^^^ Lightspeed with respect to what? There's no clear reason why the tolerance would arbitrarily grow smaller on spaceships. Just say the tolerance is a couple hundred km/s.
Also, what happens when the receivers are going at different speeds? How do you account for the effects of gravity and acceleration? What form is the information transmitted and received in?
^^ Five Words: You Fail Quantum Physics Forever
Edit: Darnit, Ninja'd
edited 28th Mar '11 7:01:26 PM by storyyeller
Blind Final Fantasy 6 Let's PlayI can't quite remember why the tolerance level decreased as speed increased. I was thinking of some value in time dilation equations that's squared.
I imagine gravity intense enough to replicate the time discrepancy between two reference frames moving at relative velocities above the tolerance level would have similar affects on transmitters. As for acceleration, the receivers will work normally so long as the relative velocities remain fairly close together.
With two receivers moving at radically different speeds, because of the discrepancy between the passage of time in the two reference frame, quantum fluctuations that interfere with the particles/waves that carry the information in FTL transmissions become exponentially more likely so as to preserve causality. At speeds far beyond the tolerance level, significant portions of transmissions will simply be missing or garbled. Eventually, when the velocity difference is to high, no information at all will be transmitted.
Information is encoded in waves/particles that I've yet to name. I'm not quite sure how they work, or how they're subject to the laws of physics. This is a signature. There are many like it, but this one is mine.
Well I can't think of anything immediately wrong with it, except for the specifications of velocities in a privileged frame.
Blind Final Fantasy 6 Let's PlayOk, I found a giant hole in the system.
Say you have two posts A and B, which are stationary with respect to each other. Meanwhile, a spaceship C is flying past B at relativistic speed. A transmits to B through the FTL mechanism, then B transmits to C using ordinary light. You've just got error free communication across arbitrary differences in velocity.
edited 28th Mar '11 9:22:42 PM by storyyeller
Blind Final Fantasy 6 Let's PlayNo it doesn't. Without the velocity constraint, you've just got standard FTL, which means causality violations.
You need to look at the link between A and C.
Blind Final Fantasy 6 Let's PlayI see, that makes sense...Well, the idea that quantum fluctuations would cause the signal of an FTL communicator to break down is based on a theory that the same would happen for a wormhole that was brought (by acceleration/deceleration) into a radically different reference frame from the one it was created in (which would violate causality), and that similar things could happen in any situation where causality would be violated...
Would it be reasonable that similar fluctuations would interfere with the radio transmission until such a time that ship C is in a similar reference frame to ship B or the information from ship A could have reached ship C at or below the speed of light?
edited 29th Mar '11 6:47:39 AM by Archereon
This is a signature. There are many like it, but this one is mine.Well I can't think of any way that would make sense, but if you want to try, good luck.
Blind Final Fantasy 6 Let's PlayThe theory I read stated that in any situation where conditions that (severely) violated casualty came to be, the likelihood of random quantum fluctuations occurring that would prevent the casualty violation skyrockets. Interestingly, in that system, if you actually managed to go back in time, that same phenomena would intervene to enforce a Stable Time Loop.
- (ex: if you went back in time and tried to kill your own grandfather, something would happen to prevent his death and the resulting paradox.)
In this case, the rate of light dispersion (which I believe is, at the quantum level at least, a probabilistic phenomena) in the radio signal sent from ship B to ship C might greatly increase, causing the signal to disperse pretty much instantly. Of course, that makes many assumptions on how the universe tracks information (it mainly accepts the idea that the information sent from ship B by radio would be recognized as the information previously sent by ansible from ship A )
edited 29th Mar '11 7:09:32 AM by Archereon
This is a signature. There are many like it, but this one is mine.Acceptable Breaks from Reality?
You exist because we allow it and you will end because we demand it.Yeah, that's the main problem. The universe does not track information.
Blind Final Fantasy 6 Let's PlaySo, basically, the FTL comm works as long as the two ends are effectively in the same frame of reference, from a relativistic standpoint? I'm not actually sure how useful that would be — you'd need to coordinate your movements incredibly strictly in order to make such a thing work on an interstellar scale. It wouldn't be impossible, but it would be really really hard, and pretty much useless on a tactical scale. Essentially, you'd need to have "mail ships" that did nothing but floated around in fairly empty space, flying in random directions at random speeds in order to "sync" with other mail ships in another system. Then they'd have to pass messages along at light speed, as has already been mentioned.
I think you can avoid the causality-breakingness of that as long as you can't travel FTL. If mail ship A notices an event E, passes a message about it to mail ship B an arbitrary distance away, and then B passes a lightspeed message to B', a ship moving outside the relativistic framework, then you're probably far enough away that it's impossible for B' to affect anything the event E anyway, which leaves causality intact. I'm not super-confident of that, though, so don't take my word for it.
edited 29th Mar '11 7:47:59 AM by NativeJovian
Really from Jupiter, but not an alien.I'm not quite sure about how that part works, I didn't read into that.
When applied to provide a solution for the grandfather paradox, this theory is called a "Post-selected model." Because causality violations creates paradoxes, unlikely but possible circumstances that would prevent that violation of causality becomes more and more likely to occur. Since relaying information from ship B to ship C is a causality violation the probability of circumstances sabotaging the transmission would remain extremely high until conditions are such that it could be made without violating causality...In theory at least. There have been experiments designed to simulate the quantum conditions time travel would cause, and they indicate that this phenomena may indeed exist.
This is a signature. There are many like it, but this one is mine.^ If FTL communication can't be observed in any way, it may as well not exist. So you're back to square one.
^^ No you can't. That doesn't work at all.
edited 29th Mar '11 8:22:29 AM by storyyeller
Blind Final Fantasy 6 Let's PlayFor the sake of the plot, I'm postulating that the universe will tolerate a certain amount of casualty violation so long as it's impact is sufficiently small. You might send information back a few picoseconds, but if you push it any further, or try to engineer circumstances that would create recursive time loops to accomplish the same thing, the attempt will fail.
The thing I'm trying to do here is find an FTL communication that doesn't "give up" special relativity and does as little damage to casualty as possible (making "practical" time travel an impossibility), since special relativity has enormous amounts of supporting evidence behind it, and there is some evidence indicating casualty may not be absolute. (Analysis of a singularity and the area surrounding it being a possible example of this.)
edited 29th Mar '11 8:29:47 AM by Archereon
This is a signature. There are many like it, but this one is mine.
What Archereon is talking about amounts to a combination of You Already Changed the Past and the Novikov Self-consistency principle
. Essentially, a paradox is impossible, therefore a paradox never occurs. Therefore, any circumstances that appear to lead to a paradox must fail. As an example, in the Grandfather Paradox, if everything else works, the bullet quantum-tunnels through your grandfather's head to maintain causality.
Also, please keep in mind that FTL signalling does not automatically mean a paradox, only a causality violation. My own story involves FTL devices being used to set up Stable Time Loops, and AFAIK, the (sketchy) math checks out.
edited 29th Mar '11 8:32:56 AM by Yej
I was under the impression that if FTL communication occurs between two reference frames that are consistent as far as relativistic Time Dilation is concerned, there is no casualty violation (though there may or may not be a special relativity violation) since there is no discrepancy between the passage of time in the reference frames.
That's correct. If they're in the same relativistic frame, then causality is preserved, because it's the differences in the frame of references that causes the break in causality in the first place.
No you can't. That doesn't work at all.
Pointing out my mistake would be more helpful than just saying I'm wrong without any explanation.
Really from Jupiter, but not an alien.
The disconnect is the fact that you're sending signals faster than time itself propagates. Causality is guaranteed to be preserved if the world lines
are the same length, not if they're only in the same reference frame.
If I'm reading that article right (and I may not be; that's way above my pay grade), then wouldn't any two objects not in the same frame of reference have different-length world lines, given that they're traveling through spacetime at different rates?
eg, if we take one of a set of twins and stick him on a rocket that zooms around near-lightspeed for a few years then returns, the "resting" twin is going to be older, meaning his world line is longer... right? But that doesn't violate causality.
Really from Jupiter, but not an alien.

Hello everyone. I've been working on a science fiction setting for a hypothetical work, which I've gone to pains to make as hard as possible within the confines of the plot. There's no FTL travel, but there absolutely MUST be Faster Than Light communication. Under my current understanding of relativity, this system I've worked out is slightly less harsh on causality than generic FTL communication, and also comes built in with a nice limitation.
FTL communication has a virtually unlimited range, and can be used to transmit immense amounts of data in short amounts of time. It is not blocked by physical objects and does not generally suffer from latency. While communicators are expensive, the cost of a transmitter pales in comparison to the dozen or so metric tons of antimatter used to fuel interstellar spacecraft.
The main weakness of Faster than Light communication is that the two communicators must be traveling at similar speeds; for information to be reliably conveyed, the transmitters' velocities relative to each other must be fairly close. At relative velocities significantly below light speed, the tolerance is a few hundred kilometers per second. As the two transmitters approach the speed of light, this value declines exponentially; by the time an object is moving at .5 c, a pair of FTL transmitters must be moving within 850 meters per second of each other to reliably communicate.
Feel free to point out any flaws in this system and suggest ideas on how to improve it. This is a signature. There are many like it, but this one is mine.