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FreakyCheeseMan
2013-02-14, 03:18 PM
So, I'm trying to come up with a consistent set of rules for time travel, such that you actually *can* change the past. So, none of the twelve-monkeys, "any change you made would already have been made" stuff. I've yet to come up with a set of rules as to how this works.

I've come up with two case studies, each of which show the wacko results of different systems.

In case study #1, Adam and Bob decide they want to kill Hitler. They decide that Adam will leave first, arrive in the past and kill hitler; Bob will leave the day before Adam, arrive shortly after Adam, and help bust Adam out of jail. Adam goes back and kills Hitler. Does Bob show up?

If so, we have a problem, because the future that Bob came from no longer exists. As such, we have time travelers showing up from no-longer-extant futures, resulting in annoying, probably exponential piles of time clones. If not, we also have a problem; when Bob left, he still had a past to go back to, as Adam had not yet messed it up. So, where does Bob go?

In case study #2, Adam and Bob get wicked drunk at a party, and Bob dares Adam to jump back in time by two minutes, into the void of space a few miles beyond Earth's atmosphere, and implies that Adam is "a scaredy-cat" if he does not. (Adam's time ship is not airtight.) An inebriated Adam takes the dare. How many Adams show up?

The sudden appearance and asphyxiation of Adam in the cold void of space is unlikely to make much difference to the party, or to Adam and Bob's unfortunate wager. However, the Adam at the party will still be subtly affected by the added gravitational pull of his time-clone in space. As such, if Adam jumps back again, it would be another, separate Adam that shows up.

Assuming no one at the party is watching the news, the rapid appearance of a moon-sized mass of drunken, dead time travelers is still unlikely to prevent the bet from taking place, until the gravitational pull becomes great enough to interfere with the celebrations, by which point it's too late anyway; the world ends with not a bang, not with a whimper, but with the sound of six billion voices screaming "What the hell?" and one solitary voice adding "God damnit, Bob."

So. How do I make a set of logical and consistent rules for time travel, that don't run into either of these problems?

Deophaun
2013-02-14, 03:39 PM
With this you're probably better off going with the multiple universe theory of time travel, as yeah, this stuff breaks down. So, when you go back in time to kill Hitler, you're actually creating a new universe where you kill Hitler. Bob can come along to, and in fact he creates two universes in doing so: one where he came back to help you out, and one where he saw WWII still happened, and figured you failed and his time was better spent making a sandwich. In fact, if Bob does show up, it might not even be the Bob you know. It might be another Bob from a similar but parallel universe, coming at the behest of another, similar but completely different Adam.

Or you can also go the Doctor Who method and say that time travelers exist outside of time, and therefor are unaffected by the changes (no ball of a billion Adams, and the time stream remains fixed while they are in it, so Bob's post-WWII reality exists until he leaves to join Adam).

FreakyCheeseMan
2013-02-14, 03:43 PM
With this you're probably better off going with the multiple universe theory of time travel, as yeah, this stuff breaks down. So, when you go back in time to kill Hitler, you're actually creating a new universe where you kill Hitler. Bob can come along to, and in fact he creates two universes in doing so: one where he came back to help you out, and one where he saw WWII still happened, and figured you failed and his time was better spent making a sandwich. In fact, if Bob does show up, it might not even be the Bob you know. It might be another Bob from a similar but parallel universe, coming at the behest of another, similar but completely different Adam.

Or you can also go the Doctor Who method and say that time travelers exist outside of time, and therefor are unaffected by the changes (no ball of a billion Adams, and the time stream remains fixed while they are in it, so Bob's post-WWII reality exists until he leaves to join Adam).

My problem isn't paradox, but time clones. If we have multiple branching time lines, then time travelers from all of them will end up in the same past, creating a recursive and exponential number of time clones.

Wait. Why would time travelers being unaffected by the changes prevent the ball of Adams?

Deophaun
2013-02-14, 03:48 PM
Wait. Why would time travelers being unaffected by the changes prevent the ball of Adams?
Because they exist outside time, which is author speak for "because I say so."

You're looking for a solution that, if found, would warrant getting published in a prestigious journal of physics. "Because I say so" is going to have to be the bedrock for your rules, not logic, as the time travel you want throws that out the window (which is why the Doctor practically has a lampshade factory in the TARDIS).

Yora
2013-02-14, 03:55 PM
I like the idea that when you time-travel, you "disconnect" from your original timeline. When you return to the moment you left, the world could be drastically different from the one you know and one in which you never existed, but you still exist with all your memories of your life and your time journey.

Depending on how far you go back and how much was changed, you might encounter a person similar to you when you return, but only if your time travel changed the timeline in a way so you never left on the time journey. If your past self in the new timeline still went on the time journey, than that was "you" and there would be no duplicate.

Essentially, you could never return to your own time. There is also no way to "fix" things to how you remember them, as every time journey only creates additional changes and there is way too much randomness involved to recreate the exact circumstances of your timeline of origin.

CarpeGuitarrem
2013-02-14, 04:01 PM
I like the idea that when you time-travel, you "disconnect" from your original timeline. When you return to the moment you left, the world could be drastically different from the one you know and one in which you never existed, but you still exist with all your memories of your life and your time journey.

Depending on how far you go back and how much was changed, you might encounter a person similar to you when you return, but only if your time travel changed the timeline in a way so you never left on the time journey. If your past self in the new timeline still went on the time journey, than that was "you" and there would be no duplicate.

Essentially, you could never return to your own time. There is also no way to "fix" things to how you remember them, as every time journey only creates additional changes and there is way too much randomness involved to recreate the exact circumstances of your timeline of origin.
One of the Fate Core expansions plays with this, I don't remember the name...and I guess it isn't very useful if you haven't backed the Kickstarter...

It should be available in not too long, though, as part of the expansions bundle.

Treblain
2013-02-14, 04:04 PM
Since you're running the show rather than coming up with rules for actual time travel, you can do narrative-based time travel by willfully concealing information about the past until the time travel determines what actually happened.

Example: Bob is in a building when it suddenly exploded. Steve travels back in time to save him. If he's successful, then he brings Bob to the present immediately, or hides him in a shed in the middle of nowhere until the past catches up, resolving the question of what Bob was up to if he was alive the whole time. If he fails, then Bob was dead all along. If you have a situation like Bob being shot to death in front of dozens of witnesses, then you have to come up with a crazier solution like clones or death-faking pills, but it works all the same.

This makes absolutely no sense in answering what actually happened in the timeline before time travel was introduced, but it works great for stories, especially ones you're improvising as you go. This way, you can change the past while having main details being fixed in place, and you never have to worry about paradoxes or memories changing.

FreakyCheeseMan
2013-02-14, 04:07 PM
Because they exist outside time, which is author speak for "because I say so."

You're looking for a solution that, if found, would warrant getting published in a prestigious journal of physics. "Because I say so" is going to have to be the bedrock for your rules, not logic, as the time travel you want throws that out the window (which is why the Doctor practically has a lampshade factory in the TARDIS).

Well, of course I'm going to have to flub the actual mechanism. But, I want to have a set of rules that will apply in a consistent way to any scenario.

So, for the Adams-in-Space one... Adam jumps back, but there's still an Adam on earth who's about to make that time jump, and now there's already an Adam in space waiting for him. It would be more difficult, but I could set it up so that each successive Adam arrives an instant later than the last.

How does Adam being unaffected by the changes alter that?

FreakyCheeseMan
2013-02-14, 04:10 PM
Since you're running the show rather than coming up with rules for actual time travel, you can do narrative-based time travel by willfully concealing information about the past until the time travel determines what actually happened.

Example: Bob is in a building when it suddenly exploded. Steve travels back in time to save him. If he's successful, then he brings Bob to the present immediately, or hides him in a shed in the middle of nowhere until the past catches up, resolving the question of what Bob was up to if he was alive the whole time. If he fails, then Bob was dead all along. If you have a situation like Bob being shot to death in front of dozens of witnesses, then you have to come up with a crazier solution like clones or death-faking pills, but it works all the same.

This makes absolutely no sense in answering what actually happened in the timeline before time travel was introduced, but it works great for stories, especially ones you're improvising as you go. This way, you can change the past while having main details being fixed in place, and you never have to worry about paradoxes or memories changing.

I'm not as worried for stories - with stories, I can always do a "The past was always the same way."

But, I have one particular thing I want to try, with time-traveling Fey trying to save their city (Avalon) from falling to a siege. My own annoying mind makes me want a consistent set of rules, but I've yet to come up with anything that would support that, without time clone singularities.

KillianHawkeye
2013-02-14, 04:20 PM
If you're looking for something more like a single timeline with rewriteable history, I suggest reading Pastwatch: The Redemption of Christopher Columbus by Orson Scott Card. His solution to sending multiple people back to different points in history was that they all had to leave the present simultaneously, because any change to past events effectively destroys the current timeline. In other words, the new timeline instantaneously overwrites the previous one. I believe it was one-way time travel, though, so maybe not what you're looking for.

It also happens to be one of the best SF novels I've ever read.

FreakyCheeseMan
2013-02-14, 04:28 PM
If you're looking for something more like a single timeline with rewriteable history, I suggest reading Pastwatch: The Redemption of Christopher Columbus by Orson Scott Card. His solution to sending multiple people back to different points in history was that they all had to leave the present simultaneously, because any change to past events effectively destroys the current timeline. In other words, the new timeline instantaneously overwrites the previous one. I believe it was one-way time travel, though, so maybe not what you're looking for.

It also happens to be one of the best SF novels I've ever read.

>_> One way time travel can be made into two-way time travel with an endowment and a stand-up freezer.

That sort of works for the first one, but it has another problem - you can never watch a time-traveler leave, because once he's left, you cease to exist.

mjlush
2013-02-14, 04:32 PM
Much to the headache of my player I really like running timetravel games

I believe the most playable version of time travel is the observer effect. ie a timetraveller cannot change the observed facts from their POV, but they can add new facts.

A time traveler could not stop JFK being shot.. but they could kidnap him and substitute a cloned replica (and probably find much too late that they have kidnapped someone elses cloned replica).

The nice thing about this approach is so long as the players play smart they have a lot of opportunity to play fast and loose. but if they try an pull a Bill and Ted and not bother to sent the time machine back to rescue themselves someone else does... much to the determent of the party :->

One thing when running a time travel game .. a detailed Campaign Chronicle is absolutely vital

Anxe
2013-02-14, 04:34 PM
Well, of course I'm going to have to flub the actual mechanism. But, I want to have a set of rules that will apply in a consistent way to any scenario.

So, for the Adams-in-Space one... Adam jumps back, but there's still an Adam on earth who's about to make that time jump, and now there's already an Adam in space waiting for him. It would be more difficult, but I could set it up so that each successive Adam arrives an instant later than the last.

How does Adam being unaffected by the changes alter that?

I'm not understanding the problem here. There's only one Adam that jumps back in time. He then dies. No other Adams are available to jump back unless you're allowing alternate universe Adams to jump into the same universe.

Grinner
2013-02-14, 04:44 PM
I'm not understanding the problem here. There's only one Adam that jumps back in time. He then dies. No other Adams are available to jump back unless you're allowing alternate universe Adams to jump into the same universe.

Adam makes the jump into the past two minutes ago, arriving at the exact point in space he left his own time (i.e. outer space) and asphyxiates.
Meanwhile, on Earth, Bob is talking Adam+1 (the second iteration of Adam) into making the jump.

Adam+1 makes the jump into the past two minutes ago, arriving at the exact point in space he left his own time (i.e. outer space) and asphyxiates.
Meanwhile, on Earth, Bob is talking Adam+2 (the third iteration of Adam) into making the jump.

Adam+2 makes the jump into the past two minutes ago, arriving at the exact point in space he left his own time (i.e. outer space) and asphyxiates.
Meanwhile, on Earth, Bob is talking Adam+3 (the fourth iteration of Adam) into making the jump.

Adam+3 makes the jump into the past two minutes ago, arriving at the exact point in space he left his own time (i.e. outer space) and asphyxiates.
Meanwhile, on Earth, Bob is talking Adam+4 (the fifth iteration of Adam) into making the jump.

Adam+4 makes the jump into the past two minutes ago, arriving at the exact point in space he left his own time (i.e. outer space) and asphyxiates.
Meanwhile, on Earth, Bob is talking Adam+5 (the sixth iteration of Adam) into making the jump.

Adam+5 makes the jump into the past two minutes ago, arriving at the exact point in space he left his own time (i.e. outer space) and asphyxiates.
Meanwhile, on Earth, Bob is talking Adam+6 (the seventh iteration of Adam) into making the jump.

Etc.

Frankly, I'd be more concerned about the effect of an infinite number of people attempting to occupy the same space at once. It could be quite...explosive.

CarpeGuitarrem
2013-02-14, 05:00 PM
Uh...no, it doesn't work like that.

Adam makes the jump into the past, where he then becomes Adam+1 and then dies. He can't jump back into the past again, because he's no longer at the party. He's in the void of space.

You're thinking of a situation like this...

Adam jumps into the past, becoming Adam+1. He does not die. Instead, he lives on and now there's two Adams from different points in time.

Adam+1 jumps back in time, becoming Adam+2, and doesn't die. Unless somehow he transports back to the exact same point in space, then...presumably they tele-frag each other and die, and the universe burps out the remains. :smallbiggrin:

But if Adam+2 transports back to another point in space, it could theoretically keep going. Except that Adam+2 is older than Adam+1 is older than Adam at the same point in the objective timeline. If Adam keeps looping back, there'll be Adams everywhere, but he'll effectively be living within the same span of years.

FreakyCheeseMan
2013-02-14, 05:04 PM
Uh...no, it doesn't work like that.

Adam makes the jump into the past, where he then becomes Adam+1 and then dies.

You're thinking of a situation like this...

Adam jumps into the past, becoming Adam+1. He does not die. Instead, he lives on and now there's two Adams from different points in time.

Adam+1 jumps back in time, becoming Adam+2, and doesn't die. Unless somehow he transports back to the exact same point in space, then...presumably they tele-frag each other and die, and the universe burps out the remains. :smallbiggrin:

But if Adam+2 transports back to another point in space, it could theoretically keep going. Except that Adam+2 is older than Adam+1 is older than Adam at the same point in the objective timeline. If Adam keeps looping back, there'll be Adams everywhere, but he'll effectively be living within the same span of years.

No....

