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Oracle_Hunter
2011-06-29, 02:27 PM
Inspired by a misreading of another thread (http://www.giantitp.com/forums/showthread.php?t=204872), I'm making one to discuss time travel in games, and which ones handle it best.

Personally, I've found Feng Shui (http://en.wikipedia.org/wiki/Feng_Shui_%28role-playing_game%29) to handle the whole thing pretty well. It uses a mixture of San Dimas Time (http://tvtropes.org/pmwiki/pmwiki.php/Main/SanDimasTime) and fixed Time Portals to keep things orderly with a healthy dose of Elastic History to prevent casual unraveling. That said, it makes specific allowances for how history can be changed and, in fact, makes that central to the structure of the world.

So, how about it? What's the most "realistic" way to have time-travel in a RPG and do you know of any interesting examples? And please, no jokes about the "one second at a time" manner of time travel :smalltongue:

randomhero00
2011-06-29, 02:49 PM
I think the most realistic form of time travel is that time is an illusion and that everything that has happened happened. We only experience time as linear because that's the only way to process it.

So if you went back in time at age 35 and killed Hitler, then that had already happened at age 6, you just haven't followed through with your stream of consciousness yet.

Oracle_Hunter
2011-06-29, 03:01 PM
I think the most realistic form of time travel is that time is an illusion and that everything that has happened happened. We only experience time as linear because that's the only way to process it.

So if you went back in time at age 35 and killed Hitler, then that had already happened at age 6, you just haven't followed through with your stream of consciousness yet.
But how does that work in a game? It's all well and good to say "predestination" and wash your hands of it but it doesn't exactly encourages characters to do anything aside from smoke and drink coffee in sidewalk cafes, moaning about their enui :smalltongue:

TheEmerged
2011-06-29, 03:13 PM
So, how about it? What's the most "realistic" way to have time-travel in a RPG and do you know of any interesting examples? And please, no jokes about the "one second at a time" manner of time travel :smalltongue:

What if I'm not joking?

Generally, I disallow this under "more trouble than it is worth" boilerplate. As a general rule (across campaign worlds) the past can only be viewed and "there is no such thing as The future" - and note the definitive article. Any attempts to see the future result in seeing what is currently the most likely future.

Oracle_Hunter
2011-06-29, 03:25 PM
What if I'm not joking?

Generally, I disallow this under "more trouble than it is worth" boilerplate. As a general rule (across campaign worlds) the past can only be viewed and "there is no such thing as The future" - and note the definitive article. Any attempts to see the future result in seeing what is currently the most likely future.
Then you should definitely consider the Feng Shui approach.

There are 4 Junctures for the Players to travel to: 69 AD, 1850 AD, 1996 AD and 2056 AD. The Time Gates to each Juncture operate on San Dimas Time and history is assumed to be generally elastic: uproot Hitler's family tree and it turns out that Bitler is running Germany during WWII. However, if you (or your organization) control certain mystic sites you can affect the outcome of history. In game, the primary example is when a group known as the Ascended took over a bunch of sites in the 12th Century (due to the opening of a new Juncture) and sparked a series of events that lead to magic being less powerful in the future. This destroyed the then-world of 1988 which had been ruled by 4 sorcerer-emperors for centuries and instead presented the world of 1988 we know as "real."

Ravens_cry
2011-06-29, 03:26 PM
The multiple universe time travel theory, "go back in time, kill your grandfather, no paradox because now you are travelling toward a future where you are not born" makes the most sense to me, or at least involves the fewest headaches.
It has it's downsides, you can never go home unless you can move parallel, cross to another streams as it were.
Still, an immutable past leads to headaches. It means the past is the best place to store antimatter in large quantities as it can never interact with matter or whoops, that would change something.

Oracle_Hunter
2011-06-29, 03:33 PM
The multiple universe time travel theory, "go back in time, kill your grandfather, no paradox because now you are travelling toward a future where you are not born" makes the most sense to me, or at least involves the fewest headaches.
It has it's downsides, you can never go home unless you can move parallel, cross to another streams as it were.
Still, an immutable past leads to headaches. It means the past is the best place to store antimatter in large quantities as it can never interact with matter or whoops, that would change something.
In Feng Shui this problem is solved via "Lateral Reincarnation:" if you kill the ancestor of someone in the current era, then their "soul" is instead placed in a different body that comes from an unbroken family line.

So if you knew Bob in 1996 and went back to 1850 and accidentally offed his ancestors, when you got back to 1996 "Bob" might look different and have a different name but would otherwise be the guy you knew before you traveled. Really, if you didn't have Time Traveler's Immunity, you probably wouldn't even know anything had happened.

Ravens_cry
2011-06-29, 03:43 PM
In Feng Shui this problem is solved via "Lateral Reincarnation:" if you kill the ancestor of someone in the current era, then their "soul" is instead placed in a different body that comes from an unbroken family line.

So if you knew Bob in 1996 and went back to 1850 and accidentally offed his ancestors, when you got back to 1996 "Bob" might look different and have a different name but would otherwise be the guy you knew before you traveled. Really, if you didn't have Time Traveler's Immunity, you probably wouldn't even know anything had happened.
Which really only works in a more spiritual setting.
Another problem with multiple universe from a gaming perspective is you need to be inhumanly good at improvisation and figuring out plausible consequences. Unlike a science fiction story, where you only need to plan as many 'branches' as appear in the story, and the actions that created them, players always do the unexpected thing.

Yora
2011-06-29, 03:45 PM
I think the only realistic type of time travel in a magical wold are locations in which an after-image of a specific event is replaying itself. The location magically recreates all the conditions and everything repeats as it did before, except for the PCs interactions with the events.
However, since it's mostly a reenactment, this does not have any influence on the present. But this does allow them to experience significant events that happened in the past.

Xefas
2011-06-29, 03:45 PM
I think the most "realistic" time travel in an RPG would be one where you cannot go back in time; only forward at an accelerated rate. Doing so would require orbiting a black hole, such that its mass causes you to move slower relative to the rest of the universe, or traveling so quickly that you push on the lightspeed barrier and have it push back, or something of that nature.

While it's not a central part of the game, you could certainly do something like that in Free Market. You could probably make a pretty penny selling tickets to the future. Don't want to wait until your favorite band's next album comes out in 6 months? Just buy a ticket to 6 months from now! Its exactly the sort of logic that works on the Donut.

Elvenoutrider
2011-06-29, 03:47 PM
An article from cracked:

http://www.cracked.com/article_18564_6-time-travel-realities-doc-brown-didnt-warn-us-about.html

Oracle_Hunter
2011-06-29, 03:51 PM
Which really only works in a more spiritual setting.
I suppose, although you can simply apply the "elastic history" principle and not call it "reincarnation" if that floats your boat :smallbiggrin:


Another problem with multiple universe from a gaming perspective is you need to be inhumanly good at improvisation and figuring out plausible consequences. Unlike a science fiction story, where you only need to plan as many 'branches' as appear in the story, and the actions that created them, players always do the unexpected thing.
Helpfully, if the DM can't easily foresee the consequences, then neither will the Players. Who can say with authority what blowing up a cotton warehouse in 1850 will do to the 1996 we know and love? In a game setting, it is the DM - and he can pick and choose any of the various Butterfly Effect modifiers he likes to have it turn out however he prefers. Sure, the Players can do more to try and "fix" history but when you use San Dimas Time you have a limited ability to perform "do overs."

Additionally, you can make the influencing of Key Events part of the system. Perhaps only a few things actually do matter in the grand scheme of things. In Feng Shui these sites are known in-game and manipulating them is a major driver of the action. For the DM, this makes the suite of probable multiverses somewhat smaller.

Yuki Akuma
2011-06-29, 03:53 PM
I think the most realistic (or at least the most awesome) time travel in an RPG I've ever encountered is in Continuum.

Because... Continuum is just awesome.

I'm bad at explaining things. >.>

Xefas
2011-06-29, 04:06 PM
I think the most realistic (or at least the most awesome) time travel in an RPG I've ever encountered is in Continuum.

I agree with this part, at the very least.

Frozen_Feet
2011-06-29, 04:13 PM
I think the most realistic version is where once you go back, you only get forward the ordinary way. You effectively "reset" universe to the point of past where you go to, then proceed to live as normal - if you want to "fast forward", you need to get yourself a cryogenic pod and go to circle the solar system with relativistic speeds or some such.

Fhaolan
2011-06-29, 04:50 PM
An additional solution to the grandfather paradox is that it's not a paradox at all.

You go back and kill your grandfather. The universe rewrites in such a manner that it was not your grandfather, but that you had a reason to go back and kill that person. Because of the rewrite, you remember the new version of history, not the prior one.

Oh, and there's the Solipsist approach. Nothing exists except the person experiencing. Prior to that person's existance there was nothing. Outside of the person's sensory perception, there is nothing. So if you went back into history and killed your grandfather, it's irrelevant because there is no such thing as history. There is only a series of experiences for the one real person. Anything is possible to happen, simply because the only rule is that reality is defined by the perception of one real individual.

Frozen_Feet
2011-06-29, 05:29 PM
I've always found the grandfather paradox a bit silly. If you've physically been transferred to the past, why should you care? All the deed does is sever its causality in regards to your original timeline, but why should it affect your body as a physical object? All your memories are nothing but fabrications now, but otherwise you should remain unharmed.

Pie Guy
2011-06-29, 06:08 PM
I've always found the grandfather paradox a bit silly. If you've physically been transferred to the past, why should you care? All the deed does is sever its causality in regards to your original timeline, but why should it affect your body as a physical object? All your memories are nothing but fabrications now, but otherwise you should remain unharmed.

Because in a singular timeline, how could you exist to go back in time to kill your grandfather? You've never existed in the first place, so then your grandfather is alive, and then you're alive to go back and kill him, but then you don't exist again. It only works out in multiple reality theory.

Frozen_Feet
2011-06-29, 06:21 PM
But whether or not your hypothetical self commits the same actions in the future, why would your physical presence be erased? The idea is that once you move something in the past, it exists in the past regardless of what causality sent it there. You effectively popped out from nowhere, and that remains regardless of whether you kill your grand parent.

Ravens_cry
2011-06-29, 06:47 PM
But whether or not your hypothetical self commits the same actions in the future, why would your physical presence be erased? The idea is that once you move something in the past, it exists in the past regardless of what causality sent it there. You effectively popped out from nowhere, and that remains regardless of whether you kill your grand parent.

In the paradox, there is only one future, one present, one past. One time actually. A young man is dead and the person who killed him can not exist if that person is killed. Hence, the paradox.

Fhaolan
2011-06-29, 07:49 PM
In the paradox, there is only one future, one present, one past. One time actually. A young man is dead and the person who killed him can not exist if that person is killed. Hence, the paradox.

The paradox assumes casualty as a hard rule. That the universe actually 'cares' that the person who killed him can not exist anymore. Unfortunately, without actual time travel there is no way to test that casualty is an actual universal constant, and that reality doesn't allow for weird stuff like this.

Ravens_cry
2011-06-29, 07:59 PM
The paradox assumes casualty as a hard rule. That the universe actually 'cares' that the person who killed him can not exist anymore. Unfortunately, without actual time travel there is no way to test that casualty is an actual universal constant, and that reality doesn't allow for weird stuff like this.

It's not that killing your grandfather would make you cease to exist, it's that to kill your grandfather in the past you need to exist but you can't exist if someone killed your grandfather before your parents were conceived. That's why it's a paradox.
All this, of course, is assuming a single timeline, but only in a single timeline is this a paradox.

Delwugor
2011-06-29, 11:17 PM
Runes, but the problem is no one knows what comes after "Future Semiconditionally Modified Subinverted Plagal Past Subjunctive Intentional".

Time travel to the future is simply explained by use of the "Ludicrous Haste" spell.

To the past requires a different much more difficult spell called "Localized Causality Disengagement". The basic premise is that there is a magical connection between cause and effect, once this connection is broken the "Ludicrous Haste" spell moves you back in time.

Alleran
2011-06-29, 11:54 PM
Personally, I like the Doctor Who version of time travel. Even if it isn't all that realistic in the end.

Wibbly wobbly timey wimey.

DragonOfUndeath
2011-06-29, 11:59 PM
I prefer the branching theory myself.
You can go back and screw up as much as you want Paradox-free and return to your own time by going back past your original point of screwing things up then go up to your original unchanged time (or one slightly different)

Tvtyrant
2011-06-30, 12:13 AM
I made a pair of cities where time can fluctuate because they have no importance; you cannot leave the cities in any time but your own but you are free to travel back and forth in time within the cities bounds. Breaking through the barrier keeping you within the town in other times takes high level magic, and thus keeps things mostly in his own time.

Welknair
2011-06-30, 12:17 AM
Inspired by a misreading of another thread (http://www.giantitp.com/forums/showthread.php?t=204872),

Did the exact same thing.


I'm a fan of the "Multiple-Worlds" model of time travel as it avoids the problem of all those nasty paradoxes. Sadly I haven't played any games in which time travel was utilized.

Ravens_cry
2011-06-30, 01:04 AM
I prefer the branching theory myself.
You can go back and screw up as much as you want Paradox-free and return to your own time by going back past your original point of screwing things up then go up to your original unchanged time (or one slightly different)
Actually, you can't return home if the branches occur on a quantum level, which is the most realistic way to look at it. Oh, you could go back to people who are exactly the same as family and friends, but you can't go back to your original timeline as there is a fundamental difference between it and the original.
You were there, and on a quantum level that makes all the difference.

Zaydos
2011-06-30, 01:07 AM
Actually, you can't return home if the branches occur on a quantum level, which is the most realistic way to look at it. Oh, you could go back to people who are exactly the same as family and friends, but you can't go back to your original timeline as there is a fundamental difference between it and the original.
You were there, and on a quantum level that makes all the difference.

And this is why you need a time machine that can just travel sidewise through time, moving straight from one branch to another... although at that point it's more of a Time And Multiple Dimensions In Space machine.

