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In Back to the Future, to get Marty back to 1985, it looks to me like the Doc connects a cable to the pole with the weather vane on top of the clock tower and fastens the other end to a light pole on the other side of the road. They need the power of the lightning to transport Marty to the future.

They know what time the lightning strikes, because the clock is stopped by the lightning and Marty has the pamphlet from the future with the time on.

Marty drives the DeLorean, the lightning strikes and goes along the cable. All goes according to plan and Marty is transported to the future.

Two questions:

  1. If the lightning strikes the pole and travels along the cable to the ground, would not the clock be bypassed and therefore not be stopped?
  2. If so, would this not constitute a time paradox?
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    Unless I am miss remembering, the clock was working when Marty returned to the future/present. Had he rechecked the paper received from the fund raiser at the start of the film it likely would have been blank or for something other than the clock.
    – Xantec
    Dec 6, 2012 at 20:01
  • Yes, I remember that now. It was working. Would it not have been a problem, though, if the clock was never stopped in the first place?
    – AJO_
    Dec 6, 2012 at 20:04
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    There's not really a way to know if it was a problem for someone else. We just know it wasn't a problem for Marty, or for Hill Valley at large, as the town was still standing. But say, for example, if someone met the love of their life at a meeting of the concerned citizens group that was trying to save the clock tower at the beginning of the movie (the lady who gave them the flyer, and how he knew when the clock stopped in the first place), it might have been a problem for them.
    – cashnwho
    Dec 6, 2012 at 20:11
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    @cashenhu Not that those two unfulfilled lovers would ever know.
    – Xantec
    Dec 6, 2012 at 20:13
  • The same goes for Marty's parents and for Biff, not to mention all the other changes Marty makes to the timeline - like performing Johnny B. Goode. Neither his parents or Biff would know about other possible lives.
    – AJO_
    Dec 6, 2012 at 20:20

3 Answers 3

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Doc Brown indirectly explained this in Back to the Future II, when he was explaining to Marty what Biff had done when he brought the Sports Almanac back to his younger self.

When the clock wasn't damaged by the lightning When Marty began to alter the past a new timeline was created wherein the clock continued to work all his changes took place. This was the timeline that Marty returned to when he went back to the future/present. He never returned to his actual original timeline, as seen at the start of the Back to the Future.

Doc Brown explains time

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  • Thanks. Good picture. On its own, the picture is almost enough to explain it. :)
    – AJO_
    Dec 6, 2012 at 20:29
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    So if the clock isn't broken in the new timeline how did that Marty know when the lightning was going to strike?
    – Michael Brown
    Dec 7, 2012 at 14:25
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    @MikeBrown I suppose he didn't. The new timeline's original Marty likely remained stuck in 1955. But it's OK, because he was replaced by Marty from the original time line.
    – Xantec
    Dec 7, 2012 at 14:50
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    The clock was broken in the alternate timeline as well. Hoylen's answer is the correct one.
    – user11651
    Jan 1, 2013 at 9:38
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    Although the clock wasn't working when Marty returned, as pointed out by @hoylen, the answer itself is still correct. An alternate timeline was created. I've updated my answer accordingly.
    – Xantec
    Jan 1, 2013 at 14:04
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Alternate timelines is the key to understanding Back to the Future -- Marty returns to an alternate 1985; not the same 1985 he left.

But the clock was definitely stopped in the alternate timeline too. When Marty returns back to the 1985, the clock was showing 10:04 (as the helicopter flies over it) even though it was 1:24am (the time the terrorists drove by on their way to the mall).

There is no paradox here, because the clock was broken in both timelines. I'm not sure why people think it was not broken. It was broken in the original timeline (the beginning of the movie) and in the alternate timeline (at the end of the movie).

The clock was not working in the alternate timeline, because Doc wrapped the cable around the hands of the clock (so he could slide down on it). So the lightning struck the clock tower and went into the clock (via the cable wrapped around the hands of the clock), as well as going down the cable to power the DeLorian.

I think we should assume the original building did not have a lightning rod, or it did not do its job properly. Otherwise, the clock would not have been broken in the original timeline at the beginning of the movie. Doc might have installed a lightning rod there to ensure the lightning will go into the cable.

What does change is the ledge in front of the clock. The ledge broke when Doc climbed across it to reattach the cable. In the original 1985, it is not broken because Doc was not up there. In the alternate 1985 it is broken (and also in 2015 which is a continuation of that alternative timeline).

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  • Yes! He did wrap the cable around the hands of the clock and the lightning could have stopped the clock and therefore there would be no time travel paradox. I'm not sure about how it works with lightning, though. The clock was broken in the alternate 1985! The ledge being broken, is to me similar to Marty destroying one of the pair of trees and in the alternate 1985, the shopping mall has a different name. He destroyed the tree and the Doc broke the ledge. When Marty arrived in 1955, the timeline split.
    – AJO_
    Dec 7, 2012 at 14:18
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The working or not working of the clock doesn't wouldn't affect the presence of the lightning. The paper with the exact time and date of the clock's stoppage only served to tell Marty and Doc the exact moment lightning hit the building. The clock's working condition wouldn't create a paradox since Marty and Doc already knew the time/date of the lightning despite the change in that timeline.

When Bif changed the timeline Marty and Doc knew, they still remembered being "out of time" with the current timeline. Therefore they would remember the time/date of the lightning regardless of the affect that lightning had on the clock.

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  • I like this answer. One could ask the same question about Back To The Future 2, but on this one I'm prepared to accept the rules of the world they created: the timeline can split.
    – AJO_
    Dec 6, 2012 at 20:27
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    The only plot hole in any of this to me is they couldn't know the exact moment the lightening struck, on the minute. An instance in a 60-second span is quite a bit of wiggle room.
    – Michael Itzoe
    Feb 18, 2013 at 19:05
  • @MichaelItzoe While it's true that the face of the clock would have been the same for 60 seconds, from the internals of the clock would have been possible to determine the exact second the strike occurred; there had been 30 years for anybody who could access the internals to find this out, hence the "at exactly" clause in the official line. I find it harder to believe it took the lightning as long as it did to propogate down the cable. The car is clearly not in contact with the cable until a substantial number of milliseconds after the lightning strike should have grounded itself out.
    – user11521
    Jul 29, 2015 at 3:02
  • @Michael Good point and I don't disagree (about the exact moment of strike), but if there was 1985 knowledge of the exact moment of the strike, wouldn't Marty have to have taken that to 1955? To our knowledge, all he took was the pamphlet that said 10:04.
    – Michael Itzoe
    Jul 29, 2015 at 14:01

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