Time travel in Back To The Future

Back To The Future is one of two films, along with Bill & Ted's Excellent Adventure, which forms the bedrock of what most people of my generation understand about time travel. Back To The Future is the gold standard of time travel stories, ranking up there with the very best of popular science fiction. It's probably the single most popular and well-known time travel story that exists.

And it uses what is probably the most widely-used model of time travel. This model is so widespread and effective that it forms a kind of universal canon. Most people, even people with no real interest in time travel, seem to have picked the rules up through osmosis. It almost seems to be the default model of time travel in most time travel fiction. It can be safely used as the basis for time travel stories, with a relatively minimal amount of explanation.


Back To The Future didn't invent the model, of course, not by decades, but it's the one that I keep coming back to and it's the one that everybody remembers.

How did Back To The Future get to this hallowed position in the science fiction canon? Because it was insanely popular, obviously, but how did it get to be popular? Well... I think hindsight is dangerous in its clarity sometimes. This is a great film, but a great film is not automatically a popular film. It's extremely easy to look back on popular and unpopular films and circle the "obvious" shortcomings of one film or positive qualities of another which led to a particular box office take; examining the characters, story and direction which gained critical approval... without giving proper credit to, say, the marketing campaign, popular taste at that specific year and month, and wild dumb luck. So I don't like to speculate.

Similarly, a popular film is not automatically a memorable one. So: what I think sets Back To The Future apart from other stories using the same model is its fine art of progressive disclosure.

Time travel is a difficult thing for a typical filmgoer to wrap their head around all in one go, and Back To The Future's model is far from the simplest there is. But the model is unfolded gradually, revealed to the audience (through its surrogate, Marty) as a discrete series of new developments. Each bullet point arises intuitively and inevitably from the previous ones, such that by the end of the film, without even realising it, the viewers have developed for themselves a full and functional understanding of what is or is not possible. The film is practically a textbook about its own universe.

Furthermore, each intuitively obvious bullet point is also clearly necessary to the story, because the alternative would be a different story with completely different themes, or even no story at all. The two are coupled: the model exists solely in order to support the story, and the whole story arises naturally from the model. Exactly which concept should come first in the writing process isn't important to the end product, although I believe that in this case it was the story. But the point is that story and model should be tailor-made for one another, they should fit like a hand and a glove.

This is not only true of time travel stories. This is a rule which can be applied to all science fiction, and arguably to all fiction not set in the present day of the real world - and even much of the rest. Every story is set somewhere, and that setting is the deliberate choice of the creator, even if it was picked by default. If the story and the universe conflict with one another, this becomes jarring, and breaks immersion, and causes the reader/listener/viewer to start thinking about the wrong things.


The model

Models of time travel can be organised into a kind of complexity hierarchy, starting with "there is no time travel" and moving up to "fixed history", where time travel is possible but nothing can ever be changed, à la Bill & Ted. Back To The Future is at the next step up on the hierarchy, and presents a "modifiable history". This is the first model where we have to start dealing with the possibility of going back in time and changing history, which means it's the first model where changing history can become a plot point in the story itself, as opposed to using time travel as a framework for some other kind of story. It's also at around this time that we start dealing with multiple timelines, although Back To The Future only lightly touches on this concept, saving that for the sequels.

The model unfolds like this.

  1. Yes, you can go back in time.

    This is obviously necessary because without it, there is no time machine and the story is not really about time travel.

    This is established nice and early in "Temporal Experiment Number One", with Einstein the dog in the Delorean's front seat. From a cinema-goer's perspective there is no substantive difference between travelling forwards and backwards in time, although anybody with a [chuckles indulgently, adjusts glasses] rudimentary grasp of relativity will understand that only travelling backwards in time actually violates causality or necessitates changes to the laws of fictional physics.

  2. But it's difficult.

    It has to be - otherwise, there would be time travellers all over the place, right? Otherwise "go back in time and fix everything" would be the kill-all, plot-breaking, story-destroyingly straightforward solution to every problem that could possibly come up, whether that problem arose directly, indirectly, or not at all from time-travelling.

    For this reason, time travel is deliberately made difficult in nearly every time travel story, and in my view these limitations often characterise and differentiate those stories. In Back To The Future the difficulty involves specific requirements of energy (plutonium/lightning) and speed. These are good choices for making the action more exciting to watch. As is well documented, early versions of the screenplay had the time machine as a stationary refrigerator, which would have had nowhere near as much style as the Delorean.

