Why does FTL violate causality?

[Phhase]

This, by itself, is why it violated causality.

How exactly does that violate causality? The way we gather information on Earth is by measuring the light that faraway bodies emit. Of course, since light has a speed, this method breaks down in temporal accuracy over great distances. But the truth, and reality exist without our observation. Out there, the stars are the way they are right now, the only reason we can't know how that is, is because we're interpreting from a material with a travel time. It doesn't seem to me like any great feat of time travel to be at a location that cannot be observed with conventional means, using theoretical means to bridge the gap of distance. ...right? Am I missing something? Completely insane?

[georgie_leech]

How exactly does that violate causality? The way we gather information on Earth is by measuring the light that faraway bodies emit. Of course, since light has a speed, this method breaks down in temporal accuracy over great distances. But the truth, and reality exist without our observation. Out there, the stars are the way they are right now, the only reason we can't know how that is, is because we're interpreting from a material with a travel time. It doesn't seem to me like any great feat of time travel to be at a location that cannot be observed with conventional means, using theoretical means to bridge the gap of distance. ...right? Am I missing something? Completely insane?

Different reference frames disagree on when "now" is relative to the star. Our reference frame isn't more true than that of one going at near light-speeds, for instance.

[NichG]

How exactly does that violate causality? The way we gather information on Earth is by measuring the light that faraway bodies emit. Of course, since light has a speed, this method breaks down in temporal accuracy over great distances. But the truth, and reality exist without our observation. Out there, the stars are the way they are right now, the only reason we can't know how that is, is because we're interpreting from a material with a travel time. It doesn't seem to me like any great feat of time travel to be at a location that cannot be observed with conventional means, using theoretical means to bridge the gap of distance. ...right? Am I missing something? Completely insane?

You're missing that the delay depends on how fast you're going as the receiver relative to the emitter, not just the distance from emitter. Therefore, by changing your velocity but remaining in place, you and a third relay can conspire to make the emitter receive a signal before it sends it.

Classically, the delay only would depend on distance in space, not distance in velocity.

[Frozen_Feet]

@Phhase: you are doing the same error of begging the question as many before you: by imagining "right now" of distant objects, you're implicitly assuming an universal reference frame where time passes at a set speed.

It doesn't. There's no such frame, "right now" is relative and true simultaneity between distant objects isn't a thing. If you introduce a no-delay mechanism between two (or more) distant reference frames, that will work out to negative delay in some third (or fourth etc.) reference frame.

[keybounce]

Lets make sure we are talking about the same thing here.

My understanding: When 1905 came around, things like the Michelson-Moorly experiment were history and "what does this mean???". The Lorenz contraction formula was known, took pages of calculus to derive, and implied the strangeness of physics that as things moved, they actually changed in shape.

Einstein decided to look at the three "obviously true" beliefs, and asked "what if we get rid of the one that seems to make the most trouble?".

So, he asked, "What if we just make these assumptions:

In an inertial reference frame:
1. The speed of light in a vacuum appears constant (the same) to all observers
2. All laws of physics appear the same to all observers.

From that, he can derive the same Lorenz contraction formula with half a page of high-school algebra, using the same setup that explains the doppler shift.

Without needing to assume that objects change in shape as they move; it suddenly becomes different observers disagreeing on the timing of things.

So, over the next 10 years, he started to ask, "What if, instead of an inertial frame of reference that requires deep space and no gravity, we use a different system of math that can describe what happens with gravity, or a rotating planet?".

That's all G.R. is. The assumption that the speed of light in a vacuum seems to be the same to everyone, as do the laws of physics.

And the conclusions? That you cannot go faster than light while remaining in the universe because you are always ... who was the ancient greek figure that was always trying to play catch-up with the turtle that started ahead of him?

---

When I started, I said that I knew that if you can make two FTL jumps with different frames of reference, the differences in the light cones that represent the frames of reference permit going back in time. I asked about "what if the universe has a special frame of reference such that any FTL jump you could make must be in this one special frame".

We know that G.R. is incomplete.
We know that it disagrees with Q.M. at the extreme.
We know that G.R. cannot describe gravity in the extremes (black holes, big bang).

Just as Newton knew that his rules did not describe Mercury, and there must be something else, so we know that G.R. is only "mostly accurate".

Now, replying to some people here:

General relativity is based on (and named after) the general principle of relativity; that the laws of physics are the same in all frames of reference.

