I was originally made uneasy about the franchise's take on time travel the first time I saw Part I in the theaters, and let that date me. I won't belabor the wealth of books, television, and films I've seen dealing with time travel, let alone more serious takes on it in science magazines and nonfiction books, beyond saying that I've seen far more than I can remember and picked up quite a few prejudices along the way. First, although some of my favorite stories about time travel have been done in Star Trek, even Harlan Ellison didn't get the bare physics of it good enough for me. Second, although I can allow for different versions of it to be possible in scifi 'theory', mixing types of time travel in one fictional universe amounts to using more than one version of physical laws at the same time, which is sloppy storytelling. Third, and most pertinent, the deus ex machina is fine for scifi stories, when taken in small doses that don't undermine that fictional universe's working rules, but when it's employed to make ad hoc fixes to dangling plotlines...I get stabby.
So, what do I have to bitch about in a twenty-five year old trilogy that I admittedly love and have watched no less than ten times? Basically, it's the 'ripple effect' that the Bobs use to explain how they can have so much going on that changes the timeline(s), yet have everything magically work out for our hero in the end. Note my ambiguity surrounding the word "timeline". Despite so much that has been written about the trilogy by well-intentioned fans and the chalked explanation that Doc. Brown gives Marty in Pt. I, there is really only ever one timeline in the films. The Bobs have implied as much in interviews, although they didn't have to. The whole ripple effect is based on this idea. I'm not talking about the particular timeline experienced by Marty as he bounces back and forth, although there is an argument for that (and not about the timeline experienced by the DeLorean either, even though that's an interesting tangent all its own). No, I mean that, in general, there is only one linear chronology from 1885 onward; the timeline shifts and fades in mysterious ways (to overly dramatic effect, thanks to the Old Biff), but nowhere is a true 'alternate' one ever created. If that were so, the ripple effect would not exist, as every newly created timeline would just carry on without regard for apparent danger of paradox. In short, for those of you who are still awake and reading, yet aren't following me well yet, the ripple effect is what happens in the trilogy when a profound change is made in the timeline that affects Marty's 'future', usually his future of 1985. For instance, when he's hit by his grandfather's car rather than his dad, and subsequently takes his dad's place as his mother's love interest, this endangers the possibility that he will ever be born. The trilogy deals with this in comical fashion by having him begin to fade away. As Doc puts it, he comes close to being "erased from existence." This is the most acute example of the ripple effect, and I'll try to show how this is logically unfounded in the plot, even given the trilogy's loose way with time-travel physics.
I mentioned alternate timelines and paradoxes, and both should be given their due for clarity. Touting quantum physics, many writers and filmmakers have opted to use the 'many worlds' idea in time travel stories. This means that every action (or decision, they're really interchangeable in effect) of objects and people splits the universe into multiples, providing a practically infinite number of simultaneously existing universes that characters can live in, some more different than others depending on the particular actions, and all diverging in appearance from their point of divergence from the 'original' (which itself may or may not exist given an equally infinite number of possible histories leading to convergent actions). In this kind of 'multiverse', anything can happen and, given enough time, theoretically every logically possible thing does happen. Back to the Future does not take place in this kind of multiverse, however. Because of the ripple effect, in BttF even big changes kind of flow down the (altered) timeline to gradually replace the 'old' future with the 'new' one. In this kind of universe, there is the danger of altering the past in a way that the ripple effect replaces the very actions that a character takes to make the change that starts the 'ripple'. In Marty's case, if he stops his own birth, he could not have gone back in time to stop his birth. This is the epitome of a Time Travel Paradox, the trope upon which countless stories have been written. In many stories, and in several places in BttF, other seemingly contradictory events are called paradoxes, but this is the true kind, a logical impossibility only shown when there is but a single timeline to be changed. In the many-worlds view, whenever a character goes back in time, an entirely new timeline is created, completely separate from the first, where he could kill himself, his father, or everyone on earth without destroying himself because he was born in a different timeline, one which he no longer affects the outcome of. There would be no need of the BttF ripple effect because both timelines could unfold with no paradox. The character does not really kill himself, he kills a different 'him'. Again, this is not the universe of BttF, so paradoxes have to be avoided.