  3. Having travelled back in time, you can change a few things, and that's fine.

    Chaos theory would suggest that as soon as a single molecule shifts out of place in 1955, all of future history would be thrown off-track. Small changes would snowball rapidly, all around the world. Marty would never be born (or maybe someone else called Marty would be born instead at about the same time), and therefore would immediately be erased from history. Depending on how rapidly the universe responded to such changes, Marty could disappear completely before he even hits the scarecrow in old man Peabody's field. Indeed, the universe could very plausibly instantaneously implode under the weight of paradox.

    This does not happen. Which is a good thing for storytelling purposes. If the universe immediately fractured into pieces there would obviously be no film - or worse, a film which would leave most of its viewers deeply confused and dissatisfied, no matter how carefully their time travel geek friends explained to them, afterwards, what had just happened.

    In fact, even better, nothing happens immediately. Even given the grace period of the "ripple effect" which we see later, in theory there would be nothing to stop Marty's limbs from starting to fade out right away. But this would make him and the audience confused and frightened, at a point in the story where the simpler concern of "being suddenly in 1955" is still front-and-centre. 1955 is a powerfully unfamiliar environment for a kid from 1985, let alone for any of us watching from the distant future present day of - at the time of writing - 2015. We need to slow things down a bit so we can get used to 1955, which is going to be the setting for most of the film.

    And so we deliberately construct our model so that it supports this. Marty is able to stumble around Hill Valley square for a while, comparing and contrasting it with 1985, meeting some significant figures and even boneheadedly kicking off a sort-of causal loop ("That's right, he's going to be the Mayor!"), all without getting into any specific trouble. The universe doesn't seem to mind too much at first that Marty's been displaced - it seems that most of history is reasonably resilient to small perturbations.

  4. But some moments in time are much more significant and delicate than others.

    When normal, non-insane people think about history, they often think about critical moments in history; moments which, if they turned out differently, would have pitched all of history onto a completely different track. This is kind of a first-order time travel story premise - your "kill Hitler, save Kennedy, prevent 9/11" moment. It's obvious and logical and easily comprehensible: "What if X didn't happen, and not-X happened instead?" is a far easier prospect to entertain with "big" events whose immediate consequences were very obvious.

    It's entirely possible to build a second-order story where we backtrack up the chain of causality a little, reaching a relatively "small" feeder event - one which, if altered very slightly, would have the chain reaction of preventing the "larger" event. This is quite easy to engineer, Rube Goldberg-style. A missed bus, due to a sandwich which took too long to eat, because of undesired mustard, because of a misheard order, because of a dropped plate, because.... In many stories, one specific feeder event is pinpointed as the sole cause of everything, to the exclusion of all the millions of other feeder events. This might be a sensible storytelling approach if you're trying to make a point about the significance of the mundane, or that any seemingly minor thing you do on any day can in theory have eternal consequences.

    A third-order story recognises the greater, more valuable, more actual truth: that every seemingly minor thing you do on every day absolutely definitely has such consequences. As a logical consequence of this, every single moment in history is of precisely equal significance.

    All of this, though, is non-intuitive, and confuses the issue. It's perfectly possible to tell stories in these frameworks, but these stories can turn out confounding, abstruse and weird, putting more of a burden on the viewers to follow and think and understand than to just relax and assume what seems logical. For a normal, non-insane person, it's much easier if most of history is "safe", but some events are "significant".

    Then we can take the next logical step:

  5. If you upset one of those critical moments, bad things can happen.

    And then at last we get to the real conflict. There must be some kind of conflict. In order for this to be a real time travel story, it has to be something which would not be possible without time travel's help. And in order for this to be a real modifiable history story, it has to be something involving modifying history - otherwise we would pick a different model. So, Marty disrupts the fateful first meeting between his future father and mother, and she falls for Marty instead... and his siblings start disappearing from his photograph, and soon he will too!

    Back To The Future is quite smart - or rather, dodges a classic pitfall - because it lowers the stakes while simultaneously raising them, by making the stakes highly personal to the protagonist. Marty isn't the most important person in the world; maybe all the universe isn't likely to self-destruct because some Californian kids were never born. But Marty himself might cease to exist, and by this point in the film we care enough about him that we don't want to see that happen.

    So you've changed history, in a way which is quickly, visibly bad...

  6. But you can set things right! (Approximately right will do.)

    As opposed to: "All is lost; there is no way to ever set things right".

    Even if we allow Marty to exist in his own past - maybe in a forked timeline of some kind, with no constraints on its future and no paradox kerplosion - the problem of chaos theory is that all events in history are, once you go back a very short while, totally dependent on chain reactions from all other events in history. Changing a single one of them results in new weather, and new humans being born, and history flipping on its head. This is easy to justify mathematically and scientifically, and makes it annoyingly difficult to tell compelling time travel stories, because even the slightest change to history can never, ever, ever be "set right".

