Is Marty McFly (Prime) The Destroyer of Worlds? Yay or Nay?

July 18, 2011 7:06 PM

I just watched the Back to the Future trilogy (for like the millionth time) and I have a question I’m hoping the Hive Mind can help me reconcile a question I have.

OK, so at the end of Back to the Future 1 (BTTF1), Marty McFly (who, for the purposes of this discussion we’ll call Marty Prime) comes back from 1955, wakes up and discovers his family are no longer the bunch of losers they were before his adventures in the past. This surprises Marty Prime. This all suggests that Marty Prime is in an alternate reality/universe from the one he originates from. This raises the question, what has happened to the alternate universe Marty (who we’ll call Marty A)? Has Marty Prime wiped Marty A from existence (perhaps allowing Marty Prime to live out his life with Jennifer at the end of BTTF3)? Or is it likely Marty A will show up at some point, meet Marty Prime and blow up a good chunk of the Universe?

It’s a similar question for BTTF2. When Marty Prime returns from the future to find yet another alternate reality where Biff is rich and Hill Valley has gone to shit, Biff’s thugs (and Biff himself) talk about this alternate reality’s Marty (who we’ll call Marty B). Has the introduction of Marty Prime into this alternate reality again wiped another alternate Marty from existence or is Marty B off having his own adventures somewhere?

Basically, does Marty Prime kill the Marty McFly of any alternate universe he enters? Is he a destroyer of worlds? Or is there some theory out there (scientific or otherwise) that actually reconciles the introduction of Marty Prime into another Marty’s world?

posted by Effigy2000 to Media & Arts (31 answers total)

Within every story featuring time travel there really exists two stories: The one about Marty McFly who went back in time and saved Doc Brown from being killed by terrorists and the (much less contemplated) one about Marty McFly who disappeared forever the night Doc Brown was killed.
posted by Jairus at 7:09 PM on July 18, 2011 [3 favorites]

This website has thoroughly covered time travel in cinema and is thoroughly entertaining:

http://www.mjyoung.net/time/back1.html

http://www.mjyoung.net/time/back2.html

http://www.mjyoung.net/time/back3.html
posted by michaelh at 7:12 PM on July 18, 2011 [10 favorites]

I've thought about this at some length, and the unfortunate conclusion I reach every time is: It's not really watertight sci-fi in any way. Like: Why does Marty McFly know Johnny B. Goode from the timeline before Chuck Berry ripped it off from him? Or really how is it reasonable that his parents lived at the same house and had children at the same time with the same names even though they lived entirely different lives?

Not to take anything away from the movie - the first one especially is jam packed with awesomeness (almost no scene is unimportant), and is wholly awesome. But I just don't see it holding up to any kind of rigorous analysis. Scant few time-travel movies do.
posted by aubilenon at 7:15 PM on July 18, 2011 [4 favorites]

There is the Many-worlds theory. Simply put - with each decision we make we form a new universe.
posted by deborah at 7:17 PM on July 18, 2011 [1 favorite]

I don't see it holding up to any kind of rigorous analysis either, but just a clue to the creator's intention - doesn't Marty have a photograph of himself that starts fading as things start looking bleak for his future parents to get together? That seems to suggest there is only one world in the BTTF universe, which is changed by the actions of the protaganists. This photograph comes from the world of Marty Prime.*

This idea from TV tropes may also provide some insight to your problem.


*or does it?
posted by randomkeystrike at 7:27 PM on July 18, 2011 [1 favorite]

Response by poster: michaelh, the link you provided seems to make the most sense, but I don't think its perfect. Here's the explanation from the site, with my edits for people to lazy to read the link and just want to stay here.

"The Marty McFly [Prime] whom we see at the end of this film does not exist. Although he returned before he left, at the moment that his other self leaves for the past, that timeline is erased. The only future which exists is the one at the end of [BTTF1]. Remember: for the Marty McFly who is seen leaving by the Marty McFly who has just returned, that more affluent existence, that altered history we just watched, is his. He is not the same McFly. And when he returns, he will reach point F, the end of the timeline he is on his way to create.

