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> I see light arrive from X at some time T1+t1 Except that your notion of T1+t1 is already going to be different from any other (non-stationary) observer's, y

> I see light arrive from X at some time T1+t1 Except that your notion of T1+t1 is already going to be different from any other (non-stationary) observer's, yes? > I do a little math to calculate how long it took the light to get there (t1) Doesn't this assume that light has a fixed speed? We know already that this ain't necessarily the case, even in a vacuum; there are plenty of things - like sufficiently strong gravity wells - that can slow down light considerably. And in the context of situations involving warp drives and wormholes and other things bending spacetime willy-nilly, that further throws a wrench in attempting to figure out how long it took light to reach you. > and now I know what T1 is More like you have a vague idea of what T1 should be, assuming a flat spacetime. My point is that spacetime curvature can already throw this out the window, and that it therefore doesn't seem like we should be taking that estimated departure time as fact. That is: if how an observer is perceiving something is indeed what actually dictates reality, then I don't see how causality can actually exist in the first place, because different observers can perceive the same events in different sequences based on their relative movement. If causality is to exist at all, even under normal conditions, then it seems like it needs to be possible for observers to be wrong, no? So maybe I need to ask this even more specifically: > The issue is that I can find a frame of reference where T2 < T1 Okay, which frame of reference would that be? Under what conditions? By what mechanisms that would apply to literally all cases of superluminal communication (even ones explicitly formulated to avoid things like relativistic relative speeds upon which thought experiments like the tachyonic antitelephone depend)?