    In Back To The Future we ignore or discard this problem. There's a clear, explicit "good enough" win condition, and some wiggle room in reaching that objective. It's not hopeless. Because, again... if it were hopeless, there would be no film.

  7. Although, setting things right is difficult too.

    It would be very possible to go too far in the opposite direction from "it's utterly hopeless". Here, you would end up with a universe which almost has a mind and a will of its own, where Fate is real, and the world has a "specific way it should be", with cosmic forces constantly trying to push Lorraine and George back together. Were that the case, all Marty would need to do is leave, or perhaps be forced to leave by events beyond his control, and everything would be fine.

    Which would be dissatisfying in the extreme. It's very, very important that Marty solves his own problem. Or, as we see in the climax of the story, he tries - but eventually, it's George's problem, and it's George who solves it.

  8. And then you can go home.

    Because a happy ending is necessary.

So what we end up with is a spacetime continuum made from discrete major events, joined together by relatively minor ones. This universe permits a certain amount of non-disruptive "time tourism", but reacts badly when certain sensitive points in history are altered. Luckily, this universe isn't so fussy about the details that it instantly swallows you whole if a single electron is displaced. More luckily, this reaction is slow enough that there's room to set things right, if you can.

And the story structure which falls out of this model is the story structure where that's exactly what happens. Our protagonist interferes with the normal course of history, with dire consequences... which he is eventually able to set right.

Words I'd use to describe this formula: powerful; versatile; intuitive; widely-understood.


So, all this essay really covers so far is the model that Back To The Future uses, and the specific conflict and story which rises up naturally out of this particular model: the A-plot. What else is there?

Well, loads. The film is like a Swiss watch, even staying only on the time travel-based plot threads.

There's the causal pseudo-loop whereby Marty invents skateboarding, which we can acknowledge and carefully set aside as fairly unremarkable. There's another loop involving Mayor Goldie Wilson, as already mentioned.

There's another loop whereby Marty invents rock & roll music, and this is much more significant on a personal level because it signals a turning point in Marty's confidence about his own musical abilities. At the start of the film he's frightened that his demo tape will be rejected. He later meets his young father, who has similar creative neuroses, which set Marty's own fears into perspective. But then he gets up on stage and plays "Johnny B. Goode" - and the crowd goes wild for it. He does this without even thinking about it. It's "an oldie" - he already knows the song is going to be a hit. From one perspective it is simply another cheap little causal loop, but from another it's the moment when he realises that he can really can be - no, he is a rock star.

There is, of course, the enormously significant B-plot: the storyline referred to in the title, whereby Marty is stranded in an unfamiliar past era with almost no hope of getting Back To The Future. This storyline is absolute dynamite all by itself, in its conception and execution, and in how it ties into the A-plot by giving Marty serious time pressure to get his parents together before the thunderstorm when he needs to leave. But that kind of story structure, of being stranded in time with no way to get "home", is not really unique to this modifiable-history model (and nor is the "Marty invents rock & roll" thread, come to that). You can tell a story like that in a fixed-history model quite easily.

Hell, if you want to spend time examining the travails of an interesting, stranded protagonist, you can stick them on a desert island in the present day. You don't throw time travel into your storyline for no reason at all. You do it because there's a particular thing in the past or the future that you want to explore, and because you need the model to support that exploration. Here, it's the generational differences between the Fifties and the Eighties - or, as we discover, generational similarities. That's why I consider the Lorraine/George/Marty storyline to be the A-plot of this film.

The final time travel-related plot thread is, I guess, the C-plot: Doc is gunned down by terrorists in the first act before Marty leaves 1985, and Marty wants to somehow rewrite that small piece of history in order to save him. This is a difficult problem, hovering in the background during most of the 1955 scenes. Marty is caught at the junction of two conflicting desires, one to set established history straight to put his future parents back together, and one to alter established history to save his friend.

Even not knowing what truly happens to him in the future, Doc refuses to hear any kind of specific warning from Marty, saying that no one should know too much about their own future, and that the consequences could be disastrous. Doc's resistance is quite rational, because both of them have seen the possibility of erasure from history with their own eyes. The conflict arises because Marty knows full well that this is no worse than what actually happens, and the Doc has absolutely nothing to lose - and none of history beyond that point is known, so nothing can be lost from altering that one moment.