DragonOfUndeath
2011-06-30, 01:10 AM
Everyone is exactly the same, have the same memories and there is no difference except on a quantum level where you put the machine in that minute you were there.
Sure technically you aren't in your original configuration but who cares about the dust patterns of a desert?

Ravens_cry
2011-06-30, 01:33 AM
And this is why you need a time machine that can just travel sidewise through time, moving straight from one branch to another... although at that point it's more of a Time And Multiple Dimensions In Space machine.
Add the XYZ axes ,and you might as well just call it a "Travel Machine."

Everyone is exactly the same, have the same memories and there is no difference except on a quantum level where you put the machine in that minute you were there.
Sure technically you aren't in your original configuration but who cares about the dust patterns of a desert?
The thing is your loved ones back home, the ones you came from, will never see you again. That might matter to some people. Unless, of course, it has the additional capabilities Zaydos mentions.

DragonOfUndeath
2011-06-30, 01:43 AM
Except when you realize there are countless branches where you aren't born, you are a different gender, your parents didn't meet etc. it doesn't quite matter as much.

Lamech
2011-06-30, 02:26 AM
The way to handle time travel in an RPG? With a lot of restrictions.
Two basic theories of time travel.
A) You can't change things, there is one time line and it will always be that way. You can go back it time but everything is pre-determined.
B) You can change things. Pretty cool right. You go back in time and shoot your grandfather now your grandfather is dead. As time now unfolds you won't be born. But hey whatever. Its all good.
Long wall of text explaining why time travel sucks.
So what can you do with each?
A) This is really hard to do with an RPG due to the need for predestination and knowing everything. But really if someone tries to cause a paradox something will need to stop them.
Of course you can do a bunch of really cool things! One a character can essentially get all the time they need if they are careful. Also if your careful you can generate infinite amounts of processing power essentially.

B) This is a fun one. You can go back in time and shoot the grandfather.
But weird conclusions are reached if you think too hard. We could get two timelines. Timeline 1 and timeline 2. Timeline 1 results in a time traveler changing time to timeline 2. And timeline 2 has someone change it to timeline 1. So which timeline is the real one when history moves forward?

You could say quantum mechanics dictates randomness so this won't happen, or chaos theory says its really unlikely. Which is good, but... that means each time you time travel you have a new timeline. And as long as the amount of mass in the system is bounded there are only so many timelines to choose from... so either 1) you have this problem 2) eventually a timeline will be reached in which it feeds back into itself, similar to the no change option or 3) a timeline with no time travel. So it appears that changing the past is still impossible in the final time line even if the past can be changed. Anyone can go back in time, its just that in the final timeline they will have spawned out of nothingness, or failed to change anything.

But it sucks least with option B. If you didn't click on the spoiler I recommend not thinking about it too hard. But time travel needs to be controlled. Have you seen the second Austin Powers movie? Remember how there were two Austins? If players have there own time machine they could do that. Or do that 500 times. Don't let them freely travel. I recommend keeping when and where they go limited. I also recommend keep the players together as a group, that way no one gets erased ever.

Then we need to know how "elastic" time is. If I go back in time and shoot Hitler will he be replaced by a Bitler? Or will world war II not have happened?
Also important, suppose I go back in time and save a bunch of people. Realistically these people will marry and date people who weren't all dead in the first time line. And then the would-be married/dating people do the same. They will take jobs shift people all over the country. And a million other ways to screw up marriages and relationships. So even if Bitler replaces Hitler a LOT of people could end up erased. Or you could decree that no most people are just fine even if the PC's save an orphanage in 1357.

Ravens_cry
2011-06-30, 02:52 AM
Except when you realize there are countless branches where you aren't born, you are a different gender, your parents didn't meet etc. it doesn't quite matter as much.
Maybe, but many people care enough about loved ones that they wouldn't want the loved ones not to see them again.
" Mom/Dad/Boyfriend/Girlfriend/Friend/Brother/Sister/Pet et cetera , you will never see me again. I will see someone who is basically you in every way and they will see me again, but you, the you I am talking will not. Ever."
I am sorry, but I wouldn't want to do that to the people I love. Or for that matter, to my enemies. The bastards aren't getting rid of me that easily!:smallamused:
@Lamech:
I always felt that the time-line was brittle, hella brittle, quantumly brittle. Of course, if you are super careful, basically stealthing while floating in a spacesuit, you might not change enough to notice. On the other hand, thanks to the butterfly effect, small, possibly microscopic changes could change things in ways we can barely conceive. Like not being conceived. After all, if it had being one sperm instead of another that fertilized the egg that became you, you wouldn't be you.And that also goes for every person born in the time between your incursion and when you started. Given how much of our modern world depends on political factors, a persons personal preference over another thing, their whims and pettiness, even tiny changes could add up to an almost completely different world.

Lamech
2011-06-30, 03:55 AM
@Lamech:
I always felt that the time-line was brittle, hella brittle, quantumly brittle. Of course, if you are super careful, basically stealthing while floating in a spacesuit, you might not change enough to notice. On the other hand, thanks to the butterfly effect, small, possibly microscopic changes could change things in ways we can barely conceive. Like not being conceived. After all, if it had being one sperm instead of another that fertilized the egg that became you, you wouldn't be you.And that also goes for every person born in the time between your incursion and when you started. Given how much of our modern world depends on political factors, a persons personal preference over another thing, their whims and pettiness, even tiny changes could add up to an almost completely different world.Yup basically.

Earthwalker
2011-06-30, 03:57 AM
How I have done time travel in games I ran.
In a superhero game (Squadron UK). The hero wakes up as a figure approachs his bed, it appears to be a copy of himself. This copy drops a pocket watch on the bed and tells the hero.
“Be at the 12th national bank tomorrow at 11am, its of vital importance, and don’t lose the watch”
The hero then propmply disappears. All the PCs got the same message and all of them went to the bank the next day, sure enough at 11am the bank is robbed, by a gang, the gang leader has some kind of belt that appears to have some limited control of time (making the gang leader fast, or freezing people in place, the pocket watches seem to grant some imunity to these time powers) the heros either defeat him or he escapes. Cue searching for his base.
<now cutting a long story short>
The heros follow a trail to a kidnapped scientist called the Time King. They rescue The Time Kings daughter and free him from the control of a mob boss. As a closing sceane the Time King give each player a pocket watch and tells them to go back in time to deliever the watches to themselves and start all this in motion again.




The other use has been in Runequest and hero gating. Where you can relive and rewrite event in the past but you are on another plane reliving the events, your changes do happen but in Runequest all things are true, so you may change the past but history as it is survives. One organization is going into hero gates and repeating their version of history so many times it is having an effect on the current time line but this level of power is kind of beyond the scope of the PCs.

Fri
2011-06-30, 04:00 AM
In Michael Crichton's Timeline and some other stories I read, time travel is basically impossible. But you can travel to alternate universes that younger or older than your home universe. It's basically dimensional travel, instead of time travel.

NichG
2011-06-30, 09:41 AM
Branching timelines is by far easier to run, but its particularly exploitive and there are some oddities about it if you want to run it 'realistically'. First of all, branching timelines means that pretty much there are no conserved quantities in the universe unless you add an additional rule - every time a branching occurs you double the amount of energy, etc that exists. 'Okay, fine, but within a universe its conserved!' you might say, but once people can timetravel, there's no reason that there aren't naturally occuring physical states that can timetravel. In fact, there must be - even if the physical state that enables timetravel is incredibly rare, you're doubling your number of universes every Planck time in order to attempt it. Some universe out there is going to get that state. If you instead say that branching doesn't occur until someone time-travels, that helps a bit, but the first time-traveler basically sets off an explosion of branchings.

So now we've got a universe that has energy flowing in from all these time travel events, what happens? Well, there's a paradox associated with theories of an infinite universe (in both size and age, since they're fundamentally related). If the universe is infinite, there is an infinite amount of light coming in from distant stars. Lets say that gets absorbed along the way - well, that just means that the absorbing things heat up a little bit, and start radiating like a blackbody, so that doesn't save us. Now in our branching timetravel universe, for every universe you travel out of, there are a multitude of other universes that a version of you is traveling out of. And since they're going down a branch, that means that they don't just redistribute and keep it even - they concentrate. So you'd end up with the past being filled by a nigh infinite number of converging time travelers, each carrying energy (in the form of mass) from the future.

Fhaolan
2011-06-30, 09:49 AM
It's not that killing your grandfather would make you cease to exist, it's that to kill your grandfather in the past you need to exist but you can't exist if someone killed your grandfather before your parents were conceived. That's why it's a paradox.
All this, of course, is assuming a single timeline, but only in a single timeline is this a paradox.

Exactly. The point I'm trying to make is that it's a paradox that may not even matter. The paradox assumes that the universe *cares* that killing your grandfather means that your parents weren't conceived, and therefore you don't exist to go back in time and kill your grandfather. There are myriad solutions to this paradox, mostly involving history re-writing in whatever means necessary to resolve it. Since we have no experience with RL time travel, we have no actual proof that such anomolies can't happen. For all we know, the universe if filled with time travellers that should not exist according to the grandfather paradox.

Heck for all we know, it means the parent your grandfather would have conceived, gets concieved via immaculate conception. Who knows? The challenge is not to get a solution to the paradox, but to enumerate all the different solutions. Last I counted, I got up to twelve before I got bored.

DragonOfUndeath
2011-06-30, 10:05 AM
Branching timelines is by far easier to run, but its particularly exploitive and there are some oddities about it if you want to run it 'realistically'. First of all, branching timelines means that pretty much there are no conserved quantities in the universe unless you add an additional rule - every time a branching occurs you double the amount of energy, etc that exists. 'Okay, fine, but within a universe its conserved!' you might say, but once people can timetravel, there's no reason that there aren't naturally occuring physical states that can timetravel. In fact, there must be - even if the physical state that enables timetravel is incredibly rare, you're doubling your number of universes every Planck time in order to attempt it. Some universe out there is going to get that state. If you instead say that branching doesn't occur until someone time-travels, that helps a bit, but the first time-traveler basically sets off an explosion of branchings.

So now we've got a universe that has energy flowing in from all these time travel events, what happens? Well, there's a paradox associated with theories of an infinite universe (in both size and age, since they're fundamentally related). If the universe is infinite, there is an infinite amount of light coming in from distant stars. Lets say that gets absorbed along the way - well, that just means that the absorbing things heat up a little bit, and start radiating like a blackbody, so that doesn't save us. Now in our branching timetravel universe, for every universe you travel out of, there are a multitude of other universes that a version of you is traveling out of. And since they're going down a branch, that means that they don't just redistribute and keep it even - they concentrate. So you'd end up with the past being filled by a nigh infinite number of converging time travelers, each carrying energy (in the form of mass) from the future.

So the amount of energy needed is immense, you are assuming that the energy is not held up in Time Particles (a type of energy similar to Stem Cells for humans, able to become anything needed), waiting to be converted at the time of a split. Since Energy has no Mass and there are Spatial Dimensions we can't see, then there may be a pool of energy somewhere being slowly drained as we branch.
This does have unfortunate implications though as to what happens when this pool dries up. But I have a way of explaining that too.
You see, Energy can't be created or destroyed right? But Entropy means that there will be the Heat Death of the Universe. What if, at that moment all the Mass converts to Energy and all the Energy reverts to Time Particles? It would refill the pool with the entire amount of energy it had taken earlier.

We now have a way of resolving the increasing mass issue without using actual infinity (just functional infinity) or paradoxi

Urpriest
2011-06-30, 10:17 AM
Realistic time travel would require the game to contain a representation of the Lorentz group. You'd really want time (turn order I suppose) to be put on an equal footing with space to get game-mechanical constructs that transform properly.

Basically, you presumably want to play a game in causal order. Provided the party sticks together, this means that you need to only determine events in the party's rear light-cone and go through resulting events as they enter the party's forward light-cone. The issue with time travel is that it means the situation is topologically nontrivial. So you'd want some sort of topological method of conflict resolution, rather than a local one like most games have. Maybe Burning Wheel? :smallwink:

Also, why yes I do have a giant pile of catgirl corpses in my basement, why do you ask?

Edit:

So the amount of energy needed is immense, you are assuming that the energy is not held up in Time Particles (a type of energy similar to Stem Cells for humans, able to become anything needed), waiting to be converted at the time of a split. Since Energy has no Mass and there are Spatial Dimensions we can't see, then there may be a pool of energy somewhere being slowly drained as we branch.
This does have unfortunate implications though as to what happens when this pool dries up. But I have a way of explaining that too.
You see, Energy can't be created or destroyed right? But Entropy means that there will be the Heat Death of the Universe. What if, at that moment all the Mass converts to Energy and all the Energy reverts to Time Particles? It would refill the pool with the entire amount of energy it had taken earlier.

We now have a way of resolving the increasing mass issue without using actual infinity (just functional infinity) or paradoxi

Gah! (The rest of the post too, but especially that).

Cespenar
2011-06-30, 10:18 AM
Best time travel theory is a dead time travel theory.

DragonOfUndeath
2011-06-30, 10:39 AM
Gah! (The rest of the post too, but especially that).

It's too negligible to really count and could exist on a completely different Dimension to us so it practically has no mass
And what's wrong with the rest of the post?

Urpriest
2011-06-30, 11:44 AM
It's too negligible to really count and could exist on a completely different Dimension to us so it practically has no mass
And what's wrong with the rest of the post?

Well lemme look at the rest of this conversation so I get a better idea of what was being discussed.


