9 ms·

Physical Warp Drives (2021)

68 points by JPLeRouzic 4y ago

JPLeRouzic 4y ago
"Conceptually, we demonstrate that any warp drive, including the Alcubierre drive, is a shell of regular or exotic material moving inertially with a certain velocity. Therefore, any warp drive requires propulsion. We show that a class of subluminal, spherically symmetric warp drive spacetimes, at least in principle, can be constructed based on the physical principles known to humanity today" https://arxiv.org/abs/2102.06824 https://arxiv.org/abs/2102.06824
- deleted 4y ago
  [deleted]
bilsbie 4y ago
Is this for real? What are we waiting for?
- sph 4y ago
  A way to create negative energy / mass. Until then, an Alcubierre drive remains just a theory
  - ben_w 4y ago
    "We present the first general model for subliminal positive-energy, spherically symmetric warp drives" There's plenty of other reasons it's still just a theory, but that isn't one here, if I read it right.
- rolenthedeep 4y ago
  Generally, the math works, and it looks like it should be possible. We've even got some ideas for how to physically build the thing. The main problem is that it requires exotic matter with negative mass or negative energy. I don't think we have any good evidence to suggest such a state of matter is possible in this universe.
  - bilsbie 4y ago
    I was under the impression this version doesn’t need exotic matter?
    - ghkbrew 4y ago
      This abstract says only that they've reduced the need for exotic matter by some orders of magnitude. Presumably it's still required. However, there were some recent papers describing warp drive geometries with only positive mass. Ex: https://arxiv.org/abs/2201.00652 https://arxiv.org/abs/2201.00652
      - ben_w 4y ago
        They also say: "We present the first general model for subliminal positive-energy, spherically symmetric warp drives" Not that I understand the physics well enough to do anything with this, but as you say, there is other work that's definitely (claiming) positive-only energy densities.
        rolenthedeep 4y ago
        My understanding is that physics allows you to create a bubble of spacetime without negative energy, but that bubble doesn't accelerate through space. The ship drags the bubble along with it at sublight speed using normal thrusters.
  - raattgift 4y ago
    > the math works, and it looks like it should be possible The math can only work (if it even does) in exceptionally constrained circumstances. The main purely-theoretical issue is that the paper's <https://arxiv.org/abs/2102.06824v2 https://arxiv.org/abs/2102.06824v2> solution is wholly spherically symmetric, and with vacuum everywhere but in the warping region. "Fig. 1: Asymptotically-flat vacuum background ... general stationary curved region with a spherical topology ... (the warping region) ... flat inner region ... ('passenger' space). ... As we discuss in Section 5, warp drive spacetimes require some form of propulsion in order to accelerate. For this reason, in physical realisations of such spacetimes, the front and rear parts are likely asymmetric". [emphasis mine] Do the maths fall apart if we deviate from total spherical symmetry, either by changing the front and rear parts, or by breaking the vaccuum condition on the inside and outside of the shell, especially if our matter is not precisely spherically distributed? They do a review of this, for limited deviations from spherical symmetry, in §4. However, the vaccum condition is what they study, not straying very far from their earlier "if [the warp region] were replaced by Minkowski spacetime, the whole spacetime would be Minkowski space." The paper declines to answer the question about the distribution of matter (i.e., not-vacuum): "all warp drive spacetimes are asymptotically-flat ... no metric which describes an accelerating warp drive solution has been presented in the literature ... metrics for ... accelerating objects and more general axisymmetric objects which preserve their shape and mass ... remains a subject for future studies". That last part suggests that the inside part might be kept appropriately symmetrical through dynamic reconfiguration of the things inside (i.e., the ship, cargo, fuel, passengers). But what about outside the warping region? In our solar system, and in our galaxy, we are not in Minkowski space. The outer part of the warp region, being asymptotically flat, might be amenable to a "thin shell" (Israel/Darmois junction formalism) with the warp bubble far enough away from everything outside that it feels no tidal forces, which for the purposes of the text above is "acceleration". What determines how far is far enough away? The authors don't attempt an answer. There are plenty of things (stars, most notably) in our part of the galaxy that will induce tides. The solar component of the lunisolar tide in