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Everyone knows that to time travel, the Delorean had to "hit 88 MPH".
Watching the movie right now though, got me wondering...

When Doc first demonstrates the time machine to Marty, he holds the brakes and revs up the drive wheels. This got me wondering... does the Delorean have to actually be MOVING at 88 MPH? Or is it somehow tied to the speedometer? What if he had the back wheels up on rollers, like they do when doing the emissions test on cars, and got the wheels (and speedometer) up to 88 while the car itself was still actually stationary?

To clarify, I am not asking why the "magic speed" was 88. There are some pretty good answers to that on the question Izkata linked to in his comment.

Nor am I asking why he needed to use a car. He didn't. He stated that in the first movie.
If you're going to build a time machine into a car, why not do it with style?
He CHOSE to use a car as his platform, and the Delorean for style.

More, I'm asking, regarding the functionality of the time machine AS BUILT into the car, what exactly was it that had to hit the magic speed? I see two possibilities...

  • the car had to be in actual spatial motion. maybe it had to do with the friction of the air over those "flux dispersal" stripes around the car or relativistic motion of the car to the surrounding environment or something.

  • only the speedometer was required to hit the target speed. the car itself could have the drive wheels lifted or on rollers of some sort, that would allow the speedometer to hit the magic speed without the car actually moving.

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Not quite a duplicate of In Back to the Future, why was the speed 88 miles per hour?, perhaps you can rephrase some of it to be more obvious? (The question title in particular) –  Izkata Feb 27 at 4:41
    
Yeah, different question... not asking why the target speed was 88... there are some good answers to that Q over on there. Let me see if I can clarify a little... –  eidylon Feb 27 at 4:42
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Or for that matter, what if the entire car was on a treadmill-like system? Also, I'd suggest changing the title to something like "Can the Delorean satisfy the '88mph' rule whilst chocked up?" –  Robotnik Feb 27 at 4:47
    
Well, I'd consider the treadmill situation to be similar to drive-wheel rollers, no? Or might there be some applicable difference? I like your title idea though, gonna use a variation of that, accounting for any speed as per the Q Izkata linked to. –  eidylon Feb 27 at 4:50
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The assertion that it's dumb just because you can make up whatever rules you want in fiction is rather specious. By that measure any question on this whole site would be off-topic. Why ask questions about aliens when you can just write them however you want? Or starships? Or stargates. With the flick of a pen you can change an alien's biology or alter the rules of hyperspace. So they're all off topic by that philosophy. Your comment doesn't come across as at all constructive. –  eidylon Feb 27 at 17:10

8 Answers 8

up vote 47 down vote accepted

I think Back to the Future 3 offers an answer. While in the past and out of gas, Marty and Doc try to get the car up to 88 mph. Two of their attempts suggest that the car needs to be moving at 88 mph, and not just the wheels spinning at the same rate as if the vehicle was moving at that speed. They drag the Delorian behind a bunch of horses, and they push the Delorian in front of a train.

Now, treadmills existed at the time (Wikipedia says they're pretty damn old). Moving a car at such a high speed takes more energy than putting the car on top of a treadmill and just spinning its wheels. Doc possesses the knowledge and skill to have tried this, so I posit that he didn't try it because it wouldn't work.

There's something about moving at 88 mph that enables time travel, it's not just a matter of spinning wheels.

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Very good point! Hadn't thought of that. Doc definitely would've had the skill to build a rudimentary treadmill or roller in 1885. In fact, probably way more than rudimentary! –  eidylon Feb 27 at 5:12
    
wouldn't the wheels still be turning just as fast in both those scenarios - it'd be hard to drag or push the car with the wheel brakes on –  HorusKol Feb 27 at 22:56
    
@HorusKol.. precisely. Which is precisely why Keen is suggesting that it is NOT related simply to the wheels rotational speed, but to the actual physical movement of the car through space. –  eidylon Sep 23 at 18:39
    
For the reasoning, you may want to look at an answer I posted to a related question: scifi.stackexchange.com/questions/18827/… –  ilinamorato Oct 1 at 18:53

It is not mentioned directly in the movie, but there is a piece of the DeLorean referred to as a "wormhole emitter" in the majority of quote-unquote "official" technical drawings/schematics/etc.

