9 ms·

What do the authors of these warp drive papers think will happen causality-wise when the proposed vehicle effectively moves faster than c? You can't do that wit

by gary_0 5y ago

What do the authors of these warp drive papers think will happen causality-wise when the proposed vehicle effectively moves faster than c? You can't do that without doing nasty things to your light cone[0] and effectively traveling into your own past. Is a prerequisite of entertaining the idea of a warp drive that we disregard causality? As far as I know, there aren't any plausible loopholes in General Relativity that allow you to pop up anywhere and anywhen you want in spacetime. [0] https://en.wikipedia.org/wiki/Light_cone https://en.wikipedia.org/wiki/Light_cone

XorNot 5y ago
Well for one thing, a speed-of-light limited warp drive would still be effectively a reaction-less drive we could use for travel in the solar system. But a decent way to beat Einstein would be find and implement an actual FTL engine: after all, if you can do it then obviously the theory isn't correct (alternately: the universe possible does not care about causality as much as we do in our little bubble of perception).
- gary_0 5y ago
  > would still be effectively a reaction-less drive True, but if the energy requirements for warp drives continue to be measured in Jupiters, reaction-mass is the least of your worries.
  - iamnotwhoiam 5y ago
    I think with that much energy the light pressure alone would get you wherever you wanted to go.
- tsimionescu 5y ago
  The whole point of warp drive FTL, and the reason it's the only kind of FTL idea taken seriously, is that it wouldn't violate GR. We already have widely accepted physical theories that contain exactly this kind of FTL 'travel': the Big Bang theory posits that space in the early universe expanded faster than the speed of light.
ctoth 5y ago
I believe the answer to this is [0], and the answer to that is [1]. I'm rather over my head here, and would love some analysis from people who know what the heck they're talking about. [0]: https://arxiv.org/abs/2105.03079 https://arxiv.org/abs/2105.03079 [1]: https://eriklentzphd.blogspot.com/2021/05/response-to-new-paper-generic-warp.html https://eriklentzphd.blogspot.com/2021/05/response-to-new-pa...
sigmoid10 5y ago
General relativity actually tells us nothing about whether causality is absolute. You can easily make up spacetime configurations that violate causality and still conform to general relativity. It's just that most people believe that causality should be protected, because... well it would be really weird if it wasn't. But it's just a conjecture and not like a theorem or anything.
- deleted 5y ago
  [deleted]
- wyager 5y ago
  It doesn't guarantee that causality is protected, but it certainly seems pretty suggestive that GR creates a big infeasibility region for positive-mass objects (outside of the future-past light cone). There are ways you could still violate causality despite this restriction, but it makes it a lot harder. There's no particular reason we couldn't have a ++++ spacetime, which wouldn't have this form of causal censorship, so I count this as one more piece of evidence towards "maybe causality violation is impossible".
- nine_k 5y ago
  AFAICT, GR is symmetric in relation of the direction of time, as are most other important physical theories. What may add the arrow of time is the second law of thermodynamics: a statistical evolution of a system is not symmetric. I wonder now how a violation of causality could either circumvent of contradict the second law.
  - jchrisa 5y ago
    This is the content I visit the orange site for. Thanks. To add something substantive, my question to the thread is: anyone here have thoughts on how these questions relate to McTaggart's unreality of time? https://en.wikipedia.org/wiki/A_series_and_B_series https://en.wikipedia.org/wiki/A_series_and_B_series
  - dleslie 5y ago
    It's not creating new energy, right? It reads to me like causality violating configurations simply move energy around time as we'd move mass through space.
TheDudeMan 5y ago
"effectively traveling into your own past" Can you explain how traveling faster than light leads to that? Perhaps you meant "observing your own past?"
- incompatible 5y ago
  I'd guess there would be frames of reference (with relative high-speed motion) in which you'd appear at the destination before you've left the origin. The laws of physics are supposed to hold regardless of which frame of reference you use.
  - javajosh 5y ago
    SR is 2 assertions, really: first, that the laws of physics are invariant in all reference frames, and second (and far stranger), that the speed of light is invariant in all reference frames. It's that second rule that must be violated for FTL to work - FTL implies a reference frame in which light can, at the very least, switch directions! That is, light from behind you looks like it's coming from the front (but still going c).
    - a1369209993 5y ago
      > SR is 2 assertions, really: first, that the laws of physics are invariant in all reference frames, and second (and far stranger), that the speed of light is invariant in all reference frames. Technically that's only one assertion (that is, the second is a strict subset of the first).
      - javajosh 5y ago
        No, it isn't. Light is unique in this way.
        AnimalMuppet 5y ago
