The fundamental workings of special relativity are interpretable to a degree; and people have interpreted it in many ways. The core problem is the concept of the light cone: when something goes someplace quicker than a light cone can transfer light there, the potential for violating causality, within the context of the traditional interpretation of special relativity's rules, will apply. In simplest terms, anything that traverses space in excess of light can violate causality. However, methods that might produce shortcuts yet do not exceed the speed of light (like wormholes) do not. Instantaneous travel fits a weird spot in the standard model, because we know of its existence (quantum teleportation), but tend to assume that it can't be used for anything, lest it potentially violate causality. However, there have been debates on how it should be interpreted for a long time, so don't assume that you must stick to the standard interpretation. Some (including me) argue for the impossibility of time travel, but the possibility for the instantaneous transmission of information (alongside other things, like imperceivable simultaneity). I've made that argument in a few threads, [url=http://www.eclipsephase.com/faster-then-light-methods]this one[/url] being the most recent.

One should be careful about trying to carry over arguments from special relativity to general relativity. Special relativity happens in Minkowski-space: an infinite smooth expanse of time and space that is just like ordinary Euclidean space except for incorporating time in a slightly different way. Light cones extend to infinity, and every observer will agree on every other observer's light cone (this means that they would all agree on who is *not* going FTL and the ordering of their history). Going straight from point A to B has one and only one solution. General relativity happens in a Riemannian manifold: spacetime is like a sheet of rubber or a lumpy, possibly looped and twisted whiteboard. The important thing is, when you look at a small spot it still looks like Minkowski-space: special relativity remains locally true. But the overall space might be finite in extent. There can be sharp corners (singularities). There can be wormholes and weirder structures connecting places with each other. The counterpart to a straight line is a geodesic, a curve of shortest length: locally straight, but it might bend because spacetime is curved (think of a "straight" line on a sphere: it is actually a great circle). There might be several ways of going from A to B by a geodesic, of different length (again, think of a sphere). Any event has a local light cone, but extending it over long distances doesn't tell you much: it is not privileged as in SR. You can still speak of something going slower than the speed of light *locally* by checking whether it always keeps inside the local light cones. Now, in SR FTL travel or communication is fairly easy to define: anything that goes to the outside of a lightcone from an event moves faster than light. It does not matter if it is by moving through the intervening points or just mysteriously appearing at another spot in spacetime. The headaches about causality show up if it is possible to make a round trip this way. There is no generally agreed on present or simultaneity: different observers will disagree, even if they are all simultaneous according to one of them. In GR there are spacetimes that have built-in time travel (the classic one is the Gödel universe, but there are many variants like the Tipler Cylinder) or make the future the past (like the cyclic universe). Most likely we do not live in one that is ultra-weird: it *roughly* looks like Minkowski space with dents and a large scale curvature over the span of the universe, but there might be local weird features. Wormholes mean that there are paths that are always locally timelike (that is, they go slower than light) that end up far away after a short proper time (that is, as measured by the clock onboard). Whether they mess up causality or not depends on other things: GR allows wormholes to go *anywhere* in time and space, there is no reason at all for them to merely go to the "present". Indeed, defining the present is just as impossible in GR as SR, made even worse by the bumpiness of spacetime. Now, causality in GR is a deep topic. If you thought SR was bad, just consider that GR allows things like event horizons where some parts of the universe will never influence another part (please don't ask what happens when you drop entangled particles into black holes!) or spacetimes that branch out or merge. The Gott universe starts with a causal loop that also produces the big bang. I think the quick answer is that GR has even less protections against grandfather paradoxes than SR, except for some half-hearted arguments that wormholes cannot form because of the lack of suitable "building materials" (the so-called null, weak, strong and dominant energy conditions), that event horizons will censor time travel and singularities (but Hawking has backed down on that one), and a few decent theorems showing that it might be topologically impossible to cause certain nontrivial spacetimes to form. Add quantum mechanics and the headache is complete. I don't have the time to lecture on it, but it is worth mentioning that there are some theorems showing that you cannot send information using quantum entanglement. Yes, no QE in reality. Except of course that there might be more devious ways of sending information than just making use of the entanglement, especially if the universe is a wibbley-wobbly-timy-wimy-spacetime-Riemannian manifold with a badly defined past and future. As I have said before, I trust relativity. I don't think we can rely on causality. And hence, just barely, FTL might be possible. But if it is, we will have time travel and CTC hypercomputation and loads of other uncomfortable things.