AnimalMuppet 6y ago
> > I see light arrive from X at some time T1+t1 > Except that your notion of T1+t1 is already going to be different from any other (non-stationary) observer's, yes? Yes, and so is my distance from my current position to X, which I use to calculate my value of t1, which I use to calculate my value of T1. All the numbers are different for all the observers. Special relativity tells us how to convert the numbers between observers. > > I do a little math to calculate how long it took the light to get there (t1) > Doesn't this assume that light has a fixed speed? We know already that this ain't necessarily the case, even in a vacuum; there are plenty of things - like sufficiently strong gravity wells - that can slow down light considerably. And in the context of situations involving warp drives and wormholes and other things bending spacetime willy-nilly, that further throws a wrench in attempting to figure out how long it took light to reach you. First, light does have a fixed speed, locally (that is, at each point within the gravitational well or whatever). Second, gravitational wells are pretty well understood in terms of their effects on light, and I'm pretty sure we could calculate the effect of the warp drive's gravitational distortion on it as well. "The effect exists" does not equal "therefore we can't do the calculation". > That is: if how an observer is perceiving something is indeed what actually dictates reality Nobody said that - at least, not until you drag quantum mechanics into the picture, and even then, there are debates about that. > If causality is to exist at all, even under normal conditions, then it seems like it needs to be possible for observers to be wrong, no? I have no idea where you got that. If causality is to exist at all, then 1) causal signals can't travel faster than the speed of light, and 2) neither can an observer. If those two hold, then we can have real causality with no observers being wrong, because all observers always observe cause-and-effect happening in the same order. > > The issue is that I can find a frame of reference where T2 < T1 > Okay, which frame of reference would that be? Under what conditions? By what mechanisms that would apply to literally all cases of superluminal communication (even ones explicitly formulated to avoid things like relativistic relative speeds upon which thought experiments like the tachyonic antitelephone depend)? Not me. I'm not going to walk you through all the equations. I'll tell you where to start, and then I'm done. Start by learning the Lorentz Transform - not just reading the equations, but understanding what they mean and how they work. That will clear up most of your misconceptions. Any basic book on Special Relativity should get you there. The Wikipedia article might, if you take the time to really work through it carefully.
- yellowapple 6y ago
  > Start by learning the Lorentz Transform Which specifically describes coordinate transformations between two reference frames in motion relative to one another, right? My whole point is that in this context the reference frames are not (significantly) moving relative to one another - that relative movement is exactly what things like wormholes and warp drives are designed specifically to avoid. So no, that doesn't clear up much at all, unfortunately. To expand on that, because I still think we're talking past each other here: > If causality is to exist at all, then 1) causal signals can't travel faster than the speed of light, and 2) neither can an observer. Okay, which wormholes, warp drives, etc. seem to satisfy: 1. The causal signals - from the reference frames of the signal, sender, and recipient - are all moving such that v is less than c, because spacetime has been warped accordingly 2. In order to observe these signals, the observer would, too, have to be part of that system of warped spacetime (i.e. inside the wormhole, or in the path of our Alcubierre pigeon, or what have you). Otherwise, how would the observer actually see anything? In any case, thanks for bearing with me here.
  - AnimalMuppet 6y ago
    Yes, the Lorentz Transform describes coordinate transforms between two reference frames in motion relative to each other. For the question of why FTL travel blows up causality, that's exactly where you should be looking. You asked how to find a frame of reference where causality was backwards, well, if you learn the Lorentz Transform your questions in that area will be answered. Now, the warp does affect things. We're now playing with general relativity, not just special. You're locally moving at less than light speed. Does that warping also fix causality? I'd want to see that proven, not just assume that it magically does so.
    - yellowapple 6y ago
      I think we're on the same page, then. Or at least close to it. > Does that warping also fix causality? I'd want to see that proven, not just assume that it magically does so. Fair. My impression is that it does fix causality, since the warping eliminates the need to (locally) move at relativistic speeds (and therefore reduces any time dilation to negligibility, and thus the opportunities for retrocausality). So I guess I'm more coming at it from the opposite direction: that if any superluminal communication violates causality (which seems to be a common assertion, both in this thread and elsewhere), then I'd want to see that proven, rather than assume it does so. At most, what I can gather is that it's mathematically possible to construct a warp bubble capable of backwards time travel, just like how it's mathematically possible to construct a wormhole that can do so; when you're mangling spacetime, that obviously opens the door for all sorts of wacky shenanigans. Possibility != certainty, though, and various online discussions (including this one) feature folks who seem to assert that all FTL travel/communication must violate causality, which leaves me wondering why - especially when, to your point, plugging the relevant numbers into the Lorentz transformation doesn't transform anything (no local movement, so v = 0, so the Lorentz factor becomes 1, and vx/c^2 = 0, therefore no time dilation - is that math right?). There's also Hawking's chronology protection conjecture. Yeah, just a conjecture, so it can obviously be disproven at some point, but from what I understand string theory seems to support it (reading the relevant papers is on my TODO list). In any case, thanks again. I don't feel like I'm closer to a definitive answer, but at least I'm left with more to think about.

AnimalMuppet 6y ago

> > I see light arrive from X at some time T1+t1 > Except that your notion of T1+t1 is already going to be different from any other (non-stationary) observer's, yes? Yes, and so is my distance from my current position to X, which I use to calculate my value of t1, which I use to calculate my value of T1. All the numbers are different for all the observers. Special relativity tells us how to convert the numbers between observers. > > I do a little math to calculate how long it took the light to get there (t1) > Doesn't this assume that light has a fixed speed? We know already that this ain't necessarily the case, even in a vacuum; there are plenty of things - like sufficiently strong gravity wells - that can slow down light considerably. And in the context of situations involving warp drives and wormholes and other things bending spacetime willy-nilly, that further throws a wrench in attempting to figure out how long it took light to reach you. First, light does have a fixed speed, locally (that is, at each point within the gravitational well or whatever). Second, gravitational wells are pretty well understood in terms of their effects on light, and I'm pretty sure we could calculate the effect of the warp drive's gravitational distortion on it as well. "The effect exists" does not equal "therefore we can't do the calculation". > That is: if how an observer is perceiving something is indeed what actually dictates reality Nobody said that - at least, not until you drag quantum mechanics into the picture, and even then, there are debates about that. > If causality is to exist at all, even under normal conditions, then it seems like it needs to be possible for observers to be wrong, no? I have no idea where you got that. If causality is to exist at all, then 1) causal signals can't travel faster than the speed of light, and 2) neither can an observer. If those two hold, then we can have real causality with no observers being wrong, because all observers always observe cause-and-effect happening in the same order. > > The issue is that I can find a frame of reference where T2 < T1 > Okay, which frame of reference would that be? Under what conditions? By what mechanisms that would apply to literally all cases of superluminal communication (even ones explicitly formulated to avoid things like relativistic relative speeds upon which thought experiments like the tachyonic antitelephone depend)? Not me. I'm not going to walk you through all the equations. I'll tell you where to start, and then I'm done. Start by learning the Lorentz Transform - not just reading the equations, but understanding what they mean and how they work. That will clear up most of your misconceptions. Any basic book on Special Relativity should get you there. The Wikipedia article might, if you take the time to really work through it carefully.

