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View Full Version : Solving the motivation-based Time Travel Paradox



Cikomyr
2012-12-12, 06:01 PM
Watching the episode of Big Bang theory with a Time Machine (S1E14 : The Nerdvana Annihilation), Sheldon explains a "Time traveling rookie mistake":

You cannot go back into the past to prevent an event when said event led to to acquiring the time traveling device in the first place.

This is the source of the Time Traveler's plight into the latest remake of the eponymous movie; the death of his fiance causes him to fall into depression, which leads him to invent time travel. But because of this, whenever he goes back into the past, his fiance just dies again.


For me, the obvious solution always have been obvious: go back in time and fake the death of your fiance. Bring her back to the future with you. Therefore, your past self will still have the motivation to create time travel.


Wouldn't help Leonard, obviously. But I was wondering if there was a flaw in my logic.

Gnoman
2012-12-12, 06:30 PM
That's an excellent way of solving the paradox if the paradox exists. I prefer the Schlock Mercenary solution, though.

Alignment
2012-12-12, 07:44 PM
In the most recent remake, I think this element was kind of silly. First she gets shot, then he goes back in time and causes her to get hit by a horse and buggy. Causing someone to die a different way (and for a few minutes longer) does constitute changing the past. He had no reason to give up. So, I'd say that your logic holds up for this kind of time travel.

In the event of You Already Changed The Past style time travel (which I prefer), it would just depend on whether or not she was really dead to begin with (ie. did future you "already" fake your spouse's death). Doing this without creating a confusing stable time loop is difficult, though. I recommend not using time machines. :P

Somewhere
2012-12-12, 07:52 PM
Logic seems like it'd hold to me.
For what it's worth, you'll see this kind of logic used in the climax of the visual novel and anime series Steins;Gate.

jseah
2012-12-12, 11:32 PM
The thing is, while you cannot change the past in consistent time travel, I find that people tend to forget that nothing physically prevents you from doing it.

Even if you did not see anyone time travel to your own room ten minutes ago, (if your machine is able to) you can set your machine to ten minutes ago and activate it.

The thing that prevents inconsistent loops from occurring is the time travel itself. What arrives from the future will cause itself to be sent (that's the only requirement for consistency).

---------------------------------

Take for example the time traveller and his girlfriend. His lover steps out onto the street (he isn't with her) and would be about to get run over by a car if she continues.

At this point, we can explore the various possibilities of what would happen:
1 - Nothing comes from the future: The time traveller later hears about his lover getting killed in a traffic accident and time travels to save her. (Inconsistent)
2 - The time traveller saves the girl and disappears back into the future: The time traveller has no motivation to go back and so doesn't. (Inconsistent)

But those two classical scenarios are not the only possible things that can happen. After all, without anything stopping her death, the time traveller will attempt to do so, leading to inconsistency (because the time traveller left when he didn't arrive). And if he only stops her death, he wouldn't time travel to save her (the time traveller arrives without leaving).

Take this for example:
3 - The time traveller saves the girl and notes where the car passes (he knows she would get hit if he hadn't saved her), then he goes to his present self and tells him to save the girl. (Consistent)


In fact, provided there is only one time machine in play, you can boil that down to a simple rule:
Anything that is trivial for someone forewarned to stop, and the time traveller has motivation to stop from happening, will not happen. The time traveller can simply stop the event and then tell himself to do it.

thubby
2012-12-12, 11:39 PM
In the most recent remake, I think this element was kind of silly. First she gets shot, then he goes back in time and causes her to get hit by a horse and buggy. Causing someone to die a different way (and for a few minutes longer) does constitute changing the past. He had no reason to give up. So, I'd say that your logic holds up for this kind of time travel.

In the event of You Already Changed The Past style time travel (which I prefer), it would just depend on whether or not she was really dead to begin with (ie. did future you "already" fake your spouse's death). Doing this without creating a confusing stable time loop is difficult, though. I recommend not using time machines. :P

the problem isnt that the past cant change. it's that he, as a time traveler, can only exist when she dies.
it's the entire reason he created the time machine. any timeline where he theoretically succeeds is one where he retroactively doesnt exist.

the much simpler solution is to go find your past self, tell them what to do, then go do whatever the heck you want. Unless you're insanely paranoid, if an older version of you walks up to you, you listen.

jseah
2012-12-13, 12:18 AM
the much simpler solution is to go find your past self, tell them what to do, then go do whatever the heck you want. Unless you're insanely paranoid, if an older version of you walks up to you, you listen.
You can also do it and tell your past self to watch you do it.

Forum Explorer
2012-12-13, 12:53 AM
the much simpler solution is to go find your past self, tell them what to do, then go do whatever the heck you want. Unless you're insanely paranoid, if an older version of you walks up to you, you listen.