Branching timelines is by far easier to run, but its particularly exploitive and there are some oddities about it if you want to run it 'realistically'. First of all, branching timelines means that pretty much there are no conserved quantities in the universe unless you add an additional rule - every time a branching occurs you double the amount of energy, etc that exists. 'Okay, fine, but within a universe its conserved!' you might say, but once people can timetravel, there's no reason that there aren't naturally occuring physical states that can timetravel. In fact, there must be - even if the physical state that enables timetravel is incredibly rare, you're doubling your number of universes every Planck time in order to attempt it. Some universe out there is going to get that state. If you instead say that branching doesn't occur until someone time-travels, that helps a bit, but the first time-traveler basically sets off an explosion of branchings.

Ok, this is ridiculous. The whole point of an Everett-style quantum mechanical multiverse is that reality is a superposition of all of these possible histories, that happen to be noninteracting. You never get more or less of conserved quantities than you started with (except due to uncertainty relations, but that's not what we're talking about). Suppose you start with a universe containing a particle with energy E, with a 50% chance of decaying into two particles of energy E/2. Then you get a universe that's a superposition of the particle having decayed and not having decayed. That universe still contains energy E, not 2E. The only reason people call it two separate universes is because from then on the two halves of the superposition don't influence eachother, it's not as if they actually contain different "stuff" in a metaphysical sense.


So now we've got a universe that has energy flowing in from all these time travel events, what happens? Well, there's a paradox associated with theories of an infinite universe (in both size and age, since they're fundamentally related). If the universe is infinite, there is an infinite amount of light coming in from distant stars. Lets say that gets absorbed along the way - well, that just means that the absorbing things heat up a little bit, and start radiating like a blackbody, so that doesn't save us. Now in our branching timetravel universe, for every universe you travel out of, there are a multitude of other universes that a version of you is traveling out of. And since they're going down a branch, that means that they don't just redistribute and keep it even - they concentrate. So you'd end up with the past being filled by a nigh infinite number of converging time travelers, each carrying energy (in the form of mass) from the future.

Light isn't just produced from nowhere you know. To the extent that energy is conserved, any energy in radiation is at the expense of energy in something else. Just because the universe as a whole contains an infinite amount of radiation does not mean it all needs to be here and now, any more than the fact that the universe contains an infinite amount of matter means that an infinite amount of matter must be here and now.


So the amount of energy needed is immense, you are assuming that the energy is not held up in Time Particles (a type of energy similar to Stem Cells for humans, able to become anything needed), waiting to be converted at the time of a split. Since Energy has no Mass and there are Spatial Dimensions we can't see, then there may be a pool of energy somewhere being slowly drained as we branch.
This does have unfortunate implications though as to what happens when this pool dries up. But I have a way of explaining that too.
You see, Energy can't be created or destroyed right? But Entropy means that there will be the Heat Death of the Universe. What if, at that moment all the Mass converts to Energy and all the Energy reverts to Time Particles? It would refill the pool with the entire amount of energy it had taken earlier.

We now have a way of resolving the increasing mass issue without using actual infinity (just functional infinity) or paradoxi

Time Particles? Seriously? I mean, it doesn't even address his point, since he's talking about the energy implicit in the repeated doubling of universes, so if these time particles existed then every time the universes divided stuff would vanish from them, which isn't what you're trying to model at all. The comparison to Stem Cells is entirely meaningless and comes out of nowhere in particular. "Energy has no Mass" may be in some literal sense true, but energy warps spacetime as surely as mass does, given that the input in the Einstein equations is the Energy-Momentum Tensor. This pool of energy you describe that's being slowly drained? You do realize it would also double every time the universe doubled, right? (Ignoring the fact that nothing doubles anyway). The Everett multiverse isn't some sort of multiple-dimensions thing, it's a quantum effect. And finally, thermodynamics does not guarantee the heat death of the universe, which is why cyclic universe theories (in particular the Ekpyrotic universe) are still viable.

Finally, a store of energy big enough to constantly double the universe every Planck time would of course not be negligible in any sense, since it would have a gravitational mass equal to that of the universe times the age of the universe divided by the Planck time, which is obscenely huge. Especially given that gravitons are probably closed strings rather than open ones (note that this last may only be true in Bosonic String Theory, I still haven't studied Superstring theory or M theory).

Shadowknight12
2011-06-30, 12:28 PM
The best take of time travel I've come with thus far (best = has had the most satisfactory results in my experience) is this:

[Disclaimer: This is based on D&D. I cannot account for how well it would adapt on other systems.]


Time travel is epic magic. Or deity-level. Or at the very least, only possible through a priceless and unique artefact of untold power. It's not something any old wizard or cleric can obtain. No, not even via Wish or Miracle.
The reason such crazy amounts of power are necessary is because every time you travel through time, you are creating a new Material Plane somewhere. You cannot travel through time, per se. What you can do, however, is create an exact duplicate of the Material Plane you came from, only in a different time period.
When this happens, you are free to do absolutely anything you want, because no paradoxes can exist, and there is no "future" or "past" to return to. Only your native Material Plane.
You can go "back in time" to change something, but it will not have an effect on the Material Plane you came from. It will have an effect on the new Material Plane that has been created. Oh, and you can't skip forward, because that will mean creating yet another Material Plane (only this time, the "model" it will based on will not be the Material Plane you originally came from, but the new one you've just created, with all the changes you implemented).
You can go "to the future," but that will be a probabilistic analysis of what would happen if current tendencies were to be maintained. Since this is done by a deity- or epic-level intellect, the analysis will be amazingly accurate... but never perfect. There are things that simply cannot be accounted for, like true chaos, emotions and other similar immaterial things. The further you "travel" into the future, the more inaccurate the analysis becomes.
The only way to see what the results of a change in a New Material Plane (without ageing, of course) would be to put oneself in stasis. Once the time spent in stasis occurs, however, you cannot go back. If you "go back in time," you are creating a new Material Plane again. Again, the advantage is that this "new Material Plane" will be modelled after the one you're currently on, not the one you originally came from.
Yes, you can encounter yourself in this new Material Plane. There is no paradox. He is, for all intents and purposes, a completely different person.
Material Planes are linked by one Transitive Plane or another (Shadow/Ethereal/Astral).


There are four explanations that can account for the existence of these Material Planes.


They are demiplanes made of illusions (either they are impossible to be disbelieved (the DC is too high), or they are 100% real Illusion(Shadow) effects (or whatever replacement for the Shadow subschool exists in your campaign)). Nothing is real there. The "other you" you encounter? An illusion. An uncanny illusion, that makes you question exactly what is so "real" about yourself, if a mere illusion can duplicate you so flawlessly.
They are mental illusions. No demiplane exists. Other than that, it is similar to the previous one, only this gives the DM more leeway to "mess with reality" as nothing really prevents the earth from turning into gelatin or the sky from being made of tessellated skulls. The best application of these effects is rarely and subtly. Keep the world "normal" but include one or two "off details" every so often, to subtly remind them that none of it is truly real.
They are demiplanes made of real stuff and created from scratch. These people are, in fact, just as real as you are, and they have souls and other very realistic things. The only difference is that they're freshly made. And I guess one could say that their treasured memories are all fake. The rest is real, though.
This reality has not been created from scratch. All you did was travel to one of the infinite alternate realities that make up the multiverse, one that matches your specifications exactly. Given infinite variety, anything is possible. Do you want a reality where you have risen to prominence? Where you're a wizard instead of a fighter? Where your beloved was not killed by the BBEG, but instead survived, and you all live happily ever after? Done, done and marvellously done.


Hmmmmm. I don't think this counts as "Time Travel" for the purposes of this thread, however. Ah well. Maybe someone will find this useful.

randomhero00
2011-06-30, 01:35 PM
But how does that work in a game? It's all well and good to say "predestination" and wash your hands of it but it doesn't exactly encourages characters to do anything aside from smoke and drink coffee in sidewalk cafes, moaning about their enui :smalltongue:

I disagree. I never said the players would know it. They wouldn't find out till the end of the game, if then.

And that doesn't actually make things predestined, just gives the illusion it is. You still make choices and do your own thing. Its just we can't comprehend time as a whole. Time as a whole is like one big giant choice.

WalkingTarget
2011-06-30, 01:40 PM
I think the most realistic (or at least the most awesome) time travel in an RPG I've ever encountered is in Continuum.

Because... Continuum is just awesome.

I'm bad at explaining things. >.>

First thing the guy who invented a time machine did: keep going forward until further progress makes the process completely internal and at-will. As soon as somebody invents time travel, it has always existed.

There are great big piles of level time that are the playground of time traveling society (Spanners) - after a certain date (and before an earlier one, coincidentally) everybody can time travel and it's pretty much a post-singularity civilization, normal people from the intervening periods (i.e. all of us - and PCs since the game doesn't allow you to play as an Inheritor as far as I know) aren't really equipped to handle how different that would be. The Continuum is organized on the principle that we need to ensure that we get humanity safely through history to that point without mucking things up. The problem is that some selfish spanners (known as Narcissists) are out for themselves and typically operate on the assumption that they're dealing with the multiple-worlds version of time travel instead and therefore don't care.

Because spanning time and winding up in the same place on Earth really involves vast distances as well, being able to travel through time also gives the ability to teleport over distances in space.

Paradox is something you have to take into consideration, the universe cares about continuity of events and the game concept of how it enforces that is to take it out on you.

Example: Jim and Bob are watching a ball game on TV. Jim's drinking a beer and Bob decides he would like one as well. Bob sees there's no beer in the fridge, so travels back in time, finds one beer and takes it forward with him to drink. Jim has now been fragged for drinking a beer that shouldn't have been in the fridge in the first place when he looked a few minutes ago (the accumulation of frag results in various penalties to actions - starting mental, then debilitatingly physical, eventually fading out like Marty McFly - ghosts, poltergeists, etc? Spanners with too much frag trying to fix things). Because Bob is Jim's friend, he's mortified that he's done this. He goes to a store, buys some beer of the same brand, puts them in the fridge with a post-it on one saying not to drink it. He spans forward to after the game, takes that last one, then goes back and puts it in the fridge after he took the one that caused the problem, but before Jim had taken it originally. There, no more paradox and they can watch the game in peace.

Time combat is when you try to arrange things for your opponents to get fragged while protecting yourself.

Because there are whole civilizations of time travelers throughout time, there's no need to run scams of your own in order to get rich. People are already doing that on your behalf, so you don't have to worry about money.

Likewise, there are experts on every time and place throughout history, including people skilled at making things, so if you need to travel a few hundred years, there's somebody who can teach you the language, get you into period clothes, and prepare you for the trip. Most spanners will join one of these groups in order to specialize in some area to be helpful to everybody else.

One of these is the Thespians. One of their jobs is to, if necessary, replace an historical figure that's gotten killed by a rogue Narcissist or something. Don't ask them how many times Hitler's been killed, it's sort of a touchy subject.

There's a bunch more, but I've rambled on enough for now. It's an interesting game, but kind of tricky to actually play, for a variety of reasons. It's also out of print and is therefore hard to find at a reasonable price, if at all.

Ravens_cry
2011-06-30, 05:41 PM
Exactly. The point I'm trying to make is that it's a paradox that may not even matter. The paradox assumes that the universe *cares* that killing your grandfather means that your parents weren't conceived, and therefore you don't exist to go back in time and kill your grandfather. There are myriad solutions to this paradox, mostly involving history re-writing in whatever means necessary to resolve it. Since we have no experience with RL time travel, we have no actual proof that such anomolies can't happen. For all we know, the universe if filled with time travellers that should not exist according to the grandfather paradox.

Heck for all we know, it means the parent your grandfather would have conceived, gets concieved via immaculate conception. Who knows? The challenge is not to get a solution to the paradox, but to enumerate all the different solutions. Last I counted, I got up to twelve before I got bored.
But what if you knew your grandfather? Actually met him. How does that work in this situation? You can't have a single timeline where you shot your grandfather in the past and where you met your grandfather in the future of the past.

Fhaolan
2011-06-30, 07:23 PM
But what if you knew your grandfather? Actually met him. How does that work in this situation? You can't have a single timeline where you shot your grandfather in the past and where you met your grandfather in the future of the past.

Why not? :smallsmile:

1) Just because you remember something doesn't mean it happened.
2) Even if it did happen, it doesn't mean it always has happened. You may be in a flip-flop where that period is indeterminate and things may or may not have/will happen/ed. [Language breakdown. I'm not aware of a RL language that has tenses to cover this.]
3) More likely: Sure, it happened. But now it didn't. So? You shouldn't exist. And? Why would the universe spend any extra energy trying to deal with the situation, when it could just leave you be, and everything you experienced simply never happened. Sure, you remember the alternate existance, but that's your problem. You now have no source point, but oh well. Stuff happens. :smallsmile:

riccaru
2011-06-30, 08:51 PM
Simply put, you can watch, but not interact. Your body is intangible and unnoticeable by any means, only watch what happens in the specific time. So it's less time travel and more time-clairvoyance.

The other option to remove paradoxes is that upon first changing time, you're erased from your own time completely. You're now an outsider in time, if you kill your grandfather, your family doesn't exist past then. If you kill Hitler (only an issue if you're in a world based on earth) then as soon as you're gone, the powers that be replace him. Anything which would effect a large enough number of people up to the point that you left from will be slightly changed to have that event still occur. WWII would still happen, just not quite the same. Hitler would have a brand new brother, exactly like what he would have been, and everything still happens. Kill someone on the Manhattan Project, and the bomb is still created by someone else. The Gods do care what happens, to a degree. Only when it has huge consequences.

NichG
2011-06-30, 11:19 PM
Ok, this is ridiculous. The whole point of an Everett-style quantum mechanical multiverse is that reality is a superposition of all of these possible histories, that happen to be noninteracting.

