Temporal Mechanics

[Username17]

There are two ways to handle time travel that make sense:

1. Closed Loop
   My presence is defined by the continuous nature of my existence from past to present in my own reference frame. Therefore, if I change the past I change the future and I change myself. This changes what I do to the past which again changes me and my actions and so on with infinite recursion sorted out in zero time until the past creates a future which creates that specific past and that is always what is visible by a historian standing after the time loop. In this model you always provoke your own time travel because if that wasn't the case you'd just go through a spiralling montage of infinite different versions of the events until that was true.
   
   
2. Non Causal
   My presence is defined by the present in the current reference frame. Therefore, if I change the past I am simply there changing the past. The future will be different, but I am still there as an actual independent object experiencing the past as my present day. In this model, once the past has been changed the future that a time traveler remembered will necessarily no longer exist. Indeed, every traveller in time comes from a future which will never happen so that any outside observer will see them as appearing in the time stream having not been caused by anything at all.

Non Causal seems more likely to me based on the observable evidence of what happens when, for example, one is confronted by a person who has experienced more time than you (as happens every time you move faster than those you meet). And while the Non Causal time travel has an elegant and plausible answer to the grandfather paradox and the free lunch paradox, there is another thing that has been plaguing me: the occassion of two time travelers.

Imagine for a second that a time travel named Abe goes back to 1970 in Detroit to bet on the horses. During this limited window there has yet to be enough crushed butterflies for the results to be appreciably different, so he can make a lot of money. But that's not important. The important thing is that in spending his period gambling and whoring in ancient times with his sports almanac, Abe has created a new future that will come into being after fifty years - that is Abe will go through his entire life, grow old and die and there will be a single future capable of creating time travel some time down the line.

So, out of this one new shining future (where interestingly enough, Abe is never born), there comes a new time traveler named Josh. He goes back in time and changes stuff - what happens to Abe's experience? I assume for the moment that he appears in 1970 with his sports almanac from a future that no longer exists and he is confronted with a 1970 that he does not recall.

What if Josh kills Abe in 1970 and then goes back even farther and creates a history where Detroit is never built and Abe doesn't stay there - is Abe still alive? Where in the time stream would he appear?

-Username17

[Crissa]

Of course, there's the non-causal split infinity, in which actions done change this here and now but once something has been done, it cannot be un-done, but it can be un-done in an alternate layer... At least, that's how Dr Who does it. The observer's paradox, I think it is?

So it's perfectly plausible for Josh to show up from a prior or latter version, and it really isn't important to save Detroit unless you want a different version of the future to go to.

Of course, that means that everyone is from the untainted version and as more and more scribbles happen to the timeline it gets more and more messy yet somehow ends up at the same place.

Or you could have the causal infinity, where time travel doesn't actually go to differennt times as much as to an alternate universe which happens to be at the point you were aiming for. That's the one in GURPS Time Travel. So you have many parallel time streams and you can only mess with other ones, but not your own.

-Crissa

[Falgund]

Another possible Mechanic is:

When you think you travel in time, actually you travel to a parallel dimension in which the current time period is the same as the one you wanted to travel. So, when you go back to your time period, in fact you just get back to your own dimension, where nothing has changed. Now, there are multiple possibilities for what happens if you time travel a second time. Do you go to the same dimension you previously modified ? To a completly new one ?

But it's true it isn't really time travel, just something that looks like it.

Personnally, i think the Closed Loop would be the more likely mechanic if time travel was possible in the real world. But i also think it is the less fun one in a gaming sense.

[rapanui]

It seems to me that for Non Causal to work, you have to "delete" everything to the 'right' (future) of the time traveler's arrival in the timeline.

So let's take Abe and Josh.

Abe goes back from 1999 to 1970.

Abe shows up in 1970, and everything after that moment is 'deleted'. He gambles, whores, and in 1999 Josh goes back to 1970. Everything to the right of 1970 is 'deleted', so Abe's experience of having whored and gambled is 'gone'. Then he gets shot by Josh, which is a real bummer for him.

Josh then decides to go further back, to 1690 to get some fine Native American bootay. Everything to the right of 1690 gets deleted, so not only did Abe never exist, he never went back in time and never got shot.

So, now let's see this from Abe's perspective. Given that everything happens as we define above, he gets into his time machine in 1999 and then promptly ceases to exist. That's got to suck.

[RandomCasualty]

The problem with non-causal is that it leads to infinite recursion.

Consider this. A time traveler travels back in time to some far away place, but only 10 minutes into the past, or some equally short amount, so as not to have the opportunity ot change anything..

Well the future is erased, but he's still there, and so is his original self, that's about to time travel to his point in space. So 10 minutes later, his original self sends himself back 10 minutes. Does this lead to the creation of clones? Is the original merely tele-fragged? What happens? Theoretically as long as the guy keeps travelling in the past in the new future, he could produce an infinite number of clones of himself, assuming the clone is incapable of making contact with his original self.

Or What about the time traveller with the plan to constantly double his money by sending $100 into the past? He then would have $200 (his original $100 plus the time travel $100) and then could send $200, then $400, etc, constantly receiving extra money out of thin air, since it's all duplicates of the same $100. With non-causal, it's possible to get infinite resources that way, because you can have the original and the future resource both exist in the same time frame. Better yet, you never deplete any power, because the actual use of the time machine never happened.