This is why I will never trust an older version of myself if they walked up to me.
http://imgs.xkcd.com/comics/time_machine.png

ThiagoMartell
2012-12-13, 01:52 AM
In Alan Moore's Twilight of Superheroes, the ending was supposed to be an older John Constantine tricking John Constantine.

MLai
2012-12-16, 08:22 AM
@ Forum Explorer:
I don't get it...


Logic seems like it'd hold to me.
For what it's worth, you'll see this kind of logic used in the climax of the visual novel and anime series Steins;Gate.
DUDE. Edit your post with SPOILER BRACKETS.

Fjolnir
2012-12-16, 01:57 PM
The thing is, while you cannot change the past in consistent time travel, I find that people tend to forget that nothing physically prevents you from doing it.

Even if you did not see anyone time travel to your own room ten minutes ago, (if your machine is able to) you can set your machine to ten minutes ago and activate it.

The thing that prevents inconsistent loops from occurring is the time travel itself. What arrives from the future will cause itself to be sent (that's the only requirement for consistency).

---------------------------------

Take for example the time traveller and his girlfriend. His lover steps out onto the street (he isn't with her) and would be about to get run over by a car if she continues.

At this point, we can explore the various possibilities of what would happen:
1 - Nothing comes from the future: The time traveller later hears about his lover getting killed in a traffic accident and time travels to save her. (Inconsistent)
2 - The time traveller saves the girl and disappears back into the future: The time traveller has no motivation to go back and so doesn't. (Inconsistent)

But those two classical scenarios are not the only possible things that can happen. After all, without anything stopping her death, the time traveller will attempt to do so, leading to inconsistency (because the time traveller left when he didn't arrive). And if he only stops her death, he wouldn't time travel to save her (the time traveller arrives without leaving).

Take this for example:
3 - The time traveller saves the girl and notes where the car passes (he knows she would get hit if he hadn't saved her), then he goes to his present self and tells him to save the girl. (Consistent)


In fact, provided there is only one time machine in play, you can boil that down to a simple rule:
Anything that is trivial for someone forewarned to stop, and the time traveller has motivation to stop from happening, will not happen. The time traveller can simply stop the event and then tell himself to do it.

But this would mean that once time travel is invented, it is invented simultaneously across all of time and possibly all of space as well since someone will eventually use it to observe everywhen...

Emperor Ing
2012-12-16, 02:02 PM
It's simple. You're supposed to go back in time and "alter" the past in the first place, setting off the series of events that result in you getting the means of time travel. You can't change the past, anything you do in it is something you're supposed to do.

CarpeGuitarrem
2012-12-16, 07:30 PM
Actually, this was done in a certain science fiction show in one of the recent seasons...

Man on Fire
2012-12-17, 04:38 AM
According to alternate dimensions theory, when you travel back in time you create alretnate timeline. So we have your original timeline and new timeline, in which you started existing in the past. Therefore all changes happen in new timeline, while original remains unchanged. So even if you undo the evens that lead to you being able to travel back in time or willing to, or even kill your own grandfather, you won't cause a paradox, because evens from your own time, that lead you to be here, remains unchanged.

jseah
2012-12-18, 03:54 AM
But this would mean that once time travel is invented, it is invented simultaneously across all of time and possibly all of space as well since someone will eventually use it to observe everywhen...Not necessarily. Some versions of time machines (the more physics-plausible ones involving extremely fast rotating singularities) can only move things around in time for the period of they have been in continuous operation.

Sort of like, you turn on the time machine at time A and stuff from the future can come out of it, but only after time A. And the machine has to stay on until whatever goes in. When you turn it off at time B, that ends your loop. If you turn it on again at time C, and off at time D, things entering between C and D cannot travel to the times between A and B.

Those are the types I think are most likely to be consistent.

DiscipleofBob
2012-12-18, 11:21 AM
With Quantum Immortality, going back in time to save the fiance would just create an alternate universe where the time traveler visited the past and saved her. He then is presented with one of two choices: go to the present of the new universe where he finds the duplicate of himself who's lived with his presumably wife and never invented a time machine and has to deal with the fact that the time traveler is intruding on someone else's life, or the time traveler goes back to his own depressing life where his fiance died.

Cikomyr
2012-12-18, 11:36 AM
With Quantum Immortality, going back in time to save the fiance would just create an alternate universe where the time traveler visited the past and saved her. He then is presented with one of two choices: go to the present of the new universe where he finds the duplicate of himself who's lived with his presumably wife and never invented a time machine and has to deal with the fact that the time traveler is intruding on someone else's life, or the time traveler goes back to his own depressing life where his fiance died.

How about kidnapping his fiance, faking her death, and bringing her back to his own timeline?

Let his past self suffer and kidnap his own gal

Anarion
2012-12-18, 12:06 PM
In fact, provided there is only one time machine in play, you can boil that down to a simple rule:
Anything that is trivial for someone forewarned to stop, and the time traveller has motivation to stop from happening, will not happen. The time traveller can simply stop the event and then tell himself to do it.