I agree that it works for the non-interacting case, since there the physically addressable quantities are some measurement over the superposition, and so you only ever get stuff that adds up to 1 - you're just subdividing things and summing them up again. However, once you throw in the possibility of travel between these states, you no longer have a non-interacting system. In this sense, I'd say that branching points in history as provoked by time travelers must be considered to be different than the separate states which are superposed in the quantum mechanical many-worlds picture. Basically, if you want it to remain linear and probability-conserving, QM would have to apply on top of the temporal structures, not as their implementation - you'd have a superposition of many different possible branched timelines, but each possible branched timeline including all branches is a single state.

jseah
2011-07-01, 01:29 AM
The paradox assumes casualty as a hard rule. That the universe actually 'cares' that the person who killed him can not exist anymore. Unfortunately, without actual time travel there is no way to test that casualty is an actual universal constant, and that reality doesn't allow for weird stuff like this.

http://en.wikipedia.org/wiki/Novikov_self-consistency_principle

You speak of this.
Essentially, you need to determine all "copies" of time travellers and what they do as they appear.

Party A arrives at a time machine, party A' steps out of it and claims X BBEG is out to blow the world up.

Ravens_cry
2011-07-01, 01:53 AM
I am sorry, but that is just SILLY. Like I mentioned earlier, if that were true, storing antimatter in the past would be the best place to store it because history never reported a vast explosion, and it only gets weirder from there.

ZakRenning
2011-07-01, 02:15 AM
What I did once was make the PCs paradoxes as soon as they left their own time.

Essentially they ceased to exist normally. They could go and kill their grandfather. It would effect who they were, but not they themselves because they essentially were no longer who they were they became products of Time and Space not of matter.

Absol197
2011-07-01, 02:48 AM
My favorite instance of time travel is from a book called To Say Nothing of the Dog. It's based on a reactive universe that actively resists paradox.

In the book, there's a phenomenon called "slippage." Slippage occurs when a time traveler would emerge in the past at a point where his presence could alter the future. His destination in both time and space is altered, and he "slips" away from his intended destination by the amount needed to prevent him from altering the future. In this model, the Universe is almost alive in a sense: it "knows" what you're thinking and what your intents are, and knows how to keep you from mucking things up. You can go back and forth in time all you want, as long as you don't get in the way of important things.

So, for the two main examples in this thread, the grandfather paradox and killing Hitler, no matter how accurately you set your time machine, you can't arrive in the correct place or time to commit the act. Both are incredibly influential to history (both world and personal), so you "slip" away from your intended destination, arriving in a place and time where your circumstances make it impossible.

In relation to RPGs, in a Superheroes game I ran once, I implemented a similar style of time travel. However, because predestination makes players feel rather helpless to affect their world, I used the following rules:

1) Distance and Direction: Time travel is one direction only: backwards. You can't visit the future. Traveling very short distances or very long distances through time is difficult. The easiest distance is about three-hundred years, and it gets exponentially harder the further or closer you try to go (there was a check involved for the character to use his power).

2) Accuracy: Being accurate with time travel was also hard. You couldn't aim for, "9:15 am, June 27th, 1415." It was more like, "We're trying for summer 1415, but we could end up as far away as 1420."

3) Position: Your relative position on the surface of the Earth remains constant during time travel. This means, if you're trying to go back really far, you need to take continental drift into account, but you don't need to worry about accidentally ending up in the void of space because the Earth is in a different position around the sun at your point of arrival. Also, you can't go from Chicago to Beijing while also travleing back in time; you stay in Chicago. So if you want to be in a specific place when you show up, you need to get there in the present first.

4) The Time-Bridge: The power that let the character travel through time was like a worm-hole--it created a bridge from his current location to the location in the past. The bridge travels through time at the normal rate, and the character can only have one bridge open at a time. So if you spend an hour in the past then return to the present, an hour has passed in the presnet, as well. Also, you can't jump to the past, then jump further back, and so on. If you want to jump back again, you need to return to the present and close the time-bridge first.

5) Slippage: As described above, slippage occurs to stop you from interrupting major events that affect world or personal histories.

6) Single/Branching Universe: In this game, the universre was capable of branching, but it strongly resisted it. Basically, it required a tremendous effort in order to cause a change in the future, because the Universe was reactive to any changes, and tried to find ways to limit them. However, with a heroic effort, it was possible to alter the future, which was part of what the charaters were trying to accoimplish. Stop the great evil by altering history, which required all of their not-inconsiderable power.

jseah
2011-07-01, 04:16 AM
I am sorry, but that is just SILLY. Like I mentioned earlier, if that were true, storing antimatter in the past would be the best place to store it because history never reported a vast explosion, and it only gets weirder from there.
The Novikov Self-Consistency principle says that you can't cause a paradox.

You can't send antimatter to the past, not if it would generate an explosion that was never seen. Since if you sent the antimatter into the past, then it would have been in the past before you had sent it.

Imagine this thought experiment:
There's a billard table. Instead of pockets, input wormholes exist at the 2 corners that send a ball entering them back in time and coming out of the output wormholes at the other two corners.

If you knock a ball into an input wormhole such that it comes out of an output wormhole to hit itself going in (and thus never going in), that apparently generates a paradox.
The Novikov principle argues that the trajectory of your ball will never be such that it generates a paradox.
One solution:
Your ball approaches the input wormhole. A ball from the future exits the output wormhole and grazes your ball. Your ball then enters the input wormhole at the correct trajectory to create the grazing ball from the future.

Urpriest
2011-07-01, 07:59 AM
I agree that it works for the non-interacting case, since there the physically addressable quantities are some measurement over the superposition, and so you only ever get stuff that adds up to 1 - you're just subdividing things and summing them up again. However, once you throw in the possibility of travel between these states, you no longer have a non-interacting system. In this sense, I'd say that branching points in history as provoked by time travelers must be considered to be different than the separate states which are superposed in the quantum mechanical many-worlds picture. Basically, if you want it to remain linear and probability-conserving, QM would have to apply on top of the temporal structures, not as their implementation - you'd have a superposition of many different possible branched timelines, but each possible branched timeline including all branches is a single state.

Ok yes, I agree that if you have travel between those branch states, then you've potentially got all sorts of problems. But if you're not using a quantum many-worlds interpretation then you have no plausible justification for the branching anyway. So yeah, I guess I agree with you: any way you implement it there will be inconsistencies.

jseah
2011-07-01, 10:35 AM
any way you implement it there will be inconsistencies.
Eg.
You have two power cells + 1 box of X.
Each power cell can power your time machine to send back into the past exactly 1 power cell + 1 box of X (or just the box)

You send back 1 power cell and 1 box of X (your other power cell powers the time machine)

In the past, timeline B, 1 power cell + 1 box of X arrives.
You now have 3 power cells and 2 boxes of X.
You send back the 2 boxes of X and 1 power cell using 2 power cells.

In the past, timeline C, 1 power cell + 2 boxes arrive. You now have 3 power cells and 3 boxes of X.
You use 3 power cells to send back the 3 boxes of X.

In the past, timeline D, 3 boxes of X arrive. You now have 2 power cells and 4 boxes of X.
You don't send anything back and enjoy cloning your boxes of X for free.

Ravens_cry
2011-07-01, 11:42 AM
*lots o' words*
Which itself is just silly. What decides between innocuous. . .and paradox inducing?Sure, a billiard ball follows(fairly) simple physics regarding trajectories to stop itself, but once things get mroe complex, like with people and the machines of people, things get more complicated. Lets go back to the "kill your grandfather." Let's say you buy, not a gun, but a sword (back in time) and have training in its use and try to kill gramps. Whati s stopping you? Does your sword just pass right through him? Or a bomb, made with period materials, a club, strangling him with your bare hands.
Seriously, this is why I prefer the many worlds hypothesis. Sure, it has its downsides for personal excursions, but at least it doesn't break your head. Mostly.

NichG
2011-07-01, 11:53 AM
There's also the constantly changing model that showed up in one of JP Hogan's books. Basically, there's one 'real' timeline, and signals from the future erase that future. No branches. There's still energy conservation issues, but you can get around that by saying that whatever means of signaling you're using does not also transmit a net amount of energy (which is still a problem, but at least now it doesn't cause the universe to collapse, it just means you have to do thermodynamics and information theory differently when time travel is going on). For instance, you could have a communication method which biases which result you get in a quantum mechanically random measurement.

Then basically the 'fact' that there was a previous future that was erased and forgotten is hidden by the fact that the signal you got is consistent with the random fluctuations inherent in quantum mechanical processes. In other words, that former universe is just an extremely unlikely but still possible statistical fluke in a random process occurring in the current 'real' universe.

Oracle_Hunter
2011-07-01, 11:58 AM
So far, most of these examples seem to fall into Determinism or Many-Worlds, both of which make Time Travel a lame device for a RPG; they're both just different ways of saying "what you do doesn't matter."

In Determinism, the Universe (i.e. the DM) figures out what - if anything - can be changed about the timestream. Typically, the answer is "nothing."

In Many-Worlds, the Universe just kicks you into a different game when you make an alteration to the time-stream. This is better, but it also means that other people making alterations to your time-stream just get kicked into a parallel stream instead of altering your personal "alpha" stream. What's the fun of time-traveling if you can't screw with other time-travelers? :smalltongue:

Ravens_cry
2011-07-01, 12:28 PM
Well, it could still be interesting if you look at the repercussions. For example, in Multi-world, if you had cross stream travel so you can get back to your own time, you basically have indefinite resources. Such a society could get goods from all times, all manner of times. This is basically the basis of H. Beam Pipers Paratime series (http://en.wikipedia.org/wiki/Paratime_series).

NichG
2011-07-01, 12:29 PM
Well, you can do a lot of stuff in both paradigms that is interesting, but neither really captures the TV-style time travel hijinks that one might think of (of course, this thread is asking for realistic time travel options).

In the determinist perspective, you can get near-infinite computing power by setting up problems so that their solution is the only self-consistent history. You can also make objects out of nothing as long as they're perfect time loops and you have a way to make the existence of the object required for self-consistency.

In the many-worlds perspective, you can duplicate anything you want.

For traditional TV-style time travel, you probably need an unrealistic system, but that may be better for gameplay. I'm in a campaign with extensive time travel, and its handled by a 'paradox score' mechanic. You accumulate paradox the more you change things, and eventually the universe decides that its easier to make things consistent by getting rid of you than by fitting events together. At this point, you continue on as if nothing had happened in your own solipsistic bubble universe, but the rest of the party sees you vanish in a flash of blue, never to return.

I'm running a game with a sort of time travel. Basically, its similar to the save game trick - when certain people in the world die, the world resets by a random amount of time. However, it also tends to jump track a little bit, so other things are randomly different, and usually slightly worse. The party has a kaleidoscope that sees these different possible worlds, and they can see various facets of the kaleidoscope shatter when this happens. Essentially, its sort of a combination of many worlds and determinism: there are many worlds, but certain things must be true for a world to 'survive', and so the number of worlds decreases towards bottlenecks. It's more groundhog's day than true time travel though.

Urpriest
2011-07-01, 12:37 PM
In Determinism, the Universe (i.e. the DM) figures out what - if anything - can be changed about the timestream. Typically, the answer is "nothing."


Not necessarily. After all, non-time travel D&D could be set in a deterministic universe as well, and the DM still doesn't decide what the players do. All that needs to happen is that the players, rather than making decisions sequentially as they go about their business, make decisions in a holistic manner, with dice rolling etc. used to resolve conflicts between competing planned trajectories. So instead of the game going linearly in time like a story, the game is more like a map that the players and DM squabble over until they decide on a consistent series of events. This is what I meant by a topological method of conflict resolution.

Aux-Ash
2011-07-01, 02:23 PM
My personal favourite approach to timetravel is that you cannot change history, you can only fail or cause it. What has happened happened (or if you succeed in the time-jump: what will happen, will happen).

If you go back in time to kill certain famous dictators, you better read up on the assassination attempts on those people and find which one you think you were behind. Not that it's going to change anything, you can obviously read right there how it went (alternatively you'll come to the conclusion you aborted the mission before that).

The alternative is of course that you caused it. If you go back in time to stop a robbing that left your beloved grandmother with permanent emotional scars you'll soon learn that the reason your grandmother has those emotional scars was because of the firefight you started/will start to stop the robbing.

The universe doesn't need to correct itself because there was never anything correct. Time happened the way they did despite and because of time-travellers here and there. All they ever needed to do to find out how it went/will go... was to read up on it.

Oracle_Hunter
2011-07-01, 02:25 PM
Not necessarily. After all, non-time travel D&D could be set in a deterministic universe as well, and the DM still doesn't decide what the players do. All that needs to happen is that the players, rather than making decisions sequentially as they go about their business, make decisions in a holistic manner, with dice rolling etc. used to resolve conflicts between competing planned trajectories. So instead of the game going linearly in time like a story, the game is more like a map that the players and DM squabble over until they decide on a consistent series of events. This is what I meant by a topological method of conflict resolution.
This is a bit separate from a Deterministic Universe vis-a-vis time travel.

The arguments in a Deterministic Universe is that you can't create paradoxes because you can't change the past. Period. If you tried to kill your grandfather in a Deterministic Universe you would be doomed to fail: either you couldn't reach him, or it is impossible to fatally wound him, or he wasn't your real grandfather anyways. It doesn't matter if you roll a natural 20 and take away all his HP - he cannot die, because if he did, you would have a paradox.

Your argument is that the "universe" in a RPG setting may have always accepted the die rolls made as being canon. When a PC rolls a 16 on a check, the Universe had already "determined" that the outcome would be a 16. Obviously this is a pure illusion from the meta-perspective but it can work fine on a non-time-travel RPG. However, if time-travel is permitted in a RPG a deterministic universe would never allow the PC to go back in time and prevent him from making that roll due to paradox.

Also: the "uncaring Universe" theory fits under the Many Universe theory in that you can't actually change the time you came from because any "change" you make splits you off into a non-interacting timeline. It is unsatisfying for similar reasons.

riccaru
2011-07-01, 02:36 PM
Or, if you want the easy cop-out, someone else is also travelling through time, and keeping everything as it was. If you go back and kill your grandfather, the other person goes back and stops you right after.:smallbiggrin:

Shadowknight12
2011-07-01, 02:42 PM
In Many-Worlds, the Universe just kicks you into a different game when you make an alteration to the time-stream. This is better, but it also means that other people making alterations to your time-stream just get kicked into a parallel stream instead of altering your personal "alpha" stream. What's the fun of time-traveling if you can't screw with other time-travelers? :smalltongue:

Time travel shouldn't be fun. Well, to me. I just can't conceive the notion of having fun while travelling in time. Too much responsibility. Too many things to learn. Too many variables to keep track of.