Earth's oceans is substantial at 1 astronomical unit from our sun. That solar component doesn't go away when a space craft is (for example) 2 a.u. away from Earth&Moon on the opposite side of the sun. Can one zip this warp bubble safely past a 1 solar mass star at a distance of 1 au? Ten au? Can the warping region or passenger region be adjusted to allow for close approaches/operation within a star system? All questions for some possible future paper. Apart from stars, we inhabit a region of space which is dotted with things you can smack into (from the very solid like rocks and iron to the very wispy, stuff will still tend to slow you down when you hit it; deceleration and acceleration are the same issue). Near luminous enough stars, radiation pressure might also induce non-negligible accelerations. Is any of that relevant? If relevant, is it "fixable"? How? Alter the shape of the warping region, or the passenger region, or both? Questions for some possible future paper. So, even if one accepts for the sake of argument that the maths are flawless and further one just assumes the existence of controllable negative energy densities, this paper's maths are designed for a spacetime which overall isn't very much like what we have within a few hundred lightyears of here. Maybe some future paper will attempt to show that a warp bubble can work in more realistic spacetimes than almost-everywhere-empty-of-matter-and-feeling-no-gravitation setting of this paper. ("Almost-everywhere" means that there is matter and curvature in the warping region, and maybe the passenger region, but nowhere else, not even a destination star a dozen light-years away).
- naasking 4y ago
  Two different groups tried this move around the same time, one proved that subliminal drives using only positive mass were possible, the other proved the contrary. Such is the difficulty of GR field equations that it's not yet clear who's correct.
NegativeLatency 4y ago
I hope that some day there’s a paper like this to look back on like we do with Lovelace and Babbages stuff (foundational for something that seemed like magic at the time) Would be cool if it happened during my lifetime though.
23skidoo 4y ago
> and allows for new metrics without the most serious issues present in the Alcubierre solution. ...like violating causality?
- geuis 4y ago
  Subliminal, which is mentioned multiple times.
  - texaslonghorn5 4y ago
    ^ this, and I'm guessing it was autocorrected (because that's what almost happened to me) but the term is subluminal
    - deleted 4y ago
      [deleted]
- usrbinbash 4y ago
  In case someone wants to read a really good explanation about this; http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti... Generally speaking, all three of FTL (travel/communication), causality and relativity cannot all be true at the same time. If you can FTL-travel/communicate (they are the same really), then you can come up with a scenario in which an observer can see an event happening, and then see the cause of that event after it happened. Extending that logic, if the observer can also move at superluminal speeds, he could prevent the cause of the event after seeing the event happen, leading to a paradox.
  - codethief 4y ago
    > Extending that logic, if the observer can also move at superluminal speeds, he could prevent the cause of the event after seeing the event happen, leading to a paradox This is a guess, i.e. one possible outcome physicists are considering. People have proposed alternative outcomes of FTL like the (in my opinion much more sensible) Novikov consistency principle, which roughly proposes that spacetime and the entities it contains (e.g. an observer's wordline) should be looked upon as a whole, in the sense that they need to be self-consistent. Spacetime is not time-dependent and does not evolve, so it does not make much sense to say "something something leads to a [spacetime] paradox".
puffoflogic 4y ago
Apropos of nothing, in 1903 the NYT confidently declared airplanes would take ten million years to develop.
- cwillu 4y ago
  It's the difference between scaleable quantum computing, and hypercomputation: one appears to be a matter of engineering, which may turn out to be completely impractical/require a million years/whatever. The other is ruled out by our understanding of the universe. Pre-Wright, flight was the former, not the latter.
  - hnews_account_1 4y ago
    Our understanding of gravity isn’t complete. General relativity looks relatively unbreakable but the smartest minds agree non particle gravity just doesn’t make sense in an otherwise QFT universe so we’re missing something deeper. Whether that will provide a way to travel FTL in space time or outside it, we don’t know. But the science is not as settled as it seems even if mere mortals cannot think up a theory to beat the truly incredible power of GR.