This creates a different kind of ontological paradox, however. Where did the knowledge come from? That is, the time traveler acted because he was told to act. He was told to act by his future self time traveling to tell himself to act. It's a stable closed loop, but it creates information from nothing. There must have been some other result that happened first and was then "corrected" by the time traveler, either leading to multiple branching universes, or a paradox.


How about kidnapping his fiance, faking her death, and bringing her back to his own timeline?

Let his past self suffer and kidnap his own gal

This one works though.

Anne McCaffrey Pern spoilers
The presence of the old generation in the Wyrs was directl caused by Lesa (I think I'm spelling that right, it's been a while) traveling back in time and bringing all the people from the previous generation to her present, thus explaining why most Wyrs had been abandoned for hundreds of years. Ironically, by doing what she did, she probably made things much worse for herself, since the decline in reputation of the dragonriders was due on part to the mysterious absence of most of them for so much time.

jseah
2012-12-18, 12:52 PM
This creates a different kind of ontological paradox, however. Where did the knowledge come from? That is, the time traveler acted because he was told to act. He was told to act by his future self time traveling to tell himself to act. It's a stable closed loop, but it creates information from nothing. There must have been some other result that happened first and was then "corrected" by the time traveler, either leading to multiple branching universes, or a paradox.
It doesn't have to be that way, information is not a conserved quantity and can come from the future. In a very real sense, the future is affecting the past... which when you think about it, in fact, time travel necessitates that very thing!
See also: Time Loop Logic

I can even illustrate a more complex scenario that has the *possibility* of a future event affecting the start time-point of a loop (and the event is *outside* the loop!) and through that, preventing itself from happening as well as preventing any of the actors, including time traveller, from knowing that it was there in the first place. Nevertheless, you can point to it as the "meta-cause" that makes the various potential loops boil down to only one possible loop.

What? Look too deep into the consistent time travel (in my case, write a story using it. Intelligently using it) and strange stuff pops out.

Fjolnir
2012-12-18, 01:12 PM
This is my problem with the terminator movies, there needs to be another one that ends with CGI'd up 80's AHNULD and JC's daddy going back in time, perhaps even a little bad taste humor during the course of the movie between father and son of approximately the same age, we know it's a stable loop because the entire theme of the movies is "the future is fixed and you can only delay it." but we never get the payoff of the win and the end being the beginning.

Cikomyr
2012-12-18, 01:21 PM
This is my problem with the terminator movies, there needs to be another one that ends with CGI'd up 80's AHNULD and JC's daddy going back in time, perhaps even a little bad taste humor during the course of the movie between father and son of approximately the same age, we know it's a stable loop because the entire theme of the movies is "the future is fixed and you can only delay it." but we never get the payoff of the win and the end being the beginning.

That's a problem you have with a franchise who's main theme for the 2nd movie was:

"Your fate is in your hand. You can prevent the death of us all if you work for it"

But then you need to justify further sequels.

Mewtarthio
2012-12-18, 04:23 PM
Attempting to make sense of the Terminator series is an exercise in futility. Each movie has its own consistent rules for time travel, and each is mostly incompatible with the other two. The first movie uses a stable time loop (you have already changed the past). The second movie uses branching timelines ("There is no fate but what we make for ourselves"). The third movie uses some weird fate-based system (you can change the past, but Destiny or something will ensure that things turn out generally the same). The fourth movie I cannot comment on, as I have sealed it away within an impenetrable memory vault; please don't make me remember it. I bet it wasn't relevant anyway. To anything.

Dienekes
2012-12-18, 05:01 PM
Huh, you were able to remember the 3rd one but sealed the 4th one away forever? I woulda reversed those two myself. 4 was nothing special but tolerable, 3 was pretty bad.

Soras Teva Gee
2012-12-18, 05:25 PM
According to alternate dimensions theory, when you travel back in time you create alretnate timeline.

Or more like they were always there to begin with. Ergo the "time travel" is only an illusion, in one reality you left that reality, in another you entered and created such an event.

For reality to exist for you to change it, it has to exist. Which would imply it has always existed back to its own big bang.

Ergo every possibility was always true already.


This creates a different kind of ontological paradox, however. Where did the knowledge come from?

Well ask yourself the question does the knowledge need to come from anywhere?

If it does then the situation can't arise ever, as "self-correction" is a feature of the stable time loop, or it is some way an open loop.

It perfectly fine to have a pool ball from the future hit itself into a time portal to its past, as long as the 'future' pool ball will after hitting the 'past' pool ball, the 'future' pool ball is picked up and taken off the table. And 'past' pool ball was put on the table with enough energy to make the whole thing work in the first place, ie in motion.