Adam jumps back in time, and into space. From space, he stares at his 2-minutes-ago self, who is now preparing to jump. That 2-minutes-ago self then jumps, but in his timeline, there's already an adam (the first jumper) in the space he's about to occupy. And so on and so on.

Yora
2013-02-14, 05:11 PM
Here is a principle of time travel that works with only one timestream and that is 100% paradox free:

There is only a single linear timestream. When you travel backward in time, you suspend yourself from timespace while the timestream is rewinding. Then you insert yourself back into timespace at the desired moment in the past.
At that point time continues normally but with the time traveler also interacting with everything else, causing the timestream to play out differently this time.
If you want to travel to the future, you again remove yourself from timespace and then just wait it out until the timestream has again reached the point you want to go to and then re-enter the timestream.

Only downside, when you travel back in time, you destroy the future. But it does allow for as many time travelers going through time as you want to, without getting a giant cluster****.

Deophaun
2013-02-14, 05:12 PM
OK, he's a solution: quantum mechanics.

You talk about how Adam 1 has a slight gravitational pull on Adam 2, so it's a new history. That means that you also get an entirely new set of quantum fluctuations occurring throughout the entire universe (they're random, so there's no reason for them to be the same quantum fluctuations the origin timeline experienced). If you subscribe to the idea that the brain is a quantum computer (which will help you out here), Adam 2 now has a different set of significant inputs that will influence his decision. He can choose not to go back in time simply because reality changed, not because of any recognizable physical phenomena.

This, by the way, means that if you go back in time to an empty corner of the universe and do absolutely nothing, you will have completely altered the course of history and the future will be utterly unrecognizable, because everyone would have made different choices just by the fact that you changed the universe's random number seed.

Now, the question is, what do we do with all the dead catgirls that this is going to generate?

Grinner
2013-02-14, 05:17 PM
OK, he's a solution: quantum mechanics.

You talk about how Adam 1 has a slight gravitational pull on Adam 2, so it's a new history. That means that you also get an entirely new set of quantum fluctuations occurring throughout the entire universe (they're random, so there's no reason for them to be the same quantum fluctuations the origin timeline experienced). If you subscribe to the idea that the brain is a quantum computer (which will help you out here), Adam 2 now has a different set of significant inputs that will influence his decision. He can choose not to go back in time simply because reality changed, not because of any recognizable physical phenomena.

This, by the way, means that if you go back in time to an empty corner of the universe and do absolutely nothing, you will have completely altered the course of history and the future will be utterly unrecognizable, because everyone would have made different choices just by the fact that you changed the universe's random number seed.

That's a fascinating take. I'll have to read up on quantum mechanics.

Now, the question is, what do we do with all the dead catgirls that this is going to generate?

Meat is meat. :smallamused:

huttj509
2013-02-14, 05:18 PM
You're making less sense than Zero Escape. And that's saying something.

FreakyCheeseMan
2013-02-14, 05:19 PM
Here is a principle of time travel that works with only one timestream and that is 100% paradox free:

There is only a single linear timestream. When you travel backward in time, you suspend yourself from timespace while the timestream is rewinding. Then you insert yourself back into timespace at the desired moment in the past.
At that point time continues normally but with the time traveler also interacting with everything else, causing the timestream to play out differently this time.
If you want to travel to the future, you again remove yourself from timespace and then just wait it out until the timestream has again reached the point you want to go to and then re-enter the timestream.

Only downside, when you travel back in time, you destroy the future. But it does allow for as many time travelers going through time as you want to, without getting a giant cluster****.

It seems like it only allows for one, at least going backward. Time traveling forward isn't interesting, cause that's what happens by default.

It also imposes some pretty heavy narrative problems. First, you can never, ever remember seeing a time ship having left; when it left, it destroyed the universe, to you can't have seen it.

Secondly, nomatter how long you wait, no time ship will ever arrive, since the arrival of a time ship is essentially creating a universe. You can *remember* time ships arriving, though.

But, yeah, it is consistent.

kieza
2013-02-14, 05:25 PM
There is one idea that I liked, in that any change you make has to be such that the observed result doesn't change. That is, you can kill Hitler, but you have to replace him with a double who hangs around for people to see until Hitler's recorded demise. Alternately, if you don't put a double in place, someone from further up the timestream gets hit by the paradox, and comes back to resuscitate Hitler after you leave.

It rules out the "kill Hitler" time-travel plot, but it lets you do others like "save the scrolls from the Library of Alexandria," "steal the Hope Diamond and replace it with a fake" or "rescue Abraham Lincoln from his assassination."

It also lets you do some fun things with time. Example: You're locked in a prison cell. So, naturally, you decide that some time after you get out, you'll come back in time to help yourself get out. You then look under the mattress and find the key to the door. You then have to arrange things so that this becomes a stable time loop: You need to obtain the key from some point along the timeline, put it under the mattress, and replace it with the version you found in the cell, all without being caught. It's complicated, but it quickly becomes hilarious when you start doing it with cruise ships. In one of my games, the players needed so many ships that they single-handedly caused the Bermuda Triangle.

FreakyCheeseMan
2013-02-14, 05:28 PM
There is one idea that I liked, in that any change you make has to be such that the observed result doesn't change. That is, you can kill Hitler, but you have to replace him with a double who hangs around for people to see until Hitler's recorded demise. Alternately, if you don't put a double in place, someone from further up the timestream gets hit by the paradox, and comes back to resuscitate Hitler after you leave.

It rules out the "kill Hitler" time-travel plot, but it lets you do others like "save the scrolls from the Library of Alexandria," "steal the Hope Diamond and replace it with a fake" or "rescue Abraham Lincoln from his assassination."

It also lets you do some fun things with time. Example: You're locked in a prison cell. So, naturally, you decide that some time after you get out, you'll come back in time to help yourself get out. You then look under the mattress and find the key to the door. You then have to arrange things so that this becomes a stable time loop: You need to obtain the key from some point along the timeline, put it under the mattress, and replace it with the version you found in the cell, all without being caught. It's complicated, but it quickly becomes hilarious when you start doing it with cruise ships. In one of my games, the players needed so many ships that they single-handedly caused the Bermuda Triangle.

That works in terms of paradox, but it doesn't really solve the second listed problem. Plus, there's the whole issue of "What is observation" and "What about things that could become observed later?".

Reltzik
2013-02-14, 05:44 PM
Multiple futures is definitely the way to go. In the case of the Darwin-Award nominee Adam, let's say that Adam is in Universe A, and call this Adam-A. Adam then jumps back in time. This causes an alteration in the universe as it would have played out, meaning that a new universe splits off. This is universe B, which has an Adam B in it. No Adam shows up in universe A. Adam A does show up in universe B, and asphyxiates. If this does have any effect on Adam B, and his time-travelling, that will in turn create universe C, which Adam-B shows up in. In short, each universe either gets 1 Adam, or none.

The tricky part is the first scenario, wherein someone attempts to travel back in time from a universe which has had a previous time traveler travel back in time. You need to watch out for two things. First, that your arrival time is exactly the same as the previously-departing time traveler. If you're too early, you spawn a new universe, which your buddy doesn't land in, and if you're too late, your buddy spawns a new universe which you don't land in. Either way, you're separated. And second, you can expect a bunch of time travelers coming back from futures that you never would have dreamt would happen, altering the course of events, and thus ensuring that those futures didn't happen. This, of course, would introduce time travel devices before they're invented.

You might be able to hand-wave a lot of this away with the idea of multiple pasts. Just as it might be possible for the present to split off into multiple possible futures, it might also be possible for multiple pasts to combine into a single present. For example, in one past, Oswald shoots Kennedy and there are conspiracy claims that it was some CIA plot gunman on the grassy knoll etc. In a DIFFERENT past, the conspiracy actually exists, there is a second gunman, but the truth gets generally dismissed as conspiracy fiction. Because both scenarios result in pretty much the same present, we can say those timelines merged together. You can use this as a device for reconnecting time-split parties.

Though your players might have fun with, "Okay, you now have a new background wherein your character is much the same, but because your buddy changed time you instead spent the past fifteen years in a zombie post-appocalypse. I'll give you five minutes to figure out the changes in your personality, history, and stats. GO."

Now, the question is, what do we do with all the dead catgirls that this is going to generate?

Sweep them under the rug and trust anorexia to keep the bulge inconspicuous, of course. Same as always.

FreakyCheeseMan
2013-02-14, 05:57 PM
So, my problem with multiple futures is that, if they share a single past (or a smaller number of pasts than futures), then you get *lots* of futures feeding time travelers into a single past, which causes problems quickly - you'll get a lot of time clones, and each of them spawns another time line, so you get even more, and then the universe collapses in a black whole of confused time-travelers.

Multiple pasts is... an interesting idea, but conceptually, hard to work. For it to solve the problem, we have to have roughly as many pasts as futures; that, however, strains imagination.

We can imagine a future in which the machines rise against us, or do not. But, we can't really imagine a past (our past) in which WWII didn't happen. So, instinctively, it seems like there'd be more futures.

Now, I know more about orders of infinity than the average bear, so I know that isn't exactly the same as impossible to overcome, but it's still tricky. A hotel with every room full can still give a room to a new guest, so long as there are infinite rooms, but I have trouble coming up with the narrative mechanisms for such to happen in a game universe.

CarpeGuitarrem
2013-02-14, 06:14 PM
No....

Adam jumps back in time, and into space. From space, he stares at his 2-minutes-ago self, who is now preparing to jump. That 2-minutes-ago self then jumps, but in his timeline, there's already an adam (the first jumper) in the space he's about to occupy. And so on and so on.

That's because...they're one and the same.

From Adam's chronology, this is how it looks.

Five Minutes Ago
Adam is getting talked into the bet

Two Minutes Ago
Adam is still getting talked into the bet

Zero Hour
Adam jumps into the past, and now becomes Future!Adam from its point of view. He is witnessed by Adam.

There is only one Adam. I only refer to him as Adam and Future!Adam to avoid confusion. Because, really, it goes like this...

Adam jumps 2 minutes into the past, and dies in the void of space...while he watches himself from 2 minutes ago getting talked into the jump. If he survives for the full two minutes, he sees himself disappear from the present. The loop closes.

His personal timeline runs right alongside the objective timeframe, but it's as if it's been snipped and pushed back some so that the end of one and the beginning of the other overlap.

Reltzik
2013-02-14, 06:31 PM
So, my problem with multiple futures is that, if they share a single past (or a smaller number of pasts than futures), then you get *lots* of futures feeding time travelers into a single past, which causes problems quickly - you'll get a lot of time clones, and each of them spawns another time line, so you get even more, and then the universe collapses in a black whole of confused time-travelers.

Multiple pasts is... an interesting idea, but conceptually, hard to work. For it to solve the problem, we have to have roughly as many pasts as futures; that, however, strains imagination.

We can imagine a future in which the machines rise against us, or do not. But, we can't really imagine a past (our past) in which WWII didn't happen. So, instinctively, it seems like there'd be more futures.

Now, I know more about orders of infinity than the average bear, so I know that isn't exactly the same as impossible to overcome, but it's still tricky. A hotel with every room full can still give a room to a new guest, so long as there are infinite rooms, but I have trouble coming up with the narrative mechanisms for such to happen in a game universe.

Warning: Semi-intense maths follow.

I've played around with this some from the mathematical side, too. Instead of thinking of a fixed universe and a changing state, consider instead a set of possible states, and time-flow as a function or Markhov chain defined on this set of states. There's too many states to count, so we'll use a sigma-algebra (probably defined in terms of observable variables) to break these states into workable subsets. We'll call these smallest subsets universes. There are infinite possible states for each universe, but we have no mechanism for distinguishing between them, so they all fall into the same element of the sigma algebra. For extra fun, the sigma-algebra can be defined in terms of variables that we, personally, can observe, rather than can be observed from anywhere in the universe. So our present universe "A" will combine the possibility that the sun just went nova with the possibility that it didn't, because we won't be able to discern the difference for over 8 minutes.

Arbitrarily, let's say that many of these sigma algebras are measurable, especially in a conditioned-probability sense. GIVEN that we're in universe B, there is a calculable probability that we WERE in universe C, D, E, or F twenty minutes ago, and similarly twenty minutes ahead of time. There are likely infinite options, but let's stick with those four past universes as examples.

This brings us back to the two subsets of universe A (sun goes splody, versus sun doesn't go.... well, okay, it does go splody, but it's just the usual trillions of fusion bombs per second splody rather than something bad). These are NOT distinguishable in the moment. They ARE distinguishable 9 minutes later. In short, we can distinguish between lots and lots more universes in the past, than we can in the present. Hindsight really IS better. This means that there are actually MORE universes in the past than in the present, because we have the tools to break them down better. (Actually, there's only one universe in the present, and the number of universes increases the further into the past we look.)

Now, let's go back in time. The probability dictates WHICH of C, D, E, or F we end up in. Let's say we randomly end up in F. We're immediately shunted into universe G, which is what would happen to F if we had a bunch of particles randomly assemble themselves under the rules of quantum mechanics into a random group of people (us) with the appropriate physical condition and memories. This is extremely low-probability from F, but given that we started in B, which has us pushing the button on our time-travel machine, it's pretty high odds for us.

Now here's the interesting part. Every substate in B has some version of us going back in time, appearing in C, D, E, or F, and getting shunted into the appropriate offshoot. If C, D, E, and F make up every possible past for universe B, then their measure is IDENTICAL to universe B. That means we've got exactly the same order of infinity of time travelers and destination universes, AND the same sense of weighting (it's not exactly a number) of time travelers and destinations.

In other words, it could work. ... you know, if we could actually build a time machine.

Grinner
2013-02-14, 06:51 PM
*snip*

I think that makes sense...Time still advances, leaving room at that point in space-time for the next Adam?

FreakyCheeseMan
2013-02-14, 06:53 PM
Math math math ma-mathy-math-math. Math!

It's been a few years since I flunked out of my math major, but I think I followed most of that. So, you're combining the notion of a time cone into this - not only is there only one present, but that present has a very narrow spatial definition, because things just a little ways aware are unknowable?

FreakyCheeseMan
2013-02-14, 06:55 PM
That's because...they're one and the same.

From Adam's chronology, this is how it looks.

Five Minutes Ago
Adam is getting talked into the bet

Two Minutes Ago
Adam is still getting talked into the bet

Zero Hour
Adam jumps into the past, and now becomes Future!Adam from its point of view. He is witnessed by Adam.

There is only one Adam. I only refer to him as Adam and Future!Adam to avoid confusion. Because, really, it goes like this...

