Grandfather paradox

The grandfather paradox is a paradox of time travel, first described by the science fiction writer René Barjavel in his 1943 book "Le Voyageur Imprudent" ("The Imprudent Traveller"). The paradox is this: Suppose a man traveled back in time and killed his biological grandfather before the latter met the traveller's grandmother. As a result, one of the traveller's parents (and by extension, the traveller himself) would never have been conceived. This would imply that he could not have travelled back in time after all, which in turn implies the grandfather would still be alive, and the traveller would have been conceived, allowing him to travel back in time and kill his grandfather. Thus each possibility seems to imply its own negation, a type of logical paradox.

An equivalent paradox is known (in philosophy) as autoinfanticide — that is, going back in time and killing oneself as a baby — though when the word was first coined in a paper by Paul Horwich it was in the malformed version autofanticide.

The grandfather paradox has been used to argue that backwards time travel must be impossible. However, other resolutions have also been advanced.

Scientific theories

Complementary time travel

Since quantum mechanics is governed by probabilities, an unmeasured entity (in this case, your historical grandfather) has numerous probable states. When that entity is measured, the number of its probable states singularises, resulting in a single outcome (in this case, ultimately, you). Therefore, since the outcome of your grandfather is known, you killing your grandfather would be incompatible with that outcome. Thus, the outcome of one's trip backwards in time must be complementary with the state from which one left.

Novikov self-consistency principle

See the Novikov self-consistency principle and Kip S. Thorne for one view on how backwards time travel could be possible without a danger of paradoxes. According to this hypothesis, the only possible timelines are those which are entirely self-consistent, so that anything a time traveler does in the past must have been part of history all along, and the time traveler can never do anything to prevent the trip back in time from being made since this would represent an inconsistency.

Parallel universes

There could be "an ensemble of parallel universes" such that when the traveller kills the grandfather, the act took place in (or resulted in the creation of) a parallel universe in which the traveller's counterpart will never be conceived as a result. However, his prior existence in the original universe is unaltered.

Examples of parallel universes postulated in physics are:

• In quantum mechanics, the many-worlds interpretation suggests that every seemingly random quantum event with a non-zero probability actually occurs in all possible ways in different "worlds", so that history is constantly branching into different alternatives. The physicist David Deutsch has argued that if backwards time travel is possible, it should result in the traveler ending up in a different branch of history than the one he departed from. See also quantum suicide and quantum immortality.

• M-theory is put forward as a hypothetical master theory that unifies the five superstring theories, although at present it is largely incomplete. One possible consequence of ideas drawn from M-theory is that multiple universes in the form of 3-dimensional membranes known as branes could exist side-by-side in a fourth large spatial dimension (which is distinct from the concept of time as a fourth dimension) - see Brane cosmology. It is theorized that when two branes collide it sends a massive ripple of heat and energy throughout the two. This is a possible explanation of what caused the big bang according to the ekpyrotic scenario and the cyclic model. However, there is currently no argument from physics that there would be one brane for each physically possible version of history as in the many-worlds interpretation, nor is there any argument that time travel would take one to a different brane.

Theories in science fiction

Parallel universes resolution

The idea of preventing paradoxes by supposing that the time traveler is taken to a parallel universe while his original history remains intact, which is discussed above in the context of science, is also common in science fiction - see Time travel as a means of creating historical divergences. Some examples of this type of story:

• Michael Crichton's novel Timeline. However, unlike most time travel stories based on parallel universes, Crichton's novel seems to imply that changes to universes which resemble our own past can affect the universe we live in. The example given is when a professor trapped in the past sends a message to his graduate students at a medieval monastery.

• In the Marvel Universe comic books, any change made to the timeline results in an alternate timeline (for example, Professor X's son Legion attempted to kill Magneto, he accidentally killed Professor X, creating the Age of Apocalypse Timeline). Some characters know this and use it to their advantage (such as Vance Astro of the Guardians of the Galaxy, whose timeline shift allowed an alternate self to become Justice.)

• In L. Sprague de Camp's Lest Darkness Fall, a modern historian is thrown back in time to the period immediately after the fall of the Roman Empire, and introduces many anachronistic technical innovations and especially printing - thus creating a timeline where the dark ages do not happen and the achievements of Classical civilization are to a great degree preserved, and boosted by many inventions appearing a millennium or more ahead of schedule. This does not, however, change the original timeline he came from, in which world culture and society are still the result of Classical civilization having collapsed and a new civilization gradually taking its place.