        Correct. In a bit more detail: Here's the grid coordinate system for the streets in my town. One intersection is (0, 0). But in the grid system used by the next town over, that same intersection is (100, 200). And that's fine - we're just shifting the origin, which changes all the numbers. The same thing happens with velocity. If I'm on the ground, "stationary" means that you're not moving with respect to the ground. If you're in an airplane, "stationary" means that you're sitting in your seat, and you see me on the ground moving toward the back of the plane at 550 MPH. And that's still fine - we just shifted the origin of velocity. Light is unique in that, when you shift the origin of velocity, it's speed is still the same number. No other velocity transforms that way. (Well... light isn't quite unique. The speed of light is unique in that way. But gravitational waves also travel at the speed of light.)
mikewarot 5y ago
We're all traveling through spacetime at the speed of light, most of our velocity is in the time dimension, the faster you translate in the x,y,z dimensions, the less is left over for the time dimension, and thus time seems to slow down. (That's how this non-physicist things about it, anyway) In a warp field, you're just somehow shrinking X,Y,Z while not doing the same to time. I see no reason to think you'd travel backwards in time.
- ehvatum 5y ago
  Your thinking assumes a preferred reference frame. https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity
  - 3np 5y ago
    Why could it not somehow hold in all reference frames?
    - ben_w 5y ago
      In relativity, the speed of light is the only constant speed; therefore, in which reference frame are you going at e.g. 10c? Perhaps the CMB is the preferred frame of reference for all motion and this problem has a trivial solution (I am not a physicist), but everyone I’ve seen explain relativity starts with “there is no preferred frame”, and the rest follows.
perihelions 5y ago
"What do the authors of these warp drive papers think will happen causality-wise" According to §3.3, they aren't sure. If I parsed it correctly, they haven't mathematically resolved whether closed timelike curves are necessary, or precluded, or neither.
- gary_0 5y ago
  The last part of that section suggests (with a citation) that "horizon formations [may] prevent the configurations from transporting an inertial observer from the subluminal regime to the superluminal regime." So it's a distinct possibility that warp drives can't actually go FTL.
stjohnswarts 5y ago
These drives don't violate causation, they warp space. Similar to the way expansion of the early universe could "move faster than the speed of light"
- api 5y ago
  What happens if you're between the drive and the destination? Do you experience spatial contraction too? If not then the spacecraft is traveling faster than 'c' with regard to everything not in its reference frame (right?). If this kind of thing is possible then the universe is considerably weirder than we thought even after a hundred years or so of QM.
deleted 5y ago
[deleted]
tarsinge 5y ago
I never understood the problem regarding causality and the light cone. Why a latency between photons and the FTL event break causality? Isn’t just a visual delay and the events aren’t still kind of “stacked”? Even if we alter photons we just break the visual ordering, the actions still happened in order.
- Arwill 5y ago
  The quantum world constantly needs to agree on what the state of the macroscopic world is, there is a limit to the speed of that agreement happening, hence the speed limit on the spread of information (speed of light). There are contradictions happening constantly in the quantum world, but they are resolved either way. Any contradiction would be resolved on quantum level even if it can be interpreted at macroscopic scale as breaking of causality. The important thing for physics would be to better clarify what time is. Why is the speed of light as much as it is, why is there time dilation related to mass and why is the time slowing down for objects travelling faster. My guess is that the quantum world needs more of its own time to agree when there are more objects (more mass) close to one another, or that a fast travelling object needs to "agree" with its surrounding more frequently, and that overhead is manifesting as slowing of its time.
jjgreen 5y ago
Only (well-written) f77 is faster than c
bronzeage 5y ago
As far as I understand, the point of warp drive is that you shorten the space time distance you travel by warping spacetime. At no point do you travel in a specifical location faster than light. You simply cover less distance by warping space. There's no causality violation. It's similar to the fact some parts of the universe travel away from us faster than light. They don't violate causality because it's space that is expanding.
jjk166 5y ago
Why would causality need to be one way? It's not unusual for (anticipation of) future events to affect the present. For example if I think it will rain, I grab an umbrella. While typically my belief that it will rain is based on something in the past (previously weather patterns for example), does it really change anything if a time traveler claiming to be me from the day after tomorrow told me it's going to rain tomorrow? We only think of causality as going from past to future because that is the way we live. We also find the idea that particles can have an indeterminate location or pop randomly into or out of existence is weird, because our minds which have evolved to experience and understand the macroscopic world have no method of really processing that, but our intuition is not reality. If we can accept that our concept of position of objects in space is merely an approximation of a more complicated reality, the same for position of events in time should not be much harder. People get bent up about paradoxes, but we have things like the Novikov self consistency principle [0] which allow us to avoid paradoxes even without one way causality. [0] https://en.wikipedia.org/wiki/Novikov_self-consistency_principle https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...