(There are broader implications which the film ignores and which I think it's reasonable for us to ignore too. Specifically, there are numerous other significant historical events coming up between 1955 and 1985, which Doc can't possibly foresee. It would be rational if Doc was in fear of unwittingly disrupting one of these, thereby endangering the existence of many people other than himself. But I feel happy discarding this consideration because, as I mentioned above, the whole film is highly personal - Doc seems to have no family or friends beside Marty, so it's reasonable to pretend that Doc is the only person at risk from his own foreknowledge.)

Of course Doc eventually decides he can trust his friend, and everything turns out fine.

This storyline somewhat reinforces the stern warning from the A-plot, which is that "there's a way that history ought to be", while being the first thing in the film to hint that the way history ought to be isn't necessarily the way that it is. Later, Marty gets home, and finds that the new status quo is much better than it was when he left. And in the final scene, Doc returns from 2015 with nothing else on his mind but setting right an unspecified future gone bad.

Except that the time machine is a dangerously powerful tool for the "righting of wrongs", as we'll soon see...


Overall Back To The Future is a film whose core concept is that while the world alters radically from generation to generation, your parents are really not so different from you. The film selects and tailors a model of time travel to explore this concept, and then goes every conceivable extra mile - laying the whole concept out brilliantly for the viewers; harnessing the core model to drive all kinds of other stories, both dramatic and comedic; never wasting a single word of dialogue; driving iconic action sequences and somehow also finding time for a genuine emotional story with a lot of heart.

I think the film may be diminishing in its effectiveness now that the eras of 1955 and 1985 have both become so alien compared to the present day. 1955 was always supposed to be a disconcerting change of pace. And the opening sequence of the film with Marty on his skateboard is almost prescient - it introduces us to the era of 1985 as if we were strangers to it, in almost exactly the same way that the film later introduces us to 1955... but even so.

It doesn't matter, though, because the Delorean is forever. Have I mentioned the Delorean? Unquestionably the greatest, most iconic, coolest-looking time machine ever imagined? Oh my goodness, the Delorean.

Discussion (31)

2015-01-08 00:22:36 by Rocky:

Thank you. I wish more people treated time travel - or SF in general - like this. The science in SF exists solely to tell the story. Yes, that means it will likely deviate from established science, and yes, that deviation will almost certainly be not entirely internally consistent (because rebuilding physics in your head from the ground up is *hard*), but if it makes for a good story, and a good telling, then it possibly deserves a little more slack and a little less nitpicking than people like to give it.

2015-01-08 00:38:38 by qntm:

It's 2015. What current car does Doc build his time machine out of? Marty goes back in time from 2015 to 1985. What genre of music does he invent?

2015-01-08 01:53:08 by Luke:

Doc might build it out of a Telsa or similar priced sports car. This still maintains the conflict in the third movie with being unable to charge the car instead of being unable to fuel it. Techno music also arose in the mid-1980s, he might invent that, I can see a scene with him and a synthesizer putting together some techno

2015-01-08 03:12:21 by Alexander Wales:

It bears mentioning that "Hot Tub Time Machine" is in many ways the spiritual successor to BTTF - in that it goes from 2010 to 1986, and invokes a lot of the same rules/tropes. I believe they invent hip hop instead of rock and roll. As for what car you'd use ... I'm not sure that there will ever be anything as iconic as the Delorean, in part because the Delorean Motor Company went extinct. More people are familiar with the Delorean from BTTF than from any other single source. The number of people who have seen a Delorean through the film far exceeds the number of people who have seen one in real life. So a modern analog would probably have to be a small, niche car or something, instead of a more obvious answer like the Tesla.

2015-01-08 03:21:00 by John:

Back to the Future took seven years to write, and the hardest part wasn't the time travel, it was when Marty and his mother are in the car and she starts kissing him. They couldn't think of a reason why she wouldn't like Marty given that he's the coolest newest thing in town. I think this model of time travel is the one that fits our intuitive understanding of the world best. If you see the timeline as an object, we as people are familiar with objects. You can do what you want with them to a point, but they are breakable if you get something wrong. But you can fix them, and sometimes make them better. Most people's objects don't explode the second you touch them, or can only exist in one possible way. It may not make the most logical sense, but it's coherent with the the experience of everyday life. Marty goes back to the 80s, he definitely invents techno, but he'd have to bring some aunty's little helper to put in the punch. "Frankie Knuckles? it's your cousin marvin, listen to this! also I love you, you're great man"

2015-01-08 04:51:56 by Lauren:

"If the universe immediately fractured into pieces there would obviously be no film - or worse, a film which would leave most of its viewers deeply confusing and dissatisfied, no matter how carefully their time travel geek friends explained to them, afterwards, what had just happened." Arthouse cinema, here I come!

