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We all know that in Back to the Future, the clock in the Hill Valley clock tower was broken when lightning struck it on Saturday, November 12, 1955, at 10:04 P.M. It was this event that was key for Doc Brown to concoct his plan to send Marty McFly back... TO THE FUTURE!

Anyway, Doc's plan is to channel the lightning into the Flux Capacitor, which should supply the 1.21 gigawatts of power needed to activate it and send Marty and the time vehicle back... TO THE FUT--ahem. The problem is, with the power being drawn away from the tower and into the Flux Capacitor, why did the clock still break?

The most reasonable explanation I can come up with is that Doc, knowing the non-breaking of the clock could create a paradox, set up some means of purposefully breaking the clock and making it look like the lightning did it. But is there a "canon" answer?

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    Jigowatts. It's Jigowatts, an intentionally not-real order of magnitude. Gigawatts is not really a rediculous amount of energy. – user1129682 May 30 '16 at 9:29
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    @user1129682 - It was supposed to be a Gigawatt, see this article--Gale and Zemeckis wrote it in the script as "Jigowatt" because they heard someone pronounce "Gigawatt" that way, they either didn't look it up or maybe they wanted to ensure the actors pronounced it with a soft G rather than a hard G. I think it wasn't the energy alone that made it seem "ridiculous" to Doc, but the fact that it had to be delivered in one fast burst to the flux capacitor. – Hypnosifl May 30 '16 at 15:36
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    Also, 1.21 Gigawatts is not too far from the energy delivered by a real bolt of lightning, the calculations here indicate it would be somewhere around 10 billion watts, or 10 Gigawatts. – Hypnosifl May 30 '16 at 15:46
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    Watts are not a unit of energy, but power, ie rate of energy transfer. A faster burst at 1.21 GW doesn't imply faster energy transfer, it just implies less energy. – stewbasic May 31 '16 at 6:00
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    I agree that it would have been in Doc's best interest to rig the cable in such a way that it would direct the lightning strike into the clock as to avoid a paradox. Doc was completely aware of the consequences of altering the future, and would do what he could to prevent that from happening. He made his point clear when he tore up Marty's letter which warned him about the night he was sent back, Doc later read it, but he was initially very cautious. There is no solid evidence that he planned to destroy the clock, but it is heavily implied. – Jason Hutchinson Aug 16 '16 at 13:33
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The power flows through the cable, that's correct. However, the wrong assumption is that the clock is not part of the circuit, because it actually is!

If you stop the video linked by Paulie at the right moment, you'll see the lightning striking the highest point of the building (which actually makes sense):

Lightning strikes the "clock"

As you see, the lightning strikes that small antenna or flag pole on top, not the pointers of the clock. The electricity then continues through the clock, over the hands and then the cable.

As such the whole energy is not diverted from the clock, but rather pulled through it directly.

If Doc wanted to protect the clock, he'd most likely been able to attach the cable to the flag pole/antenna instead, but it's also likely that this might have screwed the whole circuit, since we don't know the actual resistance of the clock compared to the cable and flux capacitor. Worst case it could have been that the clock provides a lower resistance towards ground, which would have caused most (if not all) of the electricity not going through the cable and time machine.

So the actual (simplified) circuit looks most likely like this:

Circuit schematic

Also note that despite Doc holding the cable, he's essentially short-circuited out (meaning there's essentially almost no potential difference between his hand and therefore next to no energy flowing through him), which allows him to survive this (at least on a Hollywood level of physics being applied, considering the halo around the cable, etc.). He's like a bird sitting on a power line.


Edit: After watching the scene over and over again (after reading Valorum's answer), I noticed that you can actually see that the top end of that cable is connected to another cable, which goes straight up to the antenna.

Clock close-up

So it seems like Doc intentionally used the hands of the clock as some kind of cable relief (otherwise he would have unplugged it when he attempted to slide down).

Let's have a look at the updated circuit:

Updated circuit

As you can see, this changes nothing for Doc. However, it changes a lot for the clock! Due to the clock now being parallel to the rest of the circuit, there'll be a most likely significant higher voltage and current involved now. Before the Flux Capacitor could act as some kind of current limiter, which is no longer possible. So based on the resistance within the time machine this could actually mean that most - and not just a bit - of the lightning's power interacts with the clock (read: destroys it).

