Why did Doc need to use a moving car to achieve 88mph?

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.

• Risk avoidance? Where we are going, we wont need risk avoidance! Commented Jun 21, 2012 at 16:49
• @DVK Probably, they never needed to hijack a big train.. Commented Jun 21, 2012 at 17:07
• @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)? Commented Jun 21, 2012 at 17:15
• @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. Commented Jun 21, 2012 at 17:38
• Related question: scifi.stackexchange.com/questions/14812/… Commented Aug 13, 2014 at 15:59

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.

• That is one clever answer!! Commented Aug 13, 2014 at 18:17
• Now you're thinking with portals Commented Aug 13, 2014 at 19:09
• Commented Sep 24, 2014 at 18:54
• @Hypnosifl Someone uses inordinate amounts of energy to tear a hole through space-time and you wonder why the hole is stationary?
– user40790
Commented Mar 16, 2016 at 23:21
• 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. Commented Mar 16, 2016 at 23:43

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?

• 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
Commented Jun 21, 2012 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 <handwave>the specialized tools to modify it weren't available in 1888. </handwave> Commented Jun 21, 2012 at 18:19
• 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
Commented Jun 21, 2012 at 18:23
• However, if only power had been a concern in PT1, they could have captured that lightning with much less risk and effort. Commented Jun 24, 2012 at 19:30
• Remember, he also used the DeLorean for its stainless-steel construction. Commented Sep 7, 2012 at 15:55

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.

• Magnetic field isn't a constant quantity. It's magnitude and direction vary across places.. It can't be used as reference frame. Commented Jun 21, 2012 at 23:29
• 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? Commented Jun 21, 2012 at 23:32
• 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. Commented Jun 22, 2012 at 13:28
• 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? Commented Jun 22, 2012 at 15:19
• @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.
– user56
Commented Jun 22, 2012 at 15:58

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.

• Better than those nasty magnetic fields... +1 But, can I ask gravity well of what? Every chunk of mass in the universe creates it's own gravity well. Commented Jun 21, 2012 at 23:53
• @Sachin Shekhar: Ok, then big, strong gravity well :)
– vsz
Commented Jun 22, 2012 at 13:33
• Again, two choices: Earth or Sun? Commented Jun 22, 2012 at 14:53
• And, what about speedometer reading which doesn't care about gravity well? Commented Jun 22, 2012 at 14:55
• @SachinShekhar one does not simply fall into the Sun Commented Jul 1, 2012 at 20:52

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.

• A car was definitely not needed: backtothefuture.wikia.com/wiki/Jules_Verne_Train Commented Feb 28, 2014 at 22:30
• Originally, a vehicle wasn't needed: slashfilm.com/how-back-to-the-future-almost-nuked-the-fridge Commented Aug 13, 2014 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. Commented Jun 23, 2016 at 11:44

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?

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.

• 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". Commented Feb 20, 2023 at 10:16