Adam jumps 2 minutes into the past, and dies in the void of space...while he watches himself from 2 minutes ago getting talked into the jump. If he survives for the full two minutes, he sees himself disappear from the present. The loop closes.

His personal timeline runs right alongside the objective timeframe, but it's as if it's been snipped and pushed back some so that the end of one and the beginning of the other overlap.

So, that's just a "You can't change the past, cause you already would have" case - already mentioned I was trying to avoid those, cause they so limit what you can do with time travel.

Magnema
2013-02-14, 07:05 PM
A comment: in our universe, the past isn't actually defined. Just as we can't tell the future due to the Heisenberg Uncertainty Principle, we can't tell the past, either. Physically, there isn't any difference between going into the future and going into the past other than that going into the future increases entropy, which is a result of statistics, not any particular physical phenomenon. Therefore, the past is unknowable in the same mechanism as the future.

So: This means that the only perfectly discernable time is the present. You don't need a self-consistent universe, because the past you remember (regardless of what it may be) is only one (highly probable) past - other pasts are possible, albeit unlikely.

I suppose that this is the system of "multiple pasts" that FreakyCheeseMan mentioned and dismissed as difficult to conceptualize (which I fully agree with).

In any case, essentially, this resolves the two issues as follows:

Issue 1: Adam goes back in time, kills Hitler. The next day, he remembers himself having done that - but that doesn't mean that actually happened. The exact universal conditions depends on a lot of things, but basically, either he imagined that he was from the future the whole time or Hitler was dead the whole time and neither of them knew it (say, for instance, all of their history teachers lied to them/were mistaken/whatever).

As you might or might not be able to see (I will fully admit that this is overly complicated), while this does, technically, resolve both issues, it's not helpful, because what it means is that the characters' memories are at fault... meaning that some of what the players believed happened never actually happened, and that instead that is just what they remember happening. On the other hand, with some players, that happens often enough anyway... :smalltongue:

A side note: the system that Yora presented would also resolve the problem of "where are all the time travelers?" Obviously, there can only be one time traveler in that system, and he arrived earlier - this universe will last until someone time-travels, at which point the new universe will have one (and only one) time-traveler arrive in it as well.

Incom
2013-02-14, 07:06 PM
My solution: time travel splits timeline via Delayed Ripple Effect working at the same rate as normal time.

Scenario:

Bob from our world, Timeline A, goes back in time 90 years (that's 1923 for reference). Bob then time-travels forward 90 years to see what happens in Timeline B (presumably he killed Hitler or somesuch, but the timeline splits the moment he ceases traveling so what exactly happens doesn't matter to us).

Rules:

1. If forwards time travel occurs, shouldn't cause any paradoxes right? Don't think anything particularly unusual should occur, unless I'm wrong.
2. If backwards time travel occurs, non time-travelers don't notice, nor are they affected in any way.
3. Bob goes back and does his thing. Ripple effect begins. Timeline splits and is overwritten at a rate equal to the normal passage of time. Bob is now on Timeline B.
4. Timeline A is gradually overwritten by Timeline B--but since the ripple effect moves at the same rate as everything else does, Timeline A proceeds as normal.

If someone from a later point on Timeline A travels back before 1923, they create Timeline C. Bob doesn't notice because he's still on Timeline B, but if he travels back again more than the difference he will be shunted to Timeline C.

If someone from an earlier point on Timeline A travels back to before 1923, either nothing happens (because Bob still started on Timeline A and didn't cross the ripple), or Bob comes out in 1923-B and presumably is going to do something totally different (ie create Timeline C after crossing the ripple).

If someone on Timeline A travels back less than 90 years, they create Timeline A'. Bob doesn't notice because A and A' are the same until that point, and he created a new timeline by traveling back anyway.

If someone from Timeline B travels back before 1923, Bob won't notice because he's on Timeline B, but that person will create Timeline C.

Presumably y'all get the point.

IIRC something similar was used in Doctor McNinja, but without the overwriting-timelines part.

I have to leave, but I'll try to apply this to any paradoxes if you don't.

Reltzik
2013-02-14, 07:09 PM
It's been a few years since I flunked out of my math major, but I think I followed most of that. So, you're combining the notion of a time cone into this - not only is there only one present, but that present has a very narrow spatial definition, because things just a little ways aware are unknowable?

Kinda, except the opposite. There is only one present, but that present has little definition and thus encompasses far more weight because there are so few restrictions we can place on it. We know virtually nothing about it, and thus it encompasses far more possibilities and a far greater "weight" than a narrowly-defined pass. 10 minutes from now we might check a news site and learn that so-and-so celebrity is in jail on drug charges (as happened 2 hours ago, but only posted on the news site 5 minutes in the future). The present, however, contains two possibilities from our perspective: That the celebrity is in jail, and that she isn't. We can't distinguish between them. Yet. Ten minutes from now, we can.

Grinner
2013-02-14, 07:15 PM
As you might or might not be able to see (I will fully admit that this is overly complicated), while this does, technically, resolve both issues, it's not helpful, because what it means is that the characters' memories are at fault... meaning that some of what the players believed happened never actually happened, and that instead that is just what they remember happening. On the other hand, with some players, that happens often enough anyway... :smalltongue:

So......Time travellers are just a bunch of deluded, raving lunatics?

FreakyCheeseMan
2013-02-14, 07:35 PM
Kinda, except the opposite. There is only one present, but that present has little definition and thus encompasses far more weight because there are so few restrictions we can place on it. We know virtually nothing about it, and thus it encompasses far more possibilities and a far greater "weight" than a narrowly-defined pass. 10 minutes from now we might check a news site and learn that so-and-so celebrity is in jail on drug charges (as happened 2 hours ago, but only posted on the news site 5 minutes in the future). The present, however, contains two possibilities from our perspective: That the celebrity is in jail, and that she isn't. We can't distinguish between them. Yet. Ten minutes from now, we can.

I think it might be better to keep a strict time cone definition (4-dimensional cone, expanding at light speed from a single point in space-time); that way it's completely rigorous in terms of restrictions and knowledge.

FreakyCheeseMan
2013-02-14, 07:37 PM
My solution: time travel splits timeline via Delayed Ripple Effect working at the same rate as normal time.

Scenario:

Bob from our world, Timeline A, goes back in time 90 years (that's 1923 for reference). Bob then time-travels forward 90 years to see what happens in Timeline B (presumably he killed Hitler or somesuch, but the timeline splits the moment he ceases traveling so what exactly happens doesn't matter to us).

Rules:

1. If forwards time travel occurs, shouldn't cause any paradoxes right? Don't think anything particularly unusual should occur, unless I'm wrong.
2. If backwards time travel occurs, non time-travelers don't notice, nor are they affected in any way.
3. Bob goes back and does his thing. Ripple effect begins. Timeline splits and is overwritten at a rate equal to the normal passage of time. Bob is now on Timeline B.
4. Timeline A is gradually overwritten by Timeline B--but since the ripple effect moves at the same rate as everything else does, Timeline A proceeds as normal.

If someone from a later point on Timeline A travels back before 1923, they create Timeline C. Bob doesn't notice because he's still on Timeline B, but if he travels back again more than the difference he will be shunted to Timeline C.

If someone from an earlier point on Timeline A travels back to before 1923, either nothing happens (because Bob still started on Timeline A and didn't cross the ripple), or Bob comes out in 1923-B and presumably is going to do something totally different (ie create Timeline C after crossing the ripple).

If someone on Timeline A travels back less than 90 years, they create Timeline A'. Bob doesn't notice because A and A' are the same until that point, and he created a new timeline by traveling back anyway.

If someone from Timeline B travels back before 1923, Bob won't notice because he's on Timeline B, but that person will create Timeline C.

Presumably y'all get the point.

IIRC something similar was used in Doctor McNinja, but without the overwriting-timelines part.

I have to leave, but I'll try to apply this to any paradoxes if you don't.

Okay. So, suppose that from the year 2000, bob goes back and kills kills hitler in 1920. In the year 2020, adam (from the original time line) goes back to 1930. By that point, 1930 should have been overwritten. So, does Adam emerge into a strange past where Hitler has already been killed?

2013-02-14, 07:52 PM
Okay. So, suppose that from the year 2000, bob goes back and kills kills hitler in 1920. In the year 2020, adam (from the original time line) goes back to 1930. By that point, 1930 should have been overwritten. So, does Adam emerge into a strange past where Hitler has already been killed?

Instead of a "butterfly effect" time travel I would go with a "final destination" one if you want consistency.

So he kills hitler, but all this stuff was destined to happen thereby nothing really changes, there is just someone else who takes hitlers place.

Think of the timeline as a rubber band, yes if you directly interact with it (ie drag the band) you can change stuff, but it will correct itself to the normal "position" again once you stop actively changing it.

This I think is the only way for consistency in time travel, though it also means that time travel is ultimately useless in the long run :smallbiggrin:

AttilaTheGeek
2013-02-14, 08:02 PM
The way Doctor Who does it is actually fairly simple, and self-consistent too.

There is only one timeline, but you can only be in one place at one time.

Here's what it means for our friends Adam and Bob:

Adam gets drunk and takes a bet. He tries to jump into space, but he can't, because two minutes ago he was here (in the bar), not there (in space).

Or Adam decides to go kill Hitler (http://xkcd.com/1063/), but then Bob gives him an urgent mission (http://xkcd.com/567/). At 8:05, Bob goes back to Hitler and kills him, which he can do because he didn't exist in 1945 1939. However, that will have changed the entire future up to and including the present day, so Bob might never have been born. Adam can jump forward to 8:06, but he can never again live through the time from when he was born up to 8:05, because he was there already. Note that 8:06 might look completely different from 8:05.

lunar2
2013-02-14, 08:47 PM
okay, dragonball version of time travel.

each instance of backwards time travel creates a split in the timeline.

timeline 1, year 0. trunks leaves his crapsack world, and travels 20 years into the past.

timeline 2, year -20. trunks arrives, kills frieza. informs goku about the danger in the future. returns to his present.

timeline 1, year 0 through 3. trunks returns, and trains for 3 years. then goes back to the past.

timeline 2, year -17. trunks returns, and manages to deactivate the androids. returns to his present.

timeline 1, year 3. trunks returns from the past, and eventually deactivates the androids of the present.

timeline 1, year 7. trunks prepares to return to the past to inform his friends of his success. cell kills him, steals the time machine, and goes back even further than trunks had.

timeline 3, year - 21. cell arrives, and immediately goes into hibernation.

timeline 3, year -20. trunks arrives, kills frieza, and informs goku of the future threat, then returns to his present.

timeline 4, year 0-3. trunks (who has been altered by his travel to timeline 3 instead of 2) arrives to his present, different timeline. everything is the same, except him. trains for 3 years. returns to the past.

timeline 3, year -17. a bunch of weird stuff happened, everyone got super strong. cell died. trunks returns to the future.

timeline 4, year 3. trunks returns, goes and kills the androids.

timeline 4, year 7. cell attempts to kill trunks, and gets slaughtered.

as you can see, there are no paradoxes, and no time clones. only one instance of any time traveller exists at any point in a single timeline, but multiple different time travellers can and do co-exist, as long as one of them is the creator of that timeline. yes, there are a half dozen immediate other timelines all spiralling away to infinity, but they are not plot relevant.

Tl;Dr the solution to your time travel problem is to ignore timelines that are not plot relevant.

Talakeal
2013-02-14, 09:07 PM
My 2 pence:

If you had multiple copies of the same person / object existing at the same point in time it would violate conservation of energy, and I imagine they would collapse into nothingness in the same manner as virtual particles.

If you have a closed time loop this might be temporarily allowed as sort of a temporal potential energy, but I can't imagine the universe would allow such a thing to go on indefinitely.

CarpeGuitarrem
2013-02-14, 09:57 PM
So, that's just a "You can't change the past, cause you already would have" case - already mentioned I was trying to avoid those, cause they so limit what you can do with time travel.
No, it's more of a "history is in flux", like in Doctor Who and Looper.

But whether or not you change the past is irrelevant. Future!Adam doesn't affect Adam in your scenario. In every conceivable form of time travel, Future!Adam would witness himself jumping back in time, with no infinite loop. I can't pin it, but I think you're committing Zeno's Paradox, but with Time Travel.

dps
2013-02-15, 12:39 AM
Since you're running the show rather than coming up with rules for actual time travel, you can do narrative-based time travel by willfully concealing information about the past until the time travel determines what actually happened.

Well, it's a good thing he's not trying to come up with rules for actual time travel, because according to our current knowledge of physics, the First Rule of Time Travel is also the only rule of time travel: It ain't possible (except in the sense that we're all travelling into the future via The Slow Path).

The Grue
2013-02-15, 02:33 AM
So, that's just a "You can't change the past, cause you already would have" case - already mentioned I was trying to avoid those, cause they so limit what you can do with time travel.

Then you may as well throw hopes of a consistent and logical system out the window. "You can't change the past because you already did" is, as far as I know, the closest way to reconcile time travel with our understanding of the physical universe. Your desired alternative system could just as well involve dancing pink elephants driving a great time-wheel in downtown Moscow for all the good it's going to do incorporating it into a framework that makes sense.

In fact, you're never going to have a system for time travel that is logically consistent and makes sense without waving your hands and saying "Because I said so", because time travel is a thing that inherently makes no sense. Our universe, as near as anyone knows, simply does not work that way. Effects cannot precede causes from any observer's frame of reference. Consequently, you're not going to come up with a way of explaining time travel that will ever make 100% sense to a human being from our universe. You might as well be saying that you want to develop rules for a way of consistently moving from Point A to Point B without crossing any of the space in between. Or that you want to develop consistent rules for travelling further than left.

TL;DR, by virtue of the fact that you're dealing with time travel you're already talking about impossible stuff that is never going to make sense. No matter how much thought you put into it, you're going to have to at some point step in and say "Because I say so".

Reltzik
2013-02-15, 02:48 AM
I think it might be better to keep a strict time cone definition (4-dimensional cone, expanding at light speed from a single point in space-time); that way it's completely rigorous in terms of restrictions and knowledge.

Oh, you've got that. It's just the two-directional cone that you extract conic sections out of.

.... also, I realized that time-travel is a sink, as its use would propogate time-travel to related universes. You'd have the pristine no-time-travel worlds, but frequent use of time-travel would force a user down a sink into universes where time travel becomes frequent and even commonplace. About the only thing you couldn't use time travel to change is time travel itself; the more you use, the more common it is.