Xagroth wrote:

For game rules and ejoyment, I wouldn't allow FTL travel by any means but the Pandora Gates (Stargates!) to keep the setting into an egocasting-dependant part (not to mention limit expansion and colonization). Bear in mind that it won't keep humanity confined, you can still make the travel regulary, in infomorph state into a slowed-down simulspace. FTL communication, however, is something else entirely, save for that little part where the Gates do not allow it (you need a physical cable or container for the data to flow between sides, according to Gatecrashing: WiFi is out of range XD). It would allow other... interesting possibilities, in the way the Takeshi Kovacs setting shows (and I like that setting a lot) without breaking credibility or too much restrictions to players. I mean, if FTL travel is possible and you have forking tech, there would be few reasons not to send "seed nanoprobes" to several systems, make armies of yourself and conquer the universe... Oh, wait, you can use the Pandora Gates for that. You don't need to play Minecraft live... I think I know what happened to the TITANs... Uh... who moved forward my clock by 24 hours? Oo

With enough qubits (which is an extremely large quantity), I allow for FTL egocasting. Still, this only allows for travel to places that humanity has already been to.

Xagroth wrote:

Yeah, Decivre, but the Qbit option has a slight problem: you need to physically take the Qibts there (as you mention), and then REPLACE the used ones! Which is not only expensive, but timely since they have to be taken there. If we are talking about, for example, communication between the Oort cloud and an habitat in Sun's orbit (to make a very clear example), we are talking about thousand times the distance between Earth and the Sun, meaning years to resupply!

I didn't say it was feasible for rapid transit. But if you need to get someplace quick, and you have a large cache of qubits at both your current locale and destination that are entangled… then you have an FTL option.

Ok, here is the explanation as I see it: you see the shadow of the guy before seeing the guy. It just is not the shadow, but the guy, reaching his destination before departing. Meaning you have two of the same guy, at the same time, in different places. Add FTL comms (Qbits, to be precise) and things can go very, very wrong. Like you receiving a message from your future self, inside an escape pod, telling you to kill the guy who will crash the ship. And if you kill him and the ship won't crash, the future where you send the message won't happen. As Admiral Janeway said once, "temporal paradoxes give me headache"... The only game I know that incorporates the problem of reaching the destination before departing is Warhammer 40k, but that is because of this sweet thing called The Warp (their version of the Hyperspace), where time, space, and even existence are mere suggestions almost nobody bothers following... And things want to get inside your little tin can of reality corrsing through their neighbourhood. And do... things. To you.

Phlebotinum wrote:

I get that the speed of light is essentially the speed time, and if you had away to achieve true faster than light speed; not just taking a short cut like hyper drives/wormholes or circumventing the distance like warp/fold drive. You will have outrun time.

The problem is that you think about it in terms of some intuitive concept of time. That doesn't work: it is too imprecise. One reason explanations of relativity are tricky is that they first bore the reader with lots of talk of rulers, clocks and coordinates making the concept of time exact. Then things get weird, because they start comparing what different observers moving relative to each other will see - hard to visualise, and the normal ideas of what ought to happen can be shown to be wrong. The important thing is this: *the speed of light is always the same*, no matter how fast you move or in what direction. From this simple observation you can actually derive all of relativity. Now, if you have an event sending out a blink of light, you will get a spreading sphere of light. That is the future half of the light cone: just a set of events (places and times where the light briefly arrives). The above statement means that an observer moving past will see the same set of events getting lit up, and the distance/time spread will be the same. Different observers will agree on lightcones and their speed, but not on the speed, sizes and timings of things moving at other velocities. The really neat thing is that they will still all agree on what is inside particular lightcones. And since slower than light travel (whether by walking, teleportation or any other means) will remain inside a light cone, all observers will agree on the ordering of events. But if you have FTL, then you go outside the light cone, observers will no longer agree on the ordering of departure and arrival, and causality violations become possible. http://www.physicsguy.com/ftl/html/FTL_part4.html http://forums.xkcd.com/viewtopic.php?f=18&t=51782

Phlebotinum wrote:

My solution is simple, to get convenient interstellar travel without any of the above problems. Your FTL Drive must be able to, propel ship a through space while at the same time keeping said ship fixed in time. As long as the Ship maintains it's frame of reference it's own region of time, their are no causality violations. Because for those have to happen the ship must be moving outside of time. Somebody who actually knows advanced physics give me a Yay Nay or Maybe.

