Paradox

Doc: "I foresee two possibilities. One, seeing herself thirty years in the future would put Jennifer into shock and she'd simply pass out. Or two, the encounter 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 the worst-case scenario. The destruction however might be limited merely to our own galaxy."

Marty: "Well that's a relief!"

— Doc and Marty discussing the implications of 1985 Jennifer running into her 2015 self.

A paradox, in time travel, is "a situation in which the effect of an incident contradicts or eliminates the cause of that same incident".

The classic example of this is the "Grandfather paradox". An example of this would be in which a person travels back in time and kills his/her grandfather before the latter met the traveler's grandmother. This results in the erasing of his/her own existence — and raises the question, if he/she never existed, how could he/she travel back in time to kill his/her grandfather?

According to Dr. Emmett Brown, a paradox could ultimately unravel the very fabric of the space-time continuum and destroy the entire universe (or at least the Milky Way galaxy) and had to be avoided.

Examples

The McFlys, beginning with Dave, begin to erase from existence in a photograph.

• Marty McFly nearly created a paradox when he interfered with his parents' meeting in 1955. He erased the existence of his brother Dave and his sister Linda, as seen in a photograph, and threatened his own existence. Fortunately, he still had one week to bring his parents back together in time for the Enchantment Under the Sea dance, and averted the problem at the last moment, with his father George kissing his mother Lorraine, at the dance. Had he not done so, the paradox would be easily defined: If Marty had never been born, how could he go back to 1955 to interfere with his parents' meeting in the first place?
• When Jennifer encountered her older self and fainted, a paradox was possible. If the younger Jennifer had struck her head and died after fainting, then she would never marry Marty, never have kids, and never give Doc a reason to bring 1985 Marty and 1985 Jennifer to the future in the first place. Doc manages to catch younger Jennifer as she faints and hopes that she'll conclude that she was having a dream in what was restricted action.
• Old Biff Tannen, after stealing the DeLorean time machine and giving Grays Sports Almanac to his 1955 self, returned to the year 2015 and seemed to have endangered his own existence, collapsing in pain after he climbed out of the DeLorean. Director Robert Zemeckis cut footage that showed Biff being "erased from existence" following his collapse, after pre-release screenings suggested that it would be confusing enough to distract from the plot. In commentary for the deleted scene, Bob Gale explained that Biff had done something after 1985A that negated his existence in 2015A. Gale suggested that Lorraine, in the altered timeline, had finally had enough of her abusive husband (and possibly finding out that he killed George) and shot him "around 1996". In the 1990 behind-the-scenes show The Secrets of the Back to the Future Trilogy, host Kirk Cameron came to the same conclusion. The paradox would be that Biff would not be alive in 2015 to steal the time machine back to 1955.
• The creation of the 1985A timeline also caused the ultimate time travel paradox, preventing the invention of the time machine itself. When Doc and Marty arrived at 9:00 p.m. on October 26, the DeLorean should have been first tested more than 19 hours earlier in the normal timeline, and the time machine's creation was over 25 hours late by the time they left 1985A. However, Doc was in an asylum, Marty was in Switzerland, and neither one should have been able to take a time machine to 2015 for old Biff to steal it, and thus, create the 1985A timeline. The extra Marty and Doc had to discover how to repair the timeline quickly, not knowing when they and the time machine might start to fade from existence. As Doc told Marty, "We must succeed," since they were the only ones that could prevent this paradox.
• A paradox was threatened during Marty's second trip to 1955 when Biff's gang chased Marty into the dance; the gang spotted the first Marty on the stage, and was determined to jump him, which would have prevented the first Marty from reaching the clock tower by 10:04 pm and going back to 1985, which in turn would prevent all of the events afterward, including his second trip to 1955 (during which he encountered Biff's gang, and provoked them to pursue him in the first place). As part of his mission to get back the almanac, Marty literally sandbagged the gang's plans and averted the paradox.
• Another paradox was luckily avoided in Part I, one that Marty originally thought would be beneficial. He set the DeLorean's time circuits to arrive 10 minutes earlier in 1985, but obviously didn't consider the possibility of a paradox. Had the DeLorean not stalled, he would have arrived at the mall much sooner and met his other self, causing a huge commotion, and possibly leading to him never time traveling to begin with or who knows what else. The fact that Marty is clearly in awe over seeing another him at the end of the movie confirms he did not consider a paradox. He probably didn't even know of the term, being just a normal high school student who's not too literate with scientifics. This is also stated in Part II, when Doc has to inform him of these facts.

Clocktower paradox

The paradox of the Clocktower is explained as follows: A bolt of lightning struck the tower at 10:04 pm on November 12, 1955 and stopped the clock from running. Because Doc set up cables, and diverted the charge from the lightning bolt away from the clock and down to the DeLorean, the clock should be undamaged. There should be no "Save the Clocktower" flyer to be handed to Marty in 1985, and no reason for Doc to be at the Clocktower to prevent lightning from stopping the clock. But if there is no reason for Doc to be present, then the lightning does stop the clock, and the flyer is printed, etc.

