8 ms·

Breaking the warp barrier: solitons in Einstein–Maxwell-plasma theory (2021)

60 points by danboarder 4y ago

eesmith 4y ago
From 2021? Stick it into Google Scholar. 26 citations. Abstract from https://arxiv.org/pdf/2105.03079.pdf https://arxiv.org/pdf/2105.03079.pdf from Feb. 2022: > Three recent articles have claimed that it is possible to, at least in theory, either set up positive energy warp drives satisfying the weak energy condition (WEC), or at the very least, to minimize the WEC violations. These claims are at best incomplete, since the arguments as presented only assert but do not prove the existence of one set of timelike observers, the co-moving Eulerian observers, who see relatively “nice” physics. While these particular observers might arguably see a positive energy density, the WEC requires all timelike observers to see positive energy density. Therefore, one should carefully revisit this issue. A more careful analysis shows that the situation is actually much grimmer than advertised — within the framework adopted by those three papers all physically reasonable warp drives will certainly violate the WEC, and both the strong and dominant energy conditions. Under plausible subsidiary conditions the null energy condition is also violated. While warp drives are certainly interesting examples of speculative physics, the violation of the energy conditions, at least within the framework of standard general relativity, is unavoidable. Even in modified gravity, physically reasonable warp drives will still violate the purely geometrical null convergence condition and the timelike convergence condition which, in turn, will place very strong constraints on any modified-gravity warp drive. From https://arxiv.org/pdf/2205.15950.pdf https://arxiv.org/pdf/2205.15950.pdf from May 2022: > Specifically, this last year has seen multiple misguided claims [22–24, 28–33] as to the asserted possible avoidance/amelioration of the known energy condition violations in warp drive spacetimes; violations that were first firmly established over 20 years ago. This paper is in those "multiple misguided claims".
- danboarder 4y ago
  Thanks, I updated the title to reflect publication in (2021).
api 4y ago
A physics professor told me once that pretty much all of current cosmology rules out FTL, but then he said he doesn’t 100% dismiss it due to how weird things in QM keep turning out to be. He said that if FTL were to turn out to be possible it would mean the universe is yet another order of magnitude stranger than we thought due to all the insanely weird implications it would have. One speculation he threw out was that due to causality issues if you flew FTL you might actually be going to parallel universes without realizing it until you noticed hundreds of years later that your light speed radio emissions never made it home. They wouldn’t arrive because you sent them from another universe. Would also solve the Fermi paradox via a combinatorial explosion of universe X FTL trajectory combinations. Your warp ships never meet anyone else because that would violate causality. One warp ship arrival per universe only. Edit: thought of an even weirder Fermi paradox answer based on the above. Since intelligence eventually figures out FTL, only one intelligence exists per slice of the multiverse and they will never meet along any FTL hyper-trajectory. Causality protection conjecture gone wild. Also wouldn’t it suck if your multiverse trip was impossible in reverse? Go to the Centauri system, shoot some cool selfies, and come back to a barren lifeless Venus-like Earth. Every warp drops you in a universe where you are the only sentient life.
- midasuni 4y ago
  Imagine a wormhole which dumped you from New York to LA instantly. How would that provide time travel? Ok you beat your message by a few milliseconds, HFTs love it, but that’s now different to the ping routing via Sydney. how do you get the message back to the location it left before it left? If an FTl spaceship went to a different universe then presumably a ship in orbit of Earth aiming at mars would simply vanish and never appear in mars orbit, telescopes on earth wouldn’t see it 8, 16, 30 minutes or even years later
  - jleahy 4y ago
    What if the other end of the wormhole is travelling close to the speed of light? What if one end of the wormhole is accelerating?
  - SkittyDog 4y ago
rob_c 4y ago
Yey, except the inevitable paywall from a journal... I'll let someone download the pdf, strip it of tracking and metadata and leak it.
- eesmith 4y ago
  Preprint at https://arxiv.org/abs/2006.07125 https://arxiv.org/abs/2006.07125 .
at_a_remove 4y ago
FTL always implies closed timelike curves, and that means paradoxes. Until whatever mechanism people come up to travel FTL with has a reasonable way of preventing paradoxes right there also in the paper, it isn't even worth looking at. We would like to do FTL, but the universe isn't obliged to humor us.
- ben_w 4y ago
  One thing I’ve been wondering about that, but I can’t reliably keep track of the frames and their transformations and I know I need to do that to get the answer myself: The Alcubierre metric was designed to not have any time dilation inside the bubble. Does that change anything about the paradoxes?
  - at_a_remove 4y ago
    No. There's a good physics stackexchange on it.
    - remarkEon 4y ago
      Link? Or can you give us the tldr?
    - ben_w 4y ago
      My experience of physics stackexchange is getting three mutually exclusive answers to one question, and another question of mine getting a dismissive non-answer reply that outranked the actual answer. I don't think it's meant for people like me.
- sterlind 4y ago
  I thought the "closed" part of closed timelike curves takes care of that. Like in a Feynman diagram where a photon splits and recombines into an electron/positron pair, isn't that equivalent to an electron looping through time and annihilating itself? You could argue it's just a formalism, but mathematically it still seems like (trivial) time travel. Why can't you extend that from a single particle to a system of particles? Where's the math break down?
yarvos 4y ago
I am honestly shocked at the number of comments strongly asserting the impossibility of FTL based on simultaneity arguments from special relativity. Yes, faster than light travel through space is prohibited in both special and general relativity by simultaneity. But not so for sub light speed travel (or even being stationary) on a space which itself has the property where distances between the passenger and destination are decreasing faster than the speed of light, which is precisely the class of metric solutions that the paper is investigating. The fact that certain galaxies are receding from us faster than the speed of light due to the expansion of the universe (again just changes in the metric) provides a clear example of this kind of 'FTL' travel that is permitted in general relativity.
- deleted 4y ago
  [deleted]
- 323 4y ago
  Not to mention that everybody seems certain that the current theory, Einstein GR, is the final theory and that no new radical theory is possible that could upend everything in terms of possibilities.
  - analog31 4y ago
    I've found that that belief in future violations of present-day theory is actually quite commonplace. Judging from conversations with colleagues at the lunch table, not only are violations of theory considered to be possible, but specific violations such as time travel, and warp drive, are considered to be as good as certain. Every HN thread about FTL travel includes multiple comments warning us not to treat present-day knowledge as final.
    - lumost 4y ago
      Time travel, warp drives, worm holes, and even infinite energy machines are permissible under GR. These aren’t theory violations, but either lack a physically plausible mechanism, are I observable, or require energies outside of experimental capabilities.
galaxyLogic 4y ago
If it was possible for me to travel back in time say just 1 hour, where would I end up in space? Earth is moving through the solar system and solar system through the Milky Way and Milky Way through the local cluster. So if I went back in time 1 hour I assume I would end up in empty space because one hour ago the location where I am now did not have Earth in it. Or I might end up inside Earth. Or some other planet or star. How do you time-travelers control where you end up in space?

