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janus
Post  Post subject: Time travel rules in "Back to the Future"  |  Posted: Thu Jun 30, 2016 9:55 pm

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Most of this post was originally written as a response to a thread on another forum, but being as it constituted quite a bit of work on my part, I thought I'd get the most out of my investment and post it here.

Whenever you watch any science fiction movie, you are going to have to invoke a certain degree of suspension of disbelief. You have to accept certain things as true, even if you know that they could not happen in reality. A good script shouldn't stretch that too far. At no part of the movie should you feel the urge to stand up and shout "Oh come on, that could never happen!". ( Thus while you might accept that a spider could grow to the size of an SUV, despite what the square-cube law has to say about it, if said spider were to start quoting Shakespeare, you would likely throw your popcorn at the screen and stomp out.)

Sometimes writer deal with ideas that we have no basis by which to judge them by against reality. Take FTL travel for instance, Since no one knows how such travel would work "in reality" (If it is even possible), It can only be judged against its own framework. The writer is able to create his own rules of how it works and what is and is not possible, and we can only take him to task if he breaks his own rules. If it is needed at some point of the story to limit FTL travel to some speed, you can't have them exceeding that speed at different point of the story.

Time travel is another of those areas where the rules can be open, and a great number of movies have treated it in different ways. But as long as our writer is faithfully consistent with how he deals with it we can't kick too much. ( If someone were to suddenly stand up in the theater to exclaim that time travel doesn't "really" work like that, he should be prepared to present his diploma in temporal mechanics he received in 2050 to back up his claim.)

The rules, or least not all of them, are not always specifically explained in every movie, and sometimes it is up to the movie goer to figure out what these unspoken rules are. A case in point would be the Back to the Future movies. A careful watching of the films can reveal what of the rules of time travel for the BTTF universe are. That's what the rest of this post will be about, seeing if we can work out a consistent set of rules for these movies
First off, we create an chart outlining all the various trips through time and the results of these trips, it is somewhat based on the diagram that Doc drew on the blackboard in BTTFII:
Image
Solid black lines make up events that effect what we see in the movie, while gray lines represent potential time lines that would have occurred without intervention from our characters. Red arrows are trips back in time, Blue arrows are trips forward in time and Magenta arrow represent changes that result in new time lines. No line means events play out exactly like the black time line above.

(a) Marty takes time machine to 1955.
(1) Marty's actions create new time line (Dad knocks out Biff, etc.)1
(b) Marty travels forward to 1985 of newly created time line.2
(c) Doc takes time machine to 2015.
(d) Doc returns from 2015 to 1985 (informs Marty that something is wrong with his kids' future)3
(e) Doc takes Marty and Jenifer to 2015.
(2) Doc and Marty alter events creating new future time line for 2015.4
(f) Old Biff steals time machine and goes to 1955.5
(3) Old Biff gives earlier self sport stats book, creating new time line.
(g) Old Biff returns time machine to 2015. 6
(h) Doc, Marty and Jennifer travel to the 1985 time line created by Old Biff.7
(i) Doc and Marty return to 1955.8
(4) Doc and Marty, create yet another new time line, one that should follow the same lines as the first one created by Marty.9
(j) Time machine hit by lightning, sending Doc to 1885.
(5) Actions of Doc in 1885 create new time line (tombstone, letter to Marty, Time machine in mine)10
(k) Marty travels to 1885 to get Doc.11
(6) Marty's and Doc's actions create new time line (vanished tombstone, renamed ravine.)
(l) Marty travels to new time line 1985.12

1 Marty actually creates two new timelines here, the first leads to his father and mother never marrying and his not being born, but this timeline is aborted quickly when he creates new events leading to their marriage. This also establishes Rule 1 of BTTF time travel: Travel to the past can change the course of events, and this includes your own past. An addendum to this rule is that there is a hysteresis to effect when it comes to time travelers, as they are not immediately affected by this change. This delay action allows us to side step the grandfather paradox. Marty can exist long enough in the past to effect changes and then is erased.

2 Rule 2: traveling forward in time even via time machine returns you to a future time line created by by actions in the past and not to your original time line.