  • Birds sitting on powerlines are fine, but what if the bird sits on a powerline through which a lightning strikes? From what I've seen on youtube, when lightning strikes wires, they usually evaporate, leaving a very bright trace where the wire was. So I imagine the parts of the clock and wire would get very hot. Wouldn't that burn Doc badly where he touches the clock or the cable? – user1306322 May 30 '16 at 11:47
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    @user1306322 Yes, most likely, but at that point you should also question the whole "lightning" around the cable (that's depicted), which should probably also ignite his hair and clothes. As far as I know it's more comic book physics than real physics what's shown in the movie. What we perceive as lightning is actually ionized gas (working as a temporary conductor). Considering there's an actual (good) conductor - such as the cable - you wouldn't see any lightning "walking" along, because the cable's resistance is far lower compared to the air outside (and Doc as well). – Mario May 30 '16 at 12:53
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    You probably need to work out how much of the energy is being dumped into the wormhole generator and out of our universe. – Valorum May 30 '16 at 13:36
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    @Valorum Actually makes me wonder whether the Flux Capacitor actually works like a Capacitor. Uncharged it would be a short-circuit, protecting the clock, but once fully charged it would "lock" and still fry the clock… hm… interesting how many thoughts you can put into that one little scene. :D – Mario May 30 '16 at 13:51
  • I have to give it to this one, because of the circuit diagrams. (Nice!) I also like the idea that the clock tower was only broken this time because of Doc's improvised plan to get down to ground level quickly to complete the circuit. It's like the universe knew what it was doing when lightning struck that tree. – John Sensebe May 30 '16 at 21:59
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The 1.21 gigawatts of power isn't "drawn away from the tower" at all...it's passed down an a hefty cable that's wrapped around the hands of the clock.

enter image description here

So when the power passes through the clock...it breaks as would most things hit by lightning.

The whole scene can be seen below:

  • If the lightning had not gone directly into the clock, simply being that close to the heat and force of a strike might have damaged it enough to stop working. – The Middler May 30 '16 at 4:53
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The original plan was to use the lightning conductor to draw all of the power of the strike away from the clock and channel it down a cable running into the DeLorean. This would, as you've noted, result in the clock not being damaged by the lightning.

If you go to 0:44 in the clip below, you'll see that the lightning still hits the (now disconnected) lightning rod. With nowhere to go, the electricity arcs over from the rod and hits the metal cable, around four feet below the level of the clock hands. This has the effect of transferring power downward to the car, but also upwards, through the metal hands of the clock back into the clock mechanism.

enter image description here

  • The more often I watch the scene, that's actually (almost) true. The cable not only connects the hands of the clock but it also connects the antenna with the hands (you can see it right before Doc goes down). This is actually really weird, didn't notice that before. – Mario May 30 '16 at 13:15
  • @Mario - Yeah, in the scene just before you can see that the cable runs from the rod to the clock hands, but then when the actual strike happens, the lightning arcs over. – Valorum May 30 '16 at 13:28
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The problem lies in the fact that the clock is mechanical and weight driven, not electrical (except for the later added backlit dial) because of the fact that it was built in 1885 (no electricity).

Therefore, the electrical current would have never affected the clock at all when it hit the lightning rod on the top of the building and most likely would have went straight to ground.

  • So how did it break? – Edlothiad Sep 5 '18 at 13:59
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The existing answers make very valid points, but may also be overthinking it.

Lightning is not static. It's not a small amount of energy by any stretch of the imagination. Even if the cable were to divert the lightning away from the clock, there would be an explosion. Even without an explosion, the lightning is so powerful and erratic that it would arc to every piece of metal even close to it. If it's got enough power to arc from the sky to the ground, it's going to arc from a cable to a clock that's a foot for two away.

Lightning is incredibly volatile, for lack of a better term. It's not a clean energy transfer by any means. Think of a tree. When lightning strikes a tree, the whole thing explodes and or catches fire. If it strikes a big metal antenna that's connected to a building, it will break some stuff.

The antenna was not a lightning rod, it was a conduit/conductor. It wasn't feeding directly into the ground giving it a straight, clean path like a normal lightning rod would. It branched at the top, and had a far from linear cable attached to it. It's gonna be a harsh energy transfer.

Other than that, yes, I agree that wrapping the cable around the hands of the tower was a determining factor, even in the original design, I'm pretty sure it would have broken the clock.

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    There's a reasonable answer buried in here, but you might want to note that "It's just a show" or "Because that's how the writer's wrote it" answers are considered to be unwelcome – Valorum Aug 16 '18 at 20:13
  • Welcome to SciFi.SE! Would you mind breaking this up into paragraphs so it's a little easier to read, please? – F1Krazy Aug 16 '18 at 20:48

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