The Time Travel Paradoxes of Back To the Future

brumgrunt sent in a fun little piece to get your brain going on a cloudy monday morning. Despite countless viewings of BTTF I still never thought of a few of these. "Throughout Back To The Future Part III, there has to be two Deloreans in 1885. Also, why don't George and Lorraine recognize their son? Why doesn't the time machine disappear in the alternative 1985? These and more Back To The Future paradoxes explored..."

Comments

The one they always overlook

[elrous0]

If you travel back in time to the exact same spot, just in a different time, then (unless you're REALLY precise on the exact time of day and year), you'll most likely end up floating in space. People who make time travel movies don't seem to realize that the earth moves around its axis and around the sun. The spot I'm standing on right now will be vaccum in just a few minutes.

If Marty had went back to a different time of year without a space suit, Biff would have been the least of his worries.

Re:The one they always overlook

[Drakkenmensch]

If you travel back in time to the exact same spot, just in a different time, then (unless you're REALLY precise on the exact time of day and year), you'll most likely end up floating in space. People who make time travel movies don't seem to realize that the earth moves around its axis and around the sun. The spot I'm standing on right now will be vaccum in just a few minutes.

John Carpenter is the only director I can think of who ever complements his time travel explanation (albeit for a radio signal, but still) with the earth's revolution around the sun: http://en.wikipedia.org/wiki/Prince_of_Darkness_(film)

Re:The one they always overlook

[Anonymous Coward]

in simple term, we don't know.

there are some hypothesis of what would happen if time travel were possible, based on paradoxes, quantum mechanics, or just plain old guesswork

one is that time is unchangeable. that is, the time line as we are living is the one where all the changes and manipulation that are about to happen from people in the future travelling to the pas are already happened. so if you're planning to kill your parents, you have already succeeded at it and thus you would not be there or you would fail at some point in the future before killing them

another theory based on quantum theory is that every quantum state change that may happen is happening and each time a change may or may not happen the reality splits in two reality, one where the change has happened and one where the change didn't happen. this at quantum level, but could be extended as to complex decision of human being, if those decision are driven by quantum states at some level or other. so if you get back to kill your parents, then all the universe in which you have succeeded, you have not, you traveled and you didn't travel will spawn from the common state, thus not creating a paradox: every state is perfectly stable and every outcome is valid in it's own reality

another one is that paradoxes couldn't happen, which is similar to theory n1 in that every time travel is not going to change anything but says that paradoxes would not exists, like killing your parents, because something is going to prevent you from travelling in the first time if you're going to change the time sequence. this is mostly used as a narrative device but there are also more formal proof about that involving entropy and mathematical stuff, quite hard to explain in a slashdot post and quite over my grasping to put it down on layman terms, but you can read it further in 'how to build a time machine' from paul davies which is quite a good explicative/scientific book in spite of its catchy name

Re:The one they always overlook

[JWSmythe]

    That's something I always loved about time travel. They never mention spatial teleportation. To accomplish time travel, there must be spatial teleportation to arrive in the same place on a planet. Say your time machine was in Times square, and for some reason you were spatially oriented on the center of the planet, and you attempted to travel 3 hours to the future, when you arrived, you'd arrive somewhere between Gerlach, NV to Ravendale, CA. If you aren't spatially centered on the planet, you'd find yourself about 201,000 miles behind the best place to land (like, something solid with a breathable atmosphere).

    It makes you kind of wonder, how many basement geniuses have accomplished time travel, but were never heard from again?

Re:The one they always overlook

[Graff]

We all know the answer to that one: 1.21 gigawatts!

Ahh but that just tells us the RATE of energy, not the AMOUNT of energy! The true question is how many gigawatt-hours is it?

Of course there are two DeLoreans

[rarel]

Of course there are two Deloreans. Doc's and Marty's. It's not a plot hole at all, the whole point is that they can't gut Doc's DeLorean for parts since it would create a paradox and prevent Marty from going back in time to 1885.

The cool thing is that at one point there are FOUR DeLoreans for a few hours in 1955, Marty I, Cowboy Doc, Marty 2 (with Doc) and Biff's.

The gasoline crunch

[Drakkenmensch]

Doc probably could have MacGuyvered a distillation setup to make gasoline out of petroleum, but he quickly figured that it would take him much longer than it would take for him to get murdered and so other options were needed. He just didn't bore Marty with the details and called it impossible, adding the words "in what little time we have" in his own head.

Jennifer seeing herself...