2015-01-08 05:12:27 by Ross:

" I think the reason why Back To The Future's model of time travel has taken such firm root is its fine art of progressive disclosure." Back To The Future is a good movie (and its sequels, though not as good, are also good movies). However, 1985 is about thirty or forty years too late for this to be the origin point for this time travel model. <b>You</b> may associate this model with BTTF and think that the goodness of this movie took over the whole field -- but the truth is that the model was in lots of stories before BTTF and lots of stories after. It's a good model for telling stories, with potential for lots of plot twists, and that is why it persists. Not because everyone loves Marty McFly and Doc. (Stopped reading after the second sentence of paragraph four, because I so violently disagree with the entry point to your thesis that I can't trust myself to read the rest of it.)

2015-01-08 16:58:02 by Andrew:

What is your opinion of the theory that there are two interlocking timelines when looking at the the first movie by itself? Marty A is the protagonist we see. George A is a pushover and Doc A is shot and killed. Marty A travels to Hill Valley B 1955, does his thing and travels forward to Hill Valley B 1985. Marty B is the protagonist we see very briefly. George B is a successful writer. We see Marty B at the end of the movie when Marty A has traveled to Hill Valley B 1985 (Lone Pine Mall) and sees his doppelganger leave in the Delorean. That double is Marty B. According to the theory, Marty B actually travels to Hill Valley A 1955 and has an adventure we don't see. Then when he travels back to 1985, it is Hill Valley A (Two Pines Mall). He would arrive just as Marty A, our Marty, left at the start of the film. So Marty A takes over Marty B's great life with a cool dad and living Doc. Marty B, however, would arrive in a world with a pushover dad and a dead Doc. Each lives in the world the other created. Poetic.

2015-01-08 18:05:18 by qntm:

Yeah, that's a fun one. In the version I heard, Doc B remembers the entire fiasco with Marty A and the clocktower stunt, so he makes sure that Marty B has extra plutonium. As a result, Marty B returns to the future immediately, having *dodged* the entire adventure. The other "offscreen adventure" I'm sort of curious about is what happens a day or two later, when the FBI team tracking the stolen plutonium finally unscrambles the wreckage of the exploded VW minivan in the JC Penny parking lot. Doc's a nuclear scientist collaborating with Libyan terrorists operating in the mainland US; poor Marty is their best and only lead. Knock knock...

2015-01-08 18:06:26 by qntm:

Actually, I'm not curious about that at all. That sounds terrible.

2015-01-08 21:41:51 by David Grossberg:

So, when will we see "Time travel in Homestuck"? (Actually as convoluted as it is, I think the entire thing is just fixed history, at least up to point where I was caught up maybe a year or two ago)

2015-01-08 22:44:11 by qntm:

Homestuck is longer than War & Peace. I've tried reading it a bunch of times, but the dialogue goes on for ever and ever and I just can't get near it, sorry.

2015-01-09 00:55:51 by David Grossberg:

Yeah. I was mostly joking anyway - I haven't been able to catch up myself since I resumed having a job.

2015-01-09 06:44:05 by Eldritch:

Time travel in Homestuck is the branching timelines model. You can, via a special mechanisms, deliberately branch off a new "stable" timeline; in all other cases, it approximates fixed history with minor resilience. ("Changing history", while possible, causes the resulting timeline to be "doomed" in short order; the anthropic principle results in the "correct" timeline to be the one most observers experience, which is equivalent to fixed history complete with causal loops.) In cases not covered by this model, especially when separate timelines interact, there are no rules and everything's wibbly wobbly timey wimey; you generally still can't change anything though.

2015-01-09 13:46:25 by Silhalnor:

Now's a good time to catch up though as it's on hiatus while the Homestuck game is developed. At least, I *think* it is still on hiatus. *Goes to check* AHHH it started again sometime last month! *Suddenly realizes that it isn't as exciting as expected.* ...Huh. The obsession must have drained during the hiatus. Well, I hope it doesn't break my schedule when I start reading again soon.

2015-01-09 14:12:49 by Silhalnor:

On topic... well, on topic-ish: I've been wondering if there is a sort of midway point between altering history and not altering it. I want a mechanic for a story(-ies) of mine where you can't strictly *change* history, because that would make the terrible things that will happen in the story fixable and thus less meaningful when they do happen. Can't have people being all "Well, if the dark angels from Mercury DO enslave us all we can collect the seven pieces of the MacGuffin to go back and reverse it all." But I do want to mess with the timeline. Kind of a contradiction there. I am mostly considering some sort of "history scrambling" power where you can't change events in history but you CAN move them around so that the plague in 2015 gets moved to 1346. (The main advantage of this is that any fix you might make imposes an identical cost elsewhen. Could also spark conflict with different eras trying to shift their tragedies onto other other eras too. Plus it's novel. At least, *I've* never seen it before.) I'm having a lot of trouble figuring out how to make such a system actually work in any meaningful way but it *sounds* interesting.