    - puffoflogic 4y ago
      No no you don't understand. Science was wrong and incomplete in the past but now (as everyone learns in school) The Science has been completed. What we know is infallible and what we don't know is unknowable.
cletus 4y ago
The lesson here is an idea, no matter how implausible, can have legs as long as people want it to be true. That particular idea here is FTL travel (or communication). Nearly all of these ideas spring from a basic inability to understand the domain of a function. The domain is the set of values for which the function is defined. Any other values are undefined f(x) = 1/x for example has a domain of anything but 0. So what happens is that people plug negative values into quantities like mass without any basis for what negative mass is and then use that as a basis for [insert FTL system here]. Why stop there? Why not use imaginary masses? There is no negative mass. There is no negative energy. The domain of velocity is [0,c]. Even if you ignore that, no one goes far enough to calculate the energy requirements of, for example, "folding space" or creating a wormhole. If you do the math, it turns out you need to convert a significant portion of a stellar mass into energy. The future isn't Star Trek nor Star Wars no matter how much you want it to be.
- tsimionescu 4y ago
  The article here actually presents a warp drive that doesn't require negative mass/energy, but also doesn't move at speeds greater than c. It's interesting because it seems to be permitted by all of the laws of GR and also build-able from known materials, and it could represent a new mode of propulsion. It's also important to remember that space has expanded far faster than the speed of light in the past - c is only a limit for the speed that matter or energy can move at, it's not a limit for how space-time can change shape. We know this because the observable universe is ~93 billion light-years in size, but only ~13 billion years old - so it's size has grown faster than c. I'm with you on the idea that none of this means practical controlled faster-than-light travel will ever exist, though. Jury's still out on slower-than-light folding-space drives, though.
  - cletus 4y ago
    Doesn't this warp drive depend on negative energy density (whatever that means) or has the fundamental flaw from out-of-domain values shifted in this version to something else?
    - 4y ago
Loquebantur 4y ago
Notably, energy conservation is a local property of spacetime. Meaning, if you can manipulate the metric, you should be able to create (or vanish) energy. Creating the warp-bubble clearly takes energy dependent only on its volume, not the mass contained therein. Here, with a subluminal warp drive, you could hoist matter out of a gravity well and let it fall down afterwards.
- pdonis 4y ago
  > if you can manipulate the metric, you should be able to create (or vanish) energy. No, you can't. That would violate local energy conservation. The fact that energy conservation is a local property just means that there is no well-defined global energy in a general spacetime (though there is one in some special classes of spacetimes). It does not mean that stress-energy can be created or destroyed. > with a subluminal warp drive, you could hoist matter out of a gravity well and let it fall down afterwards. But the hoisting process would require you to expend at least as much energy as you would be able to collect from the falling down afterwards.
api 4y ago
> We show that a class of subluminal, spherically symmetric warp drive spacetimes, at least in principle, can be constructed based on the physical principles known to humanity today This is awesome. Ever since I read about the Alcubierre paper and it’s followups it seemed obvious to me that while there are huge basic physicality issues with FTL those same issues don’t exist for a near or at light speed warp drive. Such a drive would not necessarily reduce the energy requirements of travel near light speed, but I wonder if it would eliminate some of the other huge problems. Things like a single dust particle annihilating you or blue shift turning all incident radiation into a gamma ray laser aimed at the front of your ship. Would it have any impact on time dilation? If you approach the speed of light conventionally you can experience the trip subjectively as near instantaneous due to time dilation. Is that still true if you are in a warp bubble? Light speed gets you to the centauri system in about 4.5 years Earth time. With time dilation the main cruise phase of the flight would seem instantaneous on board the spacecraft. (It would be years on Earth of course.)
