6 ms·

Does an Alcubierre warp drive violate causality? From my understanding, it's no different than regular FTL travel, which implies time travel, which implies viol

by imdoor 5y ago

Does an Alcubierre warp drive violate causality? From my understanding, it's no different than regular FTL travel, which implies time travel, which implies violation of causality. To me that would indicate that an Alcubierre warp drive still shouldn't be possible despite the negative energy requirement being lifted.

shepardrtc 5y ago
No, for the original Alcubierre, its warping spacetime around the information and moving that pocket. The ship or information or whatever is still traveling inside the pocket at its normal speed. So while it could be virtually instant (ignoring energy requirements), it wouldn't be time travel. However, I believe you could arrive to your destination and look back and see yourself before you warped - and maybe as you traveled - as the light catches up to you. At least that's my understanding.
- Filligree 5y ago
  No, every FTL construction can be used to violate causality. There’s no way at all around that. Fundamentally it’s a geometric problem — relativity has a hard barrier at light speed, but if you can get past that, there’s nothing at all special about infinite speed. Is the same sort of 4-momentum as standing still, and you can accelerate in any direction from there. The warp drive in the article is subliminal, though, so it doesn’t have that problem.
  - imdoor 5y ago
    Ah, i somehow read the subluminal as superluminal in the abstract and got all excited. But the subluminal restriction makes the paper's findings make more sense.
  - ben_w 5y ago
    That’s something I’ve been meaning to ask about: Does FTL really inevitably mean that, or is that a consequence of saying “no preferred frame of reference” and therefore having e.g. “100c” meaning different things to observers in relative motion?
    - QuadmasterXLII 5y ago
      You are correct, it requires “no preferred frame of reference”.
    - crooked-v 5y ago
      "No preferred frame of reference" is how the universe fundamentally works - if you remove that, you're looking at a universe that functions differently than our own in a lot of basic ways.
      - ben_w 5y ago
        Follow up question: while no preferred frame is obviously the simplest model, is it really impossible to add a preferred frame? e.g. https://youtu.be/6MfJ59lkABY?t=413 https://youtu.be/6MfJ59lkABY?t=413
  - bloopernova 5y ago
    I've always had serious trouble understanding why FTL travel will break causality. Do you know of any good layman-level explanations? Like, if I FTL from point a to point b, 10 light years apart, in my super duper warp vessel. It takes me, for the sake of argument, 10 minutes to make that journey. Now say I set off a big comms laser at point a, sending a message to point b, before I left. I don't see that laser until 10 years later. What am I missing? I know I'm missing something, but that seems straightforward to me. It's weird to butt up against that seemingly incomprehensible.
    - xucheng 5y ago
      https://youtu.be/HUMGc8hEkpc https://youtu.be/HUMGc8hEkpc
      - spoiler 5y ago
        Oh wow, this was fascinating! I was vaguely made aware of some of these concepts by a friend who studies quantum physics, but on he's never been able to explain it so "simply" (that's not doing the explanation any justice, it's not simple and some of it still went over my head, but "it clicked"). I'm eager to check out more of their channel!
    - weavejester 5y ago
      Suppose you set off in a spaceship at 80% of lightspeed, or 0.8c, travelling away from Earth. At this speed, according to relativity, time is slowed to 60% of it's 'usual' value. So for every 10 hours that pass on Earth, only 6 will appear to pass on the spaceship. However, this is only true from the perspective of someone on Earth. From the point of view of someone on the spaceship, the opposite is true. From their perspective, the spaceship is stationary, and Earth is travelling away from it at 0.8c. Therefore, for every 10 hours that pass on the spaceship, only 6 will appear to pass on Earth. Suppose there was a way of instantaneously communicating between the two. On Earth, 10 hours into the mission, mission control sends a message to the spaceship. Because of time dilation, the spaceship receives the message only 6 hours into the mission, from their perspective. The spaceship then sends a message back, and due to the same time dilation effect, the message arrives on Earth 3 hours and 36 minutes into the mission (60% of 6 hours). In other words, the reply from the spaceship will arrive 6 hours and 24 minutes before mission control sends the original message.
  - codethief 5y ago
    > No, every FTL construction can be used to violate causality. Not necessarily, compare https://en.wikipedia.org/wiki/Novikov_self-consistency_principle https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...
- deleted 5y ago
  [deleted]
- mabbo 5y ago
  > I believe you could arrive to your destination and look back and see yourself before you warped - and maybe as you traveled - as the light catches up to you That does sound like traveling faster than light, but I think there's a reasonable view that makes it not. Suppose you traveled at some very fast speed around a black hole. It bends space time in such a way that you might, at some point, be able to collect photons that could be reconstructed into an image of you earlier in time. In short, when space time is flexible and bending, there are paths in which you can see your previous self. An alcubierre drive is a means to bend space time.
