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Comment by enraged_camel | Hacker News Reader

>>You are not correct. If anything with information travels faster than the speed of light, then there are frames of reference in which that information arrived at the destination before it left the origin.

No. If you send a radio signal to Mars, and then warp to Mars to find yourself there 0.1 seconds later, you are still 0.1 seconds after the moment on which you sent the signal from the point of origin. You are traveling faster than light, in the sense that you are traveling at [Earth-Mars Distance]/[0.1 sec] speed, but you are not actually traveling backwards in time to a point where you have not sent the signal yet.

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Nothing you just said contradicts my statement, which was, "there are frames of reference in which that information arrived at the destination before it left the origin."

I suspect that you don't know anything at all about Special Relativity, in which case, there's not much more I can tell you except to go educate yourself.

Special Relativity only applies to cases where there is uniform relative movement in two or more inertial frames of reference. That is not the case with this theory - the relative movement is clearly not uniform and although I've not read the paper I'm certain that the time warp provides a non-inertial frame.

TL;DR - ignore Special Relativity completely, General Relativity is the only thing that applies here.

We're not applying SR to an observer who warps, just to two observers in inertial frames of reference.

Suppose we have guy A and guy B who have clocks. They're on spaceships which are both drifting away from each other (i.e. provide inertial frames) at speed c/root 2; at time t=0 they were in the same place. From SR it should be clear that in A's frame, his clock reads 100 at the same time as B's frame reads 50, while in B's frame, when his clock reads 50 A's clock reads 25. All this should be basic and obvious.

So suppose an event happens on A's ship at time tA=100, and he transmits this instantly (or sends a courier C on a warpship) to B at time tA=102, when B's clock reads tB=51.

From B's perspective, he's just received a message from A at time tB=51 (which is simultaneous with tA=25.5), which A is not going to send until tA=102 i.e. tB=404. Thus, B perceives a violation of causality.

(This becomes very obvious if B can send an insta-message back to A with the same technology; B sends it at time tB=54 which is simultaneous with tA=26, A receives it at tA=26, well before he sent the original message at tA=102)

So This may not be an FTL drive, but the way I always sort of pictured it is that the FTL moves you forward in time to the relative observer (so if you move distance x you move (c/x)^-1 forward in time). So if you send an FTL courier (assuming travel time of 2) from point A with times tA=102 tB=51 it would arrive at point B at times tA=52 tB=104 (A time 53 units in the future from tA), if it got sent back it would arrive at point A at tA=106 tB=53 (A time 53 units in the future from point B). The distance traveled would be c*53 distance (53 units of time for light to travel) in 2 units of time.

This also works for the mars/earth scenario (say they are 14 minutes apart). Traveling to the place that the light you see in 14 minutes will be coming from instantly would only ever be traveling forward in time. Forward time travel doesn't break causality(?)

This is always sort of how I envisioned wormholes work (and is indeed how they work on shows like star gate). You go in one end and come out the other, unless you can communicate through the wormhole it would take light (distance times c) years to tell people on the other end you succeeded.

But perhaps I'm missing something...

So as far as B's concerned, when he sends the FTL courier to A that courier will goes slower than a light-speed message? He could send a message at tB=53,tA=26.5 and it would reach A well before tA=106.

This only works if you assume the FTL drive is fixed to some sort of absolute reference frame (which happens to be the same as A's frame). At which point you've given up on relativity, and I'll bet it's possible to come up with some cute paradoxes

I realize how this violates relativity, but I don't understand your counter example (if it was one?) because I wasn't talking about absolute time frames, I'm talking about going forward in time when going FTL. And at no point in my example could B reply before A sends a courier.

It's not a counterexample as such, but it seems obviously absurd (and violating relativity) that B's FTL message to A arbitrarily goes a lot slower than A's FTL message to B, given that the situation is completely symmetrical (you can even have them both send their messages at tA=100,tB=100, which is simultaneous from the point of view of an observer C who stayed behind on the asteroid where the two spaceships initially met up).

The point I was trying to make was that your example description makes sense if the FTL drive is somehow fixed to A's reference frame, i.e. A's reference frame is some kind of absolute reference frame and therefore the situation is not symmetrical between A and B. But that introduces its own problems.

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.

Special Relativity applies ANY TIME that you transmit information from one point A in an inertial frame to another point B in an inertial frame.

If you can use your warp drive to transmit information from A to B, then Special Relativity applies.

If you cannot use your warp drive to transmit information from A to B (e.g., from Earth to Mars), then what good is it?

(I've mentioned several times in this thread that you might be able to come up with some sort of FTL drive that doesn't violate causality if you put enough constraints on it to make it useless.)

> Special Relativity applies ANY TIME that you transmit information from one point A in an inertial frame to another point B in an inertial frame.

Not true. It also implies uniformity of space-time in all inertial frames and the space between them. Since the theoretical purpose of the warp drive is to create a non-uniformity by definition SR does not apply.

Yeah, pretty much. I'm actually amazed that so many people (nessus included - as he seems to be the most vocal poster on this thread) do not know this.

All Special Relativity requires is that space be more or less flat between point A and point B. It doesn't matter if the information travels from A to B through a wormhole and then into another universe and then out the butt of Dumbo, if it ends up at B before light would, then causality has been violated.

If, on the other hand, your FTL drive bends up space all around us to be like a giant hyperspace pretzel, (1) we're going to have a lot more problems than just a little violation of causality, and (2) causality still ends up being violated via General Relativity.

Causality is a consequence of the physical laws you are currently using, not a law in themselves. It just paints a logical boundary condition around what is possible using the current set of laws that you are using.

Compare Newtonian causality where there is only one timeframe everywhere to SR causality with its local timeframes to GR which supports warping of space and time. All different and not interchangable.

Wormholes cannot exist in SR, therefore applying SR's brand of causality is equally invalid. Under GR the concept of simultaneity is relaxed: the effect is only limited to the light cone of the cause.

Special Relativity is not invalidated by GR: Special Relativity is a special case of GR that tells us what to expect in inertial frames in a flat space. As long as points A and B are in a flat enough space, then the predictions of Special Relativity about the causal nature of information transmitted between A and B BY ANY MEANS will still be true, even if there are nearby wormholes. If Special Relativity tells us that we can make a closed causal loop via the FTL signals that make it through a wormhole, then GR will not overturn this, no matter how the information made it from A to B.

I agree that if the space Between A and B is distorted enough so that the light that travels in open space between A and B is seriously affected by a curvature that extends from A to B, then it is true that you cannot use SR to make your predictions, as the premises of SR are violated. But then GR will predict that causality can be violated anyway. (It's generally easier to find ways to violate causality in GR that SR anyway, since it doesn't require FTL transmissions of information in GR to create causal loops.)

I refer you to the FTL faq, since it seems that I am not able to talk sense into people on my own:

http://www.physicsguy.com/ftl/html/FTL_part4.html

This is as good a place to start as any:

http://en.wikipedia.org/wiki/Relativity_of_simultaneity

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