- euroderf 4y ago
  > Light speed gets you to the centauri system in about 4.5 years Earth time. With time dilation the main cruise phase of the flight would seem instantaneous on board the spacecraft. (It would be years on Earth of course.) Oh? But if there is no ether, and no absolute dead reckoning for motion in the vacuum, then for all we know, the Earth system is moving at 0.8c in one direction and the spacecraft would be moving at 0.2c in the opposite direction. Or something like that.
dsp_person 4y ago
Subluminal warp drive only needing positive energy seems like cool method to get around conservation of momentum (remember EmDrive?). I wonder if it could be used at very small scales with lower energies for some applications. Another interesting reactionless drive within possibility: > "Swimming in spacetime" is a general relativistic effect, where an extended body can change its position by using cyclic deformations in shape to exploit the curvature of space, such as due to a gravitational field. https://en.wikipedia.org/wiki/Reactionless_drive#Movement_without_thrust https://en.wikipedia.org/wiki/Reactionless_drive#Movement_wi...
- pdonis 4y ago
  > Subluminal warp drive only needing positive energy seems like cool method to get around conservation of momentum No, it isn't. You can't "get around" conservation of momentum. > Swimming in spacetime Still doesn't allow you to violate conservation of momentum. The article you link to notes this.
  - naasking 4y ago
    Yes, if I recall correctly you get displacement but without any momentum.
analog31 4y ago
Analogous to the Fermi Paradox, if warp drive / time travel are possible, why have I not been visited by someone trying to sell me one?
- api 4y ago
  If either is possible it nearly proves that we must be very early for intelligence in the universe. A universe with intelligent life and FTL turns into a party pretty quick as soon as someone invents it. If we even proved it was possible at table top scale I’d be about 99% convinced we are first in at a bare minimum our galaxy. I would also argue that we should scale it up and get a presence out there fast in case someone less nice than us is also inventing one right now. (And we aren’t all that nice.)
  - kloch 4y ago
    > we must be very early for intelligence in the universe. Not under the Zoo hypothesis: https://en.wikipedia.org/wiki/Zoo_hypothesis https://en.wikipedia.org/wiki/Zoo_hypothesis
    - yellowapple 4y ago
      So basically Earth is the Milky Way's version of the Sentinel Islands. Makes me wonder if UFO sightings are from misguided extraterrestrial missionaries trying to teach us about their Lord and Savior Behemecoatyl.
  - analog31 4y ago
    My thinking is that if time travel / warp drive is possible, then words like "early" and "first" have no meaning. After all, if humans invented it, I would 100% expect someone to be abusing me with it right now. People would steal from the past to pay for the future.
  - captainmuon 4y ago
    You can always play with the factors. Maybe FTL is possible, but requires a sizable fraction of the planet's resources. Or you can eventually reach another planet, but terraforming it and building the technology for creating more FTL ships will take millions of years. Basically: FTL is fast but pointless, because getting to FTL takes longer than getting to your destination. Maybe space is a bit more emptier than you think, and FTL is a bit slower. Or maybe it is busy, but we haven't waited long enough. I mean think of a native American in the 1400s. If transoceanic travel is possible, why haven't we seen visitors from other continents yet? Finally, it is still entirely possible that "they" are already there, and there is a cover up. I don't believe it (Occam's razor), but if FTL turns out to be theoretically possible, than I think I will have to update my beliefs :-)
cletus 4y ago
This is an abstract of an article behind a paywall so we can't see this details but it is from 2021 and I managed to find an article discussing the concept [1] > In this case negative energy is not required — rather it’s gravity itself which bends spacetime and gives rise to time dilation. In essence the gravitational field makes the passage of time within the passenger area much slower than the passage of time outside of it. A few minutes for them may be thousands of years worth of space travel, yet to leverage this powerful effect would take enormous amounts of gravity. The paper calls for the compression of an entire planet down to just a thin shell surrounding the passengers. The introduction of a physical warp drive model has changed the question from “Does something like negative energy even exist?” to “How do we compress the mass of a planet down to the size of a spaceship?” And I'll pull out the most apropos sentence: > The paper calls for the compression of an entire planet down to just a thin shell surrounding the passengers Yes, no negative energy or negative energy mass, just an entire planet compressed into a thin shell that somehow doesn't further collapse. So even if this were possible, how would you accelerate or maneuver such a massive object? I've seen so many of these proposals over the years. It's all nuclear-grade hopium with a massive flaw or huge, highly theoretical unknown. Always. [1]: https://medium.com/predict/the-first-physical-warp-drive-models-921055800764 https://medium.com/predict/the-first-physical-warp-drive-mod...
