10 ms·

No, for the original Alcubierre, its warping spacetime around the information and moving that pocket. The ship or information or whatever is still traveling ins

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.

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.