• In Dragon Ball Z, Trunks travels to a parallel universe's past and saves the heroes while they remain dead in his own world several years in the future. Later, in the universe Trunks saves the heroes, Cell travels from a different universe still in which he kills Trunks there.

• In Primer, the characters Abe and Aaron are able to go back in time and kill their previous selves. They had arguments about continuity and the like, but besides that "The only thing that matters is what is happening right now."

• In Donnie Darko, the entire movie takes place within a Tangent Universe that occurs because of a rip in the space-time continuum. Within the Tangent Universe, Donnie's mother, sister, friend, and girlfriend all die, leading him to travel backwards in time to return to the normal universe, where they are still alive.

• In Metal Gear Solid 3: Snake Eater, if Naked Snake does an action which would prevent situations in the future Metal Gear timeline, such as killing Revolver Ocelot or he himself being killed, the mission becomes a failure, and Roy Campbell informs him of the "Time Paradox" that he created with his actions. This is humorously explored in "Metal Gear Raiden: Snake Eraser", where Raiden interferes with many events in the game.

Restricted action resolution

See also: Predestination paradoxes in fiction

Another resolution, of which the Novikov self-consistency principle can be taken as an example, holds that if one were to travel back in time, the laws of nature (or other intervening cause) would simply forbid the traveller from doing anything that could later result in their time travel not occurring. For example, a shot fired at the traveller's grandfather will miss, or the gun will jam, or misfire, or the grandfather will be injured but not killed, or the person killed will turn out to be not the real grandfather, or some other event will occur to prevent the attempt from succeeding. No action the traveller takes to affect change will ever succeed, as there will always be some form of "bad luck" or coincidence preventing the outcome. In effect, the traveller will be unable to change history from the state they left it. Very commonly in fiction, the time traveller does not merely fail to prevent the actions he seeks to prevent; he in fact precipitates them (see predestination paradox), usually by accident.

This theory might lead to concerns about the existence of free will (in this model, free will may be an illusion). This theory also assumes that causality must be constant: i.e. that nothing can occur in the absence of cause, whereas some theories hold that an event may remain constant even if its initial cause was subsequently eliminated.

Closely related but distinct is the notion of the time line as self-healing. The time-traveler's actions are like throwing a stone in a large lake; the ripples spread, but are soon swamped by the effect of the existing waves. For instance, a time traveller could assassinate a politician who led his country into a disastrous war, but the politician's followers would then use his murder as a pretext for the war, and the emotional effect of that would cancel out the loss of the politician's charisma. Or the traveller could prevent a car crash from killing a loved one, only to have the loved one killed by a mugger, or fall down the stairs, choke on a meal, killed by a stray bullet, etc. In some stories it is only the event that precipitated the time traveler's decision to travel back in time that cannot be substantially changed, in others all attempted changes will be "healed" in this way, and in still others the universe can heal most changes but not sufficiently drastic ones. This is also the explanation advanced by the Dr. Who roleplaying game, which supposes that Time is like a stream; you can dam it, divert it, or block it, but the overall direction it is headed will resume after a period of conflict.

It also may not be clear whether the time traveller altered the past or precipitated the future he remembers, such as a time traveller who goes back in time to persuade an artist -- whose single surviving work is famous -- to hide the rest of the works to protect them. If, on returning to his time, he finds that these works are now well-known, he knows he has changed the past. On the other hand, he may return to a future exactly as he remembers, except that a week after his return, the works are found. Were they actually destroyed, as he believed when he travelled in time, and has he preserved them? Or was their disappearance occasioned by the artist's hiding them at his urging, and the skill with which they were hidden, and so the long time to find them, stemmed from his urgency?

Examples of this set of theories include:

• Oedipus Rex, where the actions undertaken to thwart a prophecy bring it about: Cronus' swallowing of his children to prevent their usurping his power is what encouraged Zeus to overthrow him, and Oedipus's being abandoned led him to meet his mother without being aware of her identity. This, and other folk tales involving prophecies (wherein the 'time travel' is of information), form the oldest known occurrences of the predestination paradox.
• The 2002 movie version of The Time Machine, in which the main character cannot save his girlfriend by going back in time, as he only started building the time machine out of frustration at her death. This loop is not present in the original book.
• In the film Twelve Monkeys, the main character not only is unable to prevent a tragic past event from occurring, but even realizes that, as a child, he witnessed his adult self failing in the attempt. Moreover the perpetrator of some of the related events was inspired to bring them about as a result of speaking to the protagonist.
• In the final chapters of Harry Potter and the Prisoner of Azkaban, the characters travel back in time and their own actions cause a number of events which they had already experienced from another perspective, such as Harry casting a Patronus Charm which saves his (3-hour) younger self from a group of dementors.
• Douglas Adams's Hitchhiker Series makes use of this resolution for light-hearted and comic plots; for instance, an Infinite Improbability Drive, designed by life-forms, travels back in time and rearranges a group of atoms, thereby creating life.
• Harry Harrison's The Technicolor Time Machine uses this resolution for light-hearted and comic plots; when a film-maker goes back in time to make a film of the Viking colonization of America, it proves, in the end, to be the cause of the Vikings' colonization of America, with the film-maker himself appearing in the sagas they used as their source.
• In Castle Roogna, Piers Anthony has the magician Murphy persuade the time-traveler Dor to remain out of a conflict, because he might tamper with the past, but Dor's subsequent actions did affect it, and in his own time, while everything appears unaltered, a discussion points out that later disasters may have made his beneficial effect appear to have disappeared.
• In the 1969 science fiction novel Behold the Man by Michael Moorcock,the protagonist Karl Glogauer builds a time machine in order to travel back to the Holy Land 2000 years in the past. His intention is to prove the existence of Jesus Christ. When he discovers that in fact Christ actually suffered from severe physical and learning disabilities, Glogauer decides to act out his role as recorded in the Bible, faking the miracles and finally being crucified. This implies that the person called "Jesus Christ" that Glogauer had read about in the Bible, and that spurred his decision to travel back in time, was actually himself all along, an example of a predestination paradox.
• An especially vicious example is Eric Norden's novella "The Primal Solution". An elderly Jewish scientist - Holocaust survivor, who had lost his entire family - discovers a way of "mental time travel", which enables him to project his mind into the past and take over the body of the young Adolf Hitler in the Vienna of the early 1910s. Resolved to force Hitler into suicide, the vengeful professor can't resist humiliating him first and forcing him to drink sewer water in front of surprised passers by, before making him jump into the Danube - but in the moment before drowning, Hilter regains control of his body and returns home shaken. The Professor is trapped inside Hitler's mind, but is able to "hear" him think "The Jews? Why did the Jews do this to me? I have never harmed them!". Being able to access Hitler's memories, the trapped Professor suddenly realizes that until this moment the young Hitler had not at all been an Antisemite and was in fact on good terms with some Jews. Only because something inexplicable had entered Hitler's mind - something which totally hates him and is implacably bent on his destruction, and which identifies itself as being Jewish and acting on behalf of all Jews - that he would become the genocidal Hitler known to history. Never daring to tell anybody of this presence in his mind, for fear of being considered insane, Hitler would gradually develop the idea that only by killing all Jews would he be free of that haunting presence. In short - the very act intended to avert the Holocaust ends up being its direct cause.
• A similar example was featured in a modern episode of The Twilight Zone, in which a young woman goes back in time and is employed as a nanny for the infant Hitler. She abducts the baby jumps into the Danube, killing them both. However, the family maid has chased her and witnesses their deaths. Fearing that the news will send her mistress, already frail from the loss of her previous children,over the edge, she pays a homeless Gypsy mother for her child. This infant is brought back home and presented as the original. It is he, and not the actual baby Hitler who will grow into the monstrous dictator.
• In the Futurama episode "Roswell That Ends Well", the crew travels back in time, and Fry tries to protect Enos, his grandfather, from being killed, but only causes him to die in a nuclear explosion. Fry, realizing that the woman he thought to be his grandmother cannot be his grandmother (since she was Enos's fiancée), has sex with her, impregnanting her and becoming his own grandfather.

Relative timelines resolution

It could be that the universe does not have an absolute timeline that is permanently written after events happen (or, in the deterministic view, at the start of time). Instead, each particle has its own timeline and therefore, humans have their own timeline. This might be considered similar to the theory of relativity, except that it deals with a particle's history, rather than its velocity.

Physical forces affect physical particles. If your body's physical particles go back in time, you will be able to kill your grandfather (no physical forces will mystically stop you), and nothing will physically happen to you as a result, because there are no physical forces that can "figure out" what happened and this new timeline develops, because the universe simply has no mechanism for unmaking it. Your younger self does not need to be born in order to fulfill a destiny of going back in time, because there is no written-in-stone absolute timeline that needs to be followed. If you were able to find and observe the younger versions of the particles that make you up, they too would follow physical laws and hence wouldn't form into a younger version of you (because one of your parents wouldn't be there to form you).