60 points by danboarder 4y ago

From 2021? Stick it into Google Scholar. 26 citations. Abstract from https://arxiv.org/pdf/2105.03079.pdf https://arxiv.org/pdf/2105.03079.pdf from Feb. 2022: > Three recent articles have claimed that it is possible to, at least in theory, either set up positive energy warp drives satisfying the weak energy condition (WEC), or at the very least, to minimize the WEC violations. These claims are at best incomplete, since the arguments as presented only assert but do not prove the existence of one set of timelike observers, the co-moving Eulerian observers, who see relatively “nice” physics. While these particular observers might arguably see a positive energy density, the WEC requires all timelike observers to see positive energy density. Therefore, one should carefully revisit this issue. A more careful analysis shows that the situation is actually much grimmer than advertised — within the framework adopted by those three papers all physically reasonable warp drives will certainly violate the WEC, and both the strong and dominant energy conditions. Under plausible subsidiary conditions the null energy condition is also violated. While warp drives are certainly interesting examples of speculative physics, the violation of the energy conditions, at least within the framework of standard general relativity, is unavoidable. Even in modified gravity, physically reasonable warp drives will still violate the purely geometrical null convergence condition and the timelike convergence condition which, in turn, will place very strong constraints on any modified-gravity warp drive. From https://arxiv.org/pdf/2205.15950.pdf https://arxiv.org/pdf/2205.15950.pdf from May 2022: > Specifically, this last year has seen multiple misguided claims [22–24, 28–33] as to the asserted possible avoidance/amelioration of the known energy condition violations in warp drive spacetimes; violations that were first firmly established over 20 years ago. This paper is in those "multiple misguided claims".

danboarder 4y ago
Thanks, I updated the title to reflect publication in (2021).