When Marty returned to 1985, of course, the hands of the clock were still stopped at 10:04. A likely explanation is that, before rigging up the wiring, Doc considered the paradox that would be created if the clock did not stop at 10:04 on November 12, 1955, and made certain that the clock would be damaged. Besides hanging (without planning to) his entire weight from the hands of the clock, Doc also hung a loop of cable over the minute hand.

Tombstone paradox

The paradox is explained as follows: In 1955, Doc, Marty and Copernicus discover Doc's tombstone, showing that Emmett Brown died on September 7, 1885, after being shot in the back by Buford Tannen. Marty goes back to 1885 to save his friend, Doc is not killed, and the tombstone (as shown by a changing photograph) is no longer present in 1955. If there is no tombstone in 1955, Marty will not know that he must go back to 1885. But if Marty does not go back to 1885, Doc is killed, etc. Since Doc is stuck in 1885 near the end of Part III, he would have plenty of time to avert the paradox by planting a fake tomestone in place of the real one.

Marty Jr. paradox

The paradox (described below) is as follows: Doc Brown arrived on October 26, 2015 from 1985, and learned that Marty Jr. (and, later, Marlene) had been put in jail on October 21 of that future year, then went back to 1985 to get Marty's assistance in preventing the event that started a chain reaction which completely destroyed Marty's entire family. Marty, of course, prevents the disaster, and the USA Today headline changes from "YOUTH JAILED" to "GANG JAILED". As Doc notes, "future history has now been changed", but the past has been changed as well. If the event has not happened, and Doc arrives at October 26, 2015 from 1985 to find no evidence of any problems with Marty's kids, and there is no reason for Doc to go back to 1985 to tell Marty "Something has got to be done about your kids!". But if Doc does not go back to 1985 to get Marty, then Doc and Marty don't prevent Marty Jr. from getting into trouble, etc. Doc of Part II would need to find some way to alert Doc of Part I of the potential paradox, which would be difficult, in that Doc departs 2015, gets stuck in 1885, and is not able to return for many years (until building the Jules Verne train).

Lightning paradox

While Doc was connecting the wire after the lightning struck the clock tower, he survived being struck by lightning. If the lightning had killed him, he would never invent a time machine and Marty would never go to 1955 and Doc would have no reason to connect the wire from the clock tower to the streetlamps to send Marty to the future.

1986G paradox

Emmett and Edna begin dating in 1931, causing the 1986G timeline to be created. Normally, as a consequence, the entire universe would have been destroyed. This is because of the fact that if Emmett never builds the DeLorean time machine or becomes friends with Marty, then there is no reason for his older self to go back to 1931 which eventually leads to him being killed by Kid Tannen, causing Marty to go back, rescue him, and interact with Doc's younger self, causing him to perform public experiments, which leads to his public experiments, thanks to Doc taking control of his younger self's rocket car, helping Kid Tannen to be captured, and Edna being interested in young Emmett. If none of these happened, Edna would never become interested in him, and they would never begin dating (which leads to 1986G). However, if they never began dating, then Emmett WOULD build the time machine, and do all of the things stated above, so on and so forth. Ultimately, this situation should have completely obliterated the space-time continuum.

Multiple future Martys paradox

Near the end of Episode 5 of Back to the Future: The Game, three future versions of Marty appeared to tell him and Emmett to come with him, which should've obliterated the space-time continuum as each was looking at three other versions of themselves, which is what Emmett warned Marty NOT to do. The reason is simply because all of the three Martys were from different futures which leads us to the point that the future holds many different possibilities ("The future isn't written yet, no one's has! Your future is whatever you make it to make it a good one!") as said by Doc in the third movie. Since the future is not written for anybody, there is no possible way for Marty to cause a paradox, because his future selves are different future possibilities that might and on the other hand might not happen therefore they are not existing yet and cannot create a paradox. And judging from the three Martys, he could be a rock type or a bookworm. And if neither of those happen then they won't exist. It's the most confusing part of time travel. Also in Part II, when Doc and Marty go to 2015, Jennifer could marry someone instead of Marty, causing a second possibility, or Marty and Jennifer might have never had two kids which creates a third possibly and so on.

Subpoena paradox

In Back to the Future: The Game, in Episode 1, Marty delivered a subpoena to Arthur McFly from Emmett against Kid Tannen. He answered the subpoena, and as Kid's retaliation, he murdered Artie that night, creating a grandfather paradox, and causing George and Marty to begin rippling out of time. And if Marty didn't exist, how could he deliver the subpoena?

