There are two common sci-fi situations where you would have relativistic effects that are significant on a human scale (rather than being a difference in the order of milliseconds per year, or even hours per year, of perceived change). The first is, yes, black holes and other super massive and super dense objects. Time will be perceived, from the view of an observer outside a gravity well, as passing more slowly at or towards the bottom of the gravity well. This doesn't crop up much because the gravity in question (putting aside considerations like accretion discs or EM emissions) tends to pulp and/or shred people who are in a position to be significantly influenced. The second is at speeds approaching light speed. Essentially, the very fast thing experiences time more slowly than it's slower counterpart. Of course, it gets hideously complicated when you factor in relative velocities and moving frames of reference, but that's the essence of it. Spaceships in Eclipse Phase are not moving at anything near light speed, and the Gates cheat massively (you don't go at high speed, you skip the intervening space, in true wormhole fashion). Therefore, again at the human scale, time dilation is not really an issue. Also, I really didn't like Interstellar much, for reasons which don't directly pertain to this thread.

A link between two sol system pandora gates controlled entirely by one faction would allow for that faction to time travel easily. They just have to uproot one gate and fly it around a bit at relativistic speeds to get it asynchronous with the other one, then park the loose gate in orbit. Of course, the gates may have some mechanism to prevent such shenanigans. *Sigh* if I ever do find the time to run the game I've been hoping to run, it looks like I'll have to write out the Pandora gates, too. (QE comms are already gone)

I hate Timetravel plots so hard. It's the number one sign that the writer has officially Run Out Of Ideas, followed closely by "Alternate Realities". Just handwave it. If you need a reason, say that any relative velocity that would grant meaningful temporal distortion causes the Q-Bits to untangle, or the Pandora Gate to shut off, or that the possibility of time travel is actually false due to our imperfect understanding of the underlying nature of reality. A favourite is to use Causal Loop Instability/Paradoxes; If you build a machine that would let you go back in time, then either (A) You change nothing, the time loop is stable, (B) you change things so that you no longer need to use the time machine, or (C) you change things so that can't use the time machine. A is vanishingly unlikely, as even quantum effects would need to be identical. Therefore we only need to consider B and C, and it can be taken that there are vastly more examples of C than B. It is therefore statistically likely that C will occur every time. What this means is that although it's possible to create a Time Machine, it's statistically impossible to actually use it. Something will always occur to prevent it. Should the time machine be capable of travelling to a time before it's creation, it's quite likely that the effect if C will be that the time machine prohibits it's own creation. The only place I would possibly allow this stuff is with the ETI, which established a casual loop ensuring it's own creation.

Gatecrashing discusses the "wormholes used to (relatively) time travel" issue, with the conclusion being that probably if you actually got up and did all that work to produce that effect with two active Pandora Gates, anything held inside the wormhole would be obliterated by the forces involved, thus rendering the whole exercise pointless. The example on wikipedia is also one of those ones which would only work as far back as you turned it "on", so to speak, if I'm reading it right. (It also does include a theory that you couldn't bump two wormholes close enough to make it work either) Of course, the thing to remember about the Pandora Gates is that they're built by the ETI or under ETI guidance, and the ETI cross the "Sufficiently Advanced" threshold and basically have a more advanced understanding of the nature of the universe than any of use real life humans could possibly have ever. Given that Transhumanity can barely understand how to make the gates perform what is assumed to be their intended purpose, using them to generate a stable time machine is probably outside of their current grasp, and could easily be blocked by the ETI putting in safety measures to prevent the lesser species from mucking about with spacetime.

As much as I like to play with the idea, that's not exactly what unobserved means when discussing quantum effects (afaik). To say a room is unobserved essentially means that what happens inside the room has absolutely zero influence on the universe outside it. In this theoretical room, your time traveler could appear, but he cannot leave, as at that point he has become observed. It's the Dire-Half-Dragon version of the butterfly effect, because any change, no matter how small, gets magnified via recursion. As a simple example, an individual travels in time and appears to change nothing, but a photon from the sun strikes him at an infinitesimally small different angle and bounces of into a wall where it is absorbed, altering the "random" effects of it's constituent atoms. Nothing untoward happens, so the loop iterates, and again the time the random effects are a little different. This goes on and on, theoretically until infinity... until in one iteration the photon hits just right... and every atom in the wall spontaneously reaches it's half life at once, igniting in a nuclear detonation which ruins half the country and alters weather patterns for decades to come... and that final iteration is the one that we perceive.