If a player wanted me to run a zany time-travelling campaign, I'd politely bow out. I don't think my mind is capable of ignoring the monumental consequences of time travel. I would play it deathly serious.

Oracle_Hunter
2011-07-01, 02:45 PM
Time travel shouldn't be fun. Well, to me. I just can't conceive the notion of having fun while travelling in time. Too much responsibility. Too many things to learn. Too many variables to keep track of.

If a player wanted me to run a zany time-travelling campaign, I'd politely bow out. I don't think my mind is capable of ignoring the monumental consequences of time travel. I would play it deathly serious.
Who ever said it wouldn't be serious? :smallconfused:

The "fun" of time-travel is actually being able to affect the past (and therefore the future). I mean, technically when you adventure you shoulder the same responsibility - every person you kill cannot sire children and every person you save gets the opportunity to. But here you actually get to see the results in real time - so to speak.

WalkingTarget
2011-07-01, 02:50 PM
Time travel shouldn't be fun. Well, to me. I just can't conceive the notion of having fun while travelling in time. Too much responsibility. Too many things to learn. Too many variables to keep track of.

If a player wanted me to run a zany time-travelling campaign, I'd politely bow out. I don't think my mind is capable of ignoring the monumental consequences of time travel. I would play it deathly serious.

If you can find a copy somewhere, check out Continuum. Time travel is serious business. There are in-setting rules (the Five Maxims) that you, the player, are expected to have memorized. Time travel is logged so you don't overextend yourself, but also so you know when you've been. You are even expected to keep track of your Yet (the things that you know that you're supposed to do, but haven't yet).

Shadowknight12
2011-07-01, 03:00 PM
Who ever said it wouldn't be serious? :smallconfused:

The "fun" of time-travel is actually being able to affect the past (and therefore the future). I mean, technically when you adventure you shoulder the same responsibility - every person you kill cannot sire children and every person you save gets the opportunity to. But here you actually get to see the results in real time - so to speak.

You're talking about "fun with time-travel" and "screwing over other time-travellers." I assumed you were talking about an unserious and zany campaign.

I completely and utterly disagree with you. To me, the "fun" of time-travel is the fact that the focus is on the characters, not the setting itself. I would run a time-travel campaign focusing on how the characters evolve and change as they discover things about themselves. The warrior, who wants to prevent his wife's death, the thief, who wants to keep herself from making the tragic mistake that destroyed the priceless artefact she wanted to steal, the wizard who wants to know what his future holds at all costs, and/or the cleric who wants to save the temple she grew up in from destruction.

The dilemma is not "oooo I wonder what happens if I smash this OTHER priceless artefact instead!" the dilemma is "Do I really want to kill a version of me and replace him, so that I can live in this reality where my wife never died?" or "Is knowing the future really giving me peace of mind? Or is it making me more and more consumed by worry?" The point of the campaign is not to have fun with time-travel for its own sake. The point is to do what I've always done, which is to explore the character's backgrounds and motivations, and pit them against situations that awaken conflict within themselves. The time-travel aspect is not the central protagonist, it's merely a vehicle to explore conflicts that would not ordinarily arise in another type of campaign.

An alternative would be to make the universe susceptible to changes and paradoxes, and then make sure the god of time has a troupe of outsiders sent specifically to deal with people who alter time. Explain away any paradoxes as "psychic echoes" that made the characters travel in time in the first place (meaning, if you go back in time and kill your own grandfather, a psychic echo exists and induces someone else to travel back in time to kill your grandfather. You, obviously, cease to exist). I wouldn't run this, because it sounds like a darn headache, but it's a possibility if the Many-Worlds theory is not to your satisfaction.

EDIT:



If you can find a copy somewhere, check out Continuum. Time travel is serious business. There are in-setting rules (the Five Maxims) that you, the player, are expected to have memorized. Time travel is logged so you don't overextend yourself, but also so you know when you've been. You are even expected to keep track of your Yet (the things that you know that you're supposed to do, but haven't yet).

Hah, that's odd. From the description I read on this thread, it sounded completely zany and unserious. I'll see if I can find it, though. :smalltongue:

Oracle_Hunter
2011-07-01, 03:18 PM
You're talking about "fun with time-travel" and "screwing over other time-travellers." I assumed you were talking about an unserious and zany campaign.

I completely and utterly disagree with you. To me, the "fun" of time-travel is the fact that the focus is on the characters, not the setting itself. I would run a time-travel campaign focusing on how the characters evolve and change as they discover things about themselves. The warrior, who wants to prevent his wife's death, the thief, who wants to keep herself from making the tragic mistake that destroyed the priceless artefact she wanted to steal, the wizard who wants to know what his future holds at all costs, and/or the cleric who wants to save the temple she grew up in from destruction.
But... you don't need time-travel for any of that "character focused stuff." Heck, you can do the same with any sort of prophecy/divination magic in D&D. It looks like here that the "seriousness" you speak of is just giving characters a quick-and-easy way of getting a second chance at a life-altering experience instead of having to live with the consequences of what they've done.

If time travel is going to be Serious Business, it had better be good for more than getting a do-over on an exam. Feng Shui, for example, has a single time-altering event alter reality from a sorcery-dominated 20th century to the technology dominated one we know today. That sort of change isn't possible without time-travel and - more importantly - it's the sort of power that is worth fighting a war over.

YMMV, naturally :smallsmile:

WalkingTarget
2011-07-01, 03:35 PM
Hah, that's odd. From the description I read on this thread, it sounded completely zany and unserious. I'll see if I can find it, though. :smalltongue:

Heh, odd considering that was probably my post from earlier. Giving the "fun" description of the game to get people's interest might not have gotten the overall theme across.

Having only played a few sessions of it (some years ago), I don't know if it plays well with a non-serious mindset, but ours was pretty subdued.

randomhero00
2011-07-01, 03:36 PM
I think the simplest way, and probably the most enjoyable, is to just take out paradoxes and time travel as normal. In other words, you could go back in time and kill your parents, but you'd still exist. The "backwards" time travel is really the same as "future" time travel.

This is the basis of loops in time (a theory without having to create parallel worlds). In the beginning of the loop, the killing parents experiment, your parents are alive. At the end of the loop, you are alive but your parents are not. Sorry this one is complicated to explain.

Shadowknight12
2011-07-01, 03:39 PM
But... you don't need time-travel for any of that "character focused stuff." Heck, you can do the same with any sort of prophecy/divination magic in D&D. It looks like here that the "seriousness" you speak of is just giving characters a quick-and-easy way of getting a second chance at a life-altering experience instead of having to live with the consequences of what they've done.

I beg to differ. Some options do not become available without time-travel. In the example I used above, without time-travel, the warrior cannot have a life where his wife never died. He can raise her, perhaps, kill himself or move on, but the option "have a life where such a disastrous incident never happened" is just not there. And it won't be there without time-travel.

And I beg to differ on the second one, too. Saying that it's a quick-and-easy way of getting a second chance at something is like saying that when a character casts a spell to defeat the BBEG instead of "living with the consequences" of it (EDIT: Or the warrior using magic to raise his wife back from the dead instead of 'living with the consequences of it'). That argument is flawed. Furthermore, if the entire extent of the campaign was "you get to go back in time and live happily ever after" it wouldn't be much of a campaign, would it? Not without risk, consequences, their desires turning against them, new enemies rising from the shadows, etc. The situation you are suggesting is the result of a terrible DM, not the result of a flawed premise.


If time travel is going to be Serious Business, it had better be good for more than getting a do-over on an exam. Feng Shui, for example, has a single time-altering event alter reality from a sorcery-dominated 20th century to the technology dominated one we know today. That sort of change isn't possible without time-travel and - more importantly - it's the sort of power that is worth fighting a war over.

YMMV, naturally :smallsmile:

Again, you are bastardising my argument to the point of nonsense. Because you hold no appreciation for character-driven stories and prefer time-travel stories where things are epic in scope and the entire setting is changed, doesn't mean that what I'm proposing is not just as "Serious Business" as that.

EDIT:


Heh, odd considering that was probably my post from earlier. Giving the "fun" description of the game to get people's interest might not have gotten the overall theme across.

Having only played a few sessions of it (some years ago), I don't know if it plays well with a non-serious mindset, but ours was pretty subdued.

Ah, I see. Interesting.

Oracle_Hunter
2011-07-01, 04:18 PM
Again, you are bastardising my argument to the point of nonsense. Because you hold no appreciation for character-driven stories and prefer time-travel stories where things are epic in scope and the entire setting is changed, doesn't mean that what I'm proposing is not just as "Serious Business" as that.
While I don't appreciate being told I have no appreciation for character-driven stories, I'm willing to let this drop if you are.

Still, I've always been annoyed by stories in which the best thing someone can think to do with a time-travel device is to do something mundane like take extra classes (http://harrypotter.wikia.com/wiki/Time-Turner) :smallsigh:

dps
2011-07-01, 04:23 PM
I am sorry, but that is just SILLY. Like I mentioned earlier, if that were true, storing antimatter in the past would be the best place to store it because history never reported a vast explosion, and it only gets weirder from there.

Ever heard of The Big Bang?

Infernalbargain
2011-07-01, 04:43 PM
Well realistic time travel is really hard when Einstein hates anything but getting to the future "quicker".

riccaru
2011-07-01, 05:03 PM
Ever heard of The Big Bang?

Hide it 3 seconds to the left of the future:smallamused:.

Cespenar
2011-07-01, 05:11 PM
Still, I've always been annoyed by stories in which the best thing someone can think to do with a time-travel device is to do something mundane like take extra classes (http://harrypotter.wikia.com/wiki/Time-Turner) :smallsigh:

You are probably exaggerating that for effect and know what I'm going to say already, but the "taking extra classes" bit was just a build-up for its later usage.

Shadowknight12
2011-07-01, 05:14 PM
While I don't appreciate being told I have no appreciation for character-driven stories, I'm willing to let this drop if you are.

Still, I've always been annoyed by stories in which the best thing someone can think to do with a time-travel device is to do something mundane like take extra classes (http://harrypotter.wikia.com/wiki/Time-Turner) :smallsigh:

And I don't appreciate the implication that my plots are not Serious Business enough and are mere excuses for easy do-overs. I guess that makes us even. :smalltongue:

But yes, dropping it now.

That's simply because your preferences lie elsewhere. You prefer to think of the vast, sweeping, Butterfly-flapping-its-wings-and-causing-a-hurricane consequences of time-travel on a large scale. That's fine, that's a perfectly valid preference to have. For some people, the large scale severs their emotional attachment to the story. Some people can only be emotionally invested in a story if the characters remain its focus, and if the story turns around then, not if they're simply "along for the ride" and the true focus of the narrative is on exploration.

Both viewpoints are fine. Different strokes for different folks and all that.

Oracle_Hunter
2011-07-01, 05:16 PM
You are probably exaggerating that for effect and know what I'm going to say already, but the "taking extra classes" bit was just a build-up for its later usage.
I think Sluggy Freelance put it best (http://www.sluggy.com/comics/archives/daily/050918) :smalltongue:

Shadowknight12
2011-07-01, 05:25 PM
I think Sluggy Freelance put it best (http://www.sluggy.com/comics/archives/daily/050918) :smalltongue:

I'm not touching the time-turner thing with a ten-foot pole. Character-driven or not, I expect logic and coherence from plots. Blergh, that thing has become a filthy, filthy stain in character-driven time-travel.

jseah
2011-07-02, 12:38 AM
Which itself is just silly. What decides between innocuous. . .and paradox inducing?<...>Let's say you buy, not a gun, but a sword (back in time) and have training in its use and try to kill gramps. What is stopping you? <...>
You are still looking at time as if it follows you around linearly.
If you have the Novikov principle, it doesn't.

When you travel back, the time that you arrived at will have already contained you in *your own* past. And whatever you do in your own past cannot prevent you from going back in the first place.

Either you WON'T be going back with a weapon to kill your grandfather (because it's not consistent) or you didn't manage to do it.
Only self-consistent time travel ever happens at all. Something from the future arrives to make the future itself happen.
Eg. Your grandfather tells you of that time when he met this very interesting man who told him many stories. In listening to those stories, you really want to meet that man, so you time travel back to your grandfather's time and tell a young man in a bar the bunch of stories your grandfather told you. You then leave without ever finding your grandfather. Turns out the young man in the bar WAS your grandfather.

It's just same as the billard balls (read: atoms) with a whole lot more balls.



Of course, it does allow you to do things like solve computationally intractable problems. Like crack public key encryption with a computer no more powerful than a handphone. Or solve the halting problem.

You can arrange bootstrap paradoxes but not grandfather paradoxes.

NichG
2011-07-02, 09:43 AM
The funny thing with using self-consistency to solve hard problems is, if you try to solve something too hard, you get a short. Basically the way you're doing it is you're holding the universe hostage - I'm receiving a signal and if it doesn't solve the problem I won't send it back, making things inconsistent. At some point it might be easier for your computer to get hit by a cosmic ray, messing up your checking algorithm than to find the solution. Or it might be easier for the operator to have a stroke right before he pushes the button.

You could have some interesting game plots that way - distinctly, detectably 'unrandom' stuff is happening in the world, and it all points to a catastrophic experiment in time travel computation that is about to happen.

jseah
2011-07-03, 07:35 AM
NichG:
The universe isn't sentient. The self-consistency thing implies determinism. It also implies "no randomness" since the future is essentially predetermined.