A special frame of reference such as you describe would necessarily vary on those laws of physics which make it special; therefore physical laws would not be the same in all frames of reference, therefore general relativity would not hold.

Why does a privileged frame of reference require that different observers see different laws of physics?

"Everyone agrees that FTL can only be done under special circumstances. Everyone can tell that there's only one set of rules for an FTL jump. And even though different people are moving at different rates, and see a different difference between this special frame and their own, everyone can still determine that there is only one such frame, and can figure out how to calculate the difference between their frame and this one frame".

FTL is impossible because every observer measures the speed of light in a vacuum to be the same. FTL would not just break our understanding of the universe; it would break the universe. Most people do not realize how important the fundamental constants are and how closely coupled to each other they are. FTL would mean nothing is possible any more.

If I can go faster than light inside the universe -- if I can actually outpace a light beam -- then yea, time goes wonky if G.R. remains true. It's moving in the wrong direction, so time must have its sign flipped.

If I can make a jump from A to B and arrive before light, this is a different statement. Doesn't change the observation of light's speed.

I can define a reference frame in which the solar system is not moving or rotating.
I can define a reference frame in which the Milky Way is not moving or rotating.
I can define a reference frame in which the Local Group containing the Milky Way is not moving or rotating.
I can define one in which the Laniakea Supercluster (which includes the Local Group) is not moving or rotating.
I can define one in which the Centaurus Great Wall (which includes the Laniakea Supercluster) is not moving or rotating.
I can even define one in which the center of all currently known extra-galactic structures is not moving or rotating.

But I cannot define "The universe itself" and have any idea what that frame of reference is.

There is no single reference frame that is "The universe itself".

Turn it around. The assumptions of general relativity say that if you define a frame of reference, for instance "The universe itself", then I can define another one that moves relative to that one, and which is just as definitive as that one.

Your frame of reference isn't privileged. It just isn't. It's as arbitrary as any other. If an object is moving at 0.5c in your reference frame, then whatever appears motionless to you is traveling at 0.5c to that reference frame.

The same equations needed to refer to another reference frame from yours will work to refer to yours from that one.

Two things here:

1. Yes, if I define one frame, and call it "special", you can define a different one, and call that "special". But these are just arbitrary definitions by people. This has nothing to do with whether or not the universe itself has a special frame.

2. Just because you (or I) cannot define "the universe itself" and know what that means does not mean that it cannot exist.

Just because we cannot determine "the universe" does not mean that "the universe" does not exist.

[jayem]

Lets make sure we are talking about the same thing here.
When I started, I said that I knew that if you can make two FTL jumps with different frames of reference, the differences in the light cones that represent the frames of reference permit going back in time. I asked about "what if the universe has a special frame of reference such that any FTL jump you could make must be in this one special frame".

I'm sure it didn't say that when I started, must be some relativistic effect :)

My understanding (which doesn't go to G.R)

If S.R (& G.R) still mostly holds apart from that one special portal-frame, then there will inevitably be some frames where "backwards time-travel" will seem to have occurred. As such there probably will be some weird effects. In a sense that is O.K as these frames just have to accept they are 'in the wrong', but there must be some interesting consequences, especially if/as they can interact with the portal.

If S.R doesn't generally hold, then there are enough real world aspects that cause problems. Do we need to bring in an aether frame? What does the observable universe look like after we've not calculated everything wrongly? What mechanism do we bring in for Muon decay? How does an atom bomb work?

What I don't know, in the derivation it's assumed that the relativistic correction can be separated nicely and only modifies the one bit of the Gallileon transform (I think I have seen it justified). If that is not the case there may be a space for a more complex transformation. I'm pretty sure that will be ugly though.

[NichG]

If there's a preferred reference frame, you don't have conservation of momentum or energy, and as a result there would be a host of knock-on effects. Elliptical orbits would precess at a different rate than GR predicts. Space would be opaque over a characteristic distance. Etc.

You can of course make theories with spot fixes to all of these issues, but that generally means that if you have to do that, there's something wrong.

[Radar]

When I started, I said that I knew that if you can make two FTL jumps with different frames of reference, the differences in the light cones that represent the frames of reference permit going back in time. I asked about "what if the universe has a special frame of reference such that any FTL jump you could make must be in this one special frame".

