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88mph is an important speed to screw the space-time continuum. But, motion is relative...

Hey doc, can you please explain 88mph relative to what?

What we see in the movie: it was the speedometer reading which matters. So, place the car on a conveyor belt moving in opposite direction of the car having the car's speed and acceleration. The car will reach 88mph without resultant displacement and will go time warp.

To confirm, the speedometer was normal without any modification to record absolute speed (which is impossible). You can see in the first time travel (of Einstein) that the remote-controlled car was slipping on road initially but still Doc was recording increasing speed.

This means it's clear that the time circuit needed only a revolving thing in space with a tangential speed of 88mph (w.r.t. the axel which is, at rest, relative to the time circuit). In the case of the flying car from 2015, a similar setup in rocket exhaust can be assumed (motion is relative, after all).

My question: why didn't Doc use a time machine at rest with revolving parts? It would be less risky. (Note: Marty crashed twice.) Plus... instead of pushing the car with a steam train, the revolving parts could be revolved easily with a steam engine.

Paul D. Waite
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user931
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    Risk avoidance? Where we are going, we wont need risk avoidance! – DVK-on-Ahch-To Jun 21 '12 at 16:49
  • It needed velocity, not speed. So the total distance traveled vs time it took to do it had to be 88mph. They said "speed" because the target audience may not know what velocity is, and most people wouldn't pick out the difference. – Gorchestopher H Jun 21 '12 at 17:01
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    @DVK Probably, they never needed to hijack a big train.. – user931 Jun 21 '12 at 17:07
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    @Gorchestopher 1. Velocity is also a relative quantity. 2. Speed is attached with distance. Velocity is attached with displacement. 3. After reaching 88mph, the car disappeared immediately. So, what do you really mean by total distance travelled (vs time)? – user931 Jun 21 '12 at 17:15
  • The thing is that when one travels at 88mph the same way that Earth turns they have the speed of the car and the speed of the earth - 50mph. Ladies and Gentlemen, we are accelerating. NOTE: speed of the car is relative to the ground. – Darius Jun 21 '12 at 17:30
  • Taken out of BTF wiki: Temporal displacement, also known as "breaking the time barrier", was the process of time traveling. The DeLorean time machine did so by reaching a speed of 88 mph, while powered by plutonium or Mr. Fusion to create a nuclear reaction that would yield 1.21 gigawatts of power. – Darius Jun 21 '12 at 17:33
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    @SachinShekhar Like Remiel states in his question, the motion is relative to lots of things, one of those things is the magnetic feidn of earth. Another is the gravitation field of earth. Both of them being the dominant field of their respective types in the vicinity. – Gorchestopher H Jun 21 '12 at 17:38
  • @Gorchestopher Remiel's answer is all about mis-conceptions. See my comments on that answer.. – user931 Jun 21 '12 at 23:34
  • @Darius Haven't you read slipping car in question? The frame of reference is axel of wheels.. – user931 Jun 21 '12 at 23:36
  • Can anyone explain downvotes? – user931 Jun 21 '12 at 23:39
  • Sachin you don't seem to understand the physics you are criticizing others for not grasping. I question whether there is an answer to this question that you'll accept other than "you're right" – NominSim Jun 22 '12 at 03:32
  • @NominSim Give me proper physics logic.. I'd have no reason to be strict at what I am saying. I am not that type of guy. – user931 Jun 22 '12 at 04:03
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    Related question: http://scifi.stackexchange.com/questions/14812/in-back-to-the-future-why-was-the-speed-88-miles-per-hour – Donald.McLean Aug 13 '14 at 15:59
  • Note that logically, the time transport equipment had to be able to create a 'lock' on Earth otherwise a transport to 6 months ago would leave you around 300 million kilometers from Earth, and even transporting between this time and date one year into the future or past could leave you at least a couple of hundred(?) kilometers above or below ground level (given the Sun and Moon are occasionally tugging Earth into slightly different orbits). So however the Flux Capacitor achieved the lock, (IMO) a sealed case that the major object that the De Lorean had to travel relative to, was Earth. – Andrew Thompson Jun 15 '15 at 22:06
  • Originally the time machine was supposed to be like a refrigerator: http://backtothefuture.wikia.com/wiki/Back_to_the_Future_first_draft_screenplay - My guess is they turned it into a delorean, in part cause of the whole safety and kids locking themselves in refirgerators - but also to sell toys and it's more fun. The science of it may have been plopped in to make the movie more exciting. Given that - the only answers you'll get are just going to be as pieced together as the decision to change the refrigerator into a car – Kai Qing Mar 16 '16 at 23:39