- kaashif 4y ago
  > I've seen so many of these proposals over the years. It's all nuclear-grade hopium with a massive flaw or huge, highly theoretical unknown. Always. True, the planet shell thing is nuclear grade hopium. But negative energy is big bang level hopium. This is a big downgrade.
- Steuard 4y ago
  It's an article about theoretical physics published in the 21st century, so it would be very surprising if it had not been posted on the arXiv before being published in a journal. And sure enough: https://arxiv.org/abs/2102.06824 https://arxiv.org/abs/2102.06824
deleted 4y ago
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hobz22 4y ago
Will. It. Blend?
captainmuon 4y ago
I always find these kind of papers interesting. It seems like they found a more general "warp drive" metric. But after skimming it, I don't understand how they want to create it in reality. The best shot I think we have at metric engineering - unless you want to build something crazy like moving spheres and rings of ultra dense matter - is to look into materials with interesting spin-gravity coupling. We know that rotating bodies have additional gravitational interactions. The effect is tiny, but measurable in experiments (e.g. Gravity Probe B). We also know that atomic spin is not just an abstract quantum number but actual angular momentum. So there should be some kind of spin-gravity interaction, even though it would be incredibly weak. But even though the absolute force is small, if we manage to make it time dependent we could amplify the effect. Just like a changing magetic field creates an electric field, a changing "gravitomagnetic" field would create a gravitational field. This is basically the insight that took us from compasses and static electricity to the whole of electrical engineering. I am not sure such materials could even exist. And even though I have a PhD in Physics, I have no idea how one would approach this theoretically. The treatment of particle spin seems to be very rare in GR, and then you would have to marry this to solid state physics and somehow calculate how the situation changes when the spin density changes and so on... The furthest I got was to open a thread on stackexchange about it :-) but who knows, maybe someday somebody looks into this stuff: https://physics.stackexchange.com/questions/46099/materials-with-different-gravitomagnetic-permeability https://physics.stackexchange.com/questions/46099/materials-...
- t8sr 4y ago
  I was with you until the analogy to electromagnetism and time dependency. I admittedly don't have a PhD in physics (undergrad astrophysics), so I possibly won't understand your answer, but isn't gravitomagnetism more of a useful fiction than anything people actually believe models reality? I mean the equations look similar, but there's no physical justification: gravity is not a field in the QFT sense. I guess I'm asking why you'd expect there to be an analog to the interaction between electrical and magnetic fields, when the nature of the thing is so different?
  - zachf 4y ago
AdamJacobMuller 4y ago
Alcubierre warp drive are a major plot two of my favorite hard sci-fi books, Flood[1] and Ark[2] by Stephen Baxter[2]. Really excellent books. 1 https://en.wikipedia.org/wiki/Flood_(Baxter_novel) https://en.wikipedia.org/wiki/Flood_(Baxter_novel) 2 https://en.wikipedia.org/wiki/Ark_(novel) https://en.wikipedia.org/wiki/Ark_(novel) 3 https://en.wikipedia.org/wiki/Stephen_Baxter_(author) https://en.wikipedia.org/wiki/Stephen_Baxter_(author)
- nynx 4y ago
  I love Baxter’s books but haven’t read those, thank you!