Sadly, nay. The problem is that you can get causality violations if there is anybody else moving. You take a trip to Alpha Centauri with the ship and arrive "at the same time". But an observer watching from another ship would see you arrive before or after you left. This is not just a minor problem, because you can of course get onto that ship and return "at the same time" to Sol, arriving before you left. Basically, the only way to avoid causality problems with FTL in the game is to ignore them. La la la, no time travel at all... The alternative is to get involved in tricky issues of consistency.

Well I use Orion's Arm as pretty much my guide book to what's possible in the far reaches of science and engineering. And i remember vaguely one of their articles describing traverse able Wormholes and preservation of causality. So i began to think, how dose a Wormhole enable you to travel? Wormholes work in essence by letting a craft circumvent the distance between the destination and the arrival points. During such a trip the craft never actually exceeds the speed of light. So I began to think if their is version of Wormhole travel that doesn't violate causality. thin an FTL Drive that behaved like one of those Wormholes, would allow convenient interstellar. A Warp Drive, which is moving slowly into the realm of feasibility; Would. In addition many of the other dimensional drives seen across space opera, which eclipse phase technically is; albeit with a hard science slant. Essentially if your FTL Drive can move the ship without actually having to accelerate. The ship in question wouldn't begin to De-synch from local time. And this is question related to the subject of FTL travel. How dose seeing something appear to happen before it dose mean that it actually did? The classic example that I have seen given is, a ship using an FTL Drive would to an observer on earth appear to return as it left or even before or it left; thus time travel thus touching causality in a no no place. But how do we know that what we have seen is what has actually transpired? Let's use the Warp/Fold Drive, which is the FTL concept that we are most likely to wind up using if FTL travel is possible at all. A warp ship never truly travels Faster Than The speed of Light; the ship remains still and is carried along by region of distorting space time. Since the ship itself isn't truly accelerating i suppose that we mean no time dilation. Now this hypothetical Warp ship has an effective velocity of ten times light speed. This ship is sent to Proxmia Centauri at Warp-10 that's an eight month round trip. Upon arrival the crew of this ship set off a massive explosive device for SCIENCE! then the Warp drive is engaged and the astronauts begin their trek home. The Explosion set of by the astronauts won't actually mean from earth for another four year. However when that four years have past the Astronauts clearly are not at Proxmia because they are currently on their way to Barnard's Star using the Warp-Drive 1.5.

1.The Mass Effect series took the changing the speed of light solution. 2. I get that The Speed Of Light is parallel to the Speed Of Time. And that if you were to literraly travle FTL you would break free from the normal flow time. And because of the fact that their is no one abolute time every person/being occupies their own region of/frame of Reference. And that the difrance between the regions of time do not become noticeable until one nears the speed of light. Because Light moves at the same speed at which times flows the movent of light is used the arbiter of cauasality. 4. So what happens with a wormhole or a warp drive. both conceapt Involve the travler circumventing the distance between the depature point and destination. Since the travle has never actualy exceed the speed of light durring the course of their trip, then how do they violate causality? 5. The hand-wave to get around this mind twisting problem is simple. The Image of an event is not the event in and of it self. Just becasue something seems to preceded it's cause dosen't meen that it actualy has;light-echos. As for the Notorious time line jumping Criminal Alice. the person that get's out of the gate is also her. And the braching time-line Scenario is my preffered solution to the changing the history problem.

So, what happens when you drop entangled particles in a black hole?

First I am so glad that I join the Forum, where else could i get into such a discussion? And I haven't mistaken Light for Time. In fact in a previous post i said something to effect of" because the speed of light and the speed of time are the same. we use light as the arbiter of time/causality". Why is FTL Travel Supposed to equal time travel? Because of the interrelationship between speed and flow of time. Everyone exist with in their own region of time/frame of reference. however differences between the regions of time do not become noticeable until one nears the speed of light. Now why is the Speed Of Light special? Because light moves at the same rate at which time flows! Thus anything that was literally Traveling faster than the Speed Of Light has also outrun the flow of time!!! And any craft that has begun moving around outside of time, can freely move along the time. Which is time travel. Why are Wormholes allowed in Hard Science Fiction and Transhumanist Space Opera( Which is Eclipse Phase's actual genre). First because theoretical physics, says that might exist naturally and be created artificial under very specific conditions. Second because Wormholes unlike literal FTL Travel do not automatically violate causality. Why is that you ask? Because a Wormhole is link between two distant points. Travel through one would in theory be instantaneous. A craft passing through a Wormhole doesn't travel faster than the speed of light; instead it circumvents the distance between the departure point and the destination. And since a craft going through a wormhole doesn't actually travel FTL it experience few to no time related shenanigans. because it doesn't truly leave it's own region of time.