[Anonymous Coward]

Regarding Jennifer's surprise at seeing herself, the older Jennifer is surprised because they see each other at the same time. Up to the point young Jennifer sees old Jennifer, old Jennifer can't remember seeing young Jennifer because it's not a done deal yet. Time is still in the process of being changed. The movie is pretty clear that changes to the timeline are not predetermined. Old Jennifer cannot remember seeing young Jennifer until it actually happens, thus being a surprise for both parties.

Separate Time Lines

[crndg]

There are many different ways time travel can be presented in fiction, with many different sets of "rules." In my opinion, BTTF actually sticks pretty close to its own rules, except when a) absolutely necessary for the story, or b) good for a laugh (see a).

The reason future versions of people don't know what's going on right now in their being-rewritten past is because they're in a different line on Doc's chalkboard. So when Doc in 1885 writes the note to Marty, he is from a future where (when?) he didn't know he was going to be killed by Mad Dog Tannen. So he couldn't possibly know that Marty was going to need to come back and rescue him, and would need gasoline to do it.

As for why Marty's parents don't recognize him, I would say they've had years to forget the details of what Calvin Klein looked like, and years of seeing their son every day as he grew up to look like someone they haven't seen in 30 years. Think of someone you know and see often. Now look at a picture of them from a long time ago. In your mind, they may seem like they haven't changed, but they have. It's like how I still picture my dad looking like he did a while back, when I saw him more often, and am now shocked to see that he has turned into Rush Limbaugh (not literally, but eerily similar-looking).

The one good question posed by this article is about whether Marty and Jennifer would exist in 2015, after they have just gone off in the time machine w/ Doc Brown in 1985. At that point, we might think they should be removed from any future time line until they return safely to 1985. I can only surmise that when traveling to the future, the Delorean travels along the future time line it is leaving, without regard for any changes it may introduce by doing so.

Perhaps a better overall question is: what happens to all the versions of people stuck on those time lines that are then cancelled out by Doc and Marty's travels? Do they zap out of existence? Do the time lines continue on, with fake-boob Lorraine married to Biff and all the other unpleasantness? Should we be happy that everything worked out for "our" Marty, because he's the only character who is the same person we met at the beginning of the first movie?

Re:Frame of Reference Problem

[Securityemo]

Actually, this problem is moot. As it happens, the absolute frame of reference in this universe defaults to the "sleeping" "body" of Cthulhu, in the "city" of R'hyleh, "located" on the bottom of the Atlantic Ocean here on Earth. So at most it's a difference of a few centimeters.

(Seriously, this always disturb me on some visceral level when works of fiction discuss a universial frame of reference like there was such a thing? The relative frame of reference of "the universe"? Aeaeaeaeafhtagn...)

Simple explanation...

[denzacar]

Note the portal forming in front of the Delorean just before it disappears.

Delorean is PUSHED through a portal created by the flux capacitor which actually exploits naturally occurring folds in the space-time by poking a tiny hole (from the universe's point of view) in those folds for a fraction of a second.
Delorean needs to be moving at 88 mph in order to get to the other side in one piece before the portal closes.

So you see... it is more like the Star Gate than like H.G. Wells' time machine.
Now, that one should have ended floating in space upon arrival...
Unless...

What if gravity wells (being a dent in the fabric of space-time) extend like a trench instead of like a circular dent?
So, just as it holds you firmly attached to Earth instead of flying out to space while going slowly forward through space-time, Earth's gravity-trench keeps you moving along the same line up/down the trench while you are moving fast forward/backward through space-time.

There... Now your fiction can make sense.

Re:Frame of Reference Problem

[maxwell demon]

I don't even understand why this needs explanation. We all travel forward through time, and no-one needs an explanation of why we don't phase through the planet as time moves forward.

Of course we need an explanation. Fortunately we have it: Because the matter below us causes a force on us (combined gravitational, elastic and friction), keeping us at "the same" place relative to the ground. For the very same reason there's no problem with H. G. Wells' time machine, because it's at its place all the time (he constantly sees the surrounding, just in another pace; of course that sort of time travel has its own set of problems, but that's another story), so it also should be subject to this force. However the time machine in BTTF (as well as the time machines in most stories/movies/series today) basically makes a jump in time, i.e. it simply isn't there in the intermediate times, thus there's no force which would keep it in place.

Of course one could argue that since "the same place at another time" isn't exactly defined anyway, the inventor of the time machine must have built in some calculation of the relative position of earth at the destination, and manages to move the time machine to exactly that place. However, that should enable you to not only choose the time, but also the place where you appear (possibly restricted to the future/past light cone, but that covers all of the earth for any reasonable time travel; of course if you only travel a microsecond, your choices of reentry are severely limited). There's absolutely no reason then to restrict the time machine to enter at the "same" place.