That timeline will be different, but it is possible that the end will be much the same (without the surprised Marty). Doc will have his bullet proof vest (a gamble, as he did not know that they would not shoot him in the head or use armor piercing bullets), and so will be alive, if somewhat bruised. His family might be the affluent version. And since it might be so, and the story continues that it is so, it is reasonable to conclude that on the last timeline [...] time continues into the affluent future. Back to the Future, in its first part, allows the future to continue."

But as randomkeystrike just said (and I was about to say), the photo that Marty Prime took with him back to 1955, having almost faded, restores itself to exactly what it was once Marty's parents fall in love at the Enachantment Under the Sea dance. Same lower middle class clothes and everything. Unless Marty Prime's affluent family took that photo in an ironic "hey-let's dress like people poorer than us would dress" sense, I don't think it holds true that Marty Prime's original existence no longer exists.

Which, if this holds true, still raises the question of what happened to Marty A.
posted by Effigy2000 at 7:34 PM on July 18, 2011

I had the same question after an evening of watching the trilogy.. herbally enhanced as it were.. used to be something me and friends did with shocking regularity.. We basically agreed on the many worlds theory applying. The other Martys are alive and well in their own pocket universes created by the divergence of the time stream.
posted by mediocre at 7:48 PM on July 18, 2011

Best answer: It all depends on how you think causality interacts with time travel. Does time update itself instantly, does is take time for the future to catch up with the new past (and does it happen everywhere at the same time)?

If you assume that Marty Prime is, by virtue of being a time traveler, in a privileged reference frame it's not a huge stretch that he could carry an artifact from a future that has sense been modified.

Is that destroying a world? Dunno. Did I just destroy the universe where this question only got eight answers?
posted by Kid Charlemagne at 7:50 PM on July 18, 2011 [1 favorite]

Response by poster: The link michaelh provides for BTTF2 goes some way towards explaining what happened to Marty A (and, for that matter, Marty B) when Marty Prime entered his world. Explaining what happened when Marty (Prime) returned to the 1985 ruled by rich Biff, it says;

"The [Marty and Doc] we see are temporal duplicates from an alternate universe sideways in time. But under Doc's theory, there are no alternate universes in sideways time; there is only one existent timeline, and if it changes, all others vanish. So if Doc is correct, then he and Marty--and Jennifer and Einstein--cannot be here; or else, upon their arrival, their divergent selves must cease to exist so that they may continue. Neither of these solutions are satisfying. To avoid the awkwardness of these alternatives, we must adopt the theory of sideways time--popularized most recently in Sliders, and explained somewhat in Multiverser--that alternate universes do exist, created by divergence from a common history. Thus, Marty and Doc may still have (and recall) the histories of their own past, and may be here, while at the same time there is a divergent Marty and a divergent Doc (often misnomered "parallel selves"), the one in Europe and the other institutionalized. But if this is so, then there will also be a divergent Jennifer and Einstein--and even if not, even if time has been so altered that neither of them were ever born (entirely possible, even likely given the disruption in this area), they are leaving their friends in that timeline, and returning to the past to eliminate it. Doc suggests that time will change around them, bringing them back to the original future history. However, were this so, then it would also shift the divergent Doc and Marty to the "original" history, and there would be two of each of them when these two return. No, Jennifer and Einstein will be lost, trapped in a divergent universe without any way of returning home--and Doc and Marty, in a linear history time machine, will have no way of retrieving them. In fact, when under the single real history theory of Doctor Brown history is altered, Jennifer and Einstein will vanish with the alternate universe, victims of a time travel accident."

The more I read, the more I'm certain we need a BTTF4 where Doc tells Marty they need to go back one more time to fix everything they fucked up.
posted by Effigy2000 at 7:59 PM on July 18, 2011 [1 favorite]

Best answer: Oh, shut up about alternate universes and the Many Worlds Theory, already.

If time travel only creates alternate universes, why would anybody travel back in time to change anything, since it wouldn't really affect their life? More importantly, why would anybody else care what they did? Alternate universes kill the drama of time travel.