3 This actually raises an interesting issue, shouldn't Doc's traveling back from 2015 to 1985, in of itself, constitute an event that precipitates a new future time-line? After all, just this in itself could be enough to set off a whole new series of events. But apparently not, since in the next step they travel forward to a 2015 that does not significantly differ from the one he left. This might be considered an another addendum to Rule 1: There tends to be a certain momentum to the flow of events, and it takes a significant effort to affect change to the time-line.

4 We never really see much of what this future leads to other than Marty's son not being arrested. Given what we see of the kid, even with this event erased, I still don't see a bright future ahead for him.

5 Old biff travels back to the day of the storm, so he arrives after Marty has started a new time line and thus returns to this time line. If he had returned to a date before Marty appeared in 1955, this line would go back to the top original time line.

6 As per Rule 2, old Biff should return to a 2015 which resulted from the change he made to the time line. He appears to return to the original time line he left. One possible explanation is that Doc and Marty, as time travelers, act as an "anchor" in the 2015 time line that allows the time machine to return to them. However, this does not explain why later when Marty travels from 1885 to 1985, Jennifer, who also was a time traveler and was left in the timeline created by Old Biff, doesn't act as an anchor, causing Marty to return to that time line. A better explanation seems to be that, as Doc says would happen to Jennifer and Einstein, that the 2015 time line transforms around Doc and Marty, and that being time traveler's, they are protected by the hysteresis effect mentioned above. They don't notice this because it happens at night while they are returning to the time machine.

7 Since they do arrive in the 1985 created by old Biff, this seems to support the second explanation above. If they had left Old Biff's original 2015, they should have returned to the 1985 that led to it.

8 Without going back and watching the movie, I can't remember whether Doc and Marty arrive in 1955 prior to or after Biff does. As the graphic is drawn, I have them arriving after Biff does and then creating a new time line that aborts his changes. However, upon reflection, and considering the second addendum to rule 1, the new time line isn't created by Old Biff until after they arrive, so trip (i) should probably be drawn so that it goes back to the second time line (but quite frankly I'm too lazy to go back and redraw the whole diagram to fix it). Then Doc and Marty ride along with the changes made by Old Biff, and then "restore" the time line (though they are really creating yet another new time line which should closely follow the events of the first new time line created by Marty.)

9 Since this new time line is not the one that Doc originally travels to 2015 in, events should track the same way as they did before Doc and Marty interceded in 2015 since that future has been erased by subsequent changes to the time line. Thus Marty's son would get arrested if this time line were allowed to continue, which it would if Doc and Marty had returned directly to 1985 and Doc destroyed the time machine as he said he would do and the following events did not take place.

10 Another example of time lines transforming around time travelers. In the 1955 prior to the time machine being hit by lightning, there was no letter written by Doc waiting to be delivered, nor a time machine sitting in an abandoned mine. These came into being "after" Doc was accidentally sent back. But in another sense, once this new time line was generated, they had always been there. So once this new time line is established, we have a 1955 that, at one point, has 4 time machines. (1) time machine Marty uses in trip (a), (2) time machine Old Biff takes for trip (f), (3) time machine Doc and Marty use for trip (i) (4) time machine Doc seals up in the mine.

11 At this point we again have more than one time machine at the same point of time. The time machine Doc sealed up in the mine should will be there waiting to be dug out in 1955. So what did Doc do with the gasoline that was in it? We have to assume that he removed it from the gas tank, as he would not expect it to last for 70 years and he must have used it for other purposes in the intervening time before Marty showed up, otherwise they could just used that gas to fuel the DeLorean for their return to 1985.

12 Now here we have to assume that the changes affected by creating a new time line starting back in 1885 are mainly cosmetic (second addendum, Rule 2 as this 1985 does not appear to significantly differ from that which Marty created with his first trip to 1955. Marty does however make a choice here based on knowledge he gained from his trip to 2015, and decides to to participate in the race which resulted in a crash in another 1985. Thus his future(and that of his offspring) will differ greatly from that in the 2015 we saw.