We know that G.R. is incomplete.
We know that it disagrees with Q.M. at the extreme.
We know that G.R. cannot describe gravity in the extremes (black holes, big bang).

Just as Newton knew that his rules did not describe Mercury, and there must be something else, so we know that G.R. is only "mostly accurate".

Well, we do hope it is only mostly accurate, but so far we had no luck in finding any inconsistencies in it. With the birth of gravity wave detection I expect we will gather data that might give a new insight into it. We live in amazing times for science - that one is sure.

It's not that GR disagrees with QM - it is more that they describe different aspects of reality using vastly different language, so it was a big challenge to combine them. I used the past tense, since there are some good candidates for quantum gravity theories and they already bring results - one of the last papers by Hawking made a big impact, since he was able to prove that black holes do not destroy information.

Also, black holes turned out to be extremely easy to describe. In fact, they might be the simplest possible macroscopic objects.

You probably were thinking about the singularity in the black hole and at the beginning of time. I am pretty sure that such extreme and impossible to see objects are outside our current knowledge, but it does not stop people from trying to resolve them. There are some neat cyclic cosmological models which do deal with it more often then not using some version of quantum gravity.


Anyway, as for what would happen, if there was a preferred frame of reference. Short answer? We would already detect it. The thing is, if there is a special frame of reference, where laws of physic look differently, then all the other ones will be defined by how far off they are from the ideal. A good example are non-inertial reference frames - for each one of them we can strictly derive an intertial reference frame. Such asymmetry would have even more unusual consequences, since each symmetry of our reality is directly connected to some conserved quantity (Noether's theorem). It is such a fundamental thing that elementary particle models are build strictly in the language of algebraic symmetry groups. So by cutting out the invariance of law of physics to velocity of a frame of reference, you would be for example breaking conservation of spin along with it. I'm not sure, if there is a subtle way of doing this that would avoid detection frankly.

I was about to say that space-time tunnels don't have to violate causality and allow FTL travel at the same time, but it is not nearly as simple. First, the only causality-consistent way to build one would be to make both ends in one place and then tow one of them to the designated endpoint using regular means. Second, while a single space-time tunnel might not immediately make time travel possible (time dillation on one of the ends would have to accumulate to a larger time-shift then the spatial distance between the ends through regular travel) a network of such tunnels would be easily exploitable and create a time travel possibility very quickly.

If there's a preferred reference frame, you don't have conservation of momentum or energy, and as a result there would be a host of knock-on effects. Elliptical orbits would precess at a different rate than GR predicts. Space would be opaque over a characteristic distance. Etc.

You can of course make theories with spot fixes to all of these issues, but that generally means that if you have to do that, there's something wrong.

For FTL not to cause time travel one only needs to have a distinct velocity of a reference frame or in other words a unique time axis. As such, momentum and energy would be conserved, since the symmetry of translation in space and time would not be broken, but think of the poor spin...

[keybounce]

I'm sure it didn't say that when I started, must be some relativistic effect :)

I just double checked, in case I had messed up.

My question, in the context of both sci-fi stories as well as real world physics:

But why does FTL have to use the originator's frame of reference? Why wouldn't it use it's own FoR that is based on the FTL medium / whatever you travel through to make the trip?

And once you have a single constant FoR for all trips, doesn't that make time travel by FTL travel impossible?


So ... if the idea is to allow scientific fiction that can have high speed travel and some sense of a "global" clock (Star Trek?), can you say that your FTL system has some special reference frame that applies to FTL so that you don't care what light cone you saw in normal space before you turned on your FTL drive, all FTL drives operate under a special reference frame, and does that prevent the time travel issues normally associated with FTL?

[Rockphed]

If there's a preferred reference frame, you don't have conservation of momentum or energy, and as a result there would be a host of knock-on effects. Elliptical orbits would precess at a different rate than GR predicts. Space would be opaque over a characteristic distance. Etc.

You can of course make theories with spot fixes to all of these issues, but that generally means that if you have to do that, there's something wrong.

What do you mean by "opaque"? Because for some definitions the universe is opaque if you go far enough away. We are fairly certain that the opacity is related to conditions right after the big bang, but we could be wrong about that. Highly unlikely though.

[Radar]

I just double checked, in case I had messed up.

My question, in the context of both sci-fi stories as well as real world physics:

But why does FTL have to use the originator's frame of reference? Why wouldn't it use it's own FoR that is based on the FTL medium / whatever you travel through to make the trip?