2015-01-09 18:16:24 by Andrew:

That reminds me of the Aorist rods from Hitchhiker's. People used them to siphon energy from the past, which was all well and good until they themselves started having their energy siphoned by future people.

2015-01-10 01:17:51 by Useless Mage:

Eldritch: Don't forget that plot-critical events and items are perfectly able to arise from or occur in those Doomed offshoot timelines, and indeed are even encouraged/enforced to do so in order to achieve the goals of sufficiently capable beings.

2015-01-13 18:07:42 by Aegeus:

Homestuck has a pretty simple time-travel mechanic: History is immutable, any attempts to change it are either causal loops or doomed timelines. The issue is that there are multiple universes - the Medium, your homeworld, the Void between sessions, etc. - each with their own timeline. Any time you move from one universe to another, you can arrive at any time. You can jump on a meteor and end up a million years in the past, you can fly to the Green Sun and end up arriving before it's created... there's really no consistency when jumping between universes. You arrive when the plot needs you to.

2015-01-14 03:15:38 by Bauglir:

@Silhalnor If you want to do that, one possibility is to make history-meddling dependent on gaps in knowledge, so that you can change only things you didn't know about when you traveled back in time. It's awfully arbitrary because it makes the physics dependent on something subjective, but from a narrative point of view it can be pretty satisfying. You might be able to universalize the rule somehow, but I'm struggling to come up with a way of expressing "You can't change recorded history, but you can do whatever you want as long as everybody remembers it the same way a century down the line" in a coherent way that doesn't violate causality even beyond what time travel normally does.

2015-01-14 03:38:29 by Bauglir:

Re: What I just said, I suppose you could make it a paradox-aversion law that prevents any changes that would alter your knowledge at the point you engaged in time travel, but even if you handwave chaos theory you may encounter the problem of elaborate conspiracies by which time travelers manipulate their younger selves into changing the past. Okay actually that could be pretty cool if anybody had the dedication required to ensure internal consistency.

2015-02-02 21:15:48 by chirdaki:

One particularly good example of good time travel storytelling was The Infinite Man, which I saw at MIFF last year. Like sliders meets the notebook, it was very funny too but I think because of the shoestring budget they didn't market it much. Worth checking out.

2015-02-10 19:20:11 by Daniel H:

The part of the model I’m most interested in is the part you handwaved away: the ripple effect. You seem to be good at figuring out how inconsistent-seeming models actually are consistent, what can you say about this? To me, the rippling seems inconsistent because the photograph would have been taken differently if there were fewer children, Marty wouldn’t have taken a picture of no gravestone, there’s a good chance no fax will be sent (not just an indeterminate fax), and Marty’s brain would also change (why would he remember a sibling who never existed?).

2015-04-02 13:55:31 by Adam Ring:

What's to say Marty's brain wasn't changing. I always assumed that the photo is "younger" than Marty, and therefore is quicker to change than Marty himself. In this model the ripple actually flows from the present to the past. So while they don't exist in the photo anymore, they still exist before the point the photo was taken. As the ripple moves forwards Marty begins forgetting certain events later in life but still retains memories from earlier in life. Marty is affected last, because of his existence EARLIER in time (as a result of going back to 1955). Its a bit convoluted, but what did you expect.

2015-04-19 20:48:56 by TheCustodian:

Coming to this one late. Sam, your talk of a 'third-order story' reminds me a great deal of 'The Adjustment Bureau'. That was a somewhat unfortunate movie, mostly because (IMO) the overarching explanation as to why/how was a) unnecessary and b) a huge copout. But the mechanisms of it, everyone running around to make sure everything happens exactly the right way - leave the house at the right time, spill your coffee, hit the right bus (or miss the wrong bus) feels almost exactly like we're getting one-half of a somewhat cool time-travel story, with the Agents as the time-travelers (or time-communicators via Moleskine).

2015-04-27 17:37:22 by P:

Rereading this after rewatching the films. Did you just call Michael J Fox a "small pertubation"?