vbezhenar 5y ago
Why do you think that violation of causality is not possible?
- imdoor 5y ago
  I don't know enough to think that violation of causality is not possible. It just just seems very unlikely. I can't imagine how the world would look like if it wasn't the case because of the paradoxes it would imply. You seem to imply that you think differently. Care to elaborate?
  - randallsquared 5y ago
    Not the GP, but both the many worlds interpretation and superdeterminism would avoid (the issues of) causality violation, would they not, even in the presence of time travel?
  - vbezhenar 5y ago
    I have no idea either, but I'm not aware of any fundamental reasons that violation of causality is forbidden. And imagining world with all those spooky quantum effects is already almost impossible, at least for me :)
- threesongs 5y ago
  Violation of causality is equivalent to travelling back in time. Lots of problems there, and no good model for such things.
pugworthy 5y ago
...regular FTL travel... It's kind of exciting that we see phrases like this now. I mean it wasn't that long ago that the idea of a 9 story first stage rocket coming back to land (and fly again) was fantasy.
pdonis 5y ago
> Does an Alcubierre warp drive violate causality? The original Alcubierre metric does not, but that's because it only describes a single "warp bubble" that travels in one direction, never starting or stopping and never turning around. As soon as you have more than one "warp bubble", or you let a single "warp bubble" turn around, then you will have closed timelike curves, which violate causality.
XorNot 5y ago
It did recently occur to me that it's not a hard requirement that FTL be impossible, just that causality not be violated. There may be solutions to FTL where naively they could violate causality, but in practice other effects ensure that no usage of them ever will.
- captn3m0 5y ago
  The Atomic Rockets website[0] describes the issue clearly: Relativity proves that FTL travel is identical to Time travel. Time travel makes Causality impossible, since it can be used to create paradoxes. Note that to a physicist, it is not enough that time travel never happens to be used to make a paradox. The mere fact it is possible is enough to utterly destroy Causality. 1. So if you have Relativity and FTL, Causality is impossible 2. If you do not have Relativity, then FTL is not Time travel, so you can have Causality. 3. Or more mundanely you can have Relativity and Causality, but no FTL/Time travel ∴ Causality, Relativity, FTL travel: chose any two. [0]: http://www.projectrho.com/public_html/rocket/fasterlight.php#causality http://www.projectrho.com/public_html/rocket/fasterlight.php...
  - XorNot 5y ago
    This is missing the point though: it's not "optional" per se - it's that naively, if we simply go "what if FTL" then yeah, based on current physics we have a problem. But the logic doesn't then mean "can't FTL" - it's still "can't have paradoxes". An FTL system, theoretically incapable of creating paradoxes, has no such problems. An FTL system which prevents you from going faster then light in directions which allow violating causality would be totally fine, provided this was a theoretical restriction: i.e. your FTL engine just plain can't thrust in causality violating vectors, because it encounters some temporal restriction field or similar. So the problem isn't "FTL is a causality violation", it's FTL without a mechanism to prevent causality violation is probably impossible. The sci-fi conceit of it being difficult to plot efficient paths through hyperspace or whatever might well be quite a real thing, and simply related to charting causality-allowed courses through FTL-space.
  - codethief 5y ago
    > The Atomic Rockets website[0] describes the issue clearly: I don't find their description very clear. In particular, they don't even define - what "faster-than-light" is supposed to mean in a non-relativistic setting (Newtonian mechanics) where there's no such thing as a universal velocity and you're dealing with Galilean velocity transformations. - what causality is supposed to mean exactly outside the relativistic setting where you have a universal velocity which induces the causal structure. All in all, their definitions are very inter-dependent, so the "trichotomy" they present as truth doesn't make much sense to begin with. From my POV, they're not even wrong[0]. Moreover, they seem to only consider Special Relativity and completely ignore General Relativity where the speed-of-light bound is only a local bound. Basically, even if what they say were true, it would still not apply to the General Relativistic setting we're looking at here. > Time travel makes Causality impossible, since it can be used to create paradoxes This is not so clear, either, compare e.g. https://en.wikipedia.org/wiki/Novikov_self-consistency_principle https://en.wikipedia.org/wiki/Novikov_self-consistency_princ... [0]: https://en.wikipedia.org/wiki/Not_even_wrong https://en.wikipedia.org/wiki/Not_even_wrong

by imdoor 5y ago

shepardrtc 5y ago

No, for the original Alcubierre, its warping spacetime around the information and moving that pocket. The ship or information or whatever is still traveling inside the pocket at its normal speed. So while it could be virtually instant (ignoring energy requirements), it wouldn't be time travel. However, I believe you could arrive to your destination and look back and see yourself before you warped - and maybe as you traveled - as the light catches up to you. At least that's my understanding.