Urf ! my stress point index is now off the chart

I wish I'd known about Novikov's self-consistency principle beforehand. It would have made explaining things so much easier. In any case, the scenario I was considering is based upon 3 assumptions, particularly with a view towards preventing time-travel issues is RPG settings which theoretically allow it. These assumptions are: (1) the Universe is not completely deterministic, (2) there is only one timeline, and (3) Paradoxes are possible, and I used that because that's the only combination of scenarios where an issue can really occur. Let me go into a bit more depth. 1. Most simply, if the universe is deterministic then there is absolutely no problem with time travel, because nothing can ever be changed. Both past and future are set in stone, and the properties of every particle at every moment in space and time are invariable constants. It can be helpful to imagine this universe as a crystal, where each “layer” is the state of the universe in that specific moment. Causality cannot be violated, because it doesn't exist except as a quirk of perception. In this case, there could certainly be time travellers in Hitler's attic. They don't even have to be circumspect, and can do whatever they want, safe in the knowledge they are incapable of altering events in even the tiniest way. Practically, this means that you can deny your players access to time travel even if it exists in the setting, because that denial is self-justifying – the result of their time travel becomes a de-facto requirement for time travel to occur. Imagine your players are in front of a locked vault which needs a code to enter. They decide to use time travel to get through – they will send a message back in time with the code, use it to open the door, and then send that code back to themselves. Satisfied with their plan, they tell their muses to check the mesh for the message... and there's none there. Realizing they may have pushed things too far, they decide that they will set a program to brute-force the code. This will take time and set off alarms, but they once the program's done they can send the code back so that they can open the door immediately, so they can get their work inside over with and concentrate on defending the program. Happy with their alternative, they set everything in motion, and check again for a message. Once again they are disappointed, and resign themselves to doing things the hard way. Later they ask you why it didn't work, and you respond that it didn't work because the fact that they didn't receive the message in each case caused them to alter their plans so that the message was never sent, and never will be. 2. If (1) is true, then the next requirement is that the method of time travel means that the "new timeline" overwrites the old. Otherwise the timelines can be considered separate realities, and nothing in one effects the other. You can send a timetraveler back to kill Hitler, but in your reality he never arrives; essentially all you've done is murder someone. 3. If (1) and (2) are true, then you're subject to the self-consistency principle. However, when you combine that with the butterfly effect, then the universe becomes effectively determinate, allowing the solutions by (1) to apply. To expand, consider the typical example of billiard ball: Imagine we have a time-travel wormhole, and a billiard ball. If the billiard ball's path takes it into the wormhole, it's path when leaving may interfere with it's initial entry, which would result in a paradox. This is impossible, so the ball will never be on that path - it will only be on one where the ball doesn't influence it's "earlier" self, or where the interaction is self-consistant, meaning the interaction moves into the path which causes the interaction. Now, we must consider that the billiard ball has mass, and therefore exerts a gravitational effect on the universe. This means that the path of the billiard ball will always effect it's previous self - non-influential pathways don't exist. Now we consider that the billiard ball may have spin, or may an atom may decay after going through the wormhole which means it must have the same orientation. With every increasing variable, you reduce the amount of possible pathways the ball can take. Sure, the amount will still technically be infinite, but it will be a smaller infinite. Here we get to the key matter - we set the billiard ball at rest in front of the wormhole, and attach it to a device which will accelerate it on a random path when a radioactive atom decays. There are a theoretically infinite number of paths it could take, but the vast majority of those which would lead it through the wormhole are restricted. So, whilst it's theoretically possible for it to assume a self-consistent path through the wormhole, the chance is arbitrarily small. Now apply this to a time traveler, where every particle in his body needs to be on a self-consistent path. In almost all cases, this won't occur - the traveler will never be able to actually enter the wormhole. So, time-travel is only an issue in settings where it's possible to change the past, for example if the time machine essentially "resets" the universe to the point the traveler arrives, and this is what I was talking about in my earlier post. Going back to the billiard example, in this case the billiard ball enters the wormhole and resets the scenario to it's egress point. It alters the path of the "new" ball, which enters the wormhole and resets, and which then alters the path of the next ball into another path, and so on. This will occur again and again until either the ball enters a self-consistent path, or the path prevents the new ball from entering the wormhole - and again the amount of self-consistent paths in minute compared to those where the ball misses the wormhole entirely. So, for an outside observer, the most probable chain of events is that the ball is propelled towards the wormhole, and then another emerges and deflects the original... and this occurs every time. Applied to RPGs, you have the moderately surreal scenario where you can receive messages from the future, or a possible future at any rate, but are completely incapable of sending any into the past. Trippy, isn't it :P

ThatWhichNeverWas wrote:

I wish I'd known about Novikov's self-consistency principle beforehand. It would have made explaining things so much easier.