2015-06-07 23:41:21 by Jesse M.:

There's an official BTTF FAQ by scriptwriters Bob Gale and Robert Zemeckis which was written for a fan magazine, and posted at http://bttf.wikidot.com/official-bttf-faq , which gives some additional insight into the model of time travel being used. Especially interesting are the answers to questions 1.8 and 1.9, about the scene where old Biff returns to 2015, having just come from 1955 after giving his younger self the sports almanac. They say the idea was that as he returned, the original 2015 we had been seeing up until then was changed by the ripple effect to a new 2015-A, the future of the 1985-A where Biff was rich and famous, and that this shift happened around Marty, Doc, Jennifer and Einstein. They also mention the deleted scene where old Biff vanished shortly after returning, saying the idea was that Biff was supposed to be dead in 2015-A. This suggests that time travelers are immune to having their memories changed by the ripple effect even as the world changes around them (something they also suggest in the answer to 1.19), since for example Marty didn't change into the version of Marty from 1985-A who remembered Biff as his stepfather, but that time travelers can be erased if the shouldn't even exist, or shouldn't have lived to their current age. My theory is that this means, for example, that when the Marty from the timeline seen at the end of the first movie--the one who'd remember his dad as a cool science fiction author--jumped back in time to 1955, his memories instantly reset to those of the original timeline (where George McFly in 1985 was a wimp cowed by Biff). In general , I think it makes sense to assume a model for BTTF in which any trip through time should cause the time traveler's memories to reset to the "first" version of the timeline where they made that trip (the first in the second time dimension which seems to be implied by the notion of 'changing' the timeline without multiple co-existing parallel timelines). But one thing I can't decide on is whether, if there had been cameras set up in 2015-A to record what had happened earlier in the day, they would show Doc, Marty & co. arriving and being bewildered by a future where Biff had controlled the town since the 70s, and then the cameras would show their behavior suddenly changing at the moment old Biff returned from 1955 (their memories having reset at that moment to those of their experiences in the "first" version of 2015 they visited), or if the cameras would record that Doc & Marty hadn't arrived in 2015-A earlier in the day via DeLorean at all, but just sort of materialized at the moment old Biff returned from 1955.

2015-06-08 09:06:45 by Alexander:

Uhmm... There is one single thing about the past that is significant: it has happened already. You cannot go back and change the course of history because we know that i.e. WW2 has happened. If I go back this evening and kill Hitler in 1936 WW2 probably would not have happened. But we know it happened. So nobody from the future will go back and kill Hitler. It has happened and that part of the timeline is locked.

2015-06-08 19:39:52 by Jesse M.:

Alexander, as Sam suggested in the Bill & Ted article at http://qntm.org/excellent , the most plausible theory about time travel is that the laws of nature don't allow it (look up the 'chronology protection conjecture' on wikipedia), and the second most plausible theory is the one you mention, that the timeline is locked and impossible to change (look up the 'Novikov self-consistency principle' on wikipedia). But it is possible to have models of time travel where history can be changed without it leading to any logical paradoxes. I'd say the third most plausible model is the branching-parallel-history model, where for example if today I were to open up a portal to 1936 so I could kill Hitler, this would create a new parallel timeline whose history was identical to our timeline up to 1936, but diverged afterwards, and anyone who subsequently jumped into the same portal from our universe would find themselves in that same alternate universe where Hitler had been killed before the start of WWII. The FAQ I linked to in my comment above suggests that Back to the Future doesn't use this type of model though (if it did, then when Biff returned to 2015 he would end up in a different parallel 2015 then the one Marty and Doc and Jennifer were in). I think to explain BTTF's time travel logically you need a model with 2 time dimensions, the second one being a sort of "meta time" that keeps track of changes to the timeline. This sort of model, where changes to the past ripple forward through the timeline at some rate in meta-time, is used in the time-traveling strategy game "Achron", see http://www.rockpapershotgun.com/2011/09/05/wot-i-think-achron/ and http://www.gamingexcellence.com/pc/games/achron/review for an outline. I think this is a proof of concept that you can formulate a self-consistent mechanic for this type of time travel, even if it ends up looking rather contrived and therefore not at all plausible for how time travel would work in reality, if it were possible at all. Back to the Future's model would have to be even more contrived, involving some sort of "self-healing" of the timeline for minor changes (so for example as long as Marty still gets born, he still ends up in that mall parking lot to escape the Libyans in the DeLorean, even though his dad is a changed man in the altered timeline), but I think you can figure out a set of rules for BTTF's temporal mechanics that at least don't lead to any logical inconsistencies.