Wormhole emitter

Let us assume the bright light/distortion that appears in front of the car is "the wormhole" - then it follows that the DeLorean has to drive through the wormhole; and in order to prevent random objects from following you through, it would summarily need to close within fairly short order. If the wormhole is maintained just long enough for a DeLorean to pass through it at 88 MPH, well... there you have it.

While this involves outside information, I believe it is born out in the actions of the movie. Consider the following plot points:

  • In the original *Back to the Future*, Doc has a week to prepare for the Hill Valley lightning storm, and he has at least a rudimentary understanding of his own work - indeed he still has the wound from the incident that provoked the image of the flux capacitor! So we can assume that he knows his own work well enough to find an optimal solution - and in this case it involves running a time machine straight at a lightning rod. The cop that approached him during his initial setup asks for a permit for his 'weather equipment' story, which he readily produces. So, he *could* have propped the DeLorean up on a treadmill, passed it off as "highly specialized weather-sensitive equipment" and just hung out under the clock tower until 10:04 PM - but he doesn't do this, presumably because it is necessary.
  • In *Back to the Future: Part II*, the DeLorean time travels while stationary. This should throw the theory out the window - but consider this:
    1. The flux capacitor is, presumably, *a capacitor*. Its primary task is to store energy and discharge it when needed. If 1.21 jigowatts is the nominal amount of electricity required under normal circumstances, the capacitor will release it at a preset amount. Hopefully, Doc designed it to handle the occasional, ah, hiccup - meaning it should be able to store a bit more than it is intended to release. As I'm aware, capacitors have a 'working' voltage (the safe maximum intended) and a 'max' voltage (the absolute ceiling on what it can withstand), so this follows.
    2. With that in mind, the DeLorean appears to eat quite a few lightning bolts while it's up there. If the width and breadth of the wormhole are bound at all to the amount of energy required to generate it (1.21JWe) and the flux capacitor is designed to discharge at a certain rate, AND we assume that Doc did his diligence in creating a capacitor with a much higher max voltage than its working voltage, you end up with a sustained emission from the wormhole emitter. Indeed - that resultant flash and explosion seems a bit more hectic than the others, doesn't it?
    3. The oft-contemplated '66' trail in the sky would appear to be a result of the fire trails from the wheels going out as they don't really have anything to burn besides air (and it's raining to boot). The path suggests that the car was *pulled into* the much larger wormhole, or even that the wormhole was created *around* the time machine. The fire trails have always gone where the wheels 'should have been', like water thrown from a centrifuge. So the curvature of their appearance means there was *some* force acting on the fire trails/the immediate region, even as the DeLorean left 1955.
    4. **TL;DR - Multiple lightning strikes + a steady discharge from the flux capacitor = much larger wormhole than intended. Hence, the DeLorean was already inside it and didn't *need* to drive in.**
  • Finally, in *Back to the Future: Part III*, the idea is pretty much the same as the first movie; it's crucial to accelerate the DeLorean to 88 MPH. By now, the generation of the 1.21 jigowatts is well-established as academic; Mr. Fusion handles it. And unlike 1955 Doc - who may be off the hook as he's only just become familiar with his future self's final product - *this* Doc has had the DeLorean for who-knows-how-long, and wouldn't be trifled with a simple setback like being out of gas unless it was critical to drive the DeLorean through the wormhole. And I strongly suspect he's not willing to subject the unit to another overcharge (he got pretty lucky the first time - too much juice, damaged/destroyed flying circuits, yet he still landed it and in working order enough to be left in a cave for the better part of a century) so given their options, along with the implicit ticking clock of Mad Dog Tannen looking to shoot himself a temporal deviant, I'd wager he went with the safest option.