I travel back in time and kill your parents in an alternate universe. Is there any threat to you there? Nope. Do you have any real motivation to travel back in time and stop me? Nope. Because your universe hasn't changed, and "undoing" what I do is probably just creating a third alternate universe.

Saying time travel creates alternate universes is just incredibly bad writing, because it creates a story that has no fucking point. It's the the ultimate example of people who think they're smart showing off that they're dumb about the rules of good writing. The characters have to have a real stake in the action.

"Back to the Future" is a one-universe story. Changing the past changes the one universe Marty McFly lives in, and that's why the story matters to him. You're ruining it for yourself by trying to superimpose "logical" rules created by dorks who aspire to be nerds. You need to judge "Back to the Future" by its own rules, instead of rules created by mouthbreathing comic book writers who love their "Days of Future Past" characters too much.

And if you're about to say "what about paradoxes that erase the reason for going back in time," I'm going to say "shut the fuck up." It's a time machine. Would Doc Brown build a time machine if he hadn't figured out how to avoid a self-defeating paradox? Give the man his mad science props, already.
posted by faster than a speeding bulette at 8:10 PM on July 18, 2011 [22 favorites]

[Massive, MASSIVE spoilers for "Source Code"]

This is reminding me of this Duncan Jones interview (scroll down to the bottom, continue reading on page 3, sorry there's no single-page version) which shows he's really thought through all the disturbing implications of his time travel movie.
posted by AmbroseChapel at 8:22 PM on July 18, 2011

"Back to the Future" is a one-universe story.

Yeah, that's what I was thinking too.
posted by doctor_negative at 8:59 PM on July 18, 2011 [1 favorite]

I think randomkeystrike is right that it's a one-universe story, though I'm not sure what that means for Marty A.

I love Back to the Future but it really kind of falls apart when you think about it too much. Marty never seemed to belong with his original family anyway: imagine the kind of parenting they would have given him (timid, socially awkward, neurotic) and compare that with the kind of teenager he was (confident, well-adjusted, relaxed). Then at the end, regardless of what happened to Marty A, Marty Prime has now ended up in a completely different present from the one he left, with a past he doesn't know. Aren't people going to start thinking he's kind of weird, maybe even crazy, for referring to all kinds of things that didn't happen and not remembering things he was just part of?

It might just be the kind of story that fridge logic destroys.

As for the multiple worlds theory ... It seems like it would take a lot of energy to create a universe--where does it all come from? Also, what about all the choices animals make? Yes, these sound like stupid questions, but really.... If the multiple worlds theory is correct, then there must be more universes in the multiverse than there are atoms in the universe.
posted by johnofjack at 9:07 PM on July 18, 2011 [1 favorite]

"does Marty Prime kill the Marty McFly of any alternate universe he enters? Is he a destroyer of worlds? Or is there some theory out there (scientific or otherwise) that actually reconciles the introduction of Marty Prime into another Marty’s world?"

1. Physics. Marty Prime's body is physical. So is Marty A's body. If they were to co-exist or meet, nothing in physics suggests that there would be an explosion. At any rate, Marty Prime doesn't breathe fire, and Marty A doesn't have gas for blood.

2. Logic. By contrast, there is the logical problem that one person can't be two people. That is, there's a threat of contradiction: Marty is and isn't just one person. There can't be contradictions, however, so as soon as a contradiction is about to occur, something else happens: either the world vanishes, or a Marty vanishes.