2016-02-10 06:27:33 by 3(oneplus)x(cube):

Genius!!!!! hundapu ... senpai... u r great

2017-04-24 07:09:08 by mmazd:

I've been thinking recently about the time travel hierarchy thing, and the degree to which the past can be affected, and to me it looks roughly like that: (The levels are increasingly "liberal", when it comes to altering future. As a rule, a story can usually be moved further down the hierarchy without a major rewrite.) A. You can't change the past. As simple as it goes. An unfortunate consequence is that you simply cannot travel back in time. Your mere appearance in the past means altering it to a degree, via reasons similar to why teleportation would be difficult. The level A is basically every work of fiction without time travel. (Alternatively, you cannot travel to the past, but you can look into it. Provided no past photons are harmed.) B. You can't change *your* past. I.e. you can go back to 1800 and become some unknown shoemaker. But you can't become the Beethoven or the Lincoln or the Hitler. You can't leave secret message in textbooks. Any sign of your presence in the past has to become equal to noise at latest before you're born. That is, assuming, the consequences of your presence will diminish quicker than the Butterfly Effect will grow. In universes with the Butterfly Effect rapid enough, the level B is equivalent to the level A. Rather uninteresting and rarely seen in fiction as the main plot. Appears here and there when the heroes accidentally teleport to the dinosaur era and have to survive there for a set amount of time. IIRC, the level B was explored in Asimov's "The End of Eternity" (this was a minor plot, though). C. You can't change your *perception* of the past. This is almost the previous level — with a slight twist. You still can't change the past in any meaningful way. You still can't interact with your own self. You can't create temporal loops: from your point of view, all effects have to follow their causes. You can, however, alter the past in slight ways — as long as your past self doesn't notice. This one is quite limiting, so I'm not able to recall any story that would fall here in its entirety, but "Back to the Future" has elements of it (especially sending letters to the future). D. You can change your perception of the past, but not the present. This one is definitely more interesting than the last three. We break the causality, allow for stable time loops, but we have to ensure the continuity still holds. Hence, paradoxes still apply. Effectively, it allows the heroes (and probably the writers) to "retcon" the past events as long as the perceived present outcomes stay the same. This is where "Twelve Monkeys" operate. You can teleport yourself to the past, you can send secret messages and you can steal a sample of the virus. You can even meet your younger self (as long as they don't know you are them). You can't, however, stop the virus — because you know about the virus having already spread in 2035. (If not for the dream sequence, it would be a borderline level-C example.) This is also the level of "Prisoner of Azkaban". You can throw snails at your past self and you can cast spells in their field of vision — provided you won't know about it until you actually do it. And, if you haven't seen somebody die (only assumed it), you can still save them. Interestingly enough, this is also how "Predestination" works. (It is a little known movie, though, so I won't spoil it here.) E. You can change the past, the present and the future, but not without consequences. In other words, the future will get rewritten, but it can also affect you. You can't kill your grandfather and you can't kill Hitler. This is the level of "Back to the Future", "Butterfly Effect" and many others. F. You can change anything, without consequences. Finally. No need to worry about paradoxes, the Multiverse has you covered. The middle part of "Back to the Future 2" had a taste of that, dealing with a universe, where the Hill Valley is owned by Biff. (It's still compatible with the level E, as the point of divergence happened after Marty's birth.) Many stories at this level involve a "Christmas Carol"-like plot, where the heroes end up in an alternative reality they wished for, only to realize they don't like it. Or, the whole work can be set in one, see Alternative History. * * * Now, let's try to do something with our model. Let's assume you want to kill your grandfather, before he had any children. The scenario would look like that: 1. You get born. 2. You grow up. 3. You learn about time travel. 4. You travel to the past. 5. You kill your grandfather ← at this point, all the previous events still have to hold; however, if your grandfather dies without children, it will be impossible for you to be born. Hence, the story has to be set with the time travel at the level F. Let's try something different. Say, you want to kill Hitler before he becomes the ruler of the Third Reich. 1. You get born. 2. You grow up, learning about Hitler and WW2 and decide to kill him. 3. You learn about time travel. 4. You travel to the past. 5. You kill Hitler ← at this point, you might think the story might need the level F as well; after all, if Hitler dies early, you wouldn't learn about him or have a reason to kill him, voiding the point 2. There is a workaround, though: 6. After Hitler dies, you find all your contemporary ancestors. 7. You create a "The Truman Show"-like environment for the future you. 8. You ensure the young you will grow up learning about WW2 (even though it didn't happen in the new timeline). 9. You ensure the young you won't learn about the switheroo until you, in fact, take part in the time travel. 10. You return to the present. This way, the story can be set at the level E or F. But we can do even better. If your whole life before the time travel has been boring enough so it can be perfectly recreated in the Truman-like environment, then we can even enable such a story at the level D. (In that case, everything that happened to you before the time travel has been a great lie and you learn about it only during the mission.)

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