I know I'm coming late to this, but I've been thinking about the Back to the Future trilogy a lot recently and I'm not sure it entirely makes sense.
I'm going to assume you've seen these films, but here's a reminder of the relevant plot points.
Back to the Future part 1
Marty McFly accidentally time travels from the year 1985 to 1955 and unintentionally manages to prevent his parents from falling in love. Over the course of several days he attempts to reunite the pair in order to prevent himself being erased from existence, ultimately succeeding. He returns to the year 1985 to find all is more than well - his parents are now much more happy and successful, and he also manages to save his friend Doc Brown from being murdered at the hands of a group of Libyans, from whom he had stolen plutonium to power the time machine.
How the time travel works
The film essentially tests the grandfather paradox - Marty's actions in the past stop him from being born, which means he can't have travelled in the past to do that in the first place, thus creating a paradox. However, the film provides a solution by having Marty's actions change the future slowly, giving him a few days before he is erased from history, allowing him enough time to put things right again.
The implication is that time in the Back to the Future films is a single yet changeable path, unlike the alternate parallel timelines as depicted in the Marvel cinematic universe. Marty doesn't create a new reality, he changes the future and it slowly catches up to him.
Problem 1 - Marty should be erased regardless
Despite reuniting his parents, Marty should still have been erased at the end of the film because he changes the future enough that he never would have travelled back in the first place.
In the final scene we see that Doc has learned from his experience in 1955 and improved the efficiency of the time machine so that it runs on household waste. This means he wouldn't have needed to obtain plutonium from the Libyans and therefore his murder at their hands and Marty's accidental time travel wouldn't have taken place. Marty would still exist in 1985, but the version of him that travels back in time would be erased, because that version of 1985 no longer happens.
It could be argued that since the changes to 1985 happen slowly perhaps he was erased sometime between going home to bed and Doc arriving the next morning, except that he obviously notices the changes to 1985 after he returns, showing that Marty is the original version of himself and was never erased.
Problem 2 - how fast does the future change
This one's less of a problem and more of an observation. Marty's siblings are slowly erased from history ahead of him. They're both older than him, which suggests that time changes in a linear fashion, erasing them first because they were born first.
If we knew exactly how old these characters are meant to be in 1985 and could compare how long it takes for them to each be erased in the photo in 1955, we could calculate how fast the future changes in the Back to the Future universe.
Thanks to the internet we know the following about the McFly siblings in 1985.
- Marty is aged 17, born June 12th 1968, or 4,603 days after Nov 5th 1955
- Dave is aged 21, born Nov 5th 1963, or 2,922 days after Nov 5th 1955
- Linda is aged 19, born in 1966 (date not specified)
To work out how long it is between Marty breaking the timeline and the moment when he starts to be erased we can determine the following.
- Marty travels to 1955 shortly after 01:21 on Oct 26th 1985, arriving around the same time of day on Nov 5th 1955
- He prevents his parents from meeting sometime after breakfast the same day
- He returns when the clock tower is struck by lightning at 22:04 on Nov 12th
- He starts being erased at the Enchantment under the Sea Ball after sunset (17:00) but before the clock tower is struck by lightning at 22:04
This means that his stay in 1955 is just under 8 days, but the time between the change to the timeline and the start of him being erased is probably around 12 hours less than that.
For Marty we can calculate it takes about 7 and a half days (180 hours) for the change to the future to reach his birthday 13 years (4,603 days) later, meaning that the speed of time change is around 25.5 days per hour.
Confirming these numbers by doing the same calculation for Dave is harder because we don't know when he starts being erased. His head is already missing from Marty's photograph after dark on Nov 5th at Doc's house, which must be less than 24 hours after the change to the timeline. Using these numbers we can say that it takes less than 1 day (let's say 20 hours) for the future change to reach his birthday 8 years (2,922 days) later, giving a speed of 146 days per hour.
These figures are obviously different. They're also calculated on some very shaky numbers (I don't know exactly where the internet got their birthdays from), so it's unlikely that they're accurate or even useful, but they at least establish a vague baseline for the rest of the trilogy.
Problem 3 - the erasing problem
We've established that in these films changes to the future timeline happen slowly. I picture them as waves, moving ahead of Marty, washing away the future he knew and replacing it with a different one. So if the future changes slowly, how is it that as soon as Marty's parents kiss, everything is immediately restored?
It could be argued that new changes to the timeline happen slowly, but anything that restores it back to its original state happens immediately. That might make sense, except that we know from the end of the film that the original timeline isn't restored - an alternate (and better) future is created.