by gary_0 5y ago

XorNot 5y ago

Well for one thing, a speed-of-light limited warp drive would still be effectively a reaction-less drive we could use for travel in the solar system. But a decent way to beat Einstein would be find and implement an actual FTL engine: after all, if you can do it then obviously the theory isn't correct (alternately: the universe possible does not care about causality as much as we do in our little bubble of perception).

gary_0 5y ago
> would still be effectively a reaction-less drive True, but if the energy requirements for warp drives continue to be measured in Jupiters, reaction-mass is the least of your worries.
- iamnotwhoiam 5y ago
  I think with that much energy the light pressure alone would get you wherever you wanted to go.
tsimionescu 5y ago
The whole point of warp drive FTL, and the reason it's the only kind of FTL idea taken seriously, is that it wouldn't violate GR. We already have widely accepted physical theories that contain exactly this kind of FTL 'travel': the Big Bang theory posits that space in the early universe expanded faster than the speed of light.

ctoth 5y ago

I believe the answer to this is [0], and the answer to that is [1]. I'm rather over my head here, and would love some analysis from people who know what the heck they're talking about. [0]: https://arxiv.org/abs/2105.03079 https://arxiv.org/abs/2105.03079 [1]: https://eriklentzphd.blogspot.com/2021/05/response-to-new-paper-generic-warp.html https://eriklentzphd.blogspot.com/2021/05/response-to-new-pa...

sigmoid10 5y ago

General relativity actually tells us nothing about whether causality is absolute. You can easily make up spacetime configurations that violate causality and still conform to general relativity. It's just that most people believe that causality should be protected, because... well it would be really weird if it wasn't. But it's just a conjecture and not like a theorem or anything.

deleted 5y ago
[deleted]
wyager 5y ago
It doesn't guarantee that causality is protected, but it certainly seems pretty suggestive that GR creates a big infeasibility region for positive-mass objects (outside of the future-past light cone). There are ways you could still violate causality despite this restriction, but it makes it a lot harder. There's no particular reason we couldn't have a ++++ spacetime, which wouldn't have this form of causal censorship, so I count this as one more piece of evidence towards "maybe causality violation is impossible".
nine_k 5y ago
AFAICT, GR is symmetric in relation of the direction of time, as are most other important physical theories. What may add the arrow of time is the second law of thermodynamics: a statistical evolution of a system is not symmetric. I wonder now how a violation of causality could either circumvent of contradict the second law.
- jchrisa 5y ago
  This is the content I visit the orange site for. Thanks. To add something substantive, my question to the thread is: anyone here have thoughts on how these questions relate to McTaggart's unreality of time? https://en.wikipedia.org/wiki/A_series_and_B_series https://en.wikipedia.org/wiki/A_series_and_B_series
- dleslie 5y ago
  It's not creating new energy, right? It reads to me like causality violating configurations simply move energy around time as we'd move mass through space.

TheDudeMan 5y ago

"effectively traveling into your own past" Can you explain how traveling faster than light leads to that? Perhaps you meant "observing your own past?"

incompatible 5y ago
I'd guess there would be frames of reference (with relative high-speed motion) in which you'd appear at the destination before you've left the origin. The laws of physics are supposed to hold regardless of which frame of reference you use.
- javajosh 5y ago
  SR is 2 assertions, really: first, that the laws of physics are invariant in all reference frames, and second (and far stranger), that the speed of light is invariant in all reference frames. It's that second rule that must be violated for FTL to work - FTL implies a reference frame in which light can, at the very least, switch directions! That is, light from behind you looks like it's coming from the front (but still going c).
  - a1369209993 5y ago
    > SR is 2 assertions, really: first, that the laws of physics are invariant in all reference frames, and second (and far stranger), that the speed of light is invariant in all reference frames. Technically that's only one assertion (that is, the second is a strict subset of the first).
    - javajosh 5y ago
      No, it isn't. Light is unique in this way.
      - AnimalMuppet 5y ago
        Correct. In a bit more detail: Here's the grid coordinate system for the streets in my town. One intersection is (0, 0). But in the grid system used by the next town over, that same intersection is (100, 200). And that's fine - we're just shifting the origin, which changes all the numbers. The same thing happens with velocity. If I'm on the ground, "stationary" means that you're not moving with respect to the ground. If you're in an airplane, "stationary" means that you're sitting in your seat, and you see me on the ground moving toward the back of the plane at 550 MPH. And that's still fine - we just shifted the origin of velocity. Light is unique in that, when you shift the origin of velocity, it's speed is still the same number. No other velocity transforms that way. (Well... light isn't quite unique. The speed of light is unique in that way. But gravitational waves also travel at the speed of light.)