3. Philosophy. The question you need to face is whether, by "Marty", you mean something above and beyond the name "Marty" and the physical people. As far as the name and the physical people are concerned, there's no threat of contradiction. Threat of contradiction would require that you attach a philosophical notion of "Marty" to both physical bodies -- a REAL MARTY SELF -- and equate the real Marty self with each physical body. Needless to say, you don't have to do that. You can treat "Marty" as a mere name and leave it at that. That's what many Buddhists do.
posted by Eiwalker at 9:51 PM on July 18, 2011

We looked at this in my last term's metaphysics class. Theodore Sider discusses time travel paradox, although not in great detail, in both Back to The Future and The Terminator. His conclusion was that BTTF was, essentially, paradoxical and logically impossible, while the Terminator handles time travel in such a way as to avoid this type of logical impossibility. Sorry I could not find a link to the article in question online.
posted by alltomorrowsparties at 10:26 PM on July 18, 2011

I recommend Justin Rye's page on SF chronophysics. To use Rye's terms, WTTF uses an overwriting model but with timeline changes propagating in metatime. From that point of view there is only one timeline at a time (at a metatime?), so Marty is not displacing or killing anyone.
posted by zompist at 10:35 PM on July 18, 2011

(rubs eyes wearily and repeats the MST3K mantra again under his breath)

To me, asking a time travel story to avoid logical and physics problems and still be a time travel story, with the usual elements of going back to fix what coulda/shoulda been, or going forward in time with some idea of doing something and then coming back, etc. - asking that of a time travel story is kind of impossible, due to the Butterfly Effect. So whinging about conundrums authors "missed" is kind of beside the point.

The minute Marty stepped out of the Delorean, he stepped on a bug which might otherwise have stung someone, delaying them as they went home to get the metholaide, causing them to miss out on meeting the person they were going to marry; the air current created by the DeLorean ended up creating a Tsunami which wiped out the growing Japanese economy. And so on. Marty might come back to a United States of Korea where his girlfriend and his Toyota truck never existed, even if he went back right then.

What's funny (indeed, I think it's one of the funny undercurrents in the movie, which I picked up on even as a teenager watching the movie for the first time), is the author acknowledges this conundrum - after all, the main plot of the first movie involves his chance meeting with his mother, and how that threatens to upset the apple cart. And the professor talks about it as well, quite excitedly. Clearly, he doesn't blithely feel, budda-like, that it will all end up with a bunch of parallel universes, everyone where they're meant to be.

And then the following three movies involve, in varying states, Marty's finger in the dike. Marty has to act heroically, again and again, to try to undo a lot of things which presumably would never have been problems in the first place if it hadn't been for that damn time machine.

The conceit of the movie is that he CAN, of course. If it's even possible to talk logically about time travel (and I'm not sure you can)*, Marty's every action would twist up the timeline further and further. The moral, to the extent there is one, is "there's no place like home." Leave well enough alone. Yes, your parents might have been cooler and richer, but on the other hand, you might not even be here.

And THIS, ladies and gentlemen of the jury, is why introducing time travel to the Star Trek franchise in a big, continuous way was a fatal mistake that has caused them to be dead in the water on TV...



*only possible exception being the kind of relativistic travel to the future caused by FTL, where it's a one-way trip taken as a by-product of the FTL and you accept that you'll never go "home" to your time again. The depressing kind of hard sci-fi, IOW.
posted by randomkeystrike at 5:24 AM on July 19, 2011

If you play the Telltale Games episodes for BTTF, they shed light on many of these questions. Not always in a helpful way, though; actually, it kind of makes some of them a lot worse. But I won't spoil.

I'd prefer to understand it as a single-universe story where there is no Marty A; it's all Marty Prime. His time traveling adventures have left him with a different history than the memories in his head, but he's still the only Marty in existence.

That said, it's evident from the expanded universe including the game that I'm wrong.
posted by Andrhia at 6:36 AM on July 19, 2011

Well in BTTF2 the MartyA is supposed to be in a private school in Switzerland or something. Not sure about the ending of BTTF1 though.
posted by Gungho at 7:17 AM on July 19, 2011

Good question. I've been pondering this and here's an answer that makes sense to me...

Yes, Marty-Prime has entered the alternate timeline where Marty-A resides. And yes, there should now be two Martys in this timeline, who could very well meet. But that's not going to happen, and here's why...