Assuming that if nothing arrives from the future, your computer will be available to send the message, the only possible solutions are involving something being sent back.
Basically, if no freak accidents occur to destroy the device, the only solutions involve things arriving from the future.

However, freak accidents are no more likely to occur for this than anything else. The chance of your computer spontaneously getting corrupted by a cosmic ray is the same as any other computer not using this future computation.

So no. Your scenario does not happen.

Urpriest
2011-07-03, 08:13 AM
Ever heard of The Big Bang?

As a physicist, it is my duty to inform you that the Big Bang was not literally an explosion. Carry on.


NichG:
The universe isn't sentient. The self-consistency thing implies determinism. It also implies "no randomness" since the future is essentially predetermined.

Assuming that if nothing arrives from the future, your computer will be available to send the message, the only possible solutions are involving something being sent back.
Basically, if no freak accidents occur to destroy the device, the only solutions involve things arriving from the future.

However, freak accidents are no more likely to occur for this than anything else. The chance of your computer spontaneously getting corrupted by a cosmic ray is the same as any other computer not using this future computation.

So no. Your scenario does not happen.

Actually, it works precisely because of the probabilities involved. The device being disrupted is unlikely, yes, but so is reaching the solution. Techniques that exploit time travel to do computations do so by producing a random result and using consistency to force that result to be correct. But if something screwing up your pathway is more likely than you randomly producing the correct result, then most of the time that's what will happen.

Tetsubo 57
2011-07-03, 08:32 AM
Physics pretty much guarantees that time travel is impossible. So in a sci-fi game I wouldn't use it. In a fantasy game I wouldn't worry much about it. It's magic!

DragonOfUndeath
2011-07-03, 08:37 AM
Physics pretty much guarantees that time travel is impossible. So in a sci-fi game I wouldn't use it. In a fantasy game I wouldn't worry much about it. It's magic!

How does it preclude it?
I thought that since Time is a Dimension (and thus 2-way) it is possible.
We just need a way of doing it but there is no law of physics that forbids Time-Travel.

Tetsubo 57
2011-07-03, 08:55 AM
How does it preclude it?
I thought that since Time is a Dimension (and thus 2-way) it is possible.
We just need a way of doing it but there is no law of physics that forbids Time-Travel.

I recommend you read, The Fabric of the Cosmos: Space, Time, and the Texture of Reality by Brian Greene. He can do a far better job of explaining it than I can. But three short answers:

1) The energy requirements would make it impossible.
2) Overcome #1 and you couldn't change anything. Ever.
3) With a stable wormhole (see #1) and a ship moving near the speed of light you could make such a device. But it would only work from the moment that the wormhole opened and you could only go to the other end of the wormhole that is on the ship. Not really useful and see #2.

So, in a sci-fi game I would just ignore it. If someone showed up saying they were a time traveler I would assume they were either crazy or from another dimension.

Dvandemon
2011-07-03, 09:11 AM
I think the most realistic type of Time Travel would be a combination of whoniverse time-travel, Eureka timetravel and a little bit of Astral Projection time travelling such as the sort used by the Priestess girl from the Naruto movie.

It'd be sort of like a situation where someone or something (like Outsiders combining the Inevitables, History Monks and Time Lords) monitoring time and at least trying to keep it "ordered" in the sense that it doesn't get so messed up reality falls apart. You travel back in time and kill your grandfather, you become a kind of temporal undead such as The Grandfather Paradox of the Faction Paradox. If you change the past in some way, you merge with yourself once you reach your original timeline and gain a new life (there would may be a delay of gaining Knowledge of the new timeline, as Ripple effects change your memory; it'll eventually reach the point where you remember your life in this new timeline, but you know there was something before that you changed). And finally your spirit can travel to any time it wants, thanks to a Gestalt unconsciousness shared by all beings with an Int/Wis score. This allows both retro and precognition.

jseah
2011-07-03, 09:44 AM
Actually, it works precisely because of the probabilities involved. The device being disrupted is unlikely, yes, but so is reaching the solution. Techniques that exploit time travel to do computations do so by producing a random result and using consistency to force that result to be correct. But if something screwing up your pathway is more likely than you randomly producing the correct result, then most of the time that's what will happen.
No. It doesn't.

The consistency principle implies that the universe is deterministic. (and QM doesn't get around this)

When I talk about the probability of something happening, I mean on average.
In any one particular case, whether your computer gets hit by a cosmic ray or not will happen based on the conditions around your computer.

If your computer will not get hit by a cosmic ray without something arriving from the future, then the ONLY way your computer gets hit is by the something arriving causing your computer to get hit. (and in doing so, sets off the chain of events that end up sending itself back into the past - consistency)

The difficulty of solving your future computation does not affect the events around your computer. That's ridiculous. Cosmic rays do not magically bend to hit computers just because they were going to send a solution of some problem to the past.

Oracle_Hunter
2011-07-03, 09:50 AM
I recommend you read, The Fabric of the Cosmos: Space, Time, and the Texture of Reality by Brian Greene. He can do a far better job of explaining it than I can. But three short answers:

1) The energy requirements would make it impossible.
2) Overcome #1 and you couldn't change anything. Ever.
3) With a stable wormhole (see #1) and a ship moving near the speed of light you could make such a device. But it would only work from the moment that the wormhole opened and you could only go to the other end of the wormhole that is on the ship. Not really useful and see #2.

So, in a sci-fi game I would just ignore it. If someone showed up saying they were a time traveler I would assume they were either crazy or from another dimension.
You'd be surprised to hear how often scientists declare things impossible, only to be proven wrong :smalltongue:

While I'm not about to argue Mr. Greene on a point-by-point, I would like to point out that science only advances by showing the impossible to be possible.

The Random NPC
2011-07-03, 09:58 AM
Do you have the time? (http://www.aetherco.com/continuum)

DragonOfUndeath
2011-07-03, 10:11 AM
Do you have the time?

Hail fellow Spanner :smallamused:

It is quite an odd way of greeting. Repeating the question back in the same inflection.

Urpriest
2011-07-03, 12:25 PM
No. It doesn't.

The consistency principle implies that the universe is deterministic. (and QM doesn't get around this)

When I talk about the probability of something happening, I mean on average.
In any one particular case, whether your computer gets hit by a cosmic ray or not will happen based on the conditions around your computer.

If your computer will not get hit by a cosmic ray without something arriving from the future, then the ONLY way your computer gets hit is by the something arriving causing your computer to get hit. (and in doing so, sets off the chain of events that end up sending itself back into the past - consistency)

The difficulty of solving your future computation does not affect the events around your computer. That's ridiculous. Cosmic rays do not magically bend to hit computers just because they were going to send a solution of some problem to the past.

The cosmic rays don't magically bend, no. However, neither do the calculations magically randomly generate the right number. Remember, there is no direct causal reason why time travel lets you solve these problems: the effect is consistency-based, not causal. By setting up a loop, all you do is truncate a set of otherwise possible circumstances that now would give inconsistent results.

Envision two copies of your apparatus, imbedded in two spacetimes. These spacetimes are indiscernible by us, save for the fact that in one there is a wormhole allowing time travel and in the other there is not. Now due to the imperfection of our measurements (not for quantum reasons, just ordinary imprecision), there is a range of possible states of each universe. In the universe without the wormhole those states include both states in which your apparatus produces the wrong solution and states in which it produces the right one (since this process is random anyway), states in which it is disrupted by a cosmic ray and states in which it is not. And give an experimental uncertainty we can give probabilities to each of these possible states, like this: incorrect solution >> cosmic ray >> correct solution.

Now in the other spacetime, all causal influences on the apparatus are the same, except that now there is a causal loop inside the apparatus. Because of this, certain states are ruled out. Specifically, the states in which you get an incorrect solution. But because we still cannot measure the initial state with complete precision, the rest of the range of possible events remain. So we have probabilities like so: cosmic ray >> correct solution. So we are still unlikely to get the correct solution.

If you don't think this works, then what part of your analysis am I missing?

jseah
2011-07-03, 12:59 PM
The part that you are missing is that the solution isn't generated probabilistically. You don't even generate a solution at all.
Besides, there is no "state where your computer gets hit by a cosmic ray or not". It either gets hit or it doesn't. Whether I install a future problem solver or not will not change that.

The basic future problem solver works like this:

1. Receive solution from future
2. Check solution's validity
3a. If solution is correct, send solution to the past
3b. If solution is wrong, send X to the past (where X is not the received solution, "solution+1" works just as well)

In such a case, only a solution that would pass your "validity test" would ever be received by the time travel device, since only those are consistent.

The machine never generates any solution. All it does is check.
The time travel device can be said to allow the machine to "check" every single possible solution that could be sent by the time travel device; all at once. And the Novikov principle means only those that pass the check actually happen.

Of course, the drawback to this one is that the so-called solution is only as good as the test you make for it. If your test is faulty, so is your solution. =P
Another drawback is that you can only get solutions to problems that you can check easily.
Math Speak: It only reduces NP-complete problems to P problems.
- Which is one of our major unsolved problems in computer science (P=NP?) and is pretty darned huge all by itself!

Urpriest
2011-07-03, 01:13 PM
Hmm ok, looks like I misunderstood part of the procedure. I still think it's possible to reason about the possible consistent scenarios probabilistically, but I agree that it doesn't work the way I've set it up. I'll have to think about it in more detail.

Lamech
2011-07-03, 10:03 PM
Hmm ok, looks like I misunderstood part of the procedure. I still think it's possible to reason about the possible consistent scenarios probabilistically, but I agree that it doesn't work the way I've set it up. I'll have to think about it in more detail. If I understand the trick this would be an example: I want to solve the equation 100-x=0 for x and I don't understand basic algebra, but I'm sure the answer is an integer greater than 0.
So I program my time traveling computer, to do the following
If no info from future check 1.
If a number is sent from the future check that number.
If the number checked is correct send that number back.
If the number checked is in correct send back that number+1.

And then supposedly I'm going to get a 100 back right away, and my computer will check 100 and send back a 100.

At least I think that is the idea...
Now this runs into some problems. What if I program my computer to send a 1 if it gets a 0 and a 0 if it gets a 1? What if instead of getting a 100, I get a Time-Traveling-Robot that shoots me and my computer and then builds a Time-Traveling-Robot that shoots people and computers that try to trick free info out of the universe?

Ravens_cry
2011-07-03, 10:27 PM
Ever heard of The Big Bang?
What. :smallconfused:
Seriously, what are you trying to say here? How does it have anything to do with what I said?

The Random NPC
2011-07-03, 11:00 PM
The Big Bang is commonly misinterpreted as a large explosion. Since antimatter + matter = explosion, the hypothetical storage of antimatter in the past of a deterministic universe could have been the cause of the Big Bang. But since the Big Bang wasn't really an explosion, it is a false conjecture.

riccaru
2011-07-03, 11:28 PM
The Big Bang is commonly misinterpreted as a large explosion. Since antimatter + matter = explosion, the hypothetical storage of antimatter in the past of a deterministic universe could have been the cause of the Big Bang. But since the Big Bang wasn't really an explosion, it is a false conjecture.

It also happened a very long time ago. So hide the antimatter 5 minutes in the past and destroy the universe due to paradox. Congrats, you broke physics.

Ravens_cry
2011-07-03, 11:37 PM
The Big Bang is commonly misinterpreted as a large explosion. Since antimatter + matter = explosion, the hypothetical storage of antimatter in the past of a deterministic universe could have been the cause of the Big Bang. But since the Big Bang wasn't really an explosion, it is a false conjecture.

My thoughts exactly. To have an explosion you need something to explode INTO. Not only was all matter and energy compressed into the singularity, but all space as well.

Aux-Ash
2011-07-04, 01:01 AM
It also happened a very long time ago. So hide the antimatter 5 minutes in the past and destroy the universe due to paradox. Congrats, you broke physics.

But the universe wasn't destroyed 5 minutes ago. So obviously something went wrong with that plan. :smallwink:

jseah
2011-07-04, 01:12 AM
If I understand the trick this would be an example: I want to solve the equation 100-x=0 for x and I don't understand basic algebra, but I'm sure the answer is an integer greater than 0.
So I program my time traveling computer, to do the following
If no info from future check 1.
If a number is sent from the future check that number.
If the number checked is correct send that number back.
If the number checked is in correct send back that number+1.

And then supposedly I'm going to get a 100 back right away, and my computer will check 100 and send back a 100.

At least I think that is the idea...
Mostly right. Except that "If no info from future, send 1"


Now this runs into some problems. What if I program my computer to send a 1 if it gets a 0 and a 0 if it gets a 1? What if instead of getting a 100, I get a Time-Traveling-Robot that shoots me and my computer and then builds a Time-Traveling-Robot that shoots people and computers that try to trick free info out of the universe?
Consistency says you can't set up a loop with no solutions. There is always a solution.
eg. In the "get 1 send 0", "get 0 send 1" scenario, perhaps you get nothing and your computer sends nothing. (depends on how your program works, but a solution always exists)

NichG
2011-07-04, 02:45 AM
The part that you are missing is that the solution isn't generated probabilistically. You don't even generate a solution at all.
Besides, there is no "state where your computer gets hit by a cosmic ray or not". It either gets hit or it doesn't. Whether I install a future problem solver or not will not change that.

The basic future problem solver works like this:


In such a case, only a solution that would pass your "validity test" would ever be received by the time travel device, since only those are consistent.

The machine never generates any solution. All it does is check.
The time travel device can be said to allow the machine to "check" every single possible solution that could be sent by the time travel device; all at once. And the Novikov principle means only those that pass the check actually happen.

Of course, the drawback to this one is that the so-called solution is only as good as the test you make for it. If your test is faulty, so is your solution. =P
Another drawback is that you can only get solutions to problems that you can check easily.
Math Speak: It only reduces NP-complete problems to P problems.
- Which is one of our major unsolved problems in computer science (P=NP?) and is pretty darned huge all by itself!