For more fun, time-travel would create ways to violate traditional entropy.

Therefore, instances of time-travel are themselves entropic. This actually makes sense, as it would break down the distinctions between past, present, and future, INCLUDING the progression of traditional entropy.

.... okay, I should go to sleep now. Fatigue is definitely effecting my thought processes. Yes, I spelled that right.

mjlush
2013-02-15, 07:52 AM
That works in terms of paradox, but it doesn't really solve the second listed problem. Plus, there's the whole issue of "What is observation" and "What about things that could become observed later?".

The observer effect is the acceptable face of Fate. From an Omnipotent POV it gives the illusion of free will in a completely deterministic universe. Future Adam suffocating in space and has already had an effect on Past Adam and part of that effect results in him ending up in space suffocating.

Why bother playing in a game without free will? The Omnipotent have no free will. Fortunately the Players and even the GM do not have on Omnipotent POV.. when the game has finished they can look back and see how all the events interlocked then they have the Omnipotent POV (any loose ends can be hand waved by the GM or are evidence that they start up the game again :-)

From a players POV they have enormous freedom to roam and have a real effect on the game, however the GM has a useful leash to keep them in check so they can't rewrite anything that has happened up to the point.

So if they try and meet themselves when they hadn't first time round the GM is completely at liberty to come up with the event that stopped them meeting themselves. If they pull a Bill and Ted and cant be bothered to send the time machine back to rescue them... Someone else does and the time machine sent is bugged and boobytrapped to the roof :-)

Sith_Happens
2013-02-15, 08:37 AM
The way Doctor Who does it is actually fairly simple, and self-consistent too.

There is only one timeline, but you can only be in one place at one time.

Here's what it means for our friends Adam and Bob:

Adam gets drunk and takes a bet. He tries to jump into space, but he can't, because two minutes ago he was here (in the bar), not there (in space).

Or Adam decides to go kill Hitler (http://xkcd.com/1063/), but then Bob gives him an urgent mission (http://xkcd.com/567/). At 8:05, Bob goes back to Hitler and kills him, which he can do because he didn't exist in 1945 1939. However, that will have changed the entire future up to and including the present day, so Bob might never have been born. Adam can jump forward to 8:06, but he can never again live through the time from when he was born up to 8:05, because he was there already. Note that 8:06 might look completely different from 8:05.

I was actually going to mention the Doctor Who take on time travel, but mainly in terms of the other important rule of it:

While you can change the objective past, you cannot change your own subjective past.

Admittedly, this leads to the awkward situation where your ability to change the (objective) past is inversely proportional to how much you know about it, but I think that's how "fixed points" work in the series anyways: Namely, certain events are so ubiquitous and/or monumental in affecting how the rest of history plays out that it would be impossible to change them in a way that's reconcilable with your own memories.

Ozfer
2013-02-15, 08:45 AM
Its a good thing that the topic you chose isn't infinitely complicated and paradoxical :smallwink:. And no, I don't have anything valuable to bring to the table :smallredface:.

Tar Palantir
2013-02-15, 02:59 PM
One idea that I've played around with that is related to a few of the common takes goes something like this:

You have two types of people who can be in the past: displaced individuals, and time travelers. Displaced individuals are those who were brought to the past by time travelers, and normally they follow the "already changed it" model; if there is any way to maintain a consistent history from their perspective, that history is so. If a displaced individual creates an inescapably paradoxical situation (such as a grandfather paradox), then that person becomes a time traveler.

Time travelers are unaffected by changes to the timeline. Their subjective memory of the past remains intact, regardless of any changes made. They can travel through time under their own power and change whatever they wish without fear of paradox, as no change they make can affect their own past.

FreakyCheeseMan
2013-02-15, 03:25 PM
One idea that I've played around with that is related to a few of the common takes goes something like this:

You have two types of people who can be in the past: displaced individuals, and time travelers. Displaced individuals are those who were brought to the past by time travelers, and normally they follow the "already changed it" model; if there is any way to maintain a consistent history from their perspective, that history is so. If a displaced individual creates an inescapably paradoxical situation (such as a grandfather paradox), then that person becomes a time traveler.

Time travelers are unaffected by changes to the timeline. Their subjective memory of the past remains intact, regardless of any changes made. They can travel through time under their own power and change whatever they wish without fear of paradox, as no change they make can affect their own past.

Something along those lines might actually work, for the world I'm designing- as it has two types of time travelers already.

Fey are the only group that really understand time travel. They're also insane and alien, so I don't have to worry as much about their actions - if they have to act a specific way for time travel not to cause problems, I can just say that they do that as part of their general insanity.

Humans and other races sometimes get caught up with Fey, and dumped in the wrong time.

AttilaTheGeek
2013-02-15, 11:31 PM
Fey are the only group that really understand time travel. They're also insane and alien, so I don't have to worry as much about their actions - if they have to act a specific way for time travel not to cause problems, I can just say that they do that as part of their general insanity.

Humans and other races sometimes get caught up with Fey, and dumped in the wrong time.

I feel like your players are going to question your system a lot, and you must agree if you're going through this effort to make it internally consistent (or you just want an internally consistent time travel system), but basing it on "I can just say they do it because they do" isn't going to be an effective core of any system.

FreakyCheeseMan
2013-02-15, 11:39 PM
I feel like your players are going to question your system a lot, and you must agree if you're going through this effort to make it internally consistent (or you just want an internally consistent time travel system), but basing it on "I can just say they do it because they do" isn't going to be an effective core of any system.

I mean that I can have a set of time travel rules that would cause problems, if the time travelers did certain things - such as, for instance, intentionally try to break them, or try to use them to their advantage in certain ways.

If it were humans doing the time traveling, that would be a major problem, as the next question would be "Well, why hasn't anyone done that?" or "Let's go do that."

With the Fey being the only ones who time travel, though, I feel better about having them limit their own actions in weird ways - and since the players will never reach a point where they understand the Fey *that* well (Part of the whole point of the Fey is to be strange and alien), it can function from a narrative standpoint. Not the most elegant of solutions, but doable.

erikun
2013-02-16, 12:07 AM
In case study #2, Adam and Bob get wicked drunk at a party, and Bob dares Adam to jump back in time by two minutes, into the void of space a few miles beyond Earth's atmosphere, and implies that Adam is "a scaredy-cat" if he does not. (Adam's time ship is not airtight.) An inebriated Adam takes the dare. How many Adams show up?

The sudden appearance and asphyxiation of Adam in the cold void of space is unlikely to make much difference to the party, or to Adam and Bob's unfortunate wager. However, the Adam at the party will still be subtly affected by the added gravitational pull of his time-clone in space. As such, if Adam jumps back again, it would be another, separate Adam that shows up.
There is no need to make things more complicated than it already is. Just say that Adam was already there in space, and everything is self-contained.

If you don't like that answer, then have this one based on quantum mechanics/multiple universes:

Adam A is alive in Universe A, which is asphyxiated-Adam free. Adam A goes to the party. Adam A travels back in time.

Adam C is alive in Universe C, which has a dead Adam B in space. Adam C goes to the party. Adam C travels back in time. You will notice that this is functionally identical to the previous scenario - indeed, it is identical, because there is literally no difference between the two. As such, you could either say that it recurs indefinitely or that the functionally identical universes are identical and so you only have two universes: one with a dead Adam and one without.

In case study #1, Adam and Bob decide they want to kill Hitler. They decide that Adam will leave first, arrive in the past and kill hitler; Bob will leave the day before Adam, arrive shortly after Adam, and help bust Adam out of jail. Adam goes back and kills Hitler. Does Bob show up?

If so, we have a problem, because the future that Bob came from no longer exists. As such, we have time travelers showing up from no-longer-extant futures, resulting in annoying, probably exponential piles of time clones. If not, we also have a problem; when Bob left, he still had a past to go back to, as Adam had not yet messed it up. So, where does Bob go?
The easiest way is to just say that each time travel trip spawns a new universe. That is, Adam going back in time spawns a new past, where Hitler dies (and he rots in jail). Bob going back in time spawns another new past, where Adam is then broken out of jail.

Or if the logistics of that prove too complicated, then keep it simple: Adam going back in time creates one alternate history, and unless Bob decides to travel with him on the same trip, then Bob going back in time will go to a different alternate history (one where Adam never went back). Adam is stuck in jail, because Bob ends up back in a different time where Hitler is still alive and well.

Okay. So, suppose that from the year 2000, bob goes back and kills kills hitler in 1920. In the year 2020, adam (from the original time line) goes back to 1930. By that point, 1930 should have been overwritten. So, does Adam emerge into a strange past where Hitler has already been killed?
For my proposal:

If Adam opens up a "time portal", steps through and closes it, and then kills Hitler, then his return to the future takes him to a dead-Hitler future. If Adam just opens the "time portal" and fires a bullet through it (closing it without passing through) then his current timeline remains the same, and going back to 1930 will still take him to a historically-accurate (e.g. Hitler alive) past.

Note that "consistent" doesn't necessarily mean sensible. You could have consistent time travel where there is one "alternate" universe that is constantly changed but the "normal" one is not. That might actually be fun to try, although perhaps not if your players actually want to change something that way. :smalltongue:

FreakyCheeseMan
2013-02-16, 12:35 AM
There is no need to make things more complicated than it already is. Just say that Adam was already there in space, and everything is self-contained.

I want enough complexity that time travel can still be used to change the past - specifically, Fey trying to prevent the fall of their city.

If you don't like that answer, then have this one based on quantum mechanics/multiple universes:

Adam A is alive in Universe A, which is asphyxiated-Adam free. Adam A goes to the party. Adam A travels back in time.

Adam C is alive in Universe C, which has a dead Adam B in space. Adam C goes to the party. Adam C travels back in time. You will notice that this is functionally identical to the previous scenario - indeed, it is identical, because there is literally no difference between the two. As such, you could either say that it recurs indefinitely or that the functionally identical universes are identical and so you only have two universes: one with a dead Adam and one without.

Except that Adam B would be going back to a past that already contained a dead Adam A, and Adam A would be (very slightly) different from Adam B. You could say that Adam B would end up before Adam A was there, but it's trivial to re-construct the problem such that each jump goes back to a very slightly later point on the time line. So, when Adam B jumps back, they should encounter Adam A's body, and we're back where we started.

The easiest way is to just say that each time travel trip spawns a new universe. That is, Adam going back in time spawns a new past, where Hitler dies (and he rots in jail). Bob going back in time spawns another new past, where Adam is then broken out of jail.

In that case, Bob is traveling back to a past that is not his own, an arriving from a future that would never happen. If time travelers are allowed to arrive from no-longer-valid futures, then we run into issues of time traveler overcrowding, unless we find another specific method to deal with such.

Or if the logistics of that prove too complicated, then keep it simple: Adam going back in time creates one alternate history, and unless Bob decides to travel with him on the same trip, then Bob going back in time will go to a different alternate history (one where Adam never went back). Adam is stuck in jail, because Bob ends up back in a different time where Hitler is still alive and well.

...this would be easier if we had diagrams. So, I think the model you're describing works, but it does so by continuously shunting new time travelers to their own semi-private timelines. Still trying to figure out if it creates massive numbers of time-clones within the same timeline.

For my proposal:

If Adam opens up a "time portal", steps through and closes it, and then kills Hitler, then his return to the future takes him to a dead-Hitler future. If Adam just opens the "time portal" and fires a bullet through it (closing it without passing through) then his current timeline remains the same, and going back to 1930 will still take him to a historically-accurate (e.g. Hitler alive) past.

So, that question was only relevant to one very specific model, with "Ripples" that gradually overwrote time-streams once they were altered.

Note that "consistent" doesn't necessarily mean sensible. You could have consistent time travel where there is one "alternate" universe that is constantly changed but the "normal" one is not. That might actually be fun to try, although perhaps not if your players actually want to change something that way. :smalltongue:

I've worked out models like that - where every time traveler from Timeline T0 shows up in Timeline T1. Problem is, after a couple of jumps it ceases to resemble time travel, as, unless you're going back further than anyone else from your time line, you're always going to end up in a world that's already been altered. So, going from T0 to T1, you try to jump back to kill hitler, but instead you end up in a strange nuclear wasteland cause some other guy from T0 decided to jump back to the medieval days and make himself god-emperor of time. You jump back again to try to stop him, but then you're in T2, which is already mucked up by some other guy who went back to Roman times to crown themselves the Time Emperor-God. By experience, it ceases to be time travel, and just becomes random dimension-hopping.

Incom
2013-02-16, 01:37 AM
Okay. So, suppose that from the year 2000, bob goes back and kills kills hitler in 1920. In the year 2020, adam (from the original time line) goes back to 1930. By that point, 1930 should have been overwritten. So, does Adam emerge into a strange past where Hitler has already been killed?

Exactly. Adam will be quite confused, unless he knows about the effect already (presumably having learned it from time travelers that came from prior timelines that managed to reason all this out).

There's no telling beforehand what kind of past you're going to end up going back to, which dissuades ordinary people from time traveling (which helps keep your plot from being overcomplicated by too many travelers).

Any other questions?

NichG
2013-02-16, 03:26 AM
If we want a physical model of time travel, we have to make it independent of the idea of a 'person' as an element of the model. That is to say, we must avoid ideas like 'Adam can only be in one place at one time' or 'two Adams can never meet', and must consider only the matter from which the various Adams are made.

Why not then posit time travel that requires conservation of all physically conserved quantities as part of the process? If you send back an Adam to the past, you displace an Adam-sized amount of material in the process. The total mass-energy of the universe remains the same (and this could even be a basically local rule, meaning that there is no discontinuous change in any long-ranged forces such as gravity, etc)

This solves all duplication issues. Once one Adam has jumped back, any future Adams that jump back to the same place/time simply overwrite the original Adam. So you're left with a single one determined by whatever concludes the cycle, which could still be an arbitrarily complex solution to a self-consistency problem.

Of course, doing an exact local match of all conserved quantities means time travel would be impossible. So through in something like an uncertainty principle - you can cheat by X if you can fix it in a time proportional to 1/X.

The problem remains is that of overwriting. Is the future overwritten by a time travel event or not? If it is, then how is the future not being constantly overwritten? What about time travel events that occur outside of eachothers' light cones? Introducing a secondary time axis helps with this (e.g. the ripple effect) but makes ensuring conservation much more complex. So basically you're stuck with either getting rid of relativity (which, honestly, is no big tragedy for running a tabletop game - asking players to evaluate square roots to determine their initiative order is a bit cruel) or simply allowing poorly-timed attempts to fail against a lack of viable destination to arrive at - basically, whichever unlucky traveler gets there second finds themselves without a place to land.