68 points by JPLeRouzic 4y ago

"Conceptually, we demonstrate that any warp drive, including the Alcubierre drive, is a shell of regular or exotic material moving inertially with a certain velocity. Therefore, any warp drive requires propulsion. We show that a class of subluminal, spherically symmetric warp drive spacetimes, at least in principle, can be constructed based on the physical principles known to humanity today" https://arxiv.org/abs/2102.06824 https://arxiv.org/abs/2102.06824

deleted 4y ago
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bilsbie 4y ago

Is this for real? What are we waiting for?

sph 4y ago
A way to create negative energy / mass. Until then, an Alcubierre drive remains just a theory
- ben_w 4y ago
  "We present the first general model for subliminal positive-energy, spherically symmetric warp drives" There's plenty of other reasons it's still just a theory, but that isn't one here, if I read it right.
rolenthedeep 4y ago
Generally, the math works, and it looks like it should be possible. We've even got some ideas for how to physically build the thing. The main problem is that it requires exotic matter with negative mass or negative energy. I don't think we have any good evidence to suggest such a state of matter is possible in this universe.
- bilsbie 4y ago
  I was under the impression this version doesn’t need exotic matter?
  - ghkbrew 4y ago
    This abstract says only that they've reduced the need for exotic matter by some orders of magnitude. Presumably it's still required. However, there were some recent papers describing warp drive geometries with only positive mass. Ex: https://arxiv.org/abs/2201.00652 https://arxiv.org/abs/2201.00652
    - ben_w 4y ago
      They also say: "We present the first general model for subliminal positive-energy, spherically symmetric warp drives" Not that I understand the physics well enough to do anything with this, but as you say, there is other work that's definitely (claiming) positive-only energy densities.
      - rolenthedeep 4y ago
        My understanding is that physics allows you to create a bubble of spacetime without negative energy, but that bubble doesn't accelerate through space. The ship drags the bubble along with it at sublight speed using normal thrusters.
- raattgift 4y ago
  > the math works, and it looks like it should be possible The math can only work (if it even does) in exceptionally constrained circumstances. The main purely-theoretical issue is that the paper's <https://arxiv.org/abs/2102.06824v2 https://arxiv.org/abs/2102.06824v2> solution is wholly spherically symmetric, and with vacuum everywhere but in the warping region. "Fig. 1: Asymptotically-flat vacuum background ... general stationary curved region with a spherical topology ... (the warping region) ... flat inner region ... ('passenger' space). ... As we discuss in Section 5, warp drive spacetimes require some form of propulsion in order to accelerate. For this reason, in physical realisations of such spacetimes, the front and rear parts are likely asymmetric". [emphasis mine] Do the maths fall apart if we deviate from total spherical symmetry, either by changing the front and rear parts, or by breaking the vaccuum condition on the inside and outside of the shell, especially if our matter is not precisely spherically distributed? They do a review of this, for limited deviations from spherical symmetry, in §4. However, the vaccum condition is what they study, not straying very far from their earlier "if [the warp region] were replaced by Minkowski spacetime, the whole spacetime would be Minkowski space." The paper declines to answer the question about the distribution of matter (i.e., not-vacuum): "all warp drive spacetimes are asymptotically-flat ... no metric which describes an accelerating warp drive solution has been presented in the literature ... metrics for ... accelerating objects and more general axisymmetric objects which preserve their shape and mass ... remains a subject for future studies". That last part suggests that the inside part might be kept appropriately symmetrical through dynamic reconfiguration of the things inside (i.e., the ship, cargo, fuel, passengers). But what about outside the warping region? In our solar system, and in our galaxy, we are not in Minkowski space. The outer part of the warp region, being asymptotically flat, might be amenable to a "thin shell" (Israel/Darmois junction formalism) with the warp bubble far enough away from everything outside that it feels no tidal forces, which for the purposes of the text above is "acceleration". What determines how far is far enough away? The authors don't attempt an answer. There are plenty of things (stars, most notably) in our part of the galaxy that will induce tides. The solar component of the lunisolar tide in Earth's oceans is substantial at 1 astronomical unit from our sun. That solar component doesn't go away when a space craft is (for example) 2 a.u. away from Earth&