So lets say that your solution space has N solutions that need to be checked on average before you find a valid solution. If the universe is non-deterministic, thats N times your solution checker has to fail to send a false positive. If the universe is deterministic, due to chaos if your solution checker takes macroscopic time to check the solution (i.e. the process is at least somewhat ergodic), its basically non-deterministic in the sense that the specific solution being checked causes different microscopic events to occur, which means its back roughly to non-deterministic. Worse yet, if you're using time travel, it means there is some process that exists naturally that can give rise to time travel events, which means that in a quantum mechanical universe, you have a sort of bubbling vacuum where self-consistent loops spotaneously emerge, and these could perturb events in the local environment to influence your solution checker.

Lets say for example that your solution checking hardware has an output bit error rate of 10^(-100), which is far far better than any current technology. That means that if you try to solve a traveling salesman problem (for example) between more than ~100 locations, its about as likely that in one of those attempts that contained a wrong solution the hardware glitched and said that it was the correct solution as it is that you find the correct solution first. You can improve on this in principle by checking three times and taking the majority result, lowering the probability of error, but at some point there's a bottleneck where an event either occurs (send a message back) or doesn't occur (don't send a message back/send the wrong message back), and the error rate of the bottleneck is all that counts.

If you have a deterministic universe and do the check really quickly, too fast for chaos to scramble states up, you might be able to squeeze more out of it, but there would be eventual limits.

jseah
2011-07-04, 03:55 AM
So lets say that your solution space has N solutions that need to be checked on average before you find a valid solution. If the universe is non-deterministic, thats N times your solution checker has to fail to send a false positive.
Nope. The checker only ever does one check.
Even in a non-deterministic universe.


If the universe is deterministic, due to chaos if your solution checker takes macroscopic time to check the solution (i.e. the process is at least somewhat ergodic), its basically non-deterministic in the sense that the specific solution being checked causes different microscopic events to occur, which means its back roughly to non-deterministic.
Again no. Chaos theory does not imply that.
What you speak of here is the following solution:
A burst of static comes through time machine and scrambles your circuits. It does so in such a way that activates the time machine and sends some static back in time (which was the original static coming out of it)
This IS a solution.

Then you simply do it again. (and take care to eliminate that solution)

Cosmic rays do not factor in.
Hypothetically, IF/WHEN you did not send anything through the time machine, nothing external to your system interrupts the system
THEN
Anything received from the time machine must be the cause of your system sending the thing received.


Worse yet, if you're using time travel, it means there is some process that exists naturally that can give rise to time travel events, which means that in a quantum mechanical universe, you have a sort of bubbling vacuum where self-consistent loops spotaneously emerge, and these could perturb events in the local environment to influence your solution checker.
In a QM universe, ALL solutions happen. At the same time. When any event causes it to decohere (which in a macro-scale computer, will happen), it collapses into one solution which turns out to be the one you received.

To go back to the billard ball example and use electrons, when you fire the electron at the wormhole, all possible solutions will have a certain probability based off the wavefunction of the electron interfering with it's future self arriving from the exit wormhole.
When you measure where the electron goes at the end, you decohere the electron's wavefunction and the solutions collapse into the one you actually measure.

Extending this to the future problem solver is rather more difficult. Since the computer is a macro-scale device, the solution that is received will decohere basically instantly.
This reverts to the nice familiar macro-scale world I talked about above.


Lets say for example that your solution checking hardware has an output bit error rate of 10^(-100), which is far far better than any current technology. That means that if you try to solve a traveling salesman problem (for example) between more than ~100 locations, its about as likely that in one of those attempts that contained a wrong solution the hardware glitched and said that it was the correct solution as it is that you find the correct solution first.
Of course, you could get a glitch, but whether you got an error or not must be because of the solution you received from the future. (unless you built a faulty computer)

The chance of you getting an error during the checking process is exactly the same whether or not the solution that is being checked has come from the future or not.
Future problem solving does not magically make circuits misbehave.

The only way the glitches are going to be more of a problem than in any other operation is when the "solution" you receive CAUSES the glitch.
Then you just fix the glitch and try again.


Think about it another way.
If chance events not part of my computer (like cosmic rays) can be made more/less likely to happen, then it would be theoretically possible to set up a loop that is most easily interrupted by a certain chance event you want.
-> Basically, if your version is correct, one could manipulate chance events in the world through correct setup of your future problem solver!

Mono Vertigo
2011-07-04, 05:24 AM
So, what I've gathered from this thread so far:

Realistic Time Travel:
Don't.



I'll get my scarf and coat.


(Okay, the actual contribution, no matter how minor it is: I like imagining time travel not as actual time travel, but as quantum travel between parallel dimensions. Once you move to another dimension, you can never go back to any other dimension you left; shouldn't be a practical problem seeing how there could be an infinite amount of dimensions, but more of an ethical one when you consider you're not actually changing anything in your own dimension, and whatever it is you're trying to prevent will keep existing/happening in the worlds you have left.
Also, parallel dimensions don't just need to be different of each other by virtue of having small or big differences in events; many parallel universes may have "started" later or earlier than ours, where everything else is however the same. We could travel to dimensions that are currently at the year 42, 1980, 2687, etc...

The advantage of this theory? Prevents paradoxes (since you can only move to other dimensions, not actually travel in time nor go back to a previous dimension). You can kill your own grandfather... in another you's world.
Another advantage is that time stays linear, as each dimension has its own timeline that is parallel to the others, which may have begun "earlier" or "later" in comparison with any other given timeline.
The flaw lies mainly is figuring how such travel actually works. If you end up in a world where another you, that is sufficiently similar to YOU, exists... do two of you exist, with YOU having the extra memory of time travel? Is it just a transfer of consciousness, and is the other "you" killed for real when you take over? Also, do you create new dimensions when you do anything (including just traveling to another world), or was your arrival and intervention taken in account before you even thought of doing anything?
Finally, you don't actually change much, as I said, and "free will" rings a little hollow when your acts don't matter in the bigger pictures, your loved ones are still suffering in possibly an infinity of worlds, and all you really do is moving to another reality for your own convenience.

Disclaimer: I didn't study quantum physics, or plain physics, I've just read a bunch of articles on the subject with the intent to understand and questionable success. I'm aware of Einstein's theories on time and space, I just don't really get them yet, to my great sadness. This is just the opinion of someone who's trying to find a sane alternative to time travel, and who's read Bokurano*.


*don't read Bokurano unless you want to get depressed.)

Urpriest
2011-07-04, 08:45 AM
Incidentally, Time Travel works fine in a narratively powered universe. You have two types of time, narrative and experienced. Narrative time always goes forward, while time travel occurs within time as experienced. Narrative time is forced to obey narrative causality. Since narrative causality trumps experienced causality, there can be no actual Paradoxes, because any event that violates experienced causality can still obey narrative causality. Apparent Paradoxes are then resolved according to narrative rules, which are up to whoever is telling the story's discretion.

This works for most of the sillier or more "meta" fantasy universes (Discworld!), but it can also work for sci-fi, even hard sci-fi, if the universe we experience is actually a simulation. Plenty of video games contain paradox-free time travel after all.

jseah
2011-07-04, 09:05 AM
Plenty of video games contain paradox-free time travel after all.
People ought to check out Achron. (google it, I'm not sure if posting links here is admissable)

It's a rts game that allows you to send commands (and units) to past units and generally manipulate events about 5-10 minutes in the past.

It settles the grandfather paradox by not computing a consistent solution. Instead, it computes only what each of the players see as well as a bunch of "timewaves" that move at 3x speed.

Thus, creating a grandfather paradox is easy.
eg. build a unit, send it back in time, order it to kill the factory that built it.
It simply makes the simulation oscillate between two sets of events: "unit arrives, factory destroyed" and "factory is present, unit is built, unit leaves"
Of course, more complicated paradoxes can have more than 2 states and nesting paradoxes creates oscillations in oscillations. Much much headaches. =)

Nameless Ghost
2011-07-04, 09:28 AM
I scanned the first few posts, then turned to the last page and was completely lost. :smalleek:

Anyways, I'm writing a time travel novel in my spare time that essentially runs on San Dimas Time (link omitted for your sanity).

The time machine is being tracked by the people that made it, and time passes at the same rate for it and for them, so after X time has passed for the time machine (and the people using it), X time has also passed for the scientists.

The key thing here is that the time machine is the point of reference rather than any given year or event, which I've found makes things seem more believable from a narrative perspective, even if it doesn't necessarily have any real world basis (indeed, the time travel is explicitly through magic).

Zaydos
2011-07-04, 11:21 AM
Another possibility is to do like Asimov in The End of Eternity where they made what was in effect a series of miniature universe outside of our time. Each station was parallel to one section of normal time, no matter how much time passed in Eternity. So time travelers had their own time line that they couldn't travel through even if they were able to go back in time and change history.

Actually that said all you really need is a 2nd Temporal Dimension and the ability to only travel through one. We'd never notice the second one in normal life, but temporal effects (time travel, etc) take time in the 2nd Temporal Dimension to fully come to bear in the First One. You'd only notice if you were time traveling, but this gives an explanation to San Dimas Time and why history takes "time" to reassert itself.

As to how Grandfather Paradoxes affect time travelers I'd go with Time Traveler's Immunity. If you're a time traveler you're unaffected by the changes in the time line, put perhaps only if you're close enough to temporal ground 0.

NichG
2011-07-04, 11:29 AM
Nope. The checker only ever does one check.
Even in a non-deterministic universe.


At the end of the day, this is true. But in principle, your algorithm means that there are a large number of universes that 'now' 'never were', that tried all the other combinations, and they were excluded from consideration because they were inconsistent. That means that what you get is somehow chosen from a sub-set of possible, consistent universes. If there are multiple possible consistent universes, there's no information in the universe that is carried forward to determine which you get - it must be random.

So consider the billiard ball case. Lets say the ball is directly between two wormholes, one that goes back the right amount of time and to the other. It is consistent for the billiard ball to just sit there forever without anything happening to it. It is also consistent for the billiard ball to sit there, another one with the exact same mass to suddenly come out of the past wormhole, hit it head on, and knock it into the other wormhole. The result is that the billiard ball has moved back by its diameter, but the universe contains the same energy and momentum after all is said and done. Without adding in some new physics, there is nothing to prefer one sequence of events to another. If you add more wormholes, it gets even worse, since there are multiple sets of self-consistent trajectories.

In the computational version, imagine that you ask the computer to find one solution of a problem that has 3. Which does it get, and is it always the same?

This is a macroscopic version of fluctuations that happen all the time on the quantum scale. Essentially this is mathematically almost exactly the same as spontaneous pair production of a virtual positron and electron, which then annihilate. Basically field theory models all possible sets of these sorts of events as states, and then thermodynamics implies that there is always some level of occupation of those states due to the ambient temperature (by which I mean energy scale).



Think about it another way.
If chance events not part of my computer (like cosmic rays) can be made more/less likely to happen, then it would be theoretically possible to set up a loop that is most easily interrupted by a certain chance event you want.
-> Basically, if your version is correct, one could manipulate chance events in the world through correct setup of your future problem solver!

That is exactly what you can do/are doing when you set up this system. You're basically excluding all universes that do not include some form of solution of your causal loop (which is not the same as a solution to the problem you design the system to answer when the system is imperfect). So yes, with time travel you can manipulate probability fairly consistently so long as the unlikelyness is small.

jseah
2011-07-04, 01:49 PM
If there are multiple possible consistent universes, there's no information in the universe that is carried forward to determine which you get - it must be random.
Well, I don't know how to tell which consistent universe gets picked. It could be random, or there could be some underlying process (probably related to decoherence [QM] of your received solution)

What you propose, that the "harder" your solution, the more likely you are to receive trash, cannot be right.
A validity test is made to filter out solutions. True, your machine could glitch and "pass" an invalid solution, but any valid solution will be much much more likely to pass.


That is exactly what you can do/are doing when you set up this system. You're basically excluding all universes that do not include some form of solution of your causal loop (which is not the same as a solution to the problem you design the system to answer when the system is imperfect). So yes, with time travel you can manipulate probability fairly consistently so long as the unlikelyness is small.
Even this does not allow you to manipulate events unrelated to the solution you receive.

Any unrelated problem that happens in your machine, if it would not happen without your receiving future solutions, will also not happen if you do receive future solutions.
eg. cosmic rays, freak plane crash, etc.

What can and will happen is you receiving incredibly strange (and invalid) solutions from the future that manipulate your validity test or machine in order to get themselves sent back.
Spinning billard balls emerging from wormholes to make three times round the table trick shots can happen; your experiment suffering spontaneous proton decay can't happen.

I think I can accept that "more likely" solutions are the ones you are more likely to receive. After all, the QM version of the billard ball on the table involves the billard ball entangling with itself from the future. But "more likely" does not mean "easier". It just means that it is the most probable state that is consistent.

Given that your computer is a macro-scale device, any solution that will pass your validity checker the "normal" way is far far more likely to happen. After all, your validity checker is a filter that aims to make actual solutions more likely to pass than non-solutions.
Even at our current computer chip error rates, by the time your validity checker racks up a significant error rate from processing time, the time you take to check the solution has grown so long that it's still infeasible to ask the question.
- eg. running a future problem solver on a halting problem will net correct solutions if the algorithm in question halts. However, for non-halting algorithms, you can't check whether an algorithm halts or not. And for really large algorithms, you might receive a solution of "some insanely large amount of time" and be forced to run your solution checker for the next two hundred years only to have it glitch and send the wrong solution.
- eg. large EXPTIME problems aren't solved by this version of future problem solver, for precisely the above reason
- this problem comes to mind, "you receive a solution that takes incredibly long to check, so long that your machine is more likely to glitch than check it properly"
- If this is what you were talking about, then I suppose we have been talking past each other.

Hence, I would say that feasibly checkable solutions will be what you do receive if such solutions exist at all. Ask a question with no solution and you just get "trash".