This theory is similar to the parallel universes theory, except that it happens within one universe. If parallel universes cannot interact again after time travel occurs, then essentially the parallel universe resolution and the relative timelines resolution are the same as there is no way of proving a parallel universe still exists or ever did exist.

Examples include:

• Alfred Bester's short story The Men Who Murdered Mohammed, posits that, once you change the past, you create a solipsistic universe where you can make whatever changes you like, including preventing your own birth. Each time traveller has their own solipsistic time line.

• Orson Scott Card used this theory to allow his characters to travel back in time and change the history of European colonization in the New World in his novel Pastwatch: The Redemption of Christopher Columbus.
• Similarly, this model also appears in James P. Hogan's novel Thrice Upon a Time, although Hogan confusingly uses the term "universes" to describe different moments on the same timeline rather than separate timelines.
• In Poul Anderson's Time Patrol, the Patrol's purpose is to prevent such changes in time, and when they have occurred, undo the changes as neatly as possible, to revert to the "normal" timeline. Such a Time Patrol, under one name or another, is a common feature in stories using this resolution.
• In the film, The Lake House, Kate Forster communicates with Alex Wyler by means of a mysterious mail box even though she lives in 2006 and he lives in 2004. She can send information back through time and he can send it forward. Kate finds out that he died in an automobile accident and warns him not to cross the street that day. Thus forewarned, Alex does not cross the street and is able to join Kate at the mailbox shortly after she delivers the warning. Thus the past that Kate remembers has been changed (Alex hasn't died), and no harmful consequences seem to ensue from this paradox.
• In the television series Seven Days, NSA Agent Frank Parker uses a device called the chronosphere to go back in time, usually one week, to "undo" catastrophic events. This would only be possible if the relative timelines resolution holds, because if Parker succeeds, there would never have been any reason to send him back in time.
• In the essay The Theory and Practice of Time Travel, Larry Niven proposes that, after some unknown number of revisions of history, the effect of some episode of time travel will be to create a universe where time travel, although possible, is simply never discovered. Such a timeline is stable, and in it no paradoxes occur, and so need no resolution.

In some works, the replacement is not complete. Characters may "remember" their lives in the original timeline, and more drastic effects may occur. In Jasper Fforde's Thursday Next books, time travelers caused Thursday's husband to drown as a child, but Thursday remained pregnant with his child.

Destruction resolution

Some science fiction stories suggest that causing any paradox will cause the destruction of the universe, or at least the parts of space and time affected by the paradox. The plots of such stories tend to revolve around preventing paradoxes.

Examples include:

• In the Back to the Future trilogy, it is speculated by Doc Brown that an encounter between a person and their former self "could create a time paradox, the results of which could cause a chain reaction that would unravel the very fabric of the space-time continuum and destroy the entire universe. Granted, that's a worst-case scenario. The destruction might, in fact, be very localized, limited merely to our own galaxy." However, actual paradoxes are generally averted in the films, so it is never shown if Doc's speculations are correct. The filmmakers, in their commentary on the second film of the series, say that they avoided showing one full grandfather paradox, fearing that many viewers would not comprehend what was happening; however, when the character Biff returns from the 1955 to the 2015, he suddenly appears very ill, the camera stops focusing on him, and we never see him again in that time period. This is because his interference in his own past led to his death before 2015. Marty preventing his parents from meeting has the same ultimate effect of the grandfather paradox. He starts to be "erased from existence", and had he not reversed the effect by getting his parents back together, he would not have been in existence to go back in time and disrupt his parents' meeting in the first place.
• The 2005 Doctor Who series episode Father's Day provided a unique version of the destruction resolution. A paradox causes a wound in space-time, which attracts flying carnivorous monsters, Reapers. The Reapers act like bacteria around a real wound, devouring everything, starting with the youngest people and objects, until the wound is "sterilized" and the paradox resolved by its destruction. The deadly rift would have normally been prevented by the Gallifreyians, the Time Lords, but they had all been destroyed in a war with the Daleks, and the monsters were the universe's own way of preventing the event.
• John Cramer's novel Einstein's Bridge depicts a multiverse consisting of individual Bubbles, normally isolated one from another, except that certain high-energy activities can send signals between Bubbles. The calibration runs of the Superconducting Super Collider (yes, it was built in that timeline) generate a signal which is detected by two civilizations in other Bubbles, one benevolent and one hostile. Transmission of matter between Bubbles is impossible, but information and small amounts of energy can be transferred; the hostile civilization invades our universe in this way, constructing nanomachines by remote control and proceeding to assimilate our world and remake it in their image. To halt and undo the destruction of our civilization, members of the benevolent civilization and two humans construct a "time vortex," an impossible condition which induces our entire universe to unravel itself back a distance of seventeen years, before the SSC brought attention to us. The two humans are somehow pushed back into the past by the vortex, and they use their knowledge of the general course of events to acquire wealth and power and make sure the SSC is not built.