Marty-A is very much like Marty-Prime. As far as we can tell, though Marty-A has a better home life, the choices he makes are identical to Marty-Prime's in a macro sense.*

Following that logic, Marty-A's not just twiddling his thumbs in Hill Valley-A, waiting to bump into Marty-Prime. No, he took the same actions that Marty-Prime took in his own timeline: He met up with Doc and Einstein at the mall, faced off with some angry Libyans, hit 88mph and disappeared in the DeLorean. We actually see Marty-Prime arrive in this alternate timeline a few minutes before Marty-A departs, but the two Martys never meet. And they won't, not here anyway.

The question is, where did Marty-A go? If his events played out identically to Marty-Prime's, we know the answer... November 5th, 1955. The same November 5th, 1955 that Marty-Prime traveled to, because both timelines share a common past. If that's where Marty-A went, he would be arriving at the exact same moment, in the exact same physical space, as Marty-Prime.

I think it's safe to say that this didn't happen. That would be the ultimate violation of Doc's "you must never meet yourself" rule. If anything could create a universe-exploding paradox, that would probably do it. The good news is, this Doc—Doc-A, so to speak—has the requisite knowledge to prevent it from happening. He remembers the trip made by Marty-Prime and we know he's applied that knowledge because he's wearing a bulletproof vest now.

Doc-A knows better than to send Marty-A to November 5th, 1955. Seeing as how Doc's the one who inputted that date in the first place, it's not a stretch to think he'd punch in something different this time around. Maybe he sent Marty-A to a different time in the past, or to the future. We don't know; we never see it. But I like to think that Doc's more of a mastermind than he lets on, silently taking precautions behind the scenes that Marty never has any knowledge of (nor has a need to know).

*This "macro sense" idea touches on the butterfly effect point made by randomkeystrike. The suggestion is that some events are more cosmically important than others, and as long as the "big" things remain the same the butterfly effect won't occur. This keeps the world Marty-Prime arrives in very similar to the one he departed from, save a few easily compartmentalized differences. It's preposterous movie logic, but those are the rules by which BTTF plays. The important thing is, both Martys are dating Jennifer, have the same hobbies, hang out with Doc Brown, and so on.
posted by The Winsome Parker Lewis at 9:37 AM on July 19, 2011 [1 favorite]

And as for BTTF2, that alternate timeline is very different from the one at the end of BTTF1. In that one, Doc and Marty never met and Marty never traveled through time. Doc's in an insane asylum and George McFly was murdered by Biff.

However, as Gungho mentioned, the Marty native to this timeline is conveniently off at a boarding school overseas, completely out of the picture. It doesn't matter if he and Marty-Prime are in the same timeline together, if they're on opposite sides of the planet.
posted by The Winsome Parker Lewis at 9:55 AM on July 19, 2011

The question is, where did Marty-A go? If his events played out identically to Marty-Prime's, we know the answer... November 5th, 1955. The same November 5th, 1955 that Marty-Prime traveled to, because both timelines share a common past. If that's where Marty-A went, he would be arriving at the exact same moment, in the exact same physical space, as Marty-Prime.

True, they share a common past, but it only is that specific past because of the way Marty Prime disrupted it (i.e. the whole 1955 plot of Part I). Marty A is a different person, so won't he behave differently?

Now, Marty A might look exactly like Marty Prime and have the same name -- and that's not a profound philosophical paradox, that's just your standard "This is really implausible, but movies always have wildly implausible coincidences. Just accept it." But he'll be subtly different because he has the more affluent, positive upbringing (thanks to the more traditional gender roles his parents were performing when they got together!).

Considering how lucky Marty Prime was to succeed at his plan to get his parents together and get back to 1985, Marty A might fail at getting back to 1985 if he does even some little thing slightly differently. For instance, Marty Prime almost doesn't make it back because his ignition stops working, but he makes it work by slamming his head down on it in frustration. Maybe Marty A has a more refined upbringing and doesn't do that kind of thing, which would mean his car would never start and he wouldn't make it to the lightning wire in time to get back.

