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Time travel -- Back to the Future

  1. Jun 21, 2016 #1
    Hello, My friends and I have had about a 3 month dicussion going regarding time travel.We have to get this right. Its also something they have been debating on their podcast.
    The question or arguement is.

    In the movie "Back to The Future"
    Marty goes back in time from 1985 to 1955.which after many events he must go back to the future.

    The arguement is that according to our friend Ully he, Marty CANT go back in time because he goes back to HIS present. But what we argue is that hes going to the future of that time line from 1955. He says the movie title is wrong because Ully says that Marty goes back to his future. Since he cant Return to the future because it isnt his future.

    We argue his goes back in time in that universes timeline. Then in that time line goes back to the future after the 1955 event. To a future moment of that said timeline. Not martys life.
    Last edited by a moderator: Jun 21, 2016
  2. jcsd
  3. Jun 21, 2016 #2


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    For a number of reasons that is believed to be the end of the story. It's not a matter of the paradoxes and time-line decisions such as what you are looking for, it's just not physically possible. Backward time travel is science fiction and forward time travel occurs at the rate of one second per second.

    For the purposes of the movie, any explanation is equally valid. It's what is called a "vacuous truth" because any statement of the form "if A is true then B is true" is vacuously true (true but devoid of any actual meaning) if A is known to be false.
  4. Jun 21, 2016 #3


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    Welcome to PF!

    Clarification of what? You are arguing about a fictional movie plot. I'm reasonably certain none of us here wrote it.
  5. Jun 21, 2016 #4


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    There are a few ways to have time travel without paradoxes, but the more scientific of those don't allow alternative timelines.

    Related: Back to the future grammar
  6. Jun 21, 2016 #5
    I understand its a fictional story but were trying understand it with some sense behind it. Yes in the movie world anything can happen but I want to know in the scientific aspect of it if thi can be clarified.
  7. Jun 21, 2016 #6


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    The problem is that you are asking "if the laws of physics don't apply then what do the laws of physics say about <insert nonsense of your choice>".
  8. Jun 21, 2016 #7

    Well..... loose sense then.
    Some hypthetical info.
  9. Jun 21, 2016 #8
    Guess the real question is what your opinions would be.
  10. Jun 22, 2016 #9
    That the movie title is catchy, and the movie itself was entertaining.
  11. Jun 22, 2016 #10


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    Honestly, it sounds like Ully is picking nits on a unicorn. I mean, since time travel isn't a real thing, The "rules" for it in a movie can be pretty much what the writers want them to be. Besides, it is Doc Brown that first uses the term "Back to the future", and from his perspective, Marty is being sent into his future.
    Last edited: Jun 22, 2016
  12. Jun 22, 2016 #11
    The name of the film isn't Back To His Original Future. It's Back To The Future. The future is what it is. If it would solve your argument, you could change the film's title to Back To An Altered Future. Marty would be back in 1985, it just wouldn't be his 1985.
    Or it's possible you could be over thinking an old popcorn flick about a time traveling Delorean. If this sort of thing bugs you, you should probably avoid things like the Terminator franchise and especially Dr. Who.
  13. Jun 22, 2016 #12


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  14. Jun 22, 2016 #13
    I really appriciate all your guy opinions and info.!!
  15. Jun 22, 2016 #14
    I personally love dr who. But our friend is a big on using correct grammer and wordage so were trying to get him to see it as how many have commented.
  16. Jun 29, 2016 #15


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    As I said above, the writers can make up whatever rules they want as long as they don't push it to far. The best you can hope for is that that they play by their own rules consistently. The rules are not always spelled out and you sometimes have to figure them out as you go along. With that in mind, we can to work out the "Rules of time travel" for the BTTF universe.

    First we set up an info graphic plotting all of the trips made in the movies: BTTFinfo.jpg
    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 black 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 19558

    (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

    1Marty 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.

    2Rule 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.

    7Since 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.

    8Without 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.
  17. Jul 3, 2016 #16
    Have you offered to help Bing Bang Theory with the scripts for the show ;)
  18. Jul 3, 2016 #17


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    If you like these kinds of time travel stories, Robert Heinlein has a great little short story called "By His Bootstraps" which has the main character time traveling multiple times. At one point there are four different versions of the same person in the room at the same time.
  19. Jul 3, 2016 #18


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    "I sat down in the armchair to collect my thoughts and take stock of the situation. I'd lived through it twice now, first as that sleeper, on Monday, and then as the one trying to wake him, unsuccessfully, on Tuesday. The Monday me hadn't believed in the reality of the duplication, while the Tuesday me already knew it to be a fact. Here was a perfectly ordinary time loop. What then should be done in order to get the rudder fixed? Since the Monday me slept on - I remembered that on that night I had slept through to the morning undisturbed - I saw the futility of any further efforts to rouse him. The map indicated a number of other large gravitational vortices up ahead, therefore I could count on the duplication of the present within the next few days. I decided to write myself a letter and pin it to the pillow, enabling the Monday me, when he awoke, to see for himself that the dream had been no dream. ...
    "You're the Thursday me?!" I cried, running into the room.
    "Right," he said. "Here, give me a hand..."
    "Will we be able to fix the rudder this time?" I asked as together we pulled out the heavy satchel.
    "I don't know, it wasn't fixed on Thursday, ask the Friday me..." That hadn't crossed my mind!
    I quickly ran back to the bathroom door.
    "Hey there, Friday me! Has the rudder been fixed?"
    "Not on Friday," he replied. "

    Stanislaw Lem - The Star Diaries - http://english.lem.pl/works/novels/the-star-diaries/154-the-seventh-voyage

    Very amusing story!
  20. Jul 3, 2016 #19
    I love this! thanks for the tip.
  21. Jul 4, 2016 #20
    Well, the ultimate time travel story is primer. Watch the movie which you won't understand, then read some explanations on the internet, then watch the movie again to see the marvel of complexity. One of the best movies there is.
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