Or do they? By their very nature, such existential time paradoxes are impossible, so they cannot even be stumbled into, much less planned upon. Since Marty does go back in time and change the timeline, there must be some way for the paradox to be resolved, and so it is in the trilogy. But what of the ripple effect? We are given this as an explanation for how a character's past can be changed while only reshaping the singular timeline rather than branching off a new one. When Doc tried to explain branching timelines to Marty, he got it wrong for his universe. Marty never stopped his parents from meeting, kissing, or getting together to make him, he only changed the way that they did it. In fact, given that his changes did not finally erase him or his brother and sister, he was never really in danger of being erased. Why did he begin to fade, then, and why did his siblings disappear from his photo? Was the danger of a paradox enough to do it, since no paradox ever occurred? This, in the end, is the fatal flaw in the Bobs' version of time travel. We see Marty's fading as an incentive to get him (and his dad) working to set the timeline 'right', but there's no logical reason for it. What is finally special about his dad needing to meet his mother after he's hit by the car? Nothing, since he meets her anyway. What is finally special about their first kiss or his hitting Biff? Again, nothing, since Marty's future is secured in the end regardless of all the fumbling and drama. There is, after all, nothing in the entire trilogy that stops Marty from being born, so there is no reason for him to fade. More tellingly, if he did somehow stop himself from being born, by killing his dad for instance, why would he fade away gradually rather than, well...nothing really works in that case but the many-worlds view where truly separate timelines can be created where he simply wouldn't fade.
Let's go a bit further. Later, in Pt. II, all hell breaks loose in the same week during 1955, but we don't have to dwell on that. The important part here is when Doc and Marty leave Jennifer (and Einstein) in the 'bad' version of 1985. Much has been written about this, much hair-pulling on two sides of the issue. We're told that, once the timeline is set right, the world will change around Jennifer, presumably leaving her in a better 1985 and none the wiser. It's implied, if not said, that she will not be erased because she has not been mixed up in a personal paradox like Marty. But what of the other Jennifer? At the end of Pt. I, we're shown Marty watching as his duplicate races off and activates the time machine in the 'Lone Pine Mall'. It has been argued that, regardless of the changes in the timeline, these two Marty's are really one and the same, but this can't be true as long as the 'original' sees the differences that have occurred. As long as Marty's memories of 'Twin Pines Mall' world and all the differences between it and the new version do not fade, he never becomes the same person as the Marty who was born in a cool family and owns a truck. In every way that makes logical sense given what we're shown, there are two different Marty's. And if more than one Marty exists who has a different history, including the one in the 'bad' version of 1985, then a different Jennifer also exists there when the 'original' is left to sleep. What happens to this second Jennifer? If she fades away, why? It's the 'original' who is out of place. Should not the other one change along with the rest of the world and the original fade away? Either one of those must happen, or both must continue to exist until the end of the trilogy. My point is that this fudging of the rules we're given for the BttF universe, seemingly always benefiting the original versions of the characters, goes against the very idea that the whole timeline changes to suit changes made in it, since Marty (and Jennifer) never changes. But if he never changes, then his various duplicates don't either, and we have a de facto many-worlds scenario despite every effort to show that it's not.He can go on while his first duplicate from Pt. I disappears because he replaces that duplicate in the timeline, but Jennifer in no way replaces her duplicate in the 'bad' 1985 (that we're shown). So, while one Marty can finish the story, we are still left with the problem of two Jennifer's battling it out existentially.
Finally, the Bobs have said in interviews that the Old Biff fades offscreen after returning to 2015 because his duplicate was killed in the 'bad' version (I'm told, by a vengeful Lorraine in 1990 when she learns that he killed George). If so, why doesn't Doc also fade, since the 'original' Doc was killed by the Libyans? This is reaching, since we don't actually see the Old Biff fade. It brings up the question of the ripple effect again, however. None of the characters' memories change, no matter how their reality shifts around them. Unless they are given supernatural precedence by special pleading or by some esoteric 'force' like that which allows Kirk et al to stay aloof from the profound changes in "The City on the Edge of Forever", their memory retention belies the ripple effect. The inconsistencies in this wishy-washy version of time travel are subtle and easily waved away by the Bobs as a mystery akin to quantum mechanics, but I find them a pretty damning set of flaws in what otherwise is a very ingenious trilogy.
Here's a last bit of uber-geek nonsense to wrap your mind around: since the duplicate Marty disappears at the mall in the end of Pt. I and is never seen again, does he fade? If not, then there is also at least one duplicate DeLorean. ;)