FreakyCheeseMan
2013-02-16, 10:39 AM
Why not then posit time travel that requires conservation of all physically conserved quantities as part of the process? If you send back an Adam to the past, you displace an Adam-sized amount of material in the process. The total mass-energy of the universe remains the same (and this could even be a basically local rule, meaning that there is no discontinuous change in any long-ranged forces such as gravity, etc)

Alright, yeah, that works for duplication. Thanks.

Exactly. Adam will be quite confused, unless he knows about the effect already (presumably having learned it from time travelers that came from prior timelines that managed to reason all this out).

There's no telling beforehand what kind of past you're going to end up going back to, which dissuades ordinary people from time traveling (which helps keep your plot from being overcomplicated by too many travelers).

Any other questions?

Hrrm.

If your buddy jumps 30 years into the past, then you jump 40 years into the future, and then 40 years back, do you emerge in the timeline your buddy altered, or the same one you left?

I think that could work for what I'm trying to do in my world, though.

What I'm Trying To Do In My World
There used to be a super-advanced society that ruled over the entire cosmos. When they attempted a Tower-of-Babel-esque project, they brought about a cataclysm that created cracks throughout the universe, and unleashed a sentient force of entropy upon the world. All survivors of the society were corrupted, mutated or driven insane.

A number of time travelers from the old world had jumped forward, over the Cataclysm, so were protected from the worst of its effects - they still went a bit crazy, though. Using the last of the resources they brought with them, they remade themselves as the Fey, and created a city, Avalon, to which they were bound. Their old mechanisms of time travel no longer worked, but they could use the cracks caused by the cataclysm to move through time; they built Avalon across those cracks, allowing it to move as well.

The Fey were "anchored" to Avalon - things that happened to Avalon could effect them, at corresponding points in their time line. So, if Avalon were destroyed in its thousandth year, all Fey would die in their thousandth year. (I can re-work this if it's not doable, but I need some mechanism by which the Fey are bound to the city.)

Avalon itself had to keep certain appointments with the universe - at certain points in Avalon's time line, it had to be at a certain year in the timeline of the universe as a whole. At one of these appointments, Avalon fell under siege and was destroyed. Of course, being time-travelers, Fey went back and undid it - but then it fell again at another appointment, and again, and again.

Fey kept going back and trying to save the city, so the battle played out in a lot of different ways - on the event horizon of a black hole, inside a sun, beneath the waters of an iced-over planet, tearing through space at a respectable fraction of the speed of light - but sooner or later, the city kept falling, until eventually the Fey accepted that there was something else going on, and Avalon was fated to fall.

All of this happened tens of thousands of years ago, but there are still Fey around, cause, you know, time travel; their destruction is in the universe's past, but their future. Most of them have given up, and become even crazier for it. Once in a while, humans get caught up with Fey and end up back at Avalon, even during the siege.

I want it to be possible for such human time travelers to experience teh Siege in different ways - for one time traveler to go, leave, come back, and see it happening in a different way, or for two time travelers to meet and swap stories about the different sieges they witnessed.

I think the ripple system has a lot of potential, but I'm still trying to work out if it meets all the specifics (or if I just need to change some of the specifics.)

Tetraplex
2013-02-16, 11:26 AM
I've always been a fan of the Chrono Trigger method.

Changing the past changes the present and future, but anyone from those times currently travelling or in the past is immune to the changes. One timeline, with alternate past for everyone but the travelers. So yeah, Bob would have to leave with Alex for this to work. It has potential for causing problems with the PCs memories vs the new timeline too, which is a plus in my book.

Dr Bwaa
2013-02-18, 04:58 PM
I want it to be possible for such human time travelers to experience teh Siege in different ways - for one time traveler to go, leave, come back, and see it happening in a different way, or for two time travelers to meet and swap stories about the different sieges they witnessed.

It seems like, assuming people who time travel preserve their own subjective memories, the changing-the-past-spawns-alternate-universes theory would work well enough. If you just want to see the past, you jump to the past and back again pretty quickly, to avoid Rippling. Obviously as time travelers, the Fey would understand the need to be careful about jumping back on top of themselves, etc. If you go back and actually change something, you make a new universe in which that thing happened differently, but I don't see any reason why your Fey couldn't then go back to before they made the change, grab ahold of the original "thread" and fast-forward back to the "real" present.

=== === ===

I think a better argument, though, is for what you're calling "you can't change the past because you already did", but I'd name it differently. Hear me out. At it's heart, this is supposed to be an adventure for your PCs, which means (here's the big assumption) that in the endgame, they do manage to successfully change the past in some significant way. Okay now let that sink in for a second, and now realize that what you mean is that "they can't change the past now because they already did in the future."

So, personally, I think the route to take might be this: changing significant events in the past is possible, but really hard. So you can follow a "simple" version of time travel for the vast majority of the campaign--that is, you can't change your own subjective past and certain major events are "inevitable". When you do manage to change an "unchangable" event (however this happens), that's when you create an alternate timeline (whether you technically do create a "new" timeline should be mostly irrelevant since all the PCs (hopefully) are now in it. Only branching on major events should prevent the alternate-timeline pileup that you're worried about).

CarpeGuitarrem
2013-02-18, 05:48 PM
Secondary thought: why is everyone thinking about "iterations of Adam" when they should really be thinking about "iterations of the universe"? There is one version of Adam (the one who travels around in time), but there are multiple versions of the universe. Even if you don't hold with the multiverse theory for your world, you wind up with a world where the different versions of the universe "overwrite" one another.

NichG
2013-02-18, 09:03 PM
To follow up on that last comment, there is a 'third' kind of problem with time travel that is actually only half a problem and half is what is appealing about running games/stories with time travel elements.

That problem is the issue of amplification of small events (usually called the Butterfly effect, but I'm going to use it in a slightly different way than normally used). Its exciting that the party can go back 100 years and do something tiny to topple a regime or advance civilization in ways that individuals couldn't normally achieve. But if time travel exists in the universe, then the PCs aren't the only ones doing it. Most of the time travel is going to be done passively by random stuff amidst the huge bulk of matter in the universe - either things in the presence of very high-energy objects like colliding black holes, or just very rarely amidst subatomic particles.

So if a person can go back 100 years and change a regime by giving someone a flower, what happens if trillions of subatomic particles are constantly going back in time? It means that not only is the universe constantly being erased/overwritten/adjusted, but that really there is very little permanence to any actions. Conceivably, amidst some conversation over tea, the PCs have just been overwritten a billion times, and were talking about a billion different things in those different universes. Only because of the metagame knowledge of the players is it known that it hasn't happened. Its hard to tell a story if random fluctuations in the universe can constantly rewrite the entire story, so the only way this works is if there are dramatic conventions that prevent it from being a problem (e.g. the idea of historical inertia, so even if its happening all the time, all the changes are minor because there is something that makes history generally follow the same vague paths unless something very large is done to budge it).

Even allowing for historical inertia, this instability can lead to surprisingly negative reactions from players when they first realize just how tenuous things are. I had a time travel plotline once where a player realized it and said "Wait, so, with one action we just undid everything we've done in the campaign so far - everyone we made friends with, everyone we liked, they've all now been replaced with complete strangers". That moment nearly ended the campaign, since everyone just kind of detached. So its important to be careful about this, not just from a physical consistency point of view but also a narrative consistency point of view.

Incom
2013-02-18, 10:11 PM
What I'm Trying To Do In My World
There used to be a super-advanced society that ruled over the entire cosmos. When they attempted a Tower-of-Babel-esque project, they brought about a cataclysm that created cracks throughout the universe, and unleashed a sentient force of entropy upon the world. All survivors of the society were corrupted, mutated or driven insane.

A number of time travelers from the old world had jumped forward, over the Cataclysm, so were protected from the worst of its effects - they still went a bit crazy, though. Using the last of the resources they brought with them, they remade themselves as the Fey, and created a city, Avalon, to which they were bound. Their old mechanisms of time travel no longer worked, but they could use the cracks caused by the cataclysm to move through time; they built Avalon across those cracks, allowing it to move as well.

The Fey were "anchored" to Avalon - things that happened to Avalon could effect them, at corresponding points in their time line. So, if Avalon were destroyed in its thousandth year, all Fey would die in their thousandth year. (I can re-work this if it's not doable, but I need some mechanism by which the Fey are bound to the city.)

Avalon itself had to keep certain appointments with the universe - at certain points in Avalon's time line, it had to be at a certain year in the timeline of the universe as a whole. At one of these appointments, Avalon fell under siege and was destroyed. Of course, being time-travelers, Fey went back and undid it - but then it fell again at another appointment, and again, and again.

Fey kept going back and trying to save the city, so the battle played out in a lot of different ways - on the event horizon of a black hole, inside a sun, beneath the waters of an iced-over planet, tearing through space at a respectable fraction of the speed of light - but sooner or later, the city kept falling, until eventually the Fey accepted that there was something else going on, and Avalon was fated to fall.

All of this happened tens of thousands of years ago, but there are still Fey around, cause, you know, time travel; their destruction is in the universe's past, but their future. Most of them have given up, and become even crazier for it. Once in a while, humans get caught up with Fey and end up back at Avalon, even during the siege.

I want it to be possible for such human time travelers to experience teh Siege in different ways - for one time traveler to go, leave, come back, and see it happening in a different way, or for two time travelers to meet and swap stories about the different sieges they witnessed.

I think the ripple system has a lot of potential, but I'm still trying to work out if it meets all the specifics (or if I just need to change some of the specifics.)

Bouncing a version of this off you that could work with the ripple system.

The Fey have invented some way of traveling through ripples, ie. they can go from Timeline B back to Timeline A under certain circumstances (whatever those might be--you'd have to get ahead of it for starters). The reason a regular traveler can't get back through the ripples is because time traveling forward effectively means vanishing for X years and then reappearing--by which time the ripple would have moved ahead of you. In other words, regular forward time travel involves speeding up time everywhere except for the time machine. Fey time travel might involve literally exiting time entirely, which could time travel you forward around a ripple (but also would create a new one).

Re appointments: for the sake of simplicity, the important one is at Year 0 of whatever real or made-up calendar system your setting uses.

Because of Avalon's semi-extratemporeal (pretty sure I just made up that word) nature, every ripple changes the nature of the sieging force drastically. As it turns out, the real force behind it is Entropy, the arbitrary placeholder name I'm giving to the sentient force of entropy you mentioned. The seige is randomly generated based on some arrangement which gets changed every time there's a ripple--maybe the locations of certain MacGuffins, or weather patterns altered by the abrupt appearance of travelers. (Whoever suggested something about "changing the universe's random number seed" upthread, thanks.)

Turns out that someone in Avalon at Year 0 (the beginning of the seige) figured out how to travel back and prevent the Babel project from reaching completion. Entropy is trying to ensure his own continued existence by preventing people from meddling with the event which freed him.

At some point, a Fey travels way into the distant past and finds out that there's a ripple before which everything ceases to exist. This sets a hard time limit on everything (because time travel is the ultimate munchkin-ing tool, arguably). The reason: at some point after a successful Seige, Entropy's randomness will cause some event that will send him far back in time, allowing him to prevent Creation itself. If Avalon wins the Seige eventually, this becomes impossible and this last ripple is no longer a big deal.

Just kinda throwing things out and seeing what sticks. How's it looking?

FreakyCheeseMan
2013-02-18, 11:02 PM
Bouncing a version of this off you that could work with the ripple system.

The Fey have invented some way of traveling through ripples, ie. they can go from Timeline B back to Timeline A under certain circumstances (whatever those might be--you'd have to get ahead of it for starters). The reason a regular traveler can't get back through the ripples is because time traveling forward effectively means vanishing for X years and then reappearing--by which time the ripple would have moved ahead of you. In other words, regular forward time travel involves speeding up time everywhere except for the time machine. Fey time travel might involve literally exiting time entirely, which could time travel you forward around a ripple (but also would create a new one).

Re appointments: for the sake of simplicity, the important one is at Year 0 of whatever real or made-up calendar system your setting uses.

Because of Avalon's semi-extratemporeal (pretty sure I just made up that word) nature, every ripple changes the nature of the sieging force drastically. As it turns out, the real force behind it is Entropy, the arbitrary placeholder name I'm giving to the sentient force of entropy you mentioned. The seige is randomly generated based on some arrangement which gets changed every time there's a ripple--maybe the locations of certain MacGuffins, or weather patterns altered by the abrupt appearance of travelers. (Whoever suggested something about "changing the universe's random number seed" upthread, thanks.)

Turns out that someone in Avalon at Year 0 (the beginning of the seige) figured out how to travel back and prevent the Babel project from reaching completion. Entropy is trying to ensure his own continued existence by preventing people from meddling with the event which freed him.

At some point, a Fey travels way into the distant past and finds out that there's a ripple before which everything ceases to exist. This sets a hard time limit on everything (because time travel is the ultimate munchkin-ing tool, arguably). The reason: at some point after a successful Seige, Entropy's randomness will cause some event that will send him far back in time, allowing him to prevent Creation itself. If Avalon wins the Seige eventually, this becomes impossible and this last ripple is no longer a big deal.

Just kinda throwing things out and seeing what sticks. How's it looking?

Some parts look better than others.

I was thinking that forward time travel would work in the same way as reverse- so, you could overtake ripples. This allows people who get caught up in the siege to return to their own home times, rather than a timeline altered by that particular siege, and their own involvement in it.

Pretty sure I'm going to have the end of the babel project - the "Cataclysm" be a hard wall for time travel. I'm keeping that whole society as as much of an unknown as possible; having anyone actually go back there would be... weird. The project actually *was* interfered with, just before it would have reached completion - if it had gone all the way, the universe would have ended entirely, and *mumble mumble mumble* a new one would have been created. So, traveling back to the cataclysm itself would be a hard limit- problem is, nothing and no one wants to do that. The closer you get, the more dangerous things are, so the advantage of going back further kinda evens out.

As for why the Fey, despite being able to travel through time, always keep losing... that one's kinda complicated. I can explain it, but it's a whole nother wall of text, that gets into the origin story of Elves, the babel-esque project itself, and a bunch of other things.