This means timeline changes in the film are inconsistent - one change happens slowly, another happens immediately, and both differ from the original.
Miscellaneous problems
These are less to do with time travel but they still bother me slightly.
- Doc's calculations were wrong. Marty is supposed to start driving towards the clock tower when the alarm clock Doc has set goes off, so that he hits the cable at the exact moment that the lightning occurs. But the time machine stalls and Marty doesn't set off until a few seconds later, but still arrives exactly on time.
- When Marty returns to 1985 he is overtaken by the Libyans in their van on their way to kill Doc, but the time machine stalls again so Marty sets off on foot, arriving just in time to see Doc shot again. This suggests the mall is close by, but when Marty arrives in 1955 he drives on a straight road that leads from Twin Pines to Hill Valley's town centre, passing the future site of Lyons Estates (his home) on the way. From there a road sign shows that it is still 2 miles to the town centre, which he couldn't possibly run in the time available.
- At the end of the film the McFlys employ Biff Tannen for car waxing and allow him some freedom of entry to the house (as evidenced by him bringing in the parcel). If you were George McFly, would you allow someone who tried to rape your wife in high school that level of closeness?
Back to the Future part 2
Doc, Marty, and his girlfriend Jennifer travel from the improved 1985 forwards to 2015 intending to change the future for the better. While the trio are distracted, an old version of Biff Tannen steals the time machine, travels back to 1955 and gives his younger self a sports almanac that will allow him to become a millionaire through betting. Marty, Doc and Jennifer return to 1985 to find it changed for the worse - Biff now rules the town, Marty's dad has been murdered, and Doc is in a mental institution. Doc and Marty travel back to 1955 to destroy the almanac after old Biff gave it to young Biff and succeed, but the time machine is hit by lightning and Doc is sent back to 1885, setting up the final film.
How the time travel works
The second film explicitly affirms the rules set down by the first and clearly explains them, thanks to a short section in the middle where Doc draws Marty a diagram on a chalkboard. When Biff travels to 1955 he changes the future so that when Doc and Marty arrive in 1985 they find the damage has been done and can no longer return to the 2015 they knew in order to stop him, instead having to travel back to 1955 to reverse the act immediately after it happened.
Problem 1 - how does old Biff get back to the future
If Doc and Marty can't travel forward from the bad 1985 back to the original 2015 in order to stop Biff from ever stealing the time machine, how can Biff travel back to the original 2015 after changing the future in 1955?
According to Doc's theory of time travel, when Biff returned to 2015 it should be the 2015 where he became a millionaire and took over Hill Valley. The time machine would never have been returned to Doc and Marty, leaving them trapped and eventually erased from history. Old Biff and the time machine would also slowly be erased, since neither could have existed in the bad future.
Old Biff's exception to this rule could be explained by the slowness of the change to the timeline. We don't know exactly how long Old Biff spent in 1955 but even based on the faster of the two calculations for how fast the future is 'updated' it seems possible that Old Biff could return before the future changed.
When Doc and Marty return to 1985 it has already changed, which means either that the change to the future from 1955 has already passed 1985, or that the speed of the future updating rule only applies to those who travel back and make the change, not those already in the future.
In a deleted scene from the film, after old Biff returns to 2015 he is seen being erased from history, reportedly not for the reason above but because his younger self was eventually shot by Lorraine McFly after 1985. This was probably cut because if Biff was erased then his cane should have been erased with him, but the plot needed this in order to allow Doc to understand what had happened.
Problem 2 - meeting your future self
When Marty and Jennifer travel from 1985 to 2015 they encounter their future selves. Arguably this shouldn't be possible, because as soon as they leave 1985 a new future would be created where they had mysteriously disappeared. Again, this could be explained by the film's portrayal of time changing slowly in response to events, but it highlights the pointlessness of travelling forwards in time to change your future - as soon as you travel forwards in time the future starts to change, so anything you do in the future is pointless.
Problem 3 - Doc and Marty should have been erased anyway
When Doc and Marty return to the 'bad' 1985 it is implied that there is already a Doc and a Marty in that time - Doc has been committed to an asylum and Marty is in school in Switzerland. That means that Doc never invented the time machine, which means that Doc, Marty and (presumably) Jennifer are also subject to the same slow countdown to being erased from existence that Marty faced in the first film.
This doesn't happen and is never discussed and they manage to correct the future within a few days anyway, so it's probably fine.