But again, this is a movie! So many events in the 1955 portion of the first movie itself are wildly implausible. What are the chances that he would have brought a piece of paper stating the exact time, down to the minute, when lightning struck the clock tower, which happens to be right after his parents are supposed to kiss for the first time? But even if you accept that coincidence, it still makes no sense that Doc could have calculated the time the lightning would strike down to the split-second, with Marty timing his driving perfectly just based on an analogue clock! (I sure hope Marty A has the same flyer in his pocket when we see him time-travel in the end of Part I!)

In short, just by being in this movie world, we take a lot on faith. Things work out for the characters in the end. So the best assumption is that Marty A does go back to 1955, and he has to redo his whole plan since he's a different person and this is an alternate timeline (not the same one that led to the well-adjusted 1985 we see at the end of Part I -- again, it's different because Marty A is running around doing stuff, which will necessarily change that timeline). And he has to succeed, because Marty always ends up succeeding! So Marty A gets back to a happy 1985, just as Marty Prime did. Based on the exact same reasoning as the actual movie, Marty A should then see another Marty -- call him Marty B (the one he caused to be born by succeeding in getting his parents together) -- who will be driving away from the terrorists and about to travel back to 1955, etc. The whole adventure gets run ad infinitum. That's my theory, anyway. (Of course, it's theoretically possibly that one of the alternate Martys simply fails and gets stuck in 1955, but you're not supposed to think about this.)
posted by John Cohen at 10:31 AM on July 19, 2011

Best answer: True, they share a common past, but it only is that specific past because of the way Marty Prime disrupted it (i.e. the whole 1955 plot of Part I). Marty A is a different person, so won't he behave differently?

I think you're assuming that Marty-A always existed in a timeline parallel to Marty-Prime's, and that Marty-Prime simply jumped from one existing timeline into another. Like this:

                    DEPART
TIMELINE-PRIME: |-----•---------------|

                              ARRIVE
TIMELINE-A:     |---------------•-----|

But that doesn't agree with the diagram Doc sketched on the chalkboard in the second movie. What he described looked like this:

                         DEPART
TIMELINE:       |----------•
                   TIMELINE \
                    ALTERED! \   ARRIVE
                              \----•---|

There's only one timeline. This is what faster than a speeding bulette was talking about when he criticized the multiple universes and many worlds interpretations. In the BTTF canon, timelines that have been prevented from happening do not persist in an alternate reality. They just vanish. And whatever has taken place in the shifted timeline is the new real, for better or worse.

In other words, the alternate timeline did not exist before Marty-Prime changed the past. And Marty-Prime's meddling in 1955 actually CREATED the alternate timeline and, consequently, also created Marty-A. Conversely, the prime timeline ceased to exist as well (Marty-Prime and the DeLorean-Prime were the only things from that timeline that persisted after the change).

The takeaway here is that if Marty-A goes back to 1955, it would be the same 1955 that Marty-Prime went to. There aren't multiple pasts, because there's only one timeline along which everything moves (fluxing DeLoreans notwithstanding). The future end of the timeline has shifted, but the past end of it hasn't.

When you watch the scene near the end of the first movie, where Marty-Prime observes "himself" escaping the Libyans in the mall parking lot, it seems to be the same event seen earlier in the film. But you and I are speculating that the Marty he was watching was actually Marty-A, because he was actually in a different 1985 than the one he originated from (which no longer exists).

If so, it's critical that the time circuits in that DeLorean (Marty-A's) were NOT set to the same destination as Marty-Prime's when he first made the trip. Otherwise, Marty-Prime and Marty-A would materialize inside each other in the Twin Pines Ranch in 1955. At the very least, he could have traveled to the same destination a few minutes later. But remember that it wasn't Marty who programmed the destination... it was Doc Brown. If Doc had the foresight to change the minute of Marty-A's arrival in 1955, he probably would've picked a different year entirely, just to minimize the chances of the two Martys encountering each other in the past.
posted by The Winsome Parker Lewis at 1:15 PM on July 19, 2011

The problem with this question - and it is a good question - is that it supposes the existence of a fictional universe with many, many more rules than the one in BTTF actually has. It's a lot like wondering how Godzilla can move around on land or even breathe - there are probably fan theories as to how, and some of them may be quite convincing, but there is no actual answer as it is never addressed in the movie and there are no indicators of a consistent set of rules in that universe, because the people telling the story never thought it out to enough of a degree to satisfy your question.