api 4y ago

A physics professor told me once that pretty much all of current cosmology rules out FTL, but then he said he doesn’t 100% dismiss it due to how weird things in QM keep turning out to be. He said that if FTL were to turn out to be possible it would mean the universe is yet another order of magnitude stranger than we thought due to all the insanely weird implications it would have. One speculation he threw out was that due to causality issues if you flew FTL you might actually be going to parallel universes without realizing it until you noticed hundreds of years later that your light speed radio emissions never made it home. They wouldn’t arrive because you sent them from another universe. Would also solve the Fermi paradox via a combinatorial explosion of universe X FTL trajectory combinations. Your warp ships never meet anyone else because that would violate causality. One warp ship arrival per universe only. Edit: thought of an even weirder Fermi paradox answer based on the above. Since intelligence eventually figures out FTL, only one intelligence exists per slice of the multiverse and they will never meet along any FTL hyper-trajectory. Causality protection conjecture gone wild. Also wouldn’t it suck if your multiverse trip was impossible in reverse? Go to the Centauri system, shoot some cool selfies, and come back to a barren lifeless Venus-like Earth. Every warp drops you in a universe where you are the only sentient life.

midasuni 4y ago
Imagine a wormhole which dumped you from New York to LA instantly. How would that provide time travel? Ok you beat your message by a few milliseconds, HFTs love it, but that’s now different to the ping routing via Sydney. how do you get the message back to the location it left before it left? If an FTl spaceship went to a different universe then presumably a ship in orbit of Earth aiming at mars would simply vanish and never appear in mars orbit, telescopes on earth wouldn’t see it 8, 16, 30 minutes or even years later
- jleahy 4y ago
  What if the other end of the wormhole is travelling close to the speed of light? What if one end of the wormhole is accelerating?
- SkittyDog 4y ago

rob_c 4y ago

Yey, except the inevitable paywall from a journal... I'll let someone download the pdf, strip it of tracking and metadata and leak it.

eesmith 4y ago
Preprint at https://arxiv.org/abs/2006.07125 https://arxiv.org/abs/2006.07125 .

at_a_remove 4y ago

FTL always implies closed timelike curves, and that means paradoxes. Until whatever mechanism people come up to travel FTL with has a reasonable way of preventing paradoxes right there also in the paper, it isn't even worth looking at. We would like to do FTL, but the universe isn't obliged to humor us.

ben_w 4y ago
One thing I’ve been wondering about that, but I can’t reliably keep track of the frames and their transformations and I know I need to do that to get the answer myself: The Alcubierre metric was designed to not have any time dilation inside the bubble. Does that change anything about the paradoxes?
- at_a_remove 4y ago
  No. There's a good physics stackexchange on it.
  - remarkEon 4y ago
    Link? Or can you give us the tldr?
  - ben_w 4y ago
    My experience of physics stackexchange is getting three mutually exclusive answers to one question, and another question of mine getting a dismissive non-answer reply that outranked the actual answer. I don't think it's meant for people like me.
sterlind 4y ago
I thought the "closed" part of closed timelike curves takes care of that. Like in a Feynman diagram where a photon splits and recombines into an electron/positron pair, isn't that equivalent to an electron looping through time and annihilating itself? You could argue it's just a formalism, but mathematically it still seems like (trivial) time travel. Why can't you extend that from a single particle to a system of particles? Where's the math break down?

yarvos 4y ago

I am honestly shocked at the number of comments strongly asserting the impossibility of FTL based on simultaneity arguments from special relativity. Yes, faster than light travel through space is prohibited in both special and general relativity by simultaneity. But not so for sub light speed travel (or even being stationary) on a space which itself has the property where distances between the passenger and destination are decreasing faster than the speed of light, which is precisely the class of metric solutions that the paper is investigating. The fact that certain galaxies are receding from us faster than the speed of light due to the expansion of the universe (again just changes in the metric) provides a clear example of this kind of 'FTL' travel that is permitted in general relativity.

deleted 4y ago
[deleted]
323 4y ago
Not to mention that everybody seems certain that the current theory, Einstein GR, is the final theory and that no new radical theory is possible that could upend everything in terms of possibilities.
- analog31 4y ago
  I've found that that belief in future violations of present-day theory is actually quite commonplace. Judging from conversations with colleagues at the lunch table, not only are violations of theory considered to be possible, but specific violations such as time travel, and warp drive, are considered to be as good as certain. Every HN thread about FTL travel includes multiple comments warning us not to treat present-day knowledge as final.
  - lumost 4y ago
    Time travel, warp drives, worm holes, and even infinite energy machines are permissible under GR. These aren’t theory violations, but either lack a physically plausible mechanism, are I observable, or require energies outside of experimental capabilities.

galaxyLogic 4y ago

If it was possible for me to travel back in time say just 1 hour, where would I end up in space? Earth is moving through the solar system and solar system through the Milky Way and Milky Way through the local cluster. So if I went back in time 1 hour I assume I would end up in empty space because one hour ago the location where I am now did not have Earth in it. Or I might end up inside Earth. Or some other planet or star. How do you time-travelers control where you end up in space?