So -- in closing! Speed is a requirement because the 'wormhole/tachyon field/magic thingie' only stays open briefly, and so the DeLorean needs to get in and through it before it closes up. Otherwise he'd just build it into, like, a telephone booth or a refrigerator.

EDIT: Pardon my proof by Wiki, but according to http://backtothefuture.wikia.com/wiki/Tachyon_field_generator, Bob Gale has actually christened this aspect of the car the "tachyon field generator". Wormhole emitter is apparently just a fandom colloquialism for the emitter on top of the roof of the car. Back to the Future isn't exactly stop-one for major scientific accuracy, but I would submit that the idea is more or less the same; create hole in spacetime, drive through hole in spacetime before it closes, improve parents' quality of life.

I know that the actual phrase isn't used in the movies; I do not believe there is an onscreen explanation, except that it facilitates several plot points and one of the most epic high points to a movie, like, ever.

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Wow, I never thought anyone would come up with an answer that actually makes plausible sense! Well done. –  steveha Feb 27 at 7:32
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But then the question one must ask is why the wormhole is stationary relative to the Earth... –  Schilcote Feb 27 at 17:37
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Did you just type quote-unquote in text and then surrounded the following word with actual quotation marks? –  user1306322 Feb 27 at 20:17
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I wish I could accept two answers... because this one gives a GREAT discussion of how the time machine works within the framework of known information about it. However Keen's answer actually does more to give proof one way or the other as to the possibilities (such as I see them). –  eidylon Feb 28 at 0:06
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I don't suppose I can have my wormhole emitter and eat it too… :D Though it does compel me to update my question to include movie-driven events. –  Stick Feb 28 at 0:09

The Delorean does travel through time without having its wheels spinning in at least one occasion in the movies: when using the "hoverwheels" from the future that essentially transform it into a flying car. So no, the time travelling trigger is not tied to the wheel speed.

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Heh... true, which does beg the question then of why they were trying to get it up to 88 at the end of III. It didn't even have any real relativistic motion when it was struck by lightning. Which would seem to indicate that NO motion at all was actually required, either of the wheels OR the car, and Doc tied it to the speedometer strictly out of preference, not any actual requirement. –  eidylon Feb 27 at 15:13
    
@eidylon The lightning bolt is supposedly a freak incident. Or, you could argue that because the car was hovering in midair and fighting the force of gravity, it WAS travelling at 88 miles per hour! (A hard assertion to back up but what did you expect from a comment on an answer?) –  Zibbobz Feb 27 at 15:30
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IIRC when Marty was looking up after the car was struck by lightning, there were residual "tracks" of light and smoke showing that the car did a somersault in the air. It could have been that the lightning bolt gave the car enough acceleration to start moving and actually hit 88 mph in the short time it took to do the somersault. This way we don't need to introduce any magic - the car was moving at 88 mph and the energy of the lightning bolt was enough to activate the flux capacitor and send it back to 1885. –  Alex Tokarev Feb 27 at 18:44
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Except for the magic of "wow, the car experienced a huge spike in delta-v and Doc didn't get the worst case of temporal whiplash ever" –  Stick Feb 27 at 20:02

Actually, the wormhole, once created, would be moving at the same rate as the earth (since it's creation occurred on the moving earth: Try throwing a ball on an airplane). In the larger universe, the two items would be traveling at the same rate, thus seeming stationary to those around the wormhole at the time of it's creation.

Now, 88 mph could be the minimum safe speed required to make it through the wormhole before its collapse. Since the wormhole is created in front of the Delorean and, if it's (effectively) stationary, AND it's a known time before collapse, it would be pretty easy to calculate the minimum necessary speed to reach and traverse the wormhole before it closes.

The lightning bolt, of course, was simply the necessary power source to generate 1.21 gigawatts (jigawatts? :) ) needed to create the wormhole (Normally created by nuclear fusion/fission).