I STRONGLY recommend you pick up Novikov's book. I think you'd really enjoy it.

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1. Most simply, if the universe is deterministic then there is absolutely no problem with time travel, because nothing can ever be changed.

Yep. And reading Novikov (if I remember him correctly and do him justice--both of these may not be true) the universe is deterministic, at least to a greater degree. Causality means that it must be. And this isn't just the past, but the future, since as far as physics is concerned, the only difference between the two is the direction of entropy. You can appear in the past, but it means you MUST time travel. And this is where things get tough on the GM, because usually this means: a) you're predicting the character's actions in the 'past' b) you're forcing the character's actions in the future.

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In this case, there could certainly be time travellers in Hitler's attic. They don't even have to be circumspect, and can do whatever they want, safe in the knowledge they are incapable of altering events in even the tiniest way.

This is where our understandings diverge. The time travelers can do whatever they want, but only because they know whatever they will do has already been done. They know they don't kill Hitler, because their history books say Hitler doesn't die today. They can TRY to kill Hitler, but we know, somehow, they fail (and in fact, they fail even to get their giant Nazi-stomping mecha out of the attic at all). And it goes further. Not only will they fail, but no matter how small the likelihood of a self-consistent failure occuring, that likelihood is actually 100%. The mecha MUST explode the time portal, killing them all and causing a failure. Any other outcome is not self-consistent, and so we know that it didn't happen.

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Practically, this means that you can deny your players access to time travel even if it exists in the setting, because that denial is self-justifying – the result of their time travel becomes a de-facto requirement for time travel to occur.

You can, but you don't have to. Indeed, you can say 'well, you didn't receive the code because it didn't work'. You can just as easily provide the code from the future when they arrive (as long as you know you can twist their arms enough to guarantee they send it back in the future).

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2. If (1) is true, then the next requirement is that the method of time travel means that the "new timeline" overwrites the old. Otherwise the timelines can be considered separate realities, and nothing in one effects the other. You can send a timetraveler back to kill Hitler, but in your reality he never arrives; essentially all you've done is murder someone.

This is multi-world theory. While it is possible, it isn't necessary. Novikov doesn't assume it. However, I will say that, for the sake of conceptualizing this, I found it beneficial. My background is computing, so I imagine this as a time loop. First run through, Hitler survives until 1972. PCs go back in time, bomb Hitler in 1939. Time police go back to prevent that, so the PCs stage a suicide in 1945. The last scenario is self-consistent, so that is the actual reality. But here's the important bit; the other permutations aren't just alternate histories--they fail to exist altogether. There is no reality where Hitler lived to 1972, because he was secretly assassinated in 1945 (via time-travelers).

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Imagine we have a time-travel wormhole, and a billiard ball. If the billiard ball's path takes it into the wormhole, it's path when leaving may interfere with it's initial entry, which would result in a paradox. This is impossible, so the ball will never be on that path - it will only be on one where the ball doesn't influence it's "earlier" self, or where the interaction is self-consistant, meaning the interaction moves into the path which causes the interaction.

Absolutely. It's also important to realize that the billiard ball will only ever be on one path (barring Multi-World, above). Our history books don't give five versions of Hitler's death. Only one occurred, and that will not change. Similarly, the billiard ball can only have (past tense) taken one path, and only will (future) continue to take one path. Yes, it is deterministic.

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Now we consider that the billiard ball may have spin, or may an atom may decay after going through the wormhole which means it must have the same orientation. With every increasing variable, you reduce the amount of possible pathways the ball can take. Sure, the amount will still technically be infinite, but it will be a smaller infinite.

Except the billiard ball will only suffer radioactive decay in one particular way. While this is a weird loop in time, the ball is only going through the wormhole once. It's decay and spin won't be any different from a ball that doesn't go through a wormhole (and as we know, those balls don't explore multiple realities--or if they do, they don't explore more than our time-traveling ball will).

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There are a theoretically infinite number of paths it could take, but the vast majority of those which would lead it through the wormhole are restricted. So, whilst it's theoretically possible for it to assume a self-consistent path through the wormhole, the chance is arbitrarily small.

And at this point I think we're diverging. This isn't a question of 'if anything is not self-consistent, it can't time travel'. It's a case of 'by definition, it is self-consistent--because it's already happened'. If an atomic spin of A can time travel and B can't, and the ball traveled, we KNOW all of the atoms were spinning A. We know it because it time traveled. If the ball will be fired into the wormhole in 2015, we know that when it is fired, all the atoms will spin A (because we saw it come out). You cannot change this fact any more than you can change what you had for breakfast. It is deterministic for us, for the atoms, for everything. Past and future. Stepping out and rerunning the timeline, they will fall into exactly the same pattern again--yes, a one-in-a-billion chance--because we already know that's what they'll do.