yellowapple 6y ago
> Start by learning the Lorentz Transform Which specifically describes coordinate transformations between two reference frames in motion relative to one another, right? My whole point is that in this context the reference frames are not (significantly) moving relative to one another - that relative movement is exactly what things like wormholes and warp drives are designed specifically to avoid. So no, that doesn't clear up much at all, unfortunately. To expand on that, because I still think we're talking past each other here: > If causality is to exist at all, then 1) causal signals can't travel faster than the speed of light, and 2) neither can an observer. Okay, which wormholes, warp drives, etc. seem to satisfy: 1. The causal signals - from the reference frames of the signal, sender, and recipient - are all moving such that v is less than c, because spacetime has been warped accordingly 2. In order to observe these signals, the observer would, too, have to be part of that system of warped spacetime (i.e. inside the wormhole, or in the path of our Alcubierre pigeon, or what have you). Otherwise, how would the observer actually see anything? In any case, thanks for bearing with me here.
- AnimalMuppet 6y ago
  Yes, the Lorentz Transform describes coordinate transforms between two reference frames in motion relative to each other. For the question of why FTL travel blows up causality, that's exactly where you should be looking. You asked how to find a frame of reference where causality was backwards, well, if you learn the Lorentz Transform your questions in that area will be answered. Now, the warp does affect things. We're now playing with general relativity, not just special. You're locally moving at less than light speed. Does that warping also fix causality? I'd want to see that proven, not just assume that it magically does so.
  - yellowapple 6y ago
    I think we're on the same page, then. Or at least close to it. > Does that warping also fix causality? I'd want to see that proven, not just assume that it magically does so. Fair. My impression is that it does fix causality, since the warping eliminates the need to (locally) move at relativistic speeds (and therefore reduces any time dilation to negligibility, and thus the opportunities for retrocausality). So I guess I'm more coming at it from the opposite direction: that if any superluminal communication violates causality (which seems to be a common assertion, both in this thread and elsewhere), then I'd want to see that proven, rather than assume it does so. At most, what I can gather is that it's mathematically possible to construct a warp bubble capable of backwards time travel, just like how it's mathematically possible to construct a wormhole that can do so; when you're mangling spacetime, that obviously opens the door for all sorts of wacky shenanigans. Possibility != certainty, though, and various online discussions (including this one) feature folks who seem to assert that all FTL travel/communication must violate causality, which leaves me wondering why - especially when, to your point, plugging the relevant numbers into the Lorentz transformation doesn't transform anything (no local movement, so v = 0, so the Lorentz factor becomes 1, and vx/c^2 = 0, therefore no time dilation - is that math right?). There's also Hawking's chronology protection conjecture. Yeah, just a conjecture, so it can obviously be disproven at some point, but from what I understand string theory seems to support it (reading the relevant papers is on my TODO list). In any case, thanks again. I don't feel like I'm closer to a definitive answer, but at least I'm left with more to think about.