mikewarot 5y ago

We're all traveling through spacetime at the speed of light, most of our velocity is in the time dimension, the faster you translate in the x,y,z dimensions, the less is left over for the time dimension, and thus time seems to slow down. (That's how this non-physicist things about it, anyway) In a warp field, you're just somehow shrinking X,Y,Z while not doing the same to time. I see no reason to think you'd travel backwards in time.

ehvatum 5y ago
Your thinking assumes a preferred reference frame. https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity
- 3np 5y ago
  Why could it not somehow hold in all reference frames?
  - ben_w 5y ago
    In relativity, the speed of light is the only constant speed; therefore, in which reference frame are you going at e.g. 10c? Perhaps the CMB is the preferred frame of reference for all motion and this problem has a trivial solution (I am not a physicist), but everyone I’ve seen explain relativity starts with “there is no preferred frame”, and the rest follows.

perihelions 5y ago

"What do the authors of these warp drive papers think will happen causality-wise" According to §3.3, they aren't sure. If I parsed it correctly, they haven't mathematically resolved whether closed timelike curves are necessary, or precluded, or neither.

gary_0 5y ago
The last part of that section suggests (with a citation) that "horizon formations [may] prevent the configurations from transporting an inertial observer from the subluminal regime to the superluminal regime." So it's a distinct possibility that warp drives can't actually go FTL.

stjohnswarts 5y ago

These drives don't violate causation, they warp space. Similar to the way expansion of the early universe could "move faster than the speed of light"

api 5y ago
What happens if you're between the drive and the destination? Do you experience spatial contraction too? If not then the spacecraft is traveling faster than 'c' with regard to everything not in its reference frame (right?). If this kind of thing is possible then the universe is considerably weirder than we thought even after a hundred years or so of QM.

deleted 5y ago

[deleted]

tarsinge 5y ago

I never understood the problem regarding causality and the light cone. Why a latency between photons and the FTL event break causality? Isn’t just a visual delay and the events aren’t still kind of “stacked”? Even if we alter photons we just break the visual ordering, the actions still happened in order.

Arwill 5y ago
The quantum world constantly needs to agree on what the state of the macroscopic world is, there is a limit to the speed of that agreement happening, hence the speed limit on the spread of information (speed of light). There are contradictions happening constantly in the quantum world, but they are resolved either way. Any contradiction would be resolved on quantum level even if it can be interpreted at macroscopic scale as breaking of causality. The important thing for physics would be to better clarify what time is. Why is the speed of light as much as it is, why is there time dilation related to mass and why is the time slowing down for objects travelling faster. My guess is that the quantum world needs more of its own time to agree when there are more objects (more mass) close to one another, or that a fast travelling object needs to "agree" with its surrounding more frequently, and that overhead is manifesting as slowing of its time.

jjgreen 5y ago

Only (well-written) f77 is faster than c

bronzeage 5y ago

As far as I understand, the point of warp drive is that you shorten the space time distance you travel by warping spacetime. At no point do you travel in a specifical location faster than light. You simply cover less distance by warping space. There's no causality violation. It's similar to the fact some parts of the universe travel away from us faster than light. They don't violate causality because it's space that is expanding.

jjk166 5y ago

Why would causality need to be one way? It's not unusual for (anticipation of) future events to affect the present. For example if I think it will rain, I grab an umbrella. While typically my belief that it will rain is based on something in the past (previously weather patterns for example), does it really change anything if a time traveler claiming to be me from the day after tomorrow told me it's going to rain tomorrow? We only think of causality as going from past to future because that is the way we live. We also find the idea that particles can have an indeterminate location or pop randomly into or out of existence is weird, because our minds which have evolved to experience and understand the macroscopic world have no method of really processing that, but our intuition is not reality. If we can accept that our concept of position of objects in space is merely an approximation of a more complicated reality, the same for position of events in time should not be much harder. People get bent up about paradoxes, but we have things like the Novikov self consistency principle [0] which allow us to avoid paradoxes even without one way causality. [0] https://en.wikipedia.org/wiki/Novikov_self-consistency_principle https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...