Other possible paradoxes

1. In Part II, when Doc and Marty alter the events of October 21, 2015 and save Marty McFly Jr. from going to prison, Doc would have no reason to bring Marty back with him. And if Doc never took Marty out of 1985, then how could they alter the future?
2. In Part III, Marty goes to 1885 in an attempt to save Doc from being shot in the back by Buford "Mad Dog" Tannen. Marty succeeds, but in the event of Marty saving Doc from Tannen, Marty would then have no reason to go to 1885. And if he never went to 1885, how could he save Doc?
3. In Part II, Biff takes the almanac to his 1955 self. The future is changed by this as seen by Doc and Marty when they return to 1985. But Biff could never have returned to the 2015 he left as the timeline would have diverted to the changed version. 2015 would be a future of the new timeline. Everything should have changed around Doc and Marty the same way Doc explains this would happen to Jennifer.
4. In Part I, at the end when Marty watches himself travel back in time, he is seeing a very different Marty. This new Marty will have a very different understanding of the past 30 years than the original does. The effect this new Marty will have on the past will likely be very different than the original possibly causing further changes to 1985 and having the different effects on the Marty he may witness going back, and so on. Although, the effect of the timeline changing around the time traveler(s), as listed before, might explain how this is possible, as Doc gets shot and dies before Marty's eyes, then comes back to life after Marty checks on him and looks away. It could also explain why there's no car in front of Marty's house when Doc drops him off, then it's there in the morning*, as the changes were slowly taking effect from when Marty went back in time, to when he woke up in the morning, supporting what Doc says in the last episode of the 2010 game, to "Give it a little lag time," as an explanation as to why Marty and Doc arrived in 1986, the day after Marty originally left, to make sure the timeline is as it should be after they went into the past and created new events that didn't originally happen.

* Another explanation would be that someone parked the car there later that morning.

Time travel paradox

According to today's knowledge of time travel, this type of time travel isn't even possible without a time traveling space ship instead of a car. The Earth rotates around the sun, the sun rotates around the galaxy and the galaxy moves in the universe. All that being said, if the time machine were possible, the DeLorean would be in a different place and not the the same place each time it moves into the past or future. Or better put, the DeLorean would be in the same place and the earth and the universe would no longer be. Meaning, there's no way the DeLorean would end up on the train tracks in Part III from 1885 to 1985 or let alone even on the planet.

• Possible answer: The time machine is still affected by the gravitation pull of a body such as that on Earth.
• Expounding upon that: Views from inside the time machine indicate that the DeLorean time machine is traveling through a portal that only opens when the machine reaches 88 miles per hour. Since time travel hasn't been witnessed, it's plausible that the unopened time portals are held in place by the Earth's gravity or by a physical force not yet proven. The ideas behind the first paragraph, about Earth's movement in space, have been discussed in the article about spatial displacement.
• However, Back to the Future is science fiction.

Behind the scenes

• The word "paradox" is often used to describe a mystery or an unanswered question. Among the tough questions asked of Gale and Zemeckis in interviews are:

1. "When Doc takes Marty and Jennifer out of 1985 and brings them to the future, how can Old Marty and Old Jennifer (and their family) even be in the future?"
   (This paradox can be solved by the fact that since Old Marty and Old Jennifer are indeed there in the future, it means that Marty and Jennifer do in fact return to 1985 to continue their lives to get to this point in 2015. However, this fails as an explanation because, following the logic, the only future that Marty and Jennifer will live is the one at the end of the trilogy when they get back to 1985 so the events in 2015 cannot possibly ever have occurred in that way [e.g. the accident is avoided and the fax erases].)
2. "Doc Brown of 1955 learns a lot about the future from Marty. Shouldn't the Doc of 1985 remember all of those things that happened in 1955?" (More specifically, Doc of 1955 knows from Doc's letter that the DeLorean will be hit by lightning; thus, Doc of 1985 should, logically, know enough to avoid getting hit by lightning).
   (This paradox is solved by the fact that since the first temporal displacement event is the essentially the same in both the original 1985 and the new 1985, albeit the bullet-proof vest Doc is wearing, it means that the way Doc unveiled the time machine in the new 1985 just happened to play out the same exact way as the original 1985.* Having the knowledge that he learned from Marty in 1955 means that Doc Brown still knows it but doesn't make any mention of said knowledge.) * Discussed in point 7.
3. "In 1885, when Marty tells Doc they're out of gas, why don't they just go to the Delgado Mine, dig up the DeLorean where Doc hid it, and get the gas out of it?"
   (This paradox is solved by the fact that if one individual is going to store a car in a specific spot for an extended period of time, then the liquids from inside the vehicle are drained out so as to not cause internal damage and replaced when the car is going to be used at the later point of time.)

*Paradox was also the working title of the combined script for Back to the Future Part II and Back to the Future Part III, and was the title written on the clapboards during the filming.

*Paradox was also the title of the musical score played during the scene in Part II in which the time-traveling Doc Brown talked with his younger self while handing himself a wrench to attach the electrical cable to the lamppost in 1955; since Doc Brown caused a "pair o' Docs" to occur.

Though many paradoxes arise in the trilogy, Doc may be overzealous about them because, though the effects obviously happen, the risk of destroying the space-time continuum may not really exist, mearly being a fabrication of Doc's mind to give reason to fix time-lines. Evidence of such exists, because 1985 Doc was willing to correct 1955 Doc with the wrench size and give him the suggestion that he was conducting a weather experiment as well as telling him there was going to be a storm. He also gave 1955 Doc information on how to repair the DeLorean, via the letter, while he was stuck in 1885, knowing the risks.

Appearances

• Back to the Future trilogy
• Back to the Future novelizations
• Back to the Future Part II first draft screenplay
• Paradox script
• Back to the Future: The Game

See also

• Paradox on Wikipedia
• Predestination paradox