7 Answers7

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One theory is that the flux capacitor generated a portal through which the De Lorean traveled, and it could only remain open for a brief moment with the power available; by Doc's calculations, the length of that moment was exactly the amount of time it took for a De Lorean to travel its own length at 88 mph (that is, to enter the portal completely). A stationary machine would be able to generate the portal, but not enter it.

Dan C
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ilinamorato
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    That is one clever answer!! – BMWurm Aug 13 '14 at 18:17
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    Now you're thinking with portals – calccrypto Aug 13 '14 at 19:09
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    Related: http://scifi.stackexchange.com/a/50963/28117 – Geobits Sep 24 '14 at 18:54
  • Why would a device mounted on a moving car generate a portal at rest relative to the Earth, though? If the device generated a portal at rest relative to the car, then the portal would also be flying ahead of the car at 88 mph as measured on the ground, and the fact that the car was moving at 88 mph relative to the ground would be no help in passing through it quickly. – Hypnosifl Mar 16 '16 at 23:10
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    @Hypnosifl Someone uses inordinate amounts of energy to tear a hole through space-time and you wonder why the hole is stationary? –  Mar 16 '16 at 23:21
  • @Axelrod - well, the point that there is no absolute definition of "stationary" seemed like the point of SS-3's original question, since SS-3 asked "Hey doc, can you please explain 88mph relative to what?" and also said "the speedometer was normal without any modification to record absolute speed (which is impossible)." I took it that SS-3 understood the idea of there being no absolute space in physics, and thus no non-relative definition of speed, and that the question was inspired by that. – Hypnosifl Mar 16 '16 at 23:29
  • I guess you could imagine that the device was just designed to create a hole that was moving towards the front of the device at 88 mph, and that's why when the device itself was moving forward at 88 mph relative to the ground, the hole would end up at rest relative to the ground--but the problem with that idea is that then there's no reason to get the car moving relative to the ground, you could just turn on the device in a parked car and you'd get a portal moving at 88 mph relative to the ground, so the relative speed would be the same and the car would take the same time to pass through it. – Hypnosifl Mar 16 '16 at 23:31
  • But the De Lorean has to emerge from the portal having traveled through space relative to a particular point; in this case, planet Earth. Perhaps, by creating a stationary portal, he is able to "lock" the portal to a location relative to a body, allowing it to re-enter the normal flow of space-time in a (presumably) safe location. – ilinamorato Mar 16 '16 at 23:42
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    There's also the possibility that the line he began to say before the De Lorean (and Einstein) reappeared was key. He mentions the stainless steel construction, and its effect in aiding the "flux dispersal." Perhaps "flux" begins to build up in the "capacitor" while it is stationary, causing a problem that is mitigated by motion and the shedding or "dispersal" of that "flux." That would certainly fit with the animation in the scenes, and could explain elements like the flame trails and the residual charge on the cable in 1955. – ilinamorato Mar 16 '16 at 23:43
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    And 88mph cut it so close that the license plate was sheared off. – Bishop Feb 21 '17 at 23:05
  • @Bishop - Good point! – ilinamorato Feb 24 '17 at 18:24
  • @Bishop He used a float when he should have used a double – DCOPTimDowd Jun 06 '17 at 19:39
  • I'm not sure... In BTTF 2 the Doc is accidentally blasted back in time by a bolt of lightning which (so they say) caused the flying DeLorean to go into a spin of at least 88 mph... You couldn't really traverse the portal if you were spinning... – komodosp Feb 20 '23 at 10:20
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I didn't get the impression that travelling at 88mph - or any speed - was important in any way at all for the functioning of the time machine. It was just that Doc had wired up the flux capacitor to trigger when the speedometer hit 88.