Entropy's long-term motivations are also a bit weird; he's actually not that much of a "Smash everything" sort, because he needs certain stable structures in order to increase entropy in the universe. During the Chaos Wars, when he was really active, most of what he was doing was actually wiping out lesser forces of entropy, whose short-sighted destruction would actually slow things down in the long run.

FatR
2013-02-19, 04:43 AM
So, I'm trying to come up with a consistent set of rules for time travel, such that you actually *can* change the past. So, none of the twelve-monkeys, "any change you made would already have been made" stuff. I've yet to come up with a set of rules as to how this works.
This CAN'T work. Ability to change the past freely by time travel paradoxically means that the past is absolutely immutable, because at any given moment from the Universe's creation to its end, you're already retroactively living in the universe created by the final and most successful group of time travellers in existence. And all your attempts to screw with the timestream further will somehow fail or be prevented.

FreakyCheeseMan
2013-02-19, 09:05 AM
This CAN'T work. Ability to change the past freely by time travel paradoxically means that the past is absolutely immutable, because at any given moment from the Universe's creation to its end, you're already retroactively living in the universe created by the final and most successful group of time travellers in existence. And all your attempts to screw with the timestream further will somehow fail or be prevented.

Did you read over the rest of the thread? There have already been a number of proposals that bypass that problem - treating individual timelines as different from the universe's timeline, the introduction of "Ripple effects" so that time doesn't change instantaneously, allowing for the creation of multiple, branching timelines...

As for "Already retroactively living in the universe created by the final and most successful"... that's no more true than "You're already living in a universe that is cold dead sea of entropy"; even *if* that is the ultimate result, there's no reason the story couldn't be set earlier.

Finally, we're the ones designing the universe; we can specify the rules to be whatever we want. All I was asking for was a set of rules which were consistent (i.e., didn't change from situation to situation based on narrative demands), and which didn't cause certain logistical problems (i.e., time clones galor.)

mjlush
2013-02-19, 09:18 AM
This CAN'T work. Ability to change the past freely by time travel paradoxically means that the past is absolutely immutable, because at any given moment from the Universe's creation to its end, you're already retroactively living in the universe created by the final and most successful group of time travellers in existence. And all your attempts to screw with the timestream further will somehow fail or be prevented.

I rather like this idea,I could see a campaign where the object was to destroy time travel in order to over throw the omega squad travellers

CarpeGuitarrem
2013-02-19, 09:41 AM
That problem is the issue of amplification of small events (usually called the Butterfly effect, but I'm going to use it in a slightly different way than normally used). Its exciting that the party can go back 100 years and do something tiny to topple a regime or advance civilization in ways that individuals couldn't normally achieve. But if time travel exists in the universe, then the PCs aren't the only ones doing it. Most of the time travel is going to be done passively by random stuff amidst the huge bulk of matter in the universe - either things in the presence of very high-energy objects like colliding black holes, or just very rarely amidst subatomic particles.
Yeah, this is why my preferred handling of the matter is "temporal homeostasis"--time tends towards non-entropy, and it takes actual effort to introduce entropy into the timeline (i.e., changing it). So, time may indeed be constantly rewritten (i.e. "in flux"), but it generally stays the same.

FreakyCheeseMan
2013-02-19, 09:43 AM
That problem is the issue of amplification of small events (usually called the Butterfly effect, but I'm going to use it in a slightly different way than normally used). Its exciting that the party can go back 100 years and do something tiny to topple a regime or advance civilization in ways that individuals couldn't normally achieve. But if time travel exists in the universe, then the PCs aren't the only ones doing it. Most of the time travel is going to be done passively by random stuff amidst the huge bulk of matter in the universe - either things in the presence of very high-energy objects like colliding black holes, or just very rarely amidst subatomic particles.

Hmm... something like my notion of attractive forces across time lines might do something for that/ It wouldn't be enough to alter historical events, but it could keep the subatomic particles in line.

Deepbluediver
2013-02-19, 10:13 AM
Personally, I HATE timetravel. There is exactly one story I've ever read where I actually liked the way time travel was handled, and that was mostly because the characters weren't really in control of it.

Usually, if there MUST be timetravel, I rule that there is a secret, really powerful organization of anti-timetravelers who go around putting back all the things people mess up before the paradox(s) causes the multiverse to implode in on itself...again.

So for example, you go back in time to kill Hitler, they send some one to stop you. Or just replace him with a body-double before anyone notices. So all those assassination attempts that Hitler "barely survived"? Yeah, the first one was actually succesful and all the rest where just killing clones.

If you simply MUST have time travel in your storyline, I still like the idea of some outside-party working to smooth out the bumps and deal with all the tricky situations that develop.

FreakyCheeseMan
2013-02-19, 10:19 AM
Personally, I HATE timetravel. There is exactly one story I've ever read where I actually liked the way time travel was handled, and that was mostly because the characters weren't really in control of it.

Usually, if there MUST be timetravel, I rule that there is a secret, really powerful organization of anti-timetravelers who go around putting back all the things people mess up before the paradox(s) causes the multiverse to implode in on itself...again.

So for example, you go back in time to kill Hitler, they send some one to stop you. Or just replace him with a body-double before anyone notices. So all those assassination attempts that Hitler "barely survived"? Yeah, the first one was actually succesful and all the rest where just killing clones.

If you simply MUST have time travel in your storyline, I still like the idea of some outside-party working to smooth out the bumps and deal with all the tricky situations that develop.

Why the dislike, exactly? Do you just feel that it's a cheap narrative cop-out, or...?

For me, it's mostly related to a couple of racial origin stories (Elves and Gnomes), and a way to connect players with a different era of my game setting. Oh, and a way to explain why Fey are so weird and alien.

Deepbluediver
2013-02-19, 10:47 AM
Why the dislike, exactly? Do you just feel that it's a cheap narrative cop-out, or...?

Yeah, it seems to get used that way a lot, then forgotten about as soon as the moment has passed.

In game terms, it's a storybreaking ability; something so powerful that it warps the whole narrative until everything revolves around it, black-hole style. Or it just gets ignored, which makes about as much sense as ignoring the T-rex in Jurassic Park.

Time travel is VERY powerful, and if there aren't a whole heap of limits placed on how and when it can be used, then the plot starts to break down. Like taking a story about Bond-esque baddass normals (to use the TVTrope term) and having Zeus show up.
Most authors don't handle it well.

Kurald Galain
2013-02-19, 11:03 AM
So. How do I make a set of logical and consistent rules for time travel, that don't run into either of these problems?

Recommended reading material: GURPS Time Travel. Yes, even if you don't play GURPS and have no intention of playing GURPS. The whole book is a systematic analysis of different methods of time travel, and their ramifications, consequences, and loopholes.

NichG
2013-02-19, 11:20 AM
I ran a timetravel plotline once in which all the time travel events came from the future, but were initiated from the present.

Basically, the PCs had a device that could be used to receive messages from their future selves a specific time forward. However, the messages never introduced an inconsistency on their own (it would take the PCs willfully not doing something they knew they had done, which opens the door to the time travel mechanism forcing consistency by creating the situation that someone managed to hijack their device's communication ability).

Since you're always asking for interference from the future, it prevents proactive elements in the future from dominating the plotline by seeking to alter the past. Basically, once a moment passed in which the device was not used to ask for communications, that moment became fixed and all loops regarding it were closed, since any attempts to use a future known communication date to create a paradox would trip the safeguard and give control of the device to the unknown implied third party.

I think a mechanism like this avoids the elephant in the room problem of solving everything with time travel. It does not however allow the past to be altered.

Jan Mattys
2013-02-21, 02:36 AM
Time travel is VERY powerful, and if there aren't a whole heap of limits placed on how and when it can be used, then the plot starts to break down. Like taking a story about Bond-esque baddass normals (to use the TVTrope term) and having Zeus show up.
Most authors don't handle it well.

You mean like this?
http://3.bp.blogspot.com/-u6pUUytIZA8/UO-josJvxII/AAAAAAAAHnc/nq43kRCE5pw/s1600/Die%2BHard3-1.jpg

:smallbiggrin:

qwertyu63
2013-02-22, 12:27 AM
I actually have a time travel system set up... So I'll share it with you... (Please note: I have not read the thread, I just read the OP and went from there.)

It is based on a number of rules (The Laws):

1: The Law of Locking.
Persons, objects and locations can be "Locked", granting them immunity to being greatly effected by changes in time. In addition, all "locked" persons and objects hold a special temporal frame, allowing them detect "waves" before they arrive. (That is to say "wave" detectors have to be locked.) This will be expounded on in the later Laws. Time travelers and time machines are automatically locked.

2: The Law of Waves.
Changes in the past do not instantly effect the future. Changes are carried forward by a "Wave" which moves forward through time at a rate of 10 days per second (that is if I make a change 600 days before you, any wave detecting abilities and devices you have will detect the wave 60 seconds before it arrives). As a Wave moves over a certain point, that point of time changes to match what it would be with the change the wave was carrying.

[NOTE: Those persons that are "locked", or who are in a location that is "locked", retain any memories they may have. However they also obtain the memories they would have in the new timeline. Those in a locked location remain there, while those in unlocked area are moved to where they were in that new timeline. If a locked individual is dead or non-existent, they appear where they were in the old timeline (or in the nearest open space, if the spot is taken). All of this is relative to the planet, none of this will land you in deep space.]
[As to "locked" objects (Including time machines), they stay with the person that was holding or carrying them, even if that person is unlocked. If no one is carrying or holding it, it appears where it was in the other timeline.]
[Locked locations, in addition to protecting persons there from changes, also protect themselves, and all objects within. (Like a locked building will appear in every timeline at the same spot, overwriting what was already there.)]

3: The Law of Persistence.
Minor changes do not cause large changes. Small effects are smoothed over. Killing a butterfly means there is one less butterfly, not that the world explodes.

4: The Law of the Nexus.
Almost all people have many different "Nexus" points, important parts of their life story. Changes made at these points can cause large changes to that persons life, as well as to all people connected to that person (who's connections are in turn effected, and so one [the effect is reduced as there are more degrees of separation]).

[NOTE: Due to laws 3-4, major changes to peoples life can be reversed, by returning the Nexus to the original state, or at least close enough for it to smooth over.]

5: The Law of Jump.
Time travel events still occur if they are erased. If things change in way way that would make you not travel, you are unaffected.

As to the case studies in the OP:

Case 1:
In the current present, Bob goes back in time followed by Adam.

In the past, Adam arrives and kills Hitler. He is then thrown in jail. This death creates a wave which starts on its way forward. This has no effect on Bob, as he left before the wave arrives. Bob arrives, and breaks Adam out.

In the new present, the wave reaches when they left, and the memories of the new timeline pops into Bob and Adam's heads. Since they were locked, they also retain memories of the other time line. The actual time travel has been erased from time, but the 5th law makes this just fine.

Case 2:
In the current present, Bob dares Adam to be an idiot. He goes back in time.

In the past, Adam appears in space, and dies. The 3rd law smooths over the relatively small change.

In the new present, Bob dares Adam to be an idiot. He goes back in time, becoming the idiot who dies.

There is no crazy trick. A dead Adam appears 2 minutes in the past, then he goes back to die.

Any questions?

BIG EDIT: I feel the urge to create a list of various devices this system uses (All of these devices are powered by "Flow", which is power from outside time that only locked things can access and use [This is not hard-SF].):

The Chrono-sphere:
This is the classic time machine. Controls within allow you to set a target time, and then you can have the sphere jump, taking everyone and everything inside with it. The sphere itself is permanently "locked" as is anyone who ever uses it (it does not lock objects taken with it).

The Jumper-watch:
This is a more compact (and much more expensive) time machine. Dials on the watch allow the wearer to jump (yes, the watch does go with them). The sphere itself is permanently "locked" as is anyone who ever uses it (it does not lock objects taken with it).

The Bronze-mark:
This is a small coin, bronze in color. If you carrying or holding it, you are considered locked. The coin itself is also locked.

The Wave-catcher:
This is a device that detects incoming waves. It comes in 2 forms, as follows:
Chrono-pane: A large machine that takes up most of a room, and includes a huge screen, showing when the wave started, and when it is now.
Pocket-detector: A small panel, about the size of a mobile phone. It beeps when a wave is incoming, and shows the time until arrival on itself.
Both are in and of themselves locked.

The Field-holder:
The is a generator that emits a field, locking the location around it if it's on. The range can be set up to a 500 ft radius. The device itself is locked.

The Locker:
The locker is shaped kind of like an MRI machine. Any person or object can be put in the machine, which when used locks all persons or objects within.

If you prefer magic for this, just make up spells, and go from there.

The_Final_Stand
2013-02-22, 12:24 PM
With regards to time travel, I vastly prefer the Stable Time Loops (you can't change the past, you were always going to (from your point of view) and indeed had already (from an outsider viewpoint) act as you did) version. However, you want a time travel scenario that allows changing the past. In that respect, my favorite is the "Travel backwards, you create a new timeline, travel forwards, you stay on the same line" version.

It works something like this: Alex A, from Timeline A, 2013, decides to travel backwards and kill Hitler (all the cool time travelers are doing it!). He sets his time machine to 1939, and jumps. He exits in Timeline B, in 1939. Let's say he succeeds. He now jumps forward to 2013. Naturally, having killed Hitler, some things have changed. The people who would have become his parents might have hooked up at a different time because the war finished earlier. Alex B may not exist because of that.

Critically, Universe A still exists, without Alex in any way, shape or form. A funeral gets held, because he never came back from his time travel, or whatever. Let's say Bob decides to follow Alex. He jumps back too. However, he ends up in Timeline C, and never sees Alex A. He may come across Alex C, depending on how much Bob changed by time travelling, but Alex C is essentially unchanged from standard Alex A. Alex C, again depending on how much Bob changed by time travel, may be planning to jump back and kill Hitler C. And so on and so forth.

If the whole party jumps as a group, then they probably end up in the same place, at the same time, in the same universe.

I like this (change the past) model because a single, mutable time line seems somewhat stupid. Alex jumps back and kills Hitler. The future now has no need for Alex to jump back and kill Hitler. So, are there now 2 Alex's, with different memories? Yes? Where did the Alex that killed Hitler get his memories from, when there's only one timeline? Perhaps someone reckons that they will stop this guy from killing Hitler, because they never saw what he would do. What then? If he succeeds, does the Alex that killed Hitler stop having ever existed? What happens to the Alex that grew up with no need for Hitler to be killed? And so on. It's a real headache, and is if not impossible, then extraordinarily difficult to keep consistent.

cha0s4a11
2013-02-23, 01:08 AM
One way to handle time travel would be sort of like how "Timeline" handled it - it's not actually time travel, it's just parallel universe jumping with universes that are only different in that they started at different times (i.e. barring "time traveller" interference, everything in each universe would happen the same way, the same people would be born and die at the same times and experience the exact same lives, etc). So for the examples, lets just say that there's a new universe that is born every second so that you can jump a minute "into the past" by jumping to the parallel universe that started 60 seconds after your current one.

