Sure, B sees things happen in the wrong order, but I will maintain that causality is not actually violated until an effect actually disturbs a cause.
In other words, if B sends that message back to A, you have violated causality. If B just sits there and watches events happen out of order, then the causality is obscured but not broken.
> Sure, B sees things happen in the wrong order, but I will maintain that causality is not actually violated until an effect actually disturbs a cause.
You maintain incorrectly. All inertial frames are equally good in Special Relativity. If there is any inertial frame that observes events elsewhere in the universe where an effect precedes a cause, then causality has been violated.
Furthermore, whenever Special Relativity is violated like that, there is always a way to get the violation to turn up in your own frame of reference, unless you constrain things so severely that what you are proposing isn't going to be useful for much.
All inertial frames are equally valid. All inertial frames see that the effect does not munge the cause, even if it happens out of order.
I know that 'causality' is a bad term for me to use here; I'm open to suggestions. But my point is that cause and effect are still intact. Cause, with effect not inside its light cone in the past, sends information toward effect. Effect, with cause not in its light cone in the future, receives information from cause. I don't care what order those happen in, as long as the thread of causation is intact and one-way.
Yes there are 'ways' to create proper violations, but you can avoid them if you intentionally choose to.
> I know that 'causality' is a bad term for me to use here; I'm open to suggestions.
I think that you are looking for the term "closed causal curve" or "closed causal loop". If you use FTL to achieve this, this is often called a "closed spacelike curve". If you use GR without FTL to achieve this, it is often called a "closed timelike curve".
I'm not convinced that other sorts of violations of causality are any better than closed causal loops, since you can typically use the other violations to create closed causal loops. And even if you do have closed causal loops, you can decide not to go back in time and kill your grandfather, so if the issue is just what you chose to do, then closed causal loops are no worse in this regard.
In any case, whether or not you chose to go back in time and kill your grandfather, you won't succeed, since you did not succeed.
Well the important part here is that you can pretty simply have a self-consistent universe that has mildly-limited FTL (and highly-limited time travel) if nobody makes a loop.
And I'll try to use something like 'causal loop' in the future.
And what happens if you try to make a loop? Do you think it sensible that the universe provide you with the ability to make causal loops and then somehow forbids you from actually making them, even though once you've been given the ability, they are very easy to generate?
Additionally, if the universe is going to keep stepping in your way of what it has given you the ability to do, there's no reason for it to forbid you from making causal loops. All it has to do is forbid you from generating a paradox via such a loop.
I.e., you can have a self-consistent universe that has unlimited FTL and unlimited time travel as long as everyone plays nice. In fact, you can have unlimited time travel even is everybody is trying to kill their grandfather. They just won't ever succeed.
Never did I assert anywhere in this entire thread that a universe where causality is violated--or even one with closed causal loops--is impossible. They do have serious issues, however. And for anyone to propose FTL travel as a serious possibility and then ignore the causality issues as non-serious, either indicates some amount of either ignorance, disingenuousness, or rationalization.
If, on the other hand, a proponent of FTL travel plainly states that they have no problem with a universe that always steps in somehow and prevents you from succeeding in killing your own grandfather, then at least they've made their position clear, and everyone can make up their own mind as to whether that's a likely way for the universe to be.
There are other solutions, too. It might simply be infeasible to accelerate an FTL device to sufficiently relativistic speeds. Or what if there were fields coming off of FTL devices that interfered with the speed when they criss-crossed. That one would stop any sort of loop and it wouldn't require the universe to somehow specifically forbid paradoxes.
FTL devices don't have to "criss-cross" for you to be able to create causal loops. And even if you don't have any causal loops, other violations of causality violate thermodynamics. It's not as if if you've solved the problem of causal loops--which as far as I'm aware, no one has done without postulating a preferred frame of reference, which is basically rejecting Relativity outright--then you're off the hook. All violations of causality are going to make most scientists shake their heads. And, if it is infeasible to accelerate an FTL drive up to speed, then the point is moot for space travel or communication.
By 'criss-cross' I mean that your FTL devices in different reference frames need to repeatedly get close enough for causal contact. Is there another way to make a loop using simple FTL devices?
And I might not have been clear enough on acceleration. I meant you might be able to construct a device capable of FTL transport, but it might be far harder to traditionally accelerate this entire device to high speed. The FTL isn't moot, but if your FTL is only 5c and you can't accelerate your turned-off FTL device faster than .1c you're not going to make any loops with it.
In other words, in this scenario, there isn't a preferred frame of reference per-se, but there's no way for such a large and complex construct as an FTL device to get far enough away from the galaxies' default reference frame.
You might be able to cook up something, but I doubt that it will be any more plausible than positing a preferred reference frame.
Personally, I think that supporters of FTL should stick with subspace and run with it. And, hey, the author of the FTL faq even wrote a whole subspace faq too.
Maybe. I can't directly see an event disturbing a cause after the first message, but that doesn't mean there's not a way to make it do so.
Of course the cheap answer is that it violates SR (and common sense, given the complete symmetry of the situation) for A to be able to send an insta-message to B but B not able to send an insta-message to A.
Well, either one can send a single message. It's the reply from a different frame that causes trouble. In fact you can have unlimited FTL messages sent in a single reference frame. It's only when you combine multiple FTL reference frames that things get wonky.
But if B can't send an FTL-message to A because he's going to receive an FTL-message from A in what to him is the future, that's your causality violation right there.
Right, I should be more precise with the scenario. A cancelled-message is still a transfer of information. To avoid problems you have to set up the scenario such that only one ship would send a message no matter what. But it can be either ship, and you don't have to know which ahead of time.
In other words, if B sends that message back to A, you have violated causality. If B just sits there and watches events happen out of order, then the causality is obscured but not broken.