Some interesting points:
Marty is the only character introduced in the first film that remains as his "original" version throughout the films. Even the Doc he sees when he returns to 1985 is Doc v.2 (one that met Marty and knew that he was to be shot. The Doc we meet at the beginning of the movie did not have this foreknowledge). Jennifer v.2 in movie two even looks different than Jennifer v.1:wink:

The "Marty" that Marty sees at the end of the first movie when he returns to 1985, is Marty v.2. This is a Marty for which the new version of his parents, siblings and life are perfectly normal. His parents told him a totally different story of how they first met (events leading up to the kiss). You have to wonder how this would effect how this Marty reacts upon arriving in 1955. Does this Marty precipitate a whole new set of events leading to even a different time line? Maybe we are dealing with a "many worlds" type of time travel here which each version of Marty generating whole new branches of time lines in some kind of open loop. Does this Marty even go back to the same 1955 as Marty v.1 did? (I don't see how, as you can't have two different versions of Marty in 1955 at the same "time".)

We no that changes in the time line can effect the one making the changes, Marty begins to disappear when it seems that his parent won't kiss on the dance floor, but does this apply to memories as well? Will the memories of Marty v.1 slowly change until they match the memories of Marty v.2? Maybe that is the answer to the last question. Marty v.2 goes back and precipitates the events as he remembers his folks telling him it happened, thus when he returns to 1985, he returns to a 1985 not unlike the one he left and the version of Marty he see travel back to 1955 is just like him. After one Iteration we have a closed loop rather than an open one. Meanwhile, Marty v.1's memories change to match that of v.2 and he remembers no other version of how his parents met than the newest version. Kind of like time sealing up an open wound.

Of course, this is just all just mind games and mental gymnastics on my part. Nothing to be taken too seriously (they are just movies after all), and anyone can feel free to pokes hole in anything or all of what I said here.


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iNow
Post  Post subject: Re: Time travel rules in "Back to the Future"  |  Posted: Thu Jun 30, 2016 10:06 pm
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janus wrote:
If someone were to suddenly stand up in the theater to exclaim that time travel doesn't "really" work like that, he should be prepared to present his diploma in temporal mechanics he received in 2050 to back up his claim.

Nice!

janus wrote:
We no that changes in the time line can effect the one making the changes, Marty begins to disappear when it seems that his parent won't kiss on the dance floor, but does this apply to memories as well? Will the memories of Marty v.1 slowly change until they match the memories of Marty v.2? Maybe that is the answer to the last question. Marty v.2 goes back and precipitates the events as he remembers his folks telling him it happened, thus when he returns to 1985, he returns to a 1985 not unlike the one he left and the version of Marty he see travel back to 1955 is just like him. After one Iteration we have a closed loop rather than an open one. Meanwhile, Marty v.1's memories change to match that of v.2 and he remembers no other version of how his parents met than the newest version. Kind of like time sealing up an open wound.

Or, considered another way, a bit like healing a heart using bypass surgery. The blood flows via a different tube, but the end result remains the same.

Fascinating write-up, and lots of fun! Thanks for sharing it here. Curious what others think. On another note... Maybe next up we can do something with a TARDIS or what's happened in the Star Trek movies (IV: The Voyage Home and/or the newer 2009 Star Trek from JJ Abrams).

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"[Time] is one of those concepts that is profoundly resistant to a simple definition." ~C. Sagan


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marnixR
Post  Post subject: Re: Time travel rules in "Back to the Future"  |  Posted: Fri Jul 01, 2016 8:27 am
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so what you're saying that Marty is the ultimate solipsist ?
everything revolves around him, and the time lines are alternative universes selected by his actions and the actions of the figments of his imaginations

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janus
Post  Post subject: Re: Time travel rules in "Back to the Future"  |  Posted: Fri Jul 01, 2016 3:55 pm

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marnixR wrote:
so what you're saying that Marty is the ultimate solipsist ?
everything revolves around him, and the time lines are alternative universes selected by his actions and the actions of the figments of his imaginations


Well, no. Just that during the run of the movies, he is the only character who we follow from start to finish. There is a Marty v.2, we just don't know his story. We also don't know what finally happens to Marty v.1. Throughout the movies, Marty is always running just ahead of the hysteresis effect (it almost catches up to him in the first movie). So what happens when he finally settles down in 1985, do all the changes he made in the time lines catch up to him. Does he morph into Marty v.#, who's memories no longer include a whimpy father and alcoholic mom? A clue may lay in an deleted scene from the second movie which shows Old Biff stumbling up to a dumpster after returning to 2015 and fading away. In this new 2015, this version of Biff never existed. This what I alluded to above. We don't really stay with the story long enough to see how it all shakes out in the end.


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