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So, time-travel is only an issue in settings where it's possible to change the past, for example if the time machine essentially "resets" the universe to the point the traveler arrives, and this is what I was talking about in my earlier post.

I agree with everything but the 'only' :) But yes, it becomes an issue here. And if you accept paradoxes, it's pretty easy to GM it too. Your players don't send back the code? Paradox, but that's cool. Keep rolling.

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So, for an outside observer, the most probable chain of events is that the ball is propelled towards the wormhole, and then another emerges and deflects the original... and this occurs every time.

I'm a little confused here. A ball is approaching the wormhole. A future ball comes out of the egress wormhole and hits the past ball, preventing it. So where'd the future ball come from? Or is this part of the 'if we accept paradoxes/people have free will' setting? (In which case, absolutely, you're right.)

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Trippy, isn't it :P

YES!! But I love it.

ThatWhichNeverWas wrote:

nezumi.hebereke wrote:

I STRONGLY recommend you pick up Novikov's book. I think you'd really enjoy it.

I assume you mean “The River of Time”, in which case Amazon says my copy should arrive before Christmas :D

Awesome!! I was trying to do some research on time travel for something and unfortunately it's pretty much a pariah topic in the physics community. The two books I picked up was River of Time and also Kip Thorne's book on black holes (although that one is mostly focused on, understandably, black holes. There's some stuff on time dilation and singularities and such, but nothing on paradoxes.)

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There's also a fun little twist you can add to get around arm-twisting; when the message is received at the “past” end of the time-machine, it contains a CC address which directs a copy to the “forward” end. This means the PCs don't send the message – it sends itself :D

Okay, that is very cute. I am going to need to use that one.

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But that violates the scenario definition that the universe is non-deterministic :D

Okay, now I'm back on board. Yes, my assumption here is that the universe is deterministic, it just has the illusion of determinism due to lack of knowledge--which time travel then removes. If you insist on non-determinism, things do indeed get messy.

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This isn't many-worlds either, as distinct timelines are unnecessary – the past “exists” as a superposition, and the ball will have traveled down all possible paths. “Recorded” history simply narrows down the maximum variance.

Following here, but barely :P So all of these timelines exist as separate timelines, and your PCs just happen to exist in one of them (although thanks to time travel, they may elect to visit a different one). And with that, reading your initial post, I think I'm following. In that setting, yes, you can get messages from A future (not yours specifically, but perhaps like it). And you can send messages back to A past (again, not yours). Due to the fog of time travel, the future messages may be identical to messages that would be sent from your actual future. They may also be totally whacky. But you can't affect your past.

nezumi.hebereke wrote:

Following here, but barely :P So all of these timelines exist as separate timelines, and your PCs just happen to exist in one of them (although thanks to time travel, they may elect to visit a different one). And with that, reading your initial post, I think I'm following. In that setting, yes, you can get messages from A future (not yours specifically, but perhaps like it). And you can send messages back to A past (again, not yours). Due to the fog of time travel, the future messages may be identical to messages that would be sent from your actual future. They may also be totally whacky. But you can't affect your past.

You've pretty much got it :) The “can't affect your past” is a bit off, but I'm not sure I can clear that up without causing more problems :P The only thing I really have to offer at this point is a mental image; The usual visual metaphor for timelines is a series of parallel lines, each of which occasionally splits into 2 new parallel lines. In this last scenario, imagine instead that there are timelines coming up and down diagonally, crossing to create a diamond “mesh”, where each instant in time corresponds to a point where the lines meet/merge.

fafromnice wrote:

http://blog.ed.ted.com/2014/12/07/quantum-mechanics-101-demystifying-tou... just found this ... my brain cry a little not sure why entanglement can't lead to communication though

The pain means it's working :D Usually I can get pretty far before convincing myself that [matter/energy/space/time] doesn't exist, and then I have to go lie down for a bit :P Regarding entanglement, the problem is that whilst the information about an entangled particle is transmitted faster than light, there's no way for the receiver to tell that the particle has been observed, and there's no way to influence the particle to define what's transmitted. In order to use entanglement to meaningfully transmit information, you need a way do distinguish between a particle in a superposition and one that isn't, without collapsing the waveform through observation. Just to be clear, this is trying to find out if something has a value without finding out what that value is. Also known as “Ahhh oh god my precious brainmeats whyyyyy...”

Oh, and I did see Interstellar finally! I was sort of wishing TWNW was around for the after movie discussion :P (edit: And all of my comments were already posted by someone else. Oops.)

easy it has no value for its state is that of constant change :P