As a result, there aren't going to be any theories which completely sort the thing out, since certain parts of the workings of the fictional universe contradict one another when the story calls for it.
posted by FAMOUS MONSTER at 1:20 PM on July 19, 2011 [1 favorite]

Ditto what FAMOUS MONSTER said. As fun as it is to push BTTF logic to its limit, it is internally inconsistent. One thing that really bugs me is when Doc and Marty leave Jennifer on her front porch in "Hell Valley," then go back to 1955 to fix things. When they return in the third movie, the universe has changed around her and she's right where they left her—only in the new timeline. And she still remembers her time traveling adventure.

To make things right, she either should have vanished completely and been replaced by the "Jennifer-A" native to the new timeline, who has no idea what Marty and Doc have been up to... or she should have rippled into the new timeline and encountered her doppelganger there.

If there's an alternate Marty and an alternate Doc, there should've been an alternate Jennifer (and Einstein) as well. The topic is never broached.
posted by The Winsome Parker Lewis at 1:36 PM on July 19, 2011 [1 favorite]

I believe the short answer is, the Back to the Future model of time travel doesn't work.

Here's a nice accessible article in Slate that touches on these issues - Dave Goldberg's article "A Physicist Looks at the Time Traveler's Wife". It gives the main principles for how time travel should work. The key issue you are interested in is his first point:

He says we should reject time travel that requires the many-worlds interpretation of quantum mechanics.

The many-worlds interpretation provides a fertile basis for time fiction, via the ubiquitous Back to the Future model of alternate histories: Someone visits the past, teaches his father about believing in himself (or some similar nonsense), and thereby leaps into a parallel universe with a different (ideally, better) future than the first.

For a physicist, though, there's no reason to believe that a Back to the Future-style time machine is possible. For one, there's no evidence that Everett's parallel universes exist. (There's no direct evidence against them, either.) More importantly, Einstein's theory of general relativity —the branch of physics that might make time travel possible—doesn't take kindly to the idea. Every solution to Einstein's equations involves just a single universe. Maybe I'm being overly dogmatic, but I don't think it's unfair to insist that movies stay within the realm of what we currently know about how physics works.

In a rule-abiding time-travel narrative, there are no parallel universes—just a single timeline.

So, for example (SPOILER FOR HARRY POTTER IN THIS PARAGRAPH ONLY), in the Prisoner of Azkaban, Harry Potter is able to travel in time. He is rescued at one point by a mysterious figure, and later he travels into the recent past and has to rescue himself. That's a single timeline, because the mysterious rescuer (who turns out to be future-Harry) was there the first time through.


Here are a whole raft of resources for further reading if you are interested in really getting into the paradoxes of time travel. Warning, some of these get technical.

Internet Encyclopedia of Philosophy entry on Time Travel

Stanford Encyclopedia of Philosophy entry on Time
Stanford Encyclopedia of Philosophy entry on Time Machines - discusses the main paradoxes of time travel, somewhat technical; good bibliography
Stanford Encyclopedia of Philosophy entry on Time Travel and Modern Physics - quite technical

Probably the best place to start if you wanted to read actual philosophy articles about this would be with David Lewis's paper "The Paradoxes of Time Travel", 1976 (American Philosophical Quarterly 13:145-52). That paper is also in his collected papers, Philosophical Papers Vol. II. Here is a nice handout from Professor JeeLoo Liu about David Lewis's "Paradoxes of Time Travel" (pdf).