And once you have a single constant FoR for all trips, doesn't that make time travel by FTL travel impossible?


So ... if the idea is to allow scientific fiction that can have high speed travel and some sense of a "global" clock (Star Trek?), can you say that your FTL system has some special reference frame that applies to FTL so that you don't care what light cone you saw in normal space before you turned on your FTL drive, all FTL drives operate under a special reference frame, and does that prevent the time travel issues normally associated with FTL?

In essence yes, having a single universal frame of reference for FTL would prevent time travel, but it would generally have a lot of other consequences. For the purpose of an SF story it can be dealt with, but for the real world it seems we would notice.

[NichG]

What do you mean by "opaque"? Because for some definitions the universe is opaque if you go far enough away. We are fairly certain that the opacity is related to conditions right after the big bang, but we could be wrong about that. Highly unlikely though.

For example, in active fluids (in which there is a particular favored velocity), fluctuations in momentum decay exponentially as they travel through the fluid (essentially becoming reset with respect to that special reference frame). So if you're far enough away from something which is injecting a momentum perturbation into the field, you can't detect it. Photons are basically objects with energy and momentum but no rest mass, so one would find that as they propagate through space photons would exponentially lose energy and momentum with distance traveled.

Space isn't totally transparent in the standard model, but it's non-transparency takes the form of photons participating in pair production and scattering off the vacuum. So energy and momentum aren't lost, they're just redistributed over a wider angle. So I suppose that's more like space being blurry than opaque.

[Rockphed]

For example, in active fluids (in which there is a particular favored velocity), fluctuations in momentum decay exponentially as they travel through the fluid (essentially becoming reset with respect to that special reference frame). So if you're far enough away from something which is injecting a momentum perturbation into the field, you can't detect it. Photons are basically objects with energy and momentum but no rest mass, so one would find that as they propagate through space photons would exponentially lose energy and momentum with distance traveled.

Space isn't totally transparent in the standard model, but it's non-transparency takes the form of photons participating in pair production and scattering off the vacuum. So energy and momentum aren't lost, they're just redistributed over a wider angle. So I suppose that's more like space being blurry than opaque.

Thank you for clarifying.

[jayem]

[much delayed as didn't click send]

I just double checked, in case I had messed up.

I had, by forgetting the OP by the time I'd read the thread.

My question, in the context of both sci-fi stories as well as real world physics:

But why does FTL have to use the originator's frame of reference? Why wouldn't it use it's own FoR that is based on the FTL medium / whatever you travel through to make the trip?

And once you have a single constant FoR for all trips, doesn't that make time travel by FTL travel impossible?


So ... if the idea is to allow scientific fiction that can have high speed travel and some sense of a "global" clock (Star Trek?), can you say that your FTL system has some special reference frame that applies to FTL so that you don't care what light cone you saw in normal space before you turned on your FTL drive, all FTL drives operate under a special reference frame, and does that prevent the time travel issues normally associated with FTL?

It would make it impossible in that special frame. However the other frames still exist.
NB Wormholes would be (individually) in one or two frames and have been discussed in Hawking's books.

If the lorentz stuff is correct then: from the point of view of a photon going between the two wormholes (from our perspective). It is a stationary electric field and the two wormholes are coming towards it at c, and are basically contracted to be adjacent. The universe would contain the now-effects of the ship coming out the wormhole and about to go in.
Obviously for all of us close to the wormholes frame of reference differences will be tiny, but I think once we interact with people in non-wormhole frame because they now have reversed causality in their frame, by them exploiting us and the wormholes. In turn there may be some way to exploit this (though maybe only to getting FTL messages in other directions).

If not then we need to explain magnetism.

[Douglas]

It would make it impossible in that special frame. However the other frames still exist.
NB Wormholes would be (individually) in one or two frames and have been discussed in Hawking's books.

I'm pretty sure that, so long as the special frame itself isn't moving FTL, no matter what other frame you use it can't send you farther back in time than how long it would take to travel back to your starting point at light speed in the normal frame. Even if you can instantaneously teleport arbitrary distances in the special frame, you can't make circular causality by teleporting one way and then flying back non-FTL - and teleporting for both segments of the trip just sends you forward in time on the return trip, still arriving after you left. You can't make a causal loop using this method of FTL.

