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In Back to the Future: Part II, Doc Brown discovers that the McFly family is destroyed in the future, and it can all be traced back to Marty, Jr. agreeing to some proposition by Griff and his gang. So, Doc goes back to 1985 and recruits Marty to help him prevent that event from happening, which they succeed in doing.

The problem is, if they prevent the incident from occurring, how does Doc Brown know about it in the first place? The ripple effect would inevitably catch up to Doc Brown and he would have found that all is well with the future McFlys and presumably would not have investigated further. Paradox!

  • “The ripple effect would inevitably catch up to Doc Brown” — [citation needed]. – Paul D. Waite Mar 14 '16 at 15:11
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In the Back-to-the-Future-Verse, time travelers seem to be largely insulated from the results of their own actions. Marty, for example still has detailed memories about his family and their lives after he alters the timeline at the end of BTTF1.

It stands to reason that anything short of an existential paradox (where the traveler causes him or herself to cease to exist) wouldn't create a direct paradox. The Doc would still remember why he'd interfered in the lives of Marty's kids even if his actions had removed the original cause of his meddling.


The BTTF2 Novelisation glosses over this inconsistency with a masterful hand-wave. Marty finds the whole thing confusing and so should we. Try not to think about it too hard.

Doc pulled out his binocular card to get a better look at the courthouse, and what looked to Marty like a robot, with a USA Today logo on its back, taking a picture of the wreckage. Marty realised that very photo must be the one that appeared in the new version of tomorrow’s newspaper - the version they had right in front of them. But that was weird. How could something change when it hadn’t happened yet? Marty decided he still didn’t understand this time travel business at all!

Doc tucked the binocular card back in his pocket and grinned broadly.

‘Proof beyond positive that we’ve succeeded!’ he cheered. ‘Because this hoverboard incident has now occurred, Griff now goes to jail. Therefore, your son won’t go with him tonight, and that robbery will never take place! Thus, due to the ripple effect, the newspaper is now altered!’

‘The ripple effect?’ Marty asked.

Doc nodded. ‘Just as the past affects the future, the future reverberates into the past.’

Whoa. This was heavy. But Marty remembered something like this happening once before, when he had first messed things up in 1955.

‘Kind of like that picture of me and Dave and Linda,’ he asked, ‘where my brother and sister started to disappear?’

‘Precisely!’ Doc patted his young cohort enthusiastically on the shoulder. ‘Marty, we’ve succeeded! Not exactly as I planned, but no matter. Mission accomplished!' He took a step toward the alley. ‘Let’s get Jennifer and go home.’

The Official BTTF FAQ also offers the following (less helpful) info. It's certainly possible that the Doc always meddled in that event, it's just that he'd forgotten about it

There's a theory (we like to call it the "Self-Preservation Instinct of the Space-Time Continuum Theory") that says that the continuum is always trying to keep itself "on course," and when things happen to change it, it always tries to correct itself. It is much like a river, which tries to keep its overall course. Although earthquakes, fallen trees, floods, or other circumstances might disrupt it at points, the river would cut a new channel so that it would end up back at the same place. Thus, the overall physics (or metaphysics) of the space-time continuum would insure that any of Doc's memories of events that might create paradoxes would become hazy — or be erased.

  • It's not altogether clear what Marty remembers of his life before the first film by the time we get to Parts II and III. It could be that the ripple effect catches up to him as well. Indeed, it's theorized that the problem Marty has with being called "chicken" is a new trait caused by the ripple effect, since there was no sign of it in the first film. – John Sensebe Mar 14 '16 at 16:18
  • @JohnSensebe - You'll be pleased to know that the BTTF2 novelisation makes it abundantly clear that his memory of his original family is completely intact. - "Marty McFly still couldn’t believe how much had changed. Before he had gone into the past, his father had been - Marty had to face it - a wimp"" ... etc – Valorum Mar 14 '16 at 16:53
  • that just makes my head hurt. – John Sensebe Mar 14 '16 at 17:01
  • @JohnSensebe Things would have become very complicated if the Marty in the alternate timeline did not go back the night that Doc. Brown was attacked. There is a kind of predestination paradox surrounding the events in the series. backtothefuture.wikia.com/wiki/Predestination_paradox – Jason Hutchinson Mar 14 '16 at 17:34

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