Why? Who knows. But probably for the same reason that he used the DeLorean in the first place:

The way I see it, if you're gonna build a time machine into a car, why not do it with some style?

Daniel Roseman
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    This would make some logical sense, except that they still needed to go at 88 MPH in several situations where they had more than enough time and/or resources to re-configure the flux capacitor so that it didn't require such speed. – Iszi Jun 21 '12 at 18:16
  • Well, the only time that reaching 88mph was an issue (as opposed to finding the power for the flux capacitor) was in the third one, so we can assume that the specialized tools to modify it weren't available in 1888. – Daniel Roseman Jun 21 '12 at 18:19
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    Good point about BTTF3 vs BTTF1. I'd forgotten the primary issue in 1 was power, not speed. Then again, if we accept the short-lived TV series as canonical, we might find some better case examples. (I'm not totally certain, as it's been awhile since I watched that.) – Iszi Jun 21 '12 at 18:23
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    However, if only power had been a concern in PT1, they could have captured that lightning with much less risk and effort. – leftaroundabout Jun 24 '12 at 19:30
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    Remember, he also used the DeLorean for its stainless-steel construction. – Michael Itzoe Sep 07 '12 at 15:55
  • It was an issue in 1 as well. YOU try reprogramming 1985 computer chips with 1955 technology (the fact they did it in Movie 3 is a product of Doc Brown of 1885 being able to spend a lot of time drawing up schematics to figure out how - something the two did not have the luxury to do in the short time frame they had to get the power to run the time machine). – Zibbobz May 20 '14 at 15:59
  • The speedometer argument doesn't make any sense. Firstly, why does Doc seem so happy about 88 mph when he conducts the experiment for the first time? Secondly, I seriously doubt that the speedometer worked correctly when the time machine was spun by a lightning in the second film, yet it successfully went to the past. – Malcolm Feb 16 '15 at 18:54
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If it's just the speed of the moving wheels, you're right, the whole thing is stupid. The only thing that makes sense is that the DeLorean has to be moving at 88 MPH relative to some external reference frame. My guess would be that the earth's magnetic field is involved.

Stephen Collings
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    Magnetic field isn't a constant quantity. It's magnitude and direction vary across places.. It can't be used as reference frame. – user931 Jun 21 '12 at 23:29
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    Another thing: Speedometers record speed based on revolution of wheels in the frame of axel.. Haven't you read that part? The car was slipping, but still it's speed was increasing.. Do you think magnetic field tries to manage it? – user931 Jun 21 '12 at 23:32
  • I'm saying there are two possibilities. One is that all they really need is for the wheels to move at the equivalent to 88 MPH; if that's so, Doc should have just put the car on blocks and hit the gas, or come up with a much simpler vehicle. It also wouldn't explain the flying sequences, wherein the car hits 88 MPH without its wheels spinning, and travels through time. The only reasonable explanation is that the car has to be moving at 88 MPH relative to the Earth. The magnetic field is a wild-ass guess. It manes no more or less sense than any of the other science in the movies. – Stephen Collings Jun 22 '12 at 12:51
  • Again, relative to Earth or its Magnetic field don't explain slipping car etc. – user931 Jun 22 '12 at 13:05
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    Just because the speedometer was going up to 88 MPH while the tires were slipping doesn't mean the car would have actually gone back in time had they kept that going up to 88 MPH. If you spin a car's tires, it thinks it's moving, whether it is or not. The speedometer might read 88, but that's clearly not enough. The car has to be GOING 88. – Stephen Collings Jun 22 '12 at 13:28
  • The car has to be GOING 88. ~> You're talking like 88 is an absolute thing. – user931 Jun 22 '12 at 14:35
  • Speedometer was important part of time machine. If you're saying it gave useless reading, go watch the movie again... – user931 Jun 22 '12 at 14:38
  • There's a difference between "useless" and "not useful under non-standard circumstances." – Stephen Collings Jun 22 '12 at 14:40
  • Also remember, speedometers don't care about any reference frame other than axel (and which is always attached with it).. – user931 Jun 22 '12 at 14:43
  • What are non-standard and standard circumstances, then? – user931 Jun 22 '12 at 14:45
  • For a car speedometer, "standard" would typically mean that the car is on the ground and the wheels are propelling the car forward. Doing a brake stand, putting the car on blocks, hooking the axle up to a power drill, etc. would mean the speedometer's display does NOT correspond to the actual speed of the car. Meaning the speedometer is not useful for determining the speed of the car in those circumstances. – Stephen Collings Jun 22 '12 at 15:04
  • Man, your relativity concept sucks. You've have used "actual speed" word to compare with speedometer reading. What is "actual speed"? – user931 Jun 22 '12 at 15:17
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    If you're driving car, and I put ground on conveyor belt of my question. How can you determine that the car is in non-standard situation? – user931 Jun 22 '12 at 15:19
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    @SachinShekhar Given the nature of the movie the answer is obviously not going to involve correct physics. It only has to make narrative sense. Make up your mind: you asked the question, so don't go and criticize people for trying to meet you halfway with a movie-science explanation. And remember to be nice. –  Jun 22 '12 at 15:58
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I've read quite a number of "hard" sci-fis (in the sense that the science was well developed, explained in detail, and consistent) featuring some limited time travel or FTL, and in most cases they did not work at all in strong gravity wells, or had several limitations. So we can assume it was relative to the strong gravity well we are in. Maybe moving relative to that gravitational field generates something required for time travel, just as moving in a magnetic field generates current?

