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Every time the DeLorean travels in time, it appears in the same place as the place it left. But that's the same place relative to the Earth's surface. In space-time terms, that point is constantly moving: the Earth rotates on its axis at a speed of one revolution per day, and moves around the Sun at a speed of one revolution per year (a roughly circular orbit with a radius of 93 million miles). The Sun is orbiting around the centre of our galaxy, pulling its planets with it, and our galaxy is moving relative to all other galaxies.

So if the DeLorean does not move in space in absolute terms, then a time-jump of even a few minutes would place it outside the Earth's atmosphere as the planet would have moved a few miles in that time. Therefore, to prevent this, the car's computer has to calculate the precise movement of the Earth (and Hill Valley) over the required interval travelled.

Two questions:

  1. A computer powerful enough to do that kind of calculation probably couldn't have fitted into a car in 1985. So where did Doc get it from?

  2. If he could calculate the spatial displacement to remain in the same place on the Earth's surface that accurately, why couldn't he adjust those calculations to make the DeLorean appear wherever he wanted?

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    Possible duplicate of scifi.stackexchange.com/questions/37207/… – Kevin Workman Apr 1 '16 at 15:25
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    One note: in modern physics the notion of something that "does not move in space in absolute terms" is meaningless, because relativity is incompatible with any notion of absolute space. The question of what determines where in space the DeLorean reappears is still meaningful, it's just that there isn't any reason to say one place you might imagine it reappearing (say, the spot that looks like the 'same place' to an observer at rest relative to the center of the galaxy) would be more "natural" than any other. – Hypnosifl Apr 1 '16 at 15:34
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    Since I don't think the writers had any explanation in mind, and I don't think there's really anything in the movies that could be a basis for extrapolating an answer that's consistent with what we saw onscreen, I think any answer would have to be a complete invention on our part--maybe this question would be better suited to the worldbuilding stack exchange, which does allow for make-up-your-own-explanation type questions. – Hypnosifl Apr 1 '16 at 17:11
  • Doc Brown is one smart son of a gun :) – Mykewlname Dec 6 '18 at 14:09
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As you've said, the Earth is moving around the Sun, which itself is moving around the galactic centre, which itself is hurtling through space, so everything in the universe is moving about the place. The only way to get a sense of stationary is to choose a point of reference you decide is stationary and then work your maths out from there.

For instance, when you work out how far a ball will travel, and the arc it will describe as it travels, you only use the initial velocity, position and direction of the ball. You don't need to consider the movement of the Earth, or of the Sun, or of anything else, because your premise before the calculation is the ball's movement is happening in the space you have defined as "stationary" - the Earth. There is no such thing as absolute space (or time), only relative space (or time), which coincidentally is a core part of the physics which describes theoretical wormholes.

The reason the DeLorean appears in the same place it left spatially is because it creates a wormhole through time only, the entrance and exit of which is bound by the Earth's frame of reference as the stationary place since the Earth is the dominant gravity in the local area of the DeLorean's departure.

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Your premise is flawed. There is no such thing as an absolute frame of reference. All motion is relative. So the most logical answer to the question "Where in space would the DeLorean end up?" is actually what the writers went with - the same point relative to the Earth as before - for the same reason that we tend to remain on it absent time travel, the Earth's dominant gravity well. As such, no computations are necessary to move in space. The real question becomes, why have you chosen to ignore this detail about movement in time?

