the process of being refined.

In the case of the game Legacy of Kain: Soul Reaver and its sequel Soul Reaver 2, time travel to the past is possible thanks to the existence of a machine that was created, but such travel into the past is limited to the machine's existence in the timeline. This implies that a trip to the past is not possible if the machine, at that time, was not created.

The "overlap" solution

There would be a probability that an individual would travel back in time and reproduce with his mother to create himself. But this case would be an impossible paradox, since if it worked, the son would live another future different from the first and thus history would be changed. In this case there is, for example, the short story by Robert A. Heinlein All of you zombies in which this assumption is masterfully resolved without changing the future history of the character and closing the loop by performing various time travel more.

One Line Theory

It expresses that there is only one timeline and there are no parallel universes. If this line could be altered, from this alteration it could be said that & # 34; he changed direction & # 34;, and the events prior to the traveler's future never occurred and will only remain in his memory.

Theory of the multiple present

This theory shows that if a reality is altered, a kind of different time line would be created but without erasing the previous one, so that both the memories of one reality and the other are real, and the individuals coming from the They themselves are aware of it to the point of getting confused, for not knowing how to differentiate in which reality they live.

In the episode The Time Amendment (first episode of the second season) of the television series The Outer Limits, Dr. Theresa Givens, a professor college student who develops a time travel machine. Every time she returns after altering the past, she remembers both the altered timeline and the original one. By doing a lot of time travel, that deteriorates her health. In said episode, Dr. Theresa Givens says:

But there's only one problem. Whoever travels in time, remembers not only the altered timeline but also the original line. Every time I use that technique, a new universe of possibilities breaks into my head and my brain no longer resists. I think this bleeding was the last one. I don't know how long I'll be alive.

In the movie The Butterfly Effect, the protagonist has a lack of memory, and his mother takes him to the doctor, who does not discover what is wrong with him, and recommends that he write some diaries. Growing up, he goes to college and finds said diaries, reads them and finds out that he can travel to his past. But he begins to alter the events of his friends, with the idea of helping them, discovering that all he was doing was making their lives worse; he discovers that his father could also make those trips, who advises him not to. Every time he went back in time, his brain would collapse from the new information created by that trip and it would convulse his body and he would bleed from his nose.

Other versions

In the Discovery Channel series, The Universe of Stephen Hawking, Stephen Hawking invents a simpler version of a paradox called the "mad scientist paradox": in this version, a scientist creates a wormhole and is making a weapon. When he finishes assembling it he immediately turns on the wormhole, which is from a minute in the past, and he sees himself making the weapon. He shoots into the hole and the man from the past is shot dead. Since the gun is not primed and the scientist from the future fired the shot a minute after he died, then who fired the gun?

A quite simple and logical solution to the apparent aporia of the grandfather paradox is expressed by Frank Tipler: «If someone travels to the past he becomes part of the past. This is why it cannot change the past or the future». This impossibility of changing the past appears in some works of fiction, in such a way that time travelers, when trying to avoid an event, end up accidentally causing what they wanted to avoid.