Ravens_cry
2011-07-04, 02:50 PM
Here's something I have hypothesized. In a world with branching time travel and quantum timeline brittleness, the very first time travel experiments will seem to be a failure, due to any message you send back making a different branch. The oirignal branch will see nothing. If the world has a single timeline, all messages sent to that time will literally come at once, potentially creating a burst of noise. If we actually get a clear message, we will know that a) there is multiple timelines and b) we have done this "before".

TooManySecrets
2011-07-04, 03:31 PM
+1 to Continuum.

The time travel in Continuum is based off of a short story called The Men Who Killed Mohammed. Basically, the story goes that a brilliant scientists comes home to find his wife in the arms of another man and, being a brilliant scientist, decides to make a time machine to erase his wife from existence. He succeeds in making the time machine (in 7 and a half minutes). He goes back in time and kills his wife's grandfather. Nothing happens to her, so he goes back and kills his wife's grandmother. Still nothing. Fed up with this, he goes back in time and starts killing all sorts of people (including Mohammed, hence the title). He goes back in time and teaches Marie Curie nuclear fission and Paris goes up in a nuclear blast. Still nothing.

The scientist eventually notices that he's going more and more indistinct. He meets another scientist, one who was working on time travel but had disappeared years earlier, who reveals that everybody exists in their own continuum and that by going back in time the first scientist has messed up his own timeline. Now he is a mere ghost, able to go back to any time back unable to affect anything.

Interlude aside, it's all about making sure that your players have fun. Continuum, for instance, is pretty realistic with it's sort of conscious self-consistency. The counterpart to Continuum, Narcissist, goes a different path and has time-travel create alternate universes at each point of departure. Needless to say, both games can get very confusing but for very different reasons. Oh, and both Continuum and Narcissist exist in the same universe (multiverse???) and the first act of a Narcissist is escaping the Continuum universe by doing something and not doing something at the same time.

Anyways, since we don't have time machines, there is really no such thing as "realistic time travel". I guess the most realistic would be tachyon communication, but that will take a lot of work to make fun when your players probably just want to go back in time and shoot dinosaurs.

Eric Tolle
2011-07-04, 11:25 PM
Perversely, for realism I like the time travel system from Susan Cooper's "The Dark is Rising" series. That is, time travel doesn't change the past because time travelers are part of the past. Travel back in time and attend a party on the Titanic, and a sufficiently detailed record of events would have recorded your attendance.

In game terms, this means not that players can't change the past, but that they won't. So if the issue of say, assassinating Hitler comes up, simply ask the players why they fail to assassinate the bastard.

NichG
2011-07-05, 03:39 AM
Well, I don't know how to tell which consistent universe gets picked. It could be random, or there could be some underlying process (probably related to decoherence [QM] of your received solution)

What you propose, that the "harder" your solution, the more likely you are to receive trash, cannot be right.
A validity test is made to filter out solutions. True, your machine could glitch and "pass" an invalid solution, but any valid solution will be much much more likely to pass.


So I think this points out an interesting subtle difference in self-consistent theories, and how to do computation with time loops (not that we can actually get any benefit from this, but its fun)

The setup is: You create a situation where if you receive no signal or the wrong signal, you will cause an inconsistency by sending back something different than what you received (increment by one, perhaps)

Model 1: The events actually happen, but get overwritten when the signal is sent back. This repeats until a self-consistent solution is found. If you do this with an impossible situation (i.e. my protocol is that I always send back something different than what I receive) then the loop progresses an extremely large number of times until an extremely unlikely event breaks the loop, e.g. cosmic ray causing a bit flip and screwing up the transmission.

Model 2: Only one version of events ever plays out, and it represents some form of filter applied to all things that could have happened that recovers only the self consistent ones. The nature of the filter (i.e. how it decides between multiple self consistent possibilities) is unspecified and unknown at this level of the model.

So in Model 1, you can't solve sufficiently hard NP problems with this because the time inside the loop to obtain the solution is so ridiculously long that extremely unlikely things become almost certain to happen first. However, you can set up a message receiver by a computer capable of computing some problem in, say, 6 months, and send yourself the answer when you're done. You receive the answer and immediately send it back to yourself, and you've saved 6 months of computing time, so you can pretty much do anything but big NP problems instantly. In Model 1, the difficulty of the problem matters, but the number of bits needed to transmit the solution doesn't necessarily matter.

In Model 2, your limit is the number of bits it takes to represent the answer, and the difficulty of the problem is irrelevant. The number of bits now matters though, because it indexes in some sense the likelyhood or unlikelyhood of randomly guessing the solution. If 2^N is too big, you'll start to have 'weird signal' issues that jseah mentioned, where there are statistically more signals that mess up your protocol than signals that cause your protocol to be satisfied.

I can compute in some sense what happens when you set up a protocol that is inconsistent by definition in Model 1. I don't know how that would be resolved in Model 2 though.

jseah
2011-07-05, 09:00 AM
Aha! So that's where we disagree on. Yes, I was assuming a variant of model 2.

Of course, model 1 never even occurred to me until you pointed it out. This is because I doubt model 1 is correct.

For example, if I write a not-so-good future problem solver like this:

Receive solution from Future
If no solution, send 1.
Else, Check solution
If solution is correct, send solution.
Else, send solution+1.
By your model, the future problem solver in question can only solve problems whose answers are integers and greater than or equal to 1. If your question has only 1 answer and that answer is 0, in your model, the problem solver will never solve it.

However, it is obviously clear that "machine receives 0, machines checks 0, machine sends 0" is a perfectly valid and consistent universe. And nowhere does the original billard ball problem require such a mechanic of deriving the answer from iterated redo-s of the original event without anything arriving.

And that gaining the ability to manipulate cosmic rays by doing computations is just plainly absurb. A cosmic ray aiming for your computer's circuit will hit your circuit or it will not regardless of whether your circuit is computing anything. Unless your solution somehow moves your computer?
Even under your model; in each successive universe, the universe "resets" back to the original state before the reception of the next interation of solution. The cosmic ray will/will not hit your computer and as long as the solution doesn't somehow move your computer or anything like that, it will not change whether your circuit gets hit.

*********************

However, unlike your model 2, I don't think it "tries" randomly until something pops out.

Since the consistency principle is applied all the time, *only* consistent solutions happen. The inconsistent solutions are never filtered out, are never computed; they just plain don't happen. Everything happens, all at the same time, every single possible solution; all the non-solutions never happen.

Speaking in QM terms, the wavefunction of everything involved will be constrained by physical laws into all valid solutions. (and since the criteria here is consistency with physical laws, this includes "solutions" that cause glitches)
In a macro-scale device, this wavefunction is decohered and you only ever see one. Which one you ultimately see is dependent on how your wavefunction resolves itself when observed.
I think you are more likely to get the "most likely" solution. (photons in a double slit experiment are more likely to hit spots corresponding to the part of the wavefunction that has a higher probability than other parts)

The thing is that glitches are rare even among our computers; hence, any actual solution that doesn't create a glitch will have many potential tiny variants that all amount to the same thing. (eg. small variations in voltage of one particular bit that doesn't cause your computer to flip it)
Thus any actual solution will have a far higher probability in the overall wavefunction and thus you are more likely to get one.

That is, of course, provided such a solution exists. If no solution exists, then the only ones remaining are those that cause glitches. Since the total probability of a wavefunction is always 1 (ie. something HAS to happen), you just get a glitch.
Therefore, don't ask questions you know have no answers.

The interesting thing is, you can't save the time at all. If you do a future problem solver, you cannot omit the checking step. That step serves as the filter that makes non-solutions require a glitch to be sent back.
Hence, despite the fact that you *know* you have the answer, you still have to prove that you have the answer if you are going to be able to trust the answer you have received.
- IE. If you finish checking the solution and send it back like you are supposed to, the answer you have will be correct. However, if you just send it back right away, the answer will almost certainly not be correct. It's an ontological paradox after all.
That is because your actions in filtering out the non-solutions have an impact on what you *have already* received.

EDIT: by "you can't save time" I mean, you can't save the time spent on checking unless you have a better algorithm. You do save alot of time on NP problems since checking them is really easy compared to solving them.

Lamech
2011-07-05, 09:49 AM
So with the consistent universe, why instead of the computer sending back the answer can't this happen? A Terminator builds a second Terminator. Then has the second terminator go back in time, shoot you and the computer. At that point the second Terminator builds a third, which repeats the process?
Its a consistent solution. What prevents painful death? Or any other number of screwy scenario's?

NichG
2011-07-05, 10:35 AM
And that gaining the ability to manipulate cosmic rays by doing computations is just plainly absurb. A cosmic ray aiming for your computer's circuit will hit your circuit or it will not regardless of whether your circuit is computing anything. Unless your solution somehow moves your computer?
Even under your model; in each successive universe, the universe "resets" back to the original state before the reception of the next interation of solution. The cosmic ray will/will not hit your computer and as long as the solution doesn't somehow move your computer or anything like that, it will not change whether your circuit gets hit.


The argument here is that in a quantum mechanical universe, you can't necessarily separate any small piece of it and call it completely independent from everything else. The existence of a topological loop changes the eigenstates of the universe in a non-local manner, the same way that if you do something to the end of a pipe, it changes the shape of the resonant standing waves in the pipe everywhere at once. Or another way to put it, quantum mechanically you could have these four (amongst a huge number of other) universes that, as you put it, all happen:

1. Observation of a spin in Los Angeles gives the 'up' result and computer in DC returns 1
2. Observation of a spin in Los Angeles gives the 'down' result and computer in DC returns 1
3. Observation of a spin in Los Angeles gives the 'up' result and computer in DC returns 0
4. Observation of a spin in Los Angeles gives the 'down' result and computer in DC returns 0

You normally don't need to worry about that because they're nearly independent events, so you can say that the set of states the universe can take is just the product of the states each individual event can take. However, if somehow these two results could influence eachother, and you have a filter on what states are allowed, then certain combinations may be knocked out without knocking out others. Or to put it another way, in the 10^10^10 universes that are self-consistent, some of them have the cosmic ray hitting your computer anyways. The filter of self-consistency doesn't make more of them, it simply decreases the weight of a lot of other universes, so the end result is that (in a 'spooky' way) improbable events become more probable in the sense that the chance that the universe you observe contains that result increases relative to what it should've been.

A more macroscopic example would be, instead of having a computer do a deterministic computation, you have that and a guy rolling 3d6 next to it. If the answer is correct, or if the guy rolls three 1s, you send back whatever you received, otherwise you send back something other than you received. All universes in which he rolls three 1s are self-consistent, as are all universes where the computer received the right answer. However, universes containing the wrong answer in which he rolled something other than 3 1's are inconsistent. Therefore, if your personal probability of being in a self-consistent universe is the same across all self-consistent universes (i.e. its ergodic), then you've increased the chance that the guy rolls three 1s from 1/216 to 1/2 (we leave independent degrees of freedom out of the state counting, such as the minute voltage variations in the signal, since those exist in both variants of the solution)

jseah
2011-07-05, 12:49 PM
Its a consistent solution. What prevents painful death? Or any other number of screwy scenario's?
Your time machine is too small to fit terminators through. XD

The kind of time machine that is most likely used for this sort of purpose will likely be the sort that only transmits information.
eg. by varying the strength of the gravitational field / shape of space-time for whatever device you're using to send information back to the past. No material is actually transferred.


However, universes containing the wrong answer in which he rolled something other than 3 1's are inconsistent.
Only really small effects can ever be affected by this. And only those within the light cone of you receiving the signal.

Quantum decoherence means that in macro-scale, events are at least probabilistically deterministic. This is easily summarized as "what will happen, will happen".

Unless your solution somehow affects the cosmic ray, a cosmic ray that won't hit your computer still won't hit it. There is no chance that a cosmic ray will hit your computer or not. Any such chance events turn out exactly the same way regardless of whatever comes out of the wormhole unless the stuff that comes out of the wormhole affects that event.

NichG
2011-07-05, 07:20 PM
I'd argue that the kind of weird non-local effects I'm talking about are a form of quantum entanglement. The same way that if you send two spins off in different directions that you know add to 0, and measure one spin the other is instantly determined by the measurement, you'd saying 'either the answer is correct or he rolled three ones or both' and the other states are inconsistent with what you already know about the situation.

Normally its hard to maintain entanglement because of interaction with the environment. This isn't because entanglement decays on its own, but rather because the conservation law you're using to determine the relationship between the spins only holds if those spins do not interact with an external system, and so weak interactions with an external system degrade the quality of the entanglement. Self-consistency of the form we're talking about does not depend at all on whether or not there is interaction with an external system, so its a more robust thing. I guess what I'm saying is, I'd expect that you could manipulate these time travel situations if you could produce them to create macroscopic entanglement effects that do not decohere.

jseah
2011-07-05, 10:15 PM
Your model allows someone to send FTL messages outside the time travel loop.

I use a very well tweaked future problem solver to send back solutions to an unsolvable problem 5 mins ago, such that the power lines to the computer being disrupted is the easiest way to break the loop. If the point of most-likely failure is more than 5 light minutes away, you have just manipulated chance to cause an effect *outside* your light cone.

NichG
2011-07-06, 01:26 AM
Well at first glance, so does entanglement. It takes a bit of more careful analysis to tell that you can't actually send information FTL with entanglement. I suspect though that you can transmit information outside of the light cone with the time travel model I'm using though. Consider that time travel itself is basically something following a spacelike rather than timelike curve, which is basically just a trajectory that lies outside of the light cone rather than inside of it. The run of the mill 'if I can go faster than light, I can time travel' kind of time travel I guess.

If you're doing time travel via a wormhole, then it does this by causing spacetime to be topologically non-trivial, so a point is inside the light cone if you consider paths leading through the wormhole, but outside of it if you consider paths that do not traverse the wormhole. I guess thats locally a timelike curve but globally a spacelike one? I don't remember my GR well enough...