I only take it so seriously because Debate is fun! And part of the fun of being a sci-fi fan is discussing the hows and why's of a setting. That simply doesn't happen in the fandom of pure fantasy, well at least not to the same degree. Besides I Got Served and thus had to Step Up! LOL! A little background on what I've been saying in this thread. I have been lurking on sci-fi and physics forums for years. seeing the discussions of the possibility of FTL travel, theory's and concepts brought up in those threads. made only the vaguest of since to me. then after some research,watching various specials on the History Channel and Nova on gptv; and thinking a lot. I finally began to understand. For my own writing I did invent an FTL drive. My initial idea was to combine the Warp drive(which bends space) and the Hyper drive(which accesses anther dimension) into one FTL system. In essence this Negative Space Drive creates it's own HyperSpace! The NSD works by creating faults in the spacetime geometry around the ship, the Negative Space. These Faults being can be thought of as Wormtunnles. That a ship carve through space time as it goes,circumventing normal space time geometry and thus distance. And just like a Wormhole because the ship never actually achieves FTL speed no time related shenanigans occur. The OP is free to use the NSD as in anyone else really.

Arenamontanus wrote:

The headaches about causality show up if it is possible to make a round trip this way.

I think this is an important aspect. This, and the fact that you need different inertial systems to exploit FTL for time travel -- the greater the velocity difference, the more "time-leverage" you can gain. So, roughly speaking, I think you could allow FTL without causality violations if you assumed that every FTL bit x that arrives creates something like an interference bubble that prevents any other bit y "influenced" by x from travelling back to the starting point of x. This interference can be shielded only by preventing any classical information transfer from x to y. This interference bubble has a time-space size that depends on the distance and speed of the x's trip, and on the relative velocity of anyone involved in the information transfer. Basically, the larger the time gap with a non-superluminal bit, and the larger the relative velocity of the communication devices involved, the larger and longer lasting the interference bubble. If everyone transmitting information is standing immobile relative to each other, FTL communication does not violate causality, to the best of my understanding, and thus the interference bubble has radius zero. But even if folks are moving, as long as their relative speed is small, the time-radius of the bubble can be much smaller than the light-travel-time between the source and destination. For example in a sufficiently "slow moving" neighborhood like the solar system, you could talk without causality violations to places, say, several light-minutes -- even a light hour -- away, with only seconds of perceivable lag. But as soon as a 0.1c fast courier zips by ... either the courier's FTL communication systems ("QE" in Eclipse phase) fail, or the courier can't communicate even non-superluminally with the local neighborhood (unlikely!), or the courier throws out of whack every other FTL communication system in the local neighborhood. I'd be happy to spend an afternoon working out the math with anyone (with a decent understanding of special relativity) interested in deriving the interference bubble formulas precisely. I won't do it alone, because it's a waste of time, but in good company it could be fun :)

Traveler is the classic space opera Sci-Fi RPG, the setting is good (be prepared to do some reading because there is a lot of it), but the rules system is a little dated. I actually think it was better in the GURPS rules versions. The short on their jump drive is they are more akin to star wars hyper space with limited distance and a relatively fixed trip duration. Jumps can take you any where between 1 and 6 parsecs (usually 2-3), and regardless of distance take about a week sidereal. It has the usual hand wavy sci-fi explanation of using black hole like effects to leave normal space and move through an alternate dimension (jump space). Might be worth checking out the setting, many of the tropes in Eclipse phase find their origin there.

My preferred method would be to permit FTL communications ... and permit time travel. But there are a few limitations; 1) It's extremely expensive (and those people who stand to profit the most are caught up in a cost race). 2) It only effectively works across short time ranges. I can't communicate to myself an hour ago, only a few seconds ago. 3) It's prone to failure. Specifically, I'd apply Novikov's theories of time, in which apparent paradoxes will automatically fall to the stable timeline. Given the complexity of the machines we're using, that's normally going to be failure of your communicator. Once time-travel is limited, manageable and 'knowable', the PCs can handle it, just like with anything else. It does take some brain-bending, though.

nezumi.hebereke wrote:

3) It's prone to failure. Specifically, I'd apply Novikov's theories of time, in which apparent paradoxes will automatically fall to the stable timeline. Given the complexity of the machines we're using, that's normally going to be failure of your communicator.

Honestly, the Novikov self-consistency principle is more than a little bit silly. It has no proposed mechanisms behind it and seems like a magical hypothesis (for anything but the trivial case of no time travel being possible of course).