Problem 4 - Jennifer in 1985 while time changes around her
Doc and Marty leave Jennifer (and Doc's dog) in 1985 when they travel back to 1955 to stop the 'bad' future. Doc assures Marty that things will change around them and they will be perfectly safe.
This raises two questions. Firstly, what would that experience be like? It might be a subtle change not noticed as she was asleep, or it might be a more dramatic change that could have left her with lasting psychological trauma - imagine witnessing people around you fading and then reappearing, along with buildings and other objects changing or moving. We also don't know how long it would take for the timeline to change around her - she might have been trapped in the bad 1985 for days.
Secondly, was Doc telling the truth? Did he know that Jennifer would be perfectly safe or was he lying so that Marty would leave her behind, knowing that as long as they succeeded some version of her would exist in 1985 when they returned? She later remembers her trip to the future, so it seems all worked out as planned, but it's interesting to consider.
Problem 5 - more erasing problems
After Marty burns the almanac at the end of the film the book of matches and newspaper clippings that he and Doc brought from the bad 1985 are almost immediately updated to reflect the change back to the good 1985. This is inconsistent with the slow updating of the timeline introduced in the first film, where it takes nearly a week for the future change to start to erase Marty.
It is consistent with items like this returning immediately to 'normal' once the timeline has been corrected, except that again, it's unlikely that this new version of the future is the original one. At the very least, there is no mention of these two events at any other point in the films (Marty's father being honoured and Doc being commended) so we have no prior knowledge of them.
On a related note, the other headlines on Doc's newspaper are interesting. In the Biff Tannen future, Nixon was to seek a fifth term, whereas in the restored timeline the headline says Reagan will seek a second term. An interesting aside on the ripple effect of changes to the future, perhaps.
Also I love the headline on Marty's newspaper from both realities - "More rain predicted".
Back to the Future part 3
With the help of Doc's letter, Marty finds the time machine hidden in a cave since 1885 and is instructed to return to 1985 without his friend until he discovers that Doc died not long after arriving in the past. He travels back to 1885 to save him. Doc then prevents the death of Clara Clayton, who was destined to fall into a ravine, and the two begin to fall in love. Doc and Marty manage to get the time machine working but Marty returns alone to 1985 where the time machine is destroyed. The next day Doc turns up in a new time machine he built in 1885 along with his now wife Clara and their two boys.
How the time travel works
Same as the first two films, basically, although it's getting increasingly complicated.
Problem 1 - changing the future again
Once again Marty makes all kinds of changes to his future that could have disastrous consequences. Clara's life is saved, 1885 Biff is arrested and they hijack and destroy an entire train, not to mention multiple interactions with various townsfolk including Marty's ancestors. When he returns to 1985 all appears to be unchanged, but this seems highly improbable. For example, 1885 Biff is arrested at the end of the film and likely imprisoned if not executed, which might have removed all of his ancestors from the timeline. This could have dramatic consequences on Marty's future.
Problem 2 - more things being erased
Marty's girlfriend Jennifer has a piece of paper from 2015 which was used to fire future Marty from his job. The message is erased after Marty avoids a crash in 1985 that would have prevented him from pursuing a musical career, but although the message disappears, the paper still exists (the timing of the erasing is highly convenient, but we'll ignore that for the sake of the plot).
Interestingly this is consistent with the first film where the photograph of Marty and his siblings remains even though the people in it have mostly been erased. It actually seems more likely that the piece of paper would still exist than the photograph because the piece of paper might still have been created but used for a different purpose, whereas the photograph wouldn't have been taken if the people in it didn't exist. What's more interesting is the idea that the 'fired' message on the paper could be replaced by something else from the future, perhaps entirely unrelated.
This problem also applies to the top of Old Biff's cane, although we only see it once in the film and it could have been erased later.
Once Marty prevents both him and Doc from being killed at the hands of Buford Tannen, the photograph of the future tombstone immediately changes to show no tombstone, because the tombstone is broken, which makes sense. Again though, the immediate updating of the future is inconsistent with the slow rate of change from the first film, but consistent with the immediate change at the end of the first film. At this point I'm not even sure which version of the future might be occurring, so let's leave this one alone now.
Conclusion
I hope you've enjoyed my (probably flawed) nitpicking of this beloved movie trilogy. Honestly I love those films. I don't care if they don't entirely make sense or agree with each other, this was just something I wanted to think about and write down.
If you've been affected by any of the issues raised in this article, please take a deep breath and remember that it's only a film.