[StarSlayer]

Starslayer: You sure the devs are competent? They may not be

If they were competent it would already have a try catch block

[General Battuta]

Ok, in a past shoutfest I had with Battuta, it was ultimately agreed that FTL travel and/or communication wouldn't work because it would break the universe.

The thing that caused the trouble was that if something at the destination could send an FTL signal back right away, it could/would arrive "before" the original signal was sent. So now I'm thinking... could a set of rules be devised which would avoid space-time paradoxen? Perhaps something with teleportation/jumps?

Lets here some ideas!

Preferably focusing more on the high-level "rules" than the technobabble.

Er wait, why is this true? If I sent a message to someone, and they sent the FTL message back instantaneously, I would still get it after I sent the message, as I would have to wait for my message to travel to them before they sent it back.

No, you wouldn't; at least not in all IRFs observing the exchange. In some IRFs the message will arrive before it is sent, and one of the fundamental laws of the universe is that no IRF is privileged, meaning if causality is violated in one it is violated in all and spacetime asplodes.

*sigh* Please don't make me do the math out. It's not hard but it's an algebra grind. 

Er...what's an IRF?

Inertial reference frame (observation by a non-accelerating observer.)

[Liberator]

All that explains is that FTL Comms wouldn't work necessarily.  No one said FTL Travel is impossible.

[SpardaSon21]

Well, it is because FTL Communication and FTL Travel are pretty much the same thing, the transmission of information, information being anything such as the layman's definition of information to knowledge that a ship is traveling really, really fast.

[General Battuta]

All that explains is that FTL Comms wouldn't work necessarily.  No one said FTL Travel is impossible.

No, FTL comms are FTL travel. They're the same thing.

Imagine a message arriving before it's sent - that's FTL comms. Imagine a ship leaving a dockyard, traveling FTL, and then arriving back at the dockyard before it departs - and then blowing it up. That's FTL travel. Same problems.

FTL travel is not impossible in current models of physics; however it requires something like an Alcubierre drive or a wormhole and may still lead to causal connectivity between events with spacelike separation (possibly causing paradoxes).

[Bobboau]

ok, so let me see if understand the problem.

we have 3 planets

A    B            C

arranged like that were B is more or less right between A and C, but with C further away from B than A is.

B transmits a RF message to A, then A transmits some sort of FTL response back to B. from C's perspective A transmits a FTL response, then B transmits it's initial message.
so from C's frame of reference the response came before the message that caused it.

This is the problem?

[NGTM-1R]

This would suggest we have to remove the transit part to prevent paradox.

...subspace ftw?

[Bobboau]

hold on, I want to make sure I've got that right before we jump to anything else.

[Mika]

I think I need to remind you guys of one thing that leads to those things:

The speed of light is the highest velocity by which one can transmit any sort of information by the current understanding of the Physics. I'm not sure about how that works with those quantum thingies that seem to mirror each other, but never mind, I never liked quantum physics in any case.

That being said, would it instead be fruitful to discuss what would happen if there wasn't a velocity cap? Sometimes thinking of the opposite helps understanding why things are the way are now.

[General Battuta]

ok, so let me see if understand the problem.

we have 3 planets

A    B            C

arranged like that were B is more or less right between A and C, but with C further away from B than A is.

B transmits a RF message to A, then A transmits some sort of FTL response back to B. from C's perspective A transmits a FTL response, then B transmits it's initial message.
so from C's frame of reference the response came before the message that caused it.

This is the problem?

Sort of. It depends on the relative velocity of the observer and the signalers.

It's a problem because you can construct the following scenario:

Carl is sitting on a chair. Under the chair is a bomb with a radio detonator.

Carl fires a signal from an FTL radio to detonate the bomb. The signal travels to the bomb and blows it up, detonating Carl.

However, in some IRFs, the bomb detonates before the signal is sent, causing a paradox.

[Bobboau]

but that is only an apparent paradox, caused by you being in an odd position, it's like saying there is a sound activated flash bulb and you, standing far away from it, see the flash then hear the sound. it only seems like the flash went off first because the two signals are traveling at different speeds. causality only needs to hold within the frames that are relevant to the objects involved.

[General Battuta]

but that is only an apparent paradox, caused by you being in an odd position, it's like saying there is a sound activated flash bulb and you, standing far away from it, see the flash then hear the sound. it only seems like the flash went off first because the two signals are traveling at different speeds. causality only needs to hold within the frames that are relevant to the objects involved.