Filligree 5y ago
No, every FTL construction can be used to violate causality. There’s no way at all around that. Fundamentally it’s a geometric problem — relativity has a hard barrier at light speed, but if you can get past that, there’s nothing at all special about infinite speed. Is the same sort of 4-momentum as standing still, and you can accelerate in any direction from there. The warp drive in the article is subliminal, though, so it doesn’t have that problem.
- imdoor 5y ago
  Ah, i somehow read the subluminal as superluminal in the abstract and got all excited. But the subluminal restriction makes the paper's findings make more sense.
- ben_w 5y ago
  That’s something I’ve been meaning to ask about: Does FTL really inevitably mean that, or is that a consequence of saying “no preferred frame of reference” and therefore having e.g. “100c” meaning different things to observers in relative motion?
  - QuadmasterXLII 5y ago
    You are correct, it requires “no preferred frame of reference”.
  - crooked-v 5y ago
    "No preferred frame of reference" is how the universe fundamentally works - if you remove that, you're looking at a universe that functions differently than our own in a lot of basic ways.
    - ben_w 5y ago
      Follow up question: while no preferred frame is obviously the simplest model, is it really impossible to add a preferred frame? e.g. https://youtu.be/6MfJ59lkABY?t=413 https://youtu.be/6MfJ59lkABY?t=413
- bloopernova 5y ago
  I've always had serious trouble understanding why FTL travel will break causality. Do you know of any good layman-level explanations? Like, if I FTL from point a to point b, 10 light years apart, in my super duper warp vessel. It takes me, for the sake of argument, 10 minutes to make that journey. Now say I set off a big comms laser at point a, sending a message to point b, before I left. I don't see that laser until 10 years later. What am I missing? I know I'm missing something, but that seems straightforward to me. It's weird to butt up against that seemingly incomprehensible.
  - xucheng 5y ago
    https://youtu.be/HUMGc8hEkpc https://youtu.be/HUMGc8hEkpc
    - spoiler 5y ago
      Oh wow, this was fascinating! I was vaguely made aware of some of these concepts by a friend who studies quantum physics, but on he's never been able to explain it so "simply" (that's not doing the explanation any justice, it's not simple and some of it still went over my head, but "it clicked"). I'm eager to check out more of their channel!
  - weavejester 5y ago
    Suppose you set off in a spaceship at 80% of lightspeed, or 0.8c, travelling away from Earth. At this speed, according to relativity, time is slowed to 60% of it's 'usual' value. So for every 10 hours that pass on Earth, only 6 will appear to pass on the spaceship. However, this is only true from the perspective of someone on Earth. From the point of view of someone on the spaceship, the opposite is true. From their perspective, the spaceship is stationary, and Earth is travelling away from it at 0.8c. Therefore, for every 10 hours that pass on the spaceship, only 6 will appear to pass on Earth. Suppose there was a way of instantaneously communicating between the two. On Earth, 10 hours into the mission, mission control sends a message to the spaceship. Because of time dilation, the spaceship receives the message only 6 hours into the mission, from their perspective. The spaceship then sends a message back, and due to the same time dilation effect, the message arrives on Earth 3 hours and 36 minutes into the mission (60% of 6 hours). In other words, the reply from the spaceship will arrive 6 hours and 24 minutes before mission control sends the original message.
- codethief 5y ago
  > No, every FTL construction can be used to violate causality. Not necessarily, compare https://en.wikipedia.org/wiki/Novikov_self-consistency_principle https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...
deleted 5y ago
[deleted]
mabbo 5y ago
> I believe you could arrive to your destination and look back and see yourself before you warped - and maybe as you traveled - as the light catches up to you That does sound like traveling faster than light, but I think there's a reasonable view that makes it not. Suppose you traveled at some very fast speed around a black hole. It bends space time in such a way that you might, at some point, be able to collect photons that could be reconstructed into an image of you earlier in time. In short, when space time is flexible and bending, there are paths in which you can see your previous self. An alcubierre drive is a means to bend space time.