However, this is just speculation, based on a number of other works not related to the movie's universe. There is no explanation given in any of the movies, as far as I know.

vsz
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Daniel Roseman's answer was almost there.

Let's look at Doc's full answer. From the script:

MARTY

Time machine? Are you trying to tell me you built a time machine out of a DeLorean?

BROWN

(smiles, modestly)

The way I figured it, if you're gonna build a time machine into a car, why not do it with some style? Besides, the stainless steel construction made the flux dispersal—

He's interrupted before he finishes explaining, but the stainless steel body was apparently a key factor to making the final build work correctly. A Delorean was obviously not specifically needed (it was chosen partly on style), but something of similar materials (and possibly size) was.

There is no evidence that a moving/working car was specifically needed. It is more likely that a suitable frame was needed to handle the amount of energy being pushed through it.

phantom42
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  • A car was definitely not needed: http://backtothefuture.wikia.com/wiki/Jules_Verne_Train – Kapler Feb 28 '14 at 22:30
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    Originally, a vehicle wasn't needed: http://www.slashfilm.com/how-back-to-the-future-almost-nuked-the-fridge/ – Michael Itzoe Aug 13 '14 at 13:47
  • I'm guessing that the speed of the machine is less important than its kinetic energy (0.5 * m * v^2). The train at the end of BTTF3 is clearly going much less than 88 mph when it makes its time jump, as it stops almost immediately and you can't slow something that big from 88 to 0 that quickly. The DeLorean is quite a light vehicle and so needs a high velocity to generate enough kinetic energy to create the time portal/wormhole/whatever. The train, having much greater mass, needs less velocity. – Wallnut Jun 23 '16 at 11:44
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I will now speculate based on the information that @phantom42 provided. The wheels were leaving streaks of flame (melted tire rubber?) Perhaps the machine had to be in the approximate frame shape of a car, and the environmental damage required the car to be moving so that the tires didn't melt through.

And to further speculate, perhaps the mechanics of time travel required the movement relative to the material around it to maintain the integrity of the machine (why do the tires go back in time, but the road and the flag rope stay behind?

Aaron Hall
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The time machine has to travel through time while remaining in the same place relative to the motion of the Earth, since otherwise you'd wind up in deep space.

It is reasonable, therefore, to conclude that the minimum 88mph speed is also relative to the Earth. Ilinamorato's answer provides one reason why this might be the case.

Harry Johnston
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  • This - I'm surprised it doesn't have more upvotes. It also neatly answers the question "how come they don't end up in deep space since the Earth is always moving?" which is always getting asked. Also, general relativity is associated with both time dilation and gravity so it ties in there too. In any case, in any Sci-Fi there's going to be an "information gap" between real life science and the technology in the fiction - it's why the thing hasn't been invented, so we just have to accept that some things "just work". – komodosp Feb 20 '23 at 10:16