This kind of FTL does break local conservation laws, though. A traveler spontaneously appears somewhere, and the amount of mass in the universe increases. Later, but not too much later, that same traveler departs into the past (of a distant place), and the change in mass has globally balanced out.

To make circular causality, you need to travel FTL in at least two different frames. One-frame FTL lets you travel back in time in some reference frames, but never by enough to make it possible to close the loop.

[Lvl 2 Expert]

That's not quite how entanglement works. The key point is, you aren't ever actually changing the spin, you're deciding what basis in which the spin should have a defined value.

It took me a while to let this sink in, but I've found a metaphor that works for me. You'rs basically saying that the spins are not coupled, they're synchronized. They will be making the same/the exact opposite/something like that motions (if you can call it that at this level) because they're been set to the same/the opposite/complementary starting conditions. By measuring you disrupt this pattern, so the spin here no longer says anything about the spin back there. But even when it did say something about the spin back there, it can't say anything about anything that interacted with the spin back there, because it only tells me they were synchronized at some point in the past.

That would be quite an elegant solution. I'm kind of hoping they prove that one. (Also kind of not, but that's the trekkie in me talking.)

[NichG]

It took me a while to let this sink in, but I've found a metaphor that works for me. You'rs basically saying that the spins are not coupled, they're synchronized. They will be making the same/the exact opposite/something like that motions (if you can call it that at this level) because they're been set to the same/the opposite/complementary starting conditions. By measuring you disrupt this pattern, so the spin here no longer says anything about the spin back there. But even when it did say something about the spin back there, it can't say anything about anything that interacted with the spin back there, because it only tells me they were synchronized at some point in the past.

That would be quite an elegant solution. I'm kind of hoping they prove that one. (Also kind of not, but that's the trekkie in me talking.)

That describes the classical version of entanglement. The quantum version is a bit more complex, in that the spins are described as having the properties (x,y,z) such that this synchronization would exist for any one of them. However, those axes aren't independent - there's a relationship such that you can't simultaneously know x, y, and z (much like position and momentum) because they aren't actually independent degrees of freedom in reality but are related by a constraint of constant angular momentum. Or, alternately, you could look at polarization of a photon which basically has two bases that are both equivalent to describe it (you can think of it as one basis aligned with the x,y axis, and another at a 45 degree angle to that).

If you measure x and someone else measures y for example, then the measurements have nothing to do with one-another. However, if both people were to measure x or both people were to measure y, they would see that synchronization is respected along the axis they both chose to measure. The statistics of that are subtly different than the classical statistics - specifically it exists in the correlation between the measurements rather than any measurement on its own. In order to evaluate the correlation, you have to bring the results of both measurements to the same place in space-time, which is why despite there being a deviation from the classical result it does not imply that you can transmit information faster than light.

The specific statistical quantity is given by Bell's Inequality.

[Radar]

It took me a while to let this sink in, but I've found a metaphor that works for me. You'rs basically saying that the spins are not coupled, they're synchronized. They will be making the same/the exact opposite/something like that motions (if you can call it that at this level) because they're been set to the same/the opposite/complementary starting conditions. By measuring you disrupt this pattern, so the spin here no longer says anything about the spin back there. But even when it did say something about the spin back there, it can't say anything about anything that interacted with the spin back there, because it only tells me they were synchronized at some point in the past.

And to even know that there was a synchronisation, you need to examine many experiments and have resuts from both particles.

That would be quite an elegant solution. I'm kind of hoping they prove that one. (Also kind of not, but that's the trekkie in me talking.)

They actually did as long as you assume that laws of physics have to be local. Aside from using standard two-photon entanglement (better then for particles, since it allows us to avoid any possibility of interaction after creation), they did it with three-photon entanglement, for which differences between quantum (assumption that the result of an experiment is truly random within the given constrains) and hidden variable (assumption that the results of an experiment is predetermined) theory are much more drastic.

For more information see Greenberger–Horne–Zeilinger entanglement. They are doing really cool experiments on this and quantum computers.

[Lvl 2 Expert]

And to even know that there was a synchronisation, you need to examine many experiments and have resuts from both particles.

Yeah, but I knew that part. That was what I figured was so spooky about the situation. Information travels faster than the speed of light, but without a second slower than light channel it can only be useless information. That's not a natural law, that's a made up rule.

It's almost a shame that what's actually happening is kind of normal.

(Also: Thank you both for the extra info.)