vbezhenar 5y ago

Why do you think that violation of causality is not possible?

imdoor 5y ago
I don't know enough to think that violation of causality is not possible. It just just seems very unlikely. I can't imagine how the world would look like if it wasn't the case because of the paradoxes it would imply. You seem to imply that you think differently. Care to elaborate?
- randallsquared 5y ago
  Not the GP, but both the many worlds interpretation and superdeterminism would avoid (the issues of) causality violation, would they not, even in the presence of time travel?
- vbezhenar 5y ago
  I have no idea either, but I'm not aware of any fundamental reasons that violation of causality is forbidden. And imagining world with all those spooky quantum effects is already almost impossible, at least for me :)
threesongs 5y ago
Violation of causality is equivalent to travelling back in time. Lots of problems there, and no good model for such things.

pugworthy 5y ago

...regular FTL travel... It's kind of exciting that we see phrases like this now. I mean it wasn't that long ago that the idea of a 9 story first stage rocket coming back to land (and fly again) was fantasy.

pdonis 5y ago

> Does an Alcubierre warp drive violate causality? The original Alcubierre metric does not, but that's because it only describes a single "warp bubble" that travels in one direction, never starting or stopping and never turning around. As soon as you have more than one "warp bubble", or you let a single "warp bubble" turn around, then you will have closed timelike curves, which violate causality.

XorNot 5y ago

It did recently occur to me that it's not a hard requirement that FTL be impossible, just that causality not be violated. There may be solutions to FTL where naively they could violate causality, but in practice other effects ensure that no usage of them ever will.

captn3m0 5y ago
The Atomic Rockets website[0] describes the issue clearly: Relativity proves that FTL travel is identical to Time travel. Time travel makes Causality impossible, since it can be used to create paradoxes. Note that to a physicist, it is not enough that time travel never happens to be used to make a paradox. The mere fact it is possible is enough to utterly destroy Causality. 1. So if you have Relativity and FTL, Causality is impossible 2. If you do not have Relativity, then FTL is not Time travel, so you can have Causality. 3. Or more mundanely you can have Relativity and Causality, but no FTL/Time travel ∴ Causality, Relativity, FTL travel: chose any two. [0]: http://www.projectrho.com/public_html/rocket/fasterlight.php#causality http://www.projectrho.com/public_html/rocket/fasterlight.php...
- XorNot 5y ago
  This is missing the point though: it's not "optional" per se - it's that naively, if we simply go "what if FTL" then yeah, based on current physics we have a problem. But the logic doesn't then mean "can't FTL" - it's still "can't have paradoxes". An FTL system, theoretically incapable of creating paradoxes, has no such problems. An FTL system which prevents you from going faster then light in directions which allow violating causality would be totally fine, provided this was a theoretical restriction: i.e. your FTL engine just plain can't thrust in causality violating vectors, because it encounters some temporal restriction field or similar. So the problem isn't "FTL is a causality violation", it's FTL without a mechanism to prevent causality violation is probably impossible. The sci-fi conceit of it being difficult to plot efficient paths through hyperspace or whatever might well be quite a real thing, and simply related to charting causality-allowed courses through FTL-space.
- codethief 5y ago
  > The Atomic Rockets website[0] describes the issue clearly: I don't find their description very clear. In particular, they don't even define - what "faster-than-light" is supposed to mean in a non-relativistic setting (Newtonian mechanics) where there's no such thing as a universal velocity and you're dealing with Galilean velocity transformations. - what causality is supposed to mean exactly outside the relativistic setting where you have a universal velocity which induces the causal structure. All in all, their definitions are very inter-dependent, so the "trichotomy" they present as truth doesn't make much sense to begin with. From my POV, they're not even wrong[0]. Moreover, they seem to only consider Special Relativity and completely ignore General Relativity where the speed-of-light bound is only a local bound. Basically, even if what they say were true, it would still not apply to the General Relativistic setting we're looking at here. > Time travel makes Causality impossible, since it can be used to create paradoxes This is not so clear, either, compare e.g. https://en.wikipedia.org/wiki/Novikov_self-consistency_principle https://en.wikipedia.org/wiki/Novikov_self-consistency_princ... [0]: https://en.wikipedia.org/wiki/Not_even_wrong https://en.wikipedia.org/wiki/Not_even_wrong