1) Adam and Bob want to kill Hitler, they work out how far back exactly they need to be able to accomplish that goal. If they jump back the exactly same amount of time (say they decide to jump back 90 years exactly to get Hitler early), they end up in the same universe and kill Hitler together. If they are slightly off (Adam jumps back 90 years while Bob travels 90 years and a second) they end up in different universes and thus are stuck either trying to re-time jump and meet up or to kill Hitler separately and individually).

If they manage to kill Hitler, then decide to jump back 90 years to the future, nothing will change - Hitler still came to power in their initial universe. If they, however, find a cryo-freezer and Rip Van Winkle their way 90 years to the future, then they would be able to see how a universe where Hitler died early plays out.

2) Adam's time jump is just the various copies of him shifting which universe they are in. Adam in the earliest universe to start (Now know as Adam A) will exist in a universe where there are no dead Adams. Adam A jumps to a universe that started 2 minutes later and dies. The resident Adam of that universe, Adam B is in a universe with a dead Adam A, but doesn't notice, so he jumps to a universe that started 2 minutes later and dies.
etc. etc.

The various universes and parallel jumping allow for some odd permutations of events to play out fairly naturally over the extended multi-universe timeline (i.e. Adam A jumps to Adam B's universe and kills Hitler, Adam B grows up in a Hitler-less universe and so never jumps to Adam C's universe, Adam C's universe has Hitler live... etc). It could also be fairly disorienting for someone to jump into some parallel version of history that has been affected by something that they weren't aware of.

NichG
2013-02-23, 06:00 AM
Parallel universe jumping suffers from Olber's paradox (the 'if the universe were infinite, the sky could not be dark' one). Basically, if universe travel exists in one universe at one point in time then its likely to exist in most of the alternate universes that one spawns. Since the number of universes grows exponentially with time, so does the number of travelers. This is not a problem for forward jumping (because you just stay on your current line) but if you jump backwards it means that you have this exponential number of future travelers converging on a single universe, so you should be seeing a world dominated completely by incoming travelers from all the various future universes.

If you jump strictly 'across' then you don't have this problem since the number of landing spots and the number of travelers both grow at roughly the same rate, but any backwards travel suffers from this issue.

For example, if its one new universe per second, then someone jumping back one minute converges with 10^18 copies of themselves.

FreakyCheeseMan
2013-02-23, 09:18 AM
Parallel universe jumping suffers from Olber's paradox (the 'if the universe were infinite, the sky could not be dark' one). Basically, if universe travel exists in one universe at one point in time then its likely to exist in most of the alternate universes that one spawns. Since the number of universes grows exponentially with time, so does the number of travelers. This is not a problem for forward jumping (because you just stay on your current line) but if you jump backwards it means that you have this exponential number of future travelers converging on a single universe, so you should be seeing a world dominated completely by incoming travelers from all the various future universes.

If you jump strictly 'across' then you don't have this problem since the number of landing spots and the number of travelers both grow at roughly the same rate, but any backwards travel suffers from this issue.

For example, if its one new universe per second, then someone jumping back one minute converges with 10^18 copies of themselves.

This. So far, I've heard a few ways to work around this.

1: Somehow have only a single, mutable timeline. ("Ripple") models seem to work best, here.
2: Have multiple pasts, as well as multiple futures.
3: Time travel by quantum re-arrangement of local matter. Thus, if two copies of the same time traveler arrive at the same moment, the second one gets built out of the first one, so only one shows up.

qwertyu63
2013-02-23, 10:02 AM
This. So far, I've heard a few ways to work around this.

1: Somehow have only a single, mutable timeline. ("Ripple") models seem to work best, here.

I have to ask if you saw my post, which had rules for... exactly that.

cha0s4a11
2013-02-23, 10:13 AM
Parallel universe jumping suffers from Olber's paradox (the 'if the universe were infinite, the sky could not be dark' one). Basically, if universe travel exists in one universe at one point in time then its likely to exist in most of the alternate universes that one spawns. Since the number of universes grows exponentially with time,

Gotta stop you right there.

The number of universes in my model increases linearly at the rate 1 per second.
Time travel in the sense I've described essentially consists of someone jumping from one universe to another and doesn't create additional universes any more than someone flying to Las Vegas creates additional cities.

so does the number of travelers. This is not a problem for forward jumping (because you just stay on your current line) but if you jump backwards it means that you have this exponential number of future travelers converging on a single universe, so you should be seeing a world dominated completely by incoming travelers from all the various future universes.

Only if the mulitple copies all jumped to the same target destination times at different starting times (corresponding to how much later their universe started compared to the target) which is almost certain not to occur naturally, and I can't help but suspect would be an epic pain to coordinate.

If you jump strictly 'across' then you don't have this problem since the number of landing spots and the number of travelers both grow at roughly the same rate, but any backwards travel suffers from this issue.

That's rather the point of my system - Travelling back to the past of your current universe is actually impossible - any "time travel" back is universe jumping across.

FreakyCheeseMan
2013-02-23, 10:16 AM
I have to ask if you saw my post, which had rules for... exactly that.

Yeah, a few people have put forth variations on that so far - I was referencing them en masse.

So, reading over yours specifically, a few notes.

First, having time "Locked" creatures just sounds... messy. Teleporting-into-brick-walls-that-weren't-there-a-meta-second-ago messy.

Also, any of these ripple methods really need to define their units better. You can't just point to a single timeline and say that something moves at "10 days per second." To do that, you need to rigidly define at least two time axis, and describe how people move across them, both normally and during time travel.

NichG
2013-02-23, 11:03 AM
Gotta stop you right there.

The number of universes in my model increases linearly at the rate 1 per second.

This requires that there's a 'true' branch and a lot of 'fake' branches. Otherwise each branch is also making its own branches at a rate of 1 per second. Thus exponential.

Time travel in the sense I've described essentially consists of someone jumping from one universe to another and doesn't create additional universes any more than someone flying to Las Vegas creates additional cities.

The problem is that those universes all have people jumping around too. The statement 'there are multiple universes being birthed from every decision branch' combined with 'it is possible to move between them' combined with 'you can go backwards in time when doing this' creates the problem. The first statement doesn't cause a paradox on its own, because without a means to go between them there's no catastrophe. The second statement also doesn't create problems so long as there is equilibrium (e.g. people are flying out of Las Vegas at the rate they're flying in). But the third statement creates a problem because the rate of flying out and flying in are skewed by the fact that when you go backwards, there are fewer universes than when you go forwards.

It'd sort of be like if, averaged across all available transportation methods, there were 10 times as many people flying into a place as out of it, all the time. That causes the population to grow. Normally not a problem with cities, because thats just growth. But when you're talking about a 'moment in time' it gets problematic.

Only if the mulitple copies all jumped to the same target destination times at different starting times (corresponding to how much later their universe started compared to the target) which is almost certain not to occur naturally, and I can't help but suspect would be an epic pain to coordinate.

It tends to occur because those multiple copies are all in very close-together universes that have only had a short time to diverge. You might have a few seconds of deviation, but still the problem is more 'there are suddenly 10^18 more people here than there were before' than the actual simultaneous arrival. And keep in mind, that 10^18 figure is just for travel back by 1 minute. If you're talking years, then the numbers are really insane - we're talking multiple universe-masses worth of humans.

That's rather the point of my system - Travelling back to the past of your current universe is actually impossible - any "time travel" back is universe jumping across.

This sounds like you have multiple pasts, one future. Which is a model one could consider, but its conceptually a little weird (e.g. what does it actually mean that you have multiple pasts?). In this case, universes don't branch 1/second, they _merge_ 1/second. Which implies some collapsing infinity of ancient universes that are all converging to a single state. Kind of bizarre.

Or I suppose you could just have lots of disconnected universes with no birth/death relationship to others, in which case when you jump back you don't necessarily arrive in anything that looks at all like what you think the past is.

cha0s4a11
2013-02-23, 12:11 PM
This requires that there's a 'true' branch and a lot of 'fake' branches. Otherwise each branch is also making its own branches at a rate of 1 per second. Thus exponential.

There isn't any branching. Universes don't split. It's just that there's a new universe created every second (by some multi-versal framework, not by each universe individually) that, if left to its own devices, would play out in exactly the same way as all the universes created before or after would play out if left to their own devices.

It's not time travel, it's Sliders but with all of the differences not being due to just randomness but due to the influences of travellers and the natural time offset. The natural time offset just means that it can act in a manner that resembles time travel.

If you really wanted to call the first universe (which can't be influenced by travellers from universes that started earlier by definition) or the multi-versal framework that generates these universes to be the "true" universe while all the others are "fakes" you could, but I don't see what it gets you.

The problem is that those universes all have people jumping around too. The statement 'there are multiple universes being birthed from every decision branch' combined with 'it is possible to move between them' combined with 'you can go backwards in time when doing this' creates the problem.

Fortunately at no point did I say that universes are getting created by decision points. In fact I said quite the opposite.

Repeat: There is no more than one universe per second. The time travel I propose isn't time travel, it's universe jumping and no matter how you slice it if there are 60 universes each starting with 1 me, even if the me's of those 60 universes somehow contrive a way to get together for one party, it just means that one of those universes has 60 me's having a good time and the other 59 are one's where I left at some point to go to the party.

Or I suppose you could just have lots of disconnected universes with no birth/death relationship to others, in which case when you jump back you don't necessarily arrive in anything that looks at all like what you think the past is.

Yep, that's pretty much my model - the only thing that ties the universes together is that they are one second off from each other and that, if not for those pesky "time travellers" they would play out the exact same way (which may screw with notions of free-will but that's a different discussion).

If there's been a lot of mucking about by dimension jumping "time travellers" weird results may occur, but one thing that definitely won't occur is time paradoxes, or rather those paradoxes would play out by iterating different states of the paradox across the different universes (i.e. Hitler alive/dead, etc).

qwertyu63
2013-02-23, 01:57 PM
Yeah, a few people have put forth variations on that so far - I was referencing them en masse.

So, reading over yours specifically, a few notes.

First, having time "Locked" creatures just sounds... messy. Teleporting-into-brick-walls-that-weren't-there-a-meta-second-ago messy.

Also, any of these ripple methods really need to define their units better. You can't just point to a single timeline and say that something moves at "10 days per second." To do that, you need to rigidly define at least two time axis, and describe how people move across them, both normally and during time travel.

Well, Let me take these 1 at a time:
First: Remember, sudden appearance only applies if you are personally locked, in an unlocked location, but not alive/existing in the new timeline. If you only have the first 2 you are moved to where you would be in the new line. Otherwise, I would say you are "shunted" to the nearest open space. (I never thought it through that much but there you go.)

Second: When I say "moves forward through time at a rate of 10 days per second", that is from the awareness of those persons/objects that are locked. To clear this up, let me make a couple examples (In all of these examples, I am locked)

Case 1:
I make a large change, then 2 seconds later jump forward 50 days, I will arrive there before the change, then 3 seconds later the change hits. That 5 total seconds are from my point of view.

Case 2:
I am sitting in front of my wave detector machine (which is in a locked room), when someone else (who is locked) makes a change 600 days in the past. As I am also locked, we share the same "meta-time" frame. My machine has detected the wave incoming, as it also on the locked frame. 60 seconds later, the wave hits. My room, along with myself and the machine, persist in the new line, overwriting whatever was already there.

Locked things share the one "meta-time" frame, which is the frame I am talking about "moves forward through time". Jumping through time takes up no "meta-time". Any further questions?

FreakyCheeseMan
2013-02-23, 06:00 PM
Second: When I say "moves forward through time at a rate of 10 days per second", that is from the awareness of those persons/objects that are locked. To clear this up, let me make a couple examples (In all of these examples, I am locked)

Alright, let's actually do the math here..

I propose that, for any fully conceived notion of time travel, there is some number of dimensions in which it can be viewed as a static, unchanging whole. For the universe without time travel or quantum branching, this number is four- three for the spatial dimensions, one for the temporal. At ant given temporal coordinate, the other three will describe exactly the same space; the big picture does not change.

If the universe has time travel (and time is mutable), this ceases to be the case. The four-dimensional model falls apart, as that model may be different depending on the sort of time travel that has or hasn't occurred. We can compensate for this by adding additional dimensions to encompass the change, thus presenting a five-or-higher dimensional universe, which is again static and unchanging.

So, first off, does everyone agree with this proposition? That we can examine any time travel system as a static object in some number of dimensions?

So, for your universe, what would this number be? Five? Higher?

NichG
2013-02-23, 07:40 PM
Yep, that's pretty much my model - the only thing that ties the universes together is that they are one second off from each other and that, if not for those pesky "time travellers" they would play out the exact same way (which may screw with notions of free-will but that's a different discussion).

If there's been a lot of mucking about by dimension jumping "time travellers" weird results may occur, but one thing that definitely won't occur is time paradoxes, or rather those paradoxes would play out by iterating different states of the paradox across the different universes (i.e. Hitler alive/dead, etc).

I see, so extended time travel backwards basically involves doing a succession of fast hops between universes to ladder your way back. So instead of saying 'we have N universes, each a second a part' we could just say 'there is an extra continuous dimension, such that as you move in that dimension you also move in terms of the time offset of the universe'.

I think this may work, though its a very dangerous model to attempt travel in. Each individual traveler must pass through a large set of universes on their way to their final destination, and these universes will be wildly divergent once time travel has been around for awhile. When you go into the past, you're more and more likely to not be traveling to anything reminiscent of your real past, based on how long it has been since inter-universal travel was invented. I think the thing that makes this model actually work is there's still effectively a light cone, it just extends across this extra weird space whose initial conditions were offset. The divergence of universes effect is going to be really really weird though, since it basically lets you measure indirectly the moment that time travel was first invented (at least, first from the point of view of your personal reference frame, which gets kind of complex).

qwertyu63
2013-02-24, 11:53 AM
I've snipped out most of your post for space reasons.

So, first off, does everyone agree with this proposition? That we can examine any time travel system as a static object in some number of dimensions?