The interesting thing about non-causal is that time travel to the past is actually impossible and possible at the same time. That is, it never happens in any time stream, because the moment you have travel into the past, the old future gets erased. So the only actual existent timestream is the one where time travel doesn't happen.

Thus it's possible for things to come from the future, but people time travelling into the past never happens (at least not in the most revised version of history).

[shirak]

1182860044[/unixtime]]It seems to me that for Non Causal to work, you have to "delete" everything to the 'right' (future) of the time traveler's arrival in the timeline.

So let's take Abe and Josh.

Abe goes back from 1999 to 1970.

Abe shows up in 1970, and everything after that moment is 'deleted'. He gambles, whores, and in 1999 Josh goes back to 1970. Everything to the right of 1970 is 'deleted', so Abe's experience of having whored and gambled is 'gone'. Then he gets shot by Josh, which is a real bummer for him.

Josh then decides to go further back, to 1690 to get some fine Native American bootay. Everything to the right of 1690 gets deleted, so not only did Abe never exist, he never went back in time and never got shot.

So, now let's see this from Abe's perspective. Given that everything happens as we define above, he gets into his time machine in 1999 and then promptly ceases to exist. That's got to suck.

I usually visualize time as a two-dimensional surface rather than a stream. You choose a X-axis as your Now and a Y-Axis that determines your most probable future and past. Alternate versions spread out. So if you want to travel to a past point where everything is the same as your past except your body was assembled out of thin air out of the random Brownian movement of particles, you can totally do that. But it will be a pretty far-off past and it will probably lead to a completely different future. Different people travel to different past points which might or might not lead to the same Now point (0,0)

Btw, this is only seen/known by weird Time Spirits or advanced Mystics or whatever. Normal people cannot perceive more than one Now point, despite the fact that literally every point is potentially a Now point.

[tzor]

The trick of the two-dimensional time model is you effectively avoid the question in the first place because you can effectively prohibit travel in one time axis while allowing it in the other.

It should also be pointed out that the time barrier is effectively the "C" barrier. Any trans-luninal travel can under the right relativistic conditions produce a time traveling situation. The one dimensionally fixed two dimensional time dimension system implies that one cannot travel any faster than -C in relation to the fixed time dimension. (Which means a one cannot travel back one year without traveling forward one year in the fixed dimension.) Due to relativity it's not obvious to the time traveller (since the second time dimension is not experienced as the first time dimension is) but it does have important consequences for event ordering.

By the way the two dimensional time model was also referneced in Dr. Who as well. Actually at one point there are number of models used in the series at nearly the same time. A good example of the closed model is the "short circuit" caused by the non time lord - the Brigadeer - meeting his past self. The temporal short circuit generated the equivalent energy of (at least) a dozen regenerations.

[shirak]

1182865475[/unixtime]]snip

Er, I think you misunderstood me. The second dimension, the X-axis, represents how removed from the current Now point, (0,0), another point is. So at (0,1) you have you wearing a shirt with different coloration and at (0,1000) you have you talking to the King of England on the phone about the recent dinosaur problems as you type this. Of course, there may be many more dimensions than the simple two. I have often considered a third dimension to represent different people bu I have no idea how that is possible. What does it mean that I am at 1 and you at 10 or for that matter -10

[tzor]

OK I think I did. I was referring to the two dimensional time theory in which one propogates along both time dimensions but can only travel through one of the two.

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^ . . . . . . . . /[br]| . . . . . . . ./[br]|\. . . . . . . /[br]| \ . . . . . ./[br]|  \. . . . . /[br]|   \ . . . ./[br]|    \. . . /[br]|     \ . ./[br]|      \. /[br]|       \/[br]|       /[br]|      /[br]|     /[br]|    /[br]|   /[br]|  /[br]| /[br]|/________________>

Here we see a time traveler at point X going backwards in time. He continues to move forward in the Y time dimension but travels backwards in the X time dimension. In this model every point generates shadow in Y dimension and history is propogated in the X dimension.

[Fwib]

Whichever model of time-travel produces the best game is the right one.

[User3]

I kinda like the unknown future/unknown past idea, where you look at the certainty of an occurrence as being inversely proportional to its distance from the present.

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[br]P     ^[br]O  \  |  /[br]S  *\ | /*[br]S  **\|/**[br]I  **/|\**[br]B  */ | \*[br]L  /  |  \[br]E<----+------>[br]  Past|Future

It's a very self-centered view, but that fits very well with narrative flow for a story. You don't (and maybe can't) know how much your actions are altering the future (or the past!), but you can know with certainty that the present is happening exactly as it is, at least until is becomes the past (or future).

You can even view it as a possibility for an infinite futures/infinite pasts multiverse. Past Abe dies, but future Abe won't be retroactively dead from his own perspective, although he will be in Josh's world.

[Essence]

Word, Fwib. Word.

[User3]

What if time travel always prodcues the results which are most *dramatically* appropriate?

Suppose that there are fickle time gods that rule the universe and can rewrite history, causality, anything they want. Time travel machines constitute a plea for their intercession and a travel to a sort of "dream dimension." Afterwords, when you return to your own "time" the time gods make changes to it that are based in some loose way on what you did. So if you go back to assassinate hitler, hitler may retroactively always have been dead, but nothing else that you did makes any difference.

It's not so much time travel as reality revision through spirit quests, but it works for game purposes.