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    Are you imagining there is a sort of frozen phantom DeLorean occupying the space in between the moment we see it disappear and the moment we see it reappear, and it's still acted on by gravity during this time? Something a bit like the time machine in H.G. Wells' story, except zipping through intermediate times so quickly that the person on board doesn't perceive any time between hitting 88 in one time and arriving in another? If not, I don't see how gravity can explain why it remains at the same position relative to the Earth if gravity wasn't actually "carrying it" with Earth the whole time. – Hypnosifl Apr 1 '16 at 15:47
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    Well, I don't see how a DeLorean can travel through time. The movie doesn't bother to give us a believable explanation of that either. Nor is it required to. At some point you are just quibbling with the central conceit of the movie. – J Doe Apr 1 '16 at 15:53
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    It is a plot convenience that the writers didn't feel needed to be justified. You won't find any answer from them because they didn't think about it. It is never addressed in the movie. Absent that, I attempted to provide the asker with a reasonable-sounding speculative answer, because what they are asking for is speculation. – J Doe Apr 1 '16 at 15:59
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    You might as well ask, "Eighty-eight miles per hour in what frame of reference?" The surface of the Earth at the equator already moves at roughly 1,000 MPH as the Earth rotates. It travels around the Sun at about 67,000 MPH. The solar system moves at around 490,000 MPH relative to the galactic center. Clearly, the time machine's operation is tied to the Earth somehow, or these vastly greater speeds would affect it. Perhaps the Earth's gravitation is necessary for the machine to work in the first place. – John Sensebe Apr 1 '16 at 16:24
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    The time travel ideas in Back To The Future and H G Well's "The Time Machine" are quite different. In Wells' story, the Time Traveller describes to his friends how Time is simply another dimension like the three of space, so increasing or decreasing his speed in the temporal dimension would not affect his motion in the other three, and so he would "track" his position on the Earth. He only disappears from the perspective of an observer because he's moving too fast in the temporal dimension for the observer to see, like the spokes of a wheel. He observes real time, just faster than normal. – Wallnut Apr 4 '16 at 9:34
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It's unexplained in the movie(s).

I think with any sci-fi invention we can't know all the details of how exactly it works, if we did, we'd invent it in real life. Dig deep enough into any sci-fi invention and you'll have a question which concludes it can't work or at least can't work as depicted.

This is just one of the many challenges with time travel that the Doc had to overcome in the 30 years he spent inventing the time machine.

Having said that I prefer Joe's answer or a similar one which makes the Earth itself a factor of how time travel works because:

  • The Doc didn't invent a "space machine" which he could have (or would have to have) done if he had to compensate for the movement of the Earth.

  • Already the 88mph has to be relative to the surface of the Earth.

  • It ties in with the General Relativity theory which is related to gravity and time-dilation.

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The DeLorean has to travel through time and space. I don't think it's something the writers thought about at the time, but the Earth is constantly moving. The flux capacitor has to be able to know exactly where the Earth was or will be. So when the DeLorean drives 88MPH on Main St. in 1955 as it's leaving it arrives on Main St. in 1985. The only time I noticed a difference was in BttF2 when the DeLorean leaves 1985 in the beginning of the movie it looked like it was still close to the ground. But when it arrived in 2015 it appeared to have a much higher altitude.

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It's a conceit of most, if not all time travel stories, reaching back to H.G. Wells' novel "The Time Machine". Time travel is pretty much always depicted as a displacement in time while remaining spatially fixed relative to the surrounding environment (whatever that surrounding environment might be).

There are numerous time travel stories throughout the Star Trek franchise; some get rather fanciful such as the Voyager being fractured across time (episode Shattered) - different parts of the ship are at different points in time. It's a bit absurd because the ship is constantly journeying home while somehow part of it is a few years ago while under attack from aliens long ago left far behind. In every case, the destination of a trip through time is always at the same position in the current frame of reference (usually a planet) as the departure point. It's always so regardless of whether the time trip is intentional or accidental.

There was a show a few years ago titled Continuum. A group of criminals escape justice in the year 2077 by travelling back to 2012; the protagonist is swept back in time with them. They end up in more-or-less the same place (Vancouver Canada) as they started despite the 65-year time displacement.

One notable exception to the whole time travel conceit is the TARDIS of Doctor Who, which is stated to be able to travel to any point in time or space. Having the ability to travel almost instantaneously across space allows the whole question to be side-stepped; no matter how a planet (or even a spaceship) has traveled in space, the TARDIS can materialize at a convenient spot on or in it.

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