That makes perfect sense to me.

So, for your universe, what would this number be? Five? Higher?

Five. The main three, time, and "meta-time", which would be dimension 5. These waves travel through this fifth dimension, effecting the lower 4 as they go.

FreakyCheeseMan
2013-02-24, 12:22 PM
Five. The main three, time, and "meta-time", which would be dimension 5. These waves travel through this fifth dimension, effecting the lower 4 as they go.

Alright, then let's try to describe this static state. If this description goes wrong at any point, stop me and explain how it would actually look.

Start by condensing the three spacial dimensions into a single point, since this model doesn't seem to have any particularly noteworthy interactions between space and time (light-cones aren't relevant, for example.)

So, this leaves us with two axis, we'll label the X axis time, and the Y axis meta-time. Any coordinate pair represents a frozen moment of three-dimensional space.

For now, we'll ignored the time-locked elements, just try to nail down the rest of it.

In absence of time travel, this diagram should not vary along the Y axis, at all.

A single instance of time travel should move someone along the X axis, but not they Y axis - thus it's possible to travel over waves.

So, let's introduce a time event - mark two points, Departure at some (X(d), Y), and Arrival at another point (X(a), Y), assuming X(a) < X(d).

This in turn creates a wave effect, represented by a line moving up and to the right. Everything to the left of this wave (but to the right of X(a)) should represent the universe as altered by the time event. Everything to the right of this wave should represent the unaltered universe.

Now, first significant question - is Arrival really a point, or is it a line? Because until the location in meta-time where the wave reaches X(d), there's still a time launch, so, by our rules, there should be a corresponding X(a) at the same point in meta-time. So, instead of a line originating from a point, we have a stripe originating from a vertical line.

This leads to our next issue - what happens when that stripe first hits X(d)? At that point, the changes may prevent the departure from happening, so, at that point in meta-time, there would be no corresponding arrival. Or are arrivals "Locked in"? Is that Arrival "permanent", i.e., an infinite ray going upwards along the Y-axis?

>_> This would be easier with diagrams.

Jay R
2013-02-24, 01:08 PM
Within our experience, "consistent" means that there is always a straightforward progression from cause to effect.

Time travel means that cause and effect get screwed up.

I therefore conclude that "consistent time travel" cannot exist.

First, invent a consistent, complete physical and philosophical system that solves the epistemological rupture of effect before cause, and then we'll talk.

FreakyCheeseMan
2013-02-24, 01:22 PM
Within our experience, "consistent" means that there is always a straightforward progression from cause to effect.

Time travel means that cause and effect get screwed up.

I therefore conclude that "consistent time travel" cannot exist.

First, invent a consistent, complete physical and philosophical system that solves the epistemological rupture of effect before cause, and then we'll talk.

Sure, except that isn't the definition of consistent.

Consistent just means that something behaves the same way in different situations - that you can pick one set of rules, and those rules will be followed regardless of the specifics. So no "Time travel does this, so long as it supports the plot" or "Time travel works like this, unless it would cause a problem".

qwertyu63
2013-02-24, 02:08 PM
Alright, then let's try to describe this static state. If this description goes wrong at any point, stop me and explain how it would actually look.

Start by condensing the three spacial dimensions into a single point, since this model doesn't seem to have any particularly noteworthy interactions between space and time (light-cones aren't relevant, for example.)

So, this leaves us with two axis, we'll label the X axis time, and the Y axis meta-time. Any coordinate pair represents a frozen moment of three-dimensional space.

For now, we'll ignored the time-locked elements, just try to nail down the rest of it.

In absence of time travel, this diagram should not vary along the Y axis, at all.

A single instance of time travel should move someone along the X axis, but not they Y axis - thus it's possible to travel over waves.

So, let's introduce a time event - mark two points, Departure at some (X(d), Y), and Arrival at another point (X(a), Y), assuming X(a) < X(d).

This in turn creates a wave effect, represented by a line moving up and to the right. Everything to the left of this wave (but to the right of X(a)) should represent the universe as altered by the time event. Everything to the right of this wave should represent the unaltered universe.

Now, first significant question - is Arrival really a point, or is it a line? Because until the location in meta-time where the wave reaches X(d), there's still a time launch, so, by our rules, there should be a corresponding X(a) at the same point in meta-time. So, instead of a line originating from a point, we have a stripe originating from a vertical line.

This leads to our next issue - what happens when that stripe first hits X(d)? At that point, the changes may prevent the departure from happening, so, at that point in meta-time, there would be no corresponding arrival. Or are arrivals "Locked in"? Is that Arrival "permanent", i.e., an infinite ray going upwards along the Y-axis?

>_> This would be easier with diagrams.

I agree this would be way easier with charts. You've managed to confuse me here.
If I understand it correctly, I do have a note as to the portion I have underlined. The fifth law of the system handles that one.

5: The Law of Jump.
Time travel events still occur if they are erased. If things change in way way that would make you not travel, you are unaffected.

I'm going to try and chart all this, and will edit this when I have.

EDIT: Well, I have these now.

NOTE: In all of these cases, the "single, mutable timeline" is the top row

The future is right, the past is left. Time events enter at the top, and go down. Each unit of meta-time is one second up and down for locked persons. Each unit of time up and down is 10 days of time.

Before any time travel
|@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
|@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
M |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
e |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
t |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
a |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
- |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
T |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
i |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
m |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
e |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
|@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
|@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
|@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
+------------------------------------------
Time
@ is "normal time" (this is an arbitrary choice for normal)

Then I jump back
|@@@@@@@@@@@@@@@@@@@@*@@@@@@@@@@@@@@@@@@@@<
|@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
M |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
e |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
t |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
a |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
- |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
T |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
i |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
m |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
e |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
|@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
|@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
|@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
+------------------------------------------
Time
< is my exit point,
* is normal time, except I am there.

In this example, I am trying not to change events. Thanks to the 3rd law, just being here changes nothing. Then I mess up... So a few units of meta-time later...

I make a change
|@@@@@@@@@@@@@@@@@@@@%@@@@@@@@@@@@@@@@@@@@<
|@@@@@@@@@@@@@@@@@@@@*@@@@@@@@@@@@@@@@@@@@<
M |@@@@@@@@@@@@@@@@@@@@*@@@@@@@@@@@@@@@@@@@@<
e |@@@@@@@@@@@@@@@@@@@@*@@@@@@@@@@@@@@@@@@@@<
t |@@@@@@@@@@@@@@@@@@@@*@@@@@@@@@@@@@@@@@@@@<
a |@@@@@@@@@@@@@@@@@@@@*@@@@@@@@@@@@@@@@@@@@<
- |@@@@@@@@@@@@@@@@@@@@*@@@@@@@@@@@@@@@@@@@@<
T |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
i |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
m |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
e |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
|@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
|@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
|@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
+------------------------------------------
Time

% is the wave. Right now I am under it. So a few more units of meta-time passes...

The wave moves
|@@@@@@@@@@@@@@@@@@@@*#####%@@@@@@@@@@@@@@<
|@@@@@@@@@@@@@@@@@@@@*####%@@@@@@@@@@@@@@@<
M |@@@@@@@@@@@@@@@@@@@@*###%@@@@@@@@@@@@@@@@<
e |@@@@@@@@@@@@@@@@@@@@*##%@@@@@@@@@@@@@@@@@<
t |@@@@@@@@@@@@@@@@@@@@*#%@@@@@@@@@@@@@@@@@@<
a |@@@@@@@@@@@@@@@@@@@@*%@@@@@@@@@@@@@@@@@@@<
- |@@@@@@@@@@@@@@@@@@@@%@@@@@@@@@@@@@@@@@@@@<
T |@@@@@@@@@@@@@@@@@@@@*@@@@@@@@@@@@@@@@@@@@<
i |@@@@@@@@@@@@@@@@@@@@*@@@@@@@@@@@@@@@@@@@@<
m |@@@@@@@@@@@@@@@@@@@@*@@@@@@@@@@@@@@@@@@@@<
e |@@@@@@@@@@@@@@@@@@@@*@@@@@@@@@@@@@@@@@@@@<
|@@@@@@@@@@@@@@@@@@@@*@@@@@@@@@@@@@@@@@@@@<
|@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
|@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
+------------------------------------------
Time
# is "altered time"

Nothing that interesting is happening yet, but let's wait more...

The wave reaches my jump point
|@@@@@@@@@@@@@@@@@@@@*####################%
|@@@@@@@@@@@@@@@@@@@@*###################%<
M |@@@@@@@@@@@@@@@@@@@@*##################%@<
e |@@@@@@@@@@@@@@@@@@@@*#################%@@<
t |@@@@@@@@@@@@@@@@@@@@*################%@@@<
a |@@@@@@@@@@@@@@@@@@@@*###############%@@@@<
- |@@@@@@@@@@@@@@@@@@@@*##############%@@@@@<
T |@@@@@@@@@@@@@@@@@@@@*#############%@@@@@@<
i |@@@@@@@@@@@@@@@@@@@@*############%@@@@@@@<
m |@@@@@@@@@@@@@@@@@@@@*###########%@@@@@@@@<
e |@@@@@@@@@@@@@@@@@@@@*##########%@@@@@@@@@<
|@@@@@@@@@@@@@@@@@@@@*#########%@@@@@@@@@@<
|@@@@@@@@@@@@@@@@@@@@*########%@@@@@@@@@@@<
|@@@@@@@@@@@@@@@@@@@@*#######%@@@@@@@@@@@@<
+------------------------------------------
Time

Now this right here is the cause for 99% percent of what people would call paradoxes. In this case, let's say my change results in my original self not jumping. The 5th law states that I am unaffected.

Marcelinari
2013-02-24, 02:21 PM
I'd like to try a solution to the second problem, which may or may not already have been suggested because I just skipped the last page and a half.

At T-0 seconds: (Alpha Timeline)
Adam!Alpha is talked into traveling into the past in a stable orbit above earth precisely 2 minutes ago. There is no dead Adam currently in orbit.

At T-2 minutes: (Beta Timeline)
Adam!Alpha appears in stable orbit, becoming subject to rapid decompression and dying horribly. This subtly alters the constituency of the universe and causes some unusual effects far, far down the temporal road.

At T-0 seconds: (Beta Timeline)
Adam!Beta is talked into traveling into the past in a stable orbit above earth precisely 2 minutes ago. There is currently a dead Adam in orbit.

At T-2 minutes: (Gamma Timeline)
Adam!Beta appears in stable orbit in (probably) a slightly different position than Adam!Alpha did - however, there is no sign of any other Adams, as the sequence of events leading to the appearance of Adam!Alpha never happened. The Alpha Timeline and all of its immediate constituents are redacted, and replaced by the appropriate effects of either the Beta or Gamma Timelines.

At T-0 seconds: (Gamma Timeline)
Adam!Gamma is talked into traveling into the past in a stable orbit above earth precisely 2 minutes ago. There is currently a dead Adam in orbit. Et cetera, et cetera. You can fill in the rest.

To try and explain how I reached the conclusion - to have NI Adams pop up in orbit in the same timeline seems ridiculous. There is some level of causality at work, and that requires temporal continuity between timelines - the Adams have to continue through the timelines at normal time flow. The iterations must happen in order, with one affecting the next, Alpha causing Beta, Beta causing Gamma.

However, because the causal chain leading to the appearance of Adam!Alpha in stable orbit is overridden by the causal chain leading Adam!Beta to appear in stable orbit, it makes no sense for Adam!Alpha to exist, and thus he is redacted, expunged from existence to stop physicists and philosophers from having their brains spontaneously combust. The universe is apparently kind.

Does this work? It seems logical to me, but it might be predicated on faulty logic, so point that out if you see it, please.

TuggyNE
2013-02-24, 07:04 PM
Sure, except that isn't the definition of consistent.

Consistent just means that something behaves the same way in different situations - that you can pick one set of rules, and those rules will be followed regardless of the specifics. So no "Time travel does this, so long as it supports the plot" or "Time travel works like this, unless it would cause a problem".

There are two different definitions of consistency you could reasonably use here. One is the database sense: information is maintained in a logically consistent state following certain defined rules, and has no opportunity to be corrupted by synchronicity problems or interruptions of service. The other is the narrative sense: whatever rules there are are applied uniformly to the story.

Abandoning the first (at least in principle) is probably essential to making useful time travel, because you're guaranteed to have synchronicity errors of all kinds. However, it's reasonable to keep the second if you can manage it.

FreakyCheeseMan
2013-02-24, 07:32 PM
There are two different definitions of consistency you could reasonably use here. One is the database sense: information is maintained in a logically consistent state following certain defined rules, and has no opportunity to be corrupted by synchronicity problems or interruptions of service. The other is the narrative sense: whatever rules there are are applied uniformly to the story.

Abandoning the first (at least in principle) is probably essential to making useful time travel, because you're guaranteed to have synchronicity errors of all kinds. However, it's reasonable to keep the second if you can manage it.

For the first one, you just have to accept that you're dealing with a higher-dimensional view of the universe. Consider the time-wave theories that have been offered, in which the past changes at a given rate. We can view these - at least, the more simple ones - as unchanging five-dimensional objects, in the same way that we view our own world as an unchanging four dimensional object.

Deathra13
2013-02-24, 11:20 PM
Dunno if its been mentioned but to avoid that particular paradox of the party/adam moon, simply dont allow more then one of the same individual to exist. If they jump to the time when they would exist have the two merge, including memories, thus in the case of adam jumping back two minutes, rather then dying in space he merges with current adam who now remembers jumping back in space and merging with himself and suffers some rather severe deja vu. If the character jumps to before they exist, they can muck about all they want until the point they come into existance.

NichG
2013-02-25, 04:48 AM
The real acid test for any of these models is basically: could you write a computer game that uses it?

If you can't tell a computer how to run the thing, then there's likely some place where the rules will break. Ripple models work well here - there's an RTS game called Achron that implements one.

One interesting thing that hasn't been brought up but is meaningful along these lines - for a computer, keeping track of random seeds is really tricky if you have random processes in the world and have time travel. Basically, inserting a new element that asks for a random number causes all the other random seeds to shift, which can change events non-locally. So you either have to have something that is completely deterministic, or be very clever with local random seeds such that random numbers are always generated at a constant rate (because if interactions change the rate of random number generation, that also shifts the seed).

In a tabletop sense, its basically the statement 'identifying which die roll corresponds to which die roll across two diverging timelines is nontrivial, so its hard to just say 'all other die rolls remain the same''.