PhilPapers.org listing of philosophical papers on time travel (some but not all of these are online in freely accessible versions. you can sometimes find additional ones that are posted at the authors' websites, if you search the web for a direct link to the author's university webpage, as I've done with the ted Sider papers below:)

Ted Sider's paper "A New Grandfather Paradox?"
Ted Sider's 2002 paper "Time Travel, Coincidences, and Counterfactuals" (pdf)
Ted Sider's 2005 paper "Travel in A-Time and B-Time" (pdf)
Sider has a paper on David Lewis's article about time travel, but I couldn't find a full text version online. If you have university access you can probably get the full journal article via the link in PhilPapers above.
posted by LobsterMitten at 1:57 PM on July 19, 2011 [2 favorites]

Response by poster: I think I have it. And it's a team effort!

The way I see it now presumes that faster than a speeding bulett is right; there's no parralel universes in BTTF. Given this critical assumption, here is what I think makes the most sense (in a fictional universe that makes little sense).

Marty Prime visits the Twin Pines Mall and goes back to 1955. He has his adventures there and then, having altered the future, returns to an altered 1985 minutes before he left it. At the Lone Pine Mall, Marty Prime witnesses a one-week-younger Marty Prime (who has grown up in the altered future he, unknowingly, created) leave to go back to 1955 to create the future Marty Prime has returned, and will return, to. See The Winsome Parker Lewis' comment for a good explanatory diagram.

This means that there is no Marty-A to worry about or even a seperate Marty-Prime to define. There is only ever a Marty who was first born into a world where his parents were lower-middle class yobs and whose existence changed around him after he unknowingly created it himself.

The only thing that dosent make sense is why Jennifer dosent realise that things have changed around her. However, as Kid Charlemagne has said, you could argue that given Marty is a time traveller, it gives him a privledged reference point. Indeed, once Jennifer becomes a time traveller herself, she becomes aware of changes that occur during BTTF2 and remembers those, so that sorta fits.

I think that solves the question. Unless someone sees a hole in that explanation I'm not currently seeing?
posted by Effigy2000 at 3:29 PM on July 19, 2011

At the Lone Pine Mall, Marty Prime witnesses a one-week-younger Marty Prime (who has grown up in the altered future he, unknowingly, created....

But if that Marty Prime grew up in the new, better future, why would he go back? There'd be nothing for him to fix.
posted by Dormant Gorilla at 8:17 PM on July 19, 2011

Dormant Gorilla, I suppose he'd got back for the same reason Marty-Prime went back: Not to fix things, but to escape the Libyan terrorists shooting RPGs at him. The whole going-to-1955 thing was just an accidental side effect of using a time machine as a getaway vehicle. Marty did end up fixing stuff for his parents, but only because—whoops—he stumbled into a time he didn't belong in and had to clean up his mess. It's not like he set out to do that when he met Doc at the mall.
posted by The Winsome Parker Lewis at 9:25 PM on July 19, 2011

You're right, I completely forgot about that.
posted by Dormant Gorilla at 10:09 AM on July 20, 2011

I think you're assuming that Marty-A always existed in a timeline parallel to Marty-Prime's, and that Marty-Prime simply jumped from one existing timeline into another.

Yes, I was wrong. I agree with you about the problem of Marty A going back to the same place/time as Marty Prime, though I'm not sure I made the mistake you're imputing to me. I just hadn't thought about the problem of Marty A encountering Marty Prime clearly enough. Part II makes it clear that once a new timeline is created, that is the whole new reality, and other timelines no longer exist. (It feels weird to talk about this using normal verb tenses.) That's why we see Marty-the-protagonist-of-Part-II (wearing a black leather jacket, and hoping to steal the book from Biff) walking by Marty-the-protagonist-of-Part-I (wearing orange and denim, playing Johnny B. Goode, etc.). Marty from Part I has to be still there in 1955 even "after" Marty goes back in Part II. By the same logic, Marty A will be in the same place, or very close to, Marty Prime when he goes back.

I also find it implausible that the new Doc deliberately sets the time machine so Marty A goes back to a different year, since this would trap Marty in the wrong time without his family or girlfriend for the rest of his life. While you might say Doc would prefer this to disrupting the space-time continuum, the characters are always willing to dispense with this concern when it suits their personal interests.
posted by John Cohen at 3:30 PM on July 20, 2011