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I would assume there's also some mechanism linked to the actual movement of the vehicle to ensure that the wormhole does not get created until the vehicle has reached the minimum traversal speed. If not, Houston we have a problem :) –  Stan Feb 28 at 14:29

Regarding the issue of the "standing start" at the end of the second film - There's really no reason to believe that it was a standing start. If the lightning fired the time circuits then it could just have easily fired the propulsion units.

As other answers have noted, the "66" track in the air indicates that the car somersaulted at least once. There's nothing that says that the 88mph has to be in a straight line, especially when you're talking about something that's essentially a four-dimensional construct. The wormhole (let's stick with that idea) lies in a direction that is not "up" or "down" in any conventional sense. As long as the car is in motion and moving "forward" relative to its own point of view, it should be able to enter and traverse the wormhole. Doing loopdeyloops at the time isn't really relevant as long as it has the requisite velocity. The emitter keeps the car oriented "toward" the wormhole.

From Doc's point of view, he's hovering, shouting at Marty, the suddenly WHAM, he's spinning out of control and once he puts on whatever passes for brakes in a hover-car, he looks around and says,"Oh, boy!".

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The car's acceleration was precisely timed so that it would hit 88 miles per hour at precisely 1:20 AM. The wheels spinning up on the pavement were so that the DeLorean itself would reach the required velocity at the correct time.

It's not exactly clear why the car couldn't simply start driving from 0, but if I had to guess, it'd be because the DeLorean isn't actually all that much of a performance monster (from personal experience). It takes a certain amount of engine revving to get up to speed, and Doc was doing this in a mall parking lot: at this point in the movie, where we're going, we do need roads.

By letting the DeLorean's tires spin, he was building up the required engine power so that, when he disengaged the brake, it could accelerate to 88 in a shorter amount of space. Doc couldn't just save the tires and put it in neutral until the time came to put it in drive because that would have wrecked the transmission (thank you @eidylon).

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While that might've saved rubber, wouldn't it have risked destroying the Delorean's transmission to do a neuty-drop? Tires are a heck of a lot cheaper and easier to replace than a transmission. –  eidylon Feb 27 at 6:54
    
Acceleration would be way easier to calculate based on tires that aren't slipping v/s the rate with spinning tires. Without the tires spinning, acceleration on a level surface (same environmental conditions, etc) would be constant. But with spinning tires, you'd have to compensate for coefficient of friction on the specific surface, including debris, etc. I'm inclined to think that, just like choosing a car to begin with, "smoking tires" were chosen because it looks cool. ;) –  dannysauer Feb 28 at 1:02
    
While not from an in-universe perspective, I agree about it being primarily because it looks cool (before @eidylon's comment about the transmission, that was my explanation). I'd say it was likely a degree of practicality, though, if not from a calculation standpoint, then from a spatial one. Amending my answer again. –  Stuart P. Bentley Feb 28 at 9:09

I think that what makes the most sense is that for some technobabble reason it has to go 88 mph in the reference frame of some very large mass, such as the Earth. I.e. the flux capacitor somehow makes energy from the speed difference between it and a large mass. So treadmills, etc. wouldn't work.

Of course that doesn't explain how it could time travel at the end of part 2, just from being hit by lightning. Perhaps the time travel just needs lots of power, which the flux capacitor can create from a speed difference of 88 mph between it and a large mass, or which can be provided by a bolt of lightning? But then again that doesn't explain why at the end of part 1 it needs 88 mph and a bolt of lightning.

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Considering that Earth (and thus, the car) is moving through space at ~107,000 km/h, I find it unlikely that the speed of the vehicle has bearing on its time traveling status.

A car is doing more work when actually accelerating along the ground (As opposed to spinning it's wheels in place). Perhaps the flux capacitor is physically linked to the drive-train in a manner that creates a wormhole from the mechanical work the engine is doing.

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