Observation resolution

Some speculations suggest that, under no circumstances whatsoever you would be able to "kill your grandfather". The only result of the time travel would be your knowledge that you've caused some event in the past. For example, if a protagonist kills his grandfather, it would turn out that the victim is not his grandfather at all.

Other examples

The Grandfather Paradox, or similar, is also used in the following:

• A well known, typical example of the paradox is the first Back to the Future film: Marty McFly travels to 1955 and accidentally prevents his mother's romantic attraction to his father. While trying to find out a way to return to his own year, he observes that his siblings begin to fade out from a picture he was carrying with him since he averted their own birth. Towards the end of the film, he starts to fade from reality as well, until he manages to make his parents fall in love "again".
• Robert A. Heinlein's short story "—All You Zombies—" in which a time travel agent becomes his own father - and his own mother, as well as bringing the two of them together. This does not invoke the normal paradox dynamics ("if X happened, then X cannot have happened; and if X did not happen then X must have happened"). Instead something is created from nothing (a person with no ancestors but himself) but the final outcome is self consistent.
• Spider Robinson's short story Father Paradox
• Connie Willis's novel To Say Nothing of the Dog
• The Terminator series of movies
• The Butterfly Effect films
• Star Trek episode The City On The Edge Of Forever - McCoy, with an accidental overdose on medicine, went through a time portal and saved the life of a woman, altering history in such a way that the Federation was never created. Kirk and Spock must find the point at which McCoy altered time and reverse the change, by allowing the woman to die. In the Star Trek: Deep Space Nine episode "Trials and Tribble-ations", which used footage from the original series to create a time travel episode with Kirk and crew in it, Julian Bashir flirts with a girl he later realizes may be his ancestor and, in a spoof of the paradox, begins to worry that he may be faced with the choice of becoming his own progeny or ceasing to exist.
• The popular webtoon Bonus Stage episode 87, Bonus Stages, where main character, Phil Argus, goes back in time to the first episode of the series, and kills his old self, putting a slightly different twist on the paradox, and destroying the rest of the Bonus Stage universe.
• The online machinima comedy series Red vs. Blue - The character Church travels back in time to prevent his own death, and to avert the events and disasters that put the characters into their current situation. He ends up causing all of the problems and situations of the first 48 episodes — including his own death — entirely by accident.
• The Red Dwarf episode, "Tikka To Ride", episode 7.1, in which the future version of the crew destroy the present crew; therefore, the future crew no longer exist and therefore are unable to go back in time and kill themselves, hence they survive. Also in this episode, the crew accidentally kill Lee Harvey Oswald, preventing the JFK assassination, and destroying life on earth in an atomic war. Ultimately, they convince a slightly older and politically destroyed JFK to go back in time to assassinate himself from the Grassy Knoll, to ensure the survival of the planet and his own legacy.
• The Red Dwarf episode, "Ouroboros", episode 7.3, in which Lister discovers he is his own father and travels back in time to place his son, aka himself, under the pool table where his adoptive parents first found him.
• In Metal Gear Solid 3: Snake Eater, after Revolver Ocelot and the Ocelot Unit are knocked out, it is possible to shoot Ocelot. This however results in an instant game over screen, with the words "TIME PARADOX" replacing "GAME OVER". This occurs due to the fact that Ocelot is a vital factor in the Metal Gear Solid games, and that none of them would have happened if Ocelot died. Also, if the main character, Naked Snake, dies in any way, the "TIME PARADOX" message also replaces the "SNAKE IS DEAD" screen. This is due to the fact that Naked Snake is actually Big Boss, the father of Solid Snake, who is the most critical character in the previous Metal Gear games.
• The Invader Zim episode, Bad, Bad Rubber Piggy, in which Zim tries to use a space-time object replacement device to send a Hunter-Destroyer robot to destroy Dib before he becomes Zim's enemy. GIR warns Zim that if Dib doesn't become Zim's enemy, Zim won't send the robot, and then Dib won't be destroyed and will become Zim's enemy, and then Zim will send the robot, Dib will be destroyed and won't become Zim's enemy (Gir keeps telling it until his head explodes). The paradox never occurs because Zim had failed at destroying Dib.
• K. A. Applegate's Megamorphs #3: Elfangor's Secret (of the Animorphs series) featured a storyline wherein the Animorphs are shown an alternate universe where slavery and disease are rampant, which will become reality unless they can follow a certain alien foe, Visser Four, through time in order to stop him. Visser Four travels to world events such as Washington's crossing of the Delaware and D-Day attempting and succeeding in altering the timeline with the Time Matrix to make it favorable for the invading Yeerks. The successful Yeerk abandons his host, and the Animorphs take that opportunity to ask the dying human when his parents met with the intention of preventing his birth to save the timeline, which they do.
• In Lost Highway the main character Fred seems to be stuck in endless "grandfather paradox" in which he kills his wife, her "friends" and lover.
• In The Legend of Zelda: Oracle of Ages, when Link gets possessed Impa to the area where Nayru is singing, Veran emerges from Impa and possesses Nayru. After Ralph puts away his sword and steps away from possessed Nayru, possessed Nayru goes back in time and turns almost everyone into stones and possesses Queen Ambi to make the Black Tower, thus causing a paradox to change the future. Link goes back in time to fix the paradox.
• In The Legend of Zelda: Ocarina of Time, Link learns the Song of Storms from the man in the windmill in Kakariko Village. Later, he must teach this song to the Windmill man in the past, for a certain event to happen, an event that occurred because of Link's meddling, but what the player hasn't been able to do yet, because the player didn't know the Song of Storms. The event is triggered by teaching the Windmill Guy the Song of Storms. The song is in fact spawned by a time paradox. This could alternately be explained with the sequel The Legend of Zelda: Majora's Mask, in which the song is learned again through another source.
• In Harry Potter and the Prisoner of Azkaban, Harry sees a figure on the other side of the lake (who he believed to be his deceased father) which saves him and Sirius Black with a Patronus Charm. Later he travels back in time and discovers that the figure was himself. He casts the Patronus and saves himself and Sirius. In fact, only because Harry saw himself already conjure the Patronus was he able to muster the self confidence to be able to conjure the Patronus.
• In Prince of Persia In the first part of the new Prince of Persia trilogy, the Prince releases the Sands of Time. The second game reveals that whoever releases the Sands must die. The Prince attempts to reverse his fate by going back in time to before the sands were made, at which point he discovers that he himself makes the Sands. He finally overcomes his fate when he puts on a mask that sends him back in time again, stopping his action that made the sands. As the Prince acts in paradox (if the sands were never created, he wouldn't be trying to destroy them), he is pursued by a beast called the Dahaka, a guardian of the timeline who ensures consistency through force.

Other considerations

Consideration of the grandfather paradox has led some to the idea that time travel is by its very nature paradoxical and therefore logically impossible, on the same order as round squares. For example, the philosopher Bradley Dowden made this sort of argument in the textbook Logical Reasoning, where he wrote:

Nobody has ever built a time machine that could take a person back to an earlier time. Nobody should be seriously trying to build one, either, because a good argument exists for why the machine can never be built. The argument goes like this. Suppose you did have a time machine right now, and you could step into it and travel back to some earlier time. Your actions in that time might then prevent your grandparents from ever having met one another. This would make you not born, and thus not step into the time machine. So, the claim that there could be a time machine is self-contradictory.

However, most philosophers and scientists agree that time travel into the past need not be logically impossible as long as there is no possibility of changing the past, as suggested, for example, by the Novikov self-consistency principle. Bradley Dowden himself revised the view above after being convinced of this in an exchange with the philosopher Norman Swartz.

Consideration of the possibility of backwards time travel in a hypothetical universe described by a Gödel metric led famed logician Kurt Gödel to assert that time might itself be a sort of illusion. He seems to have been suggesting something along the lines of the block time view in which time does not really "flow" but is just another dimension like space, with all events at all times being fixed within this 4-dimensional "block".

See also

• Chronology protection conjecture
• Time loop
• Predestination paradox
• Ontological paradox
• Time travel in fiction
• I'm My Own Grandpa (song)