Nope - this is a common misconception, but it's wrong. It's not analogous to the thunder/lightning situation at all. (For one - though it's not the key reason - the thunder will never, ever, ever precede the lightning.)

Causality must hold in all IRFs because no IRF is privileged. If causality breaks in one IRFs, and the laws of physics must be the same in all IRFs (which they must be, by one of the fundamental axioms of relativity), then it breaks in all IRFs.

So far this discussion is going exactly the way the one with Aardwolf did. I wish I'd taken notes. 

[Bobboau]

I know it does not jive with relativity and we are walking down well traveled streets.

my question is what if causality only had to hold within reference frames relevant to the objects involved? or why is it that appearance of causality is so important? the isse is the bomb appears to go off before the signal because the speed of light is assumed (with volumes of evidence to support the assumption) to be the speed of propagation for everything in the universe.

keep in mind that an inertial reference frame is nothing more than a mathematical model, it might well be the case that FTL movement is completely possible but our models are only valid given that no FTL motion is present. So using the details of those models that break down with FTL to say FTL is not possible only points to the fact that that particular model does not work if FTL is present, it speaks nothing to the accuracy of the model (which, FTL aside, is otherwise almost perfect).

[Bobboau]

as an asside

Never say never guys

you know what I hate, when I see unqualified statements like "the amount of energy required would be enormous, equivalent to converting the entire mass of Jupiter to energy" that much energy would get us what? moving a speck of dust one inch at 2C? moving a galaxy five quadrillion lightyears in a billionth of a second? is it dependent on mass? volume? distance? speed?
just pisses me right off....

[General Battuta]

I know it does not jive with relativity and we are walking down well traveled streets.

my question is what if causality only had to hold within reference frames relevant to the objects involved? or why is it that appearance of causality is so important? the isse is the bomb appears to go off before the signal because the speed of light is assumed (with volumes of evidence to support the assumption) to be the speed of propagation for everything in the universe.

keep in mind that an inertial reference frame is nothing more than a mathematical model, it might well be the case that FTL movement is completely possible but our models are only valid given that no FTL motion is present. So using the details of those models that break down with FTL to say FTL is not possible only points to the fact that that particular model does not work if FTL is present, it speaks nothing to the accuracy of the model (which, FTL aside, is otherwise almost perfect).

We've accelerated things to lightspeed and pumped enough energy into them that (if relativistic dynamics regarding IRFs did not hold true) they should have exceeded C easily.

They didn't. Instead they behaved exactly like relativity predicts. I know you're well aware of this (very well-informed and reasonable post by the way), but while it's interesting to postulate 'what if causality only had to hold within reference frames relevant to the objects involved?', it doesn't seem to work that way in real life - in part because of the laws of physics could be different in different reference frames then it would be easily testable.

This is the first postulate:

Under transitions between inertial reference frames, the equations of all fundamental laws of physics stay form-invariant, while all the numerical constants entering these equations preserve their values. Thus, if a fundamental physical law is expressed with a mathematical equation in one inertial frame, it must be expressed by an identical equation in any other inertial frame, provided both frames are parameterised with charts of the same type.

If different IRFs had different physical laws we could have detected it trivially by now.

[Bobboau]

From experiment, we have determined that simply pumping kinetic energy into something will only accelerate it asymptoticly close to the speed of light, yes, but I don't think that directly addresses the issue of causality. Right now I believe the discussion is on the level of "depending on where you observe a series of interactions, if FTL is possible then the effect would precede the cause". Most of the causality violations I see in the examples seem to be an artifact of 'what is observed is what happened' that is to say that it assumes light to be the fastest thing so when the light from events reach an observer it is assumed that the order of events observed was the order in which they happened even if some of those events are FTL in nature.

I actually just reread your bomb example. if the triggering action was the reception of the bomb by the FTL radio, then in what reference frame would the bomb exploding precede the detection of the FTL signal?

[Liberator]

If your outside references do not have the capability of receiving/transmitting FTL signals, then they cannot observe the FTL transaction and then the discussion of IRFs is moot.  This is also all assuming that you do not "shortcut" the LSC by some sort of dimensional hopping thru an environment where Lightspeed is not the highest speed possible.  Most speculation leans in this direction.  In SciFi NO ONE does FTL in Real Space.

[Bobboau]

at least not since flash gordon.

[NGTM-1R]

at least not since flash gordon.

Mass Effect called.

Granted they were exploiting loopholes, but...

[Bobboau]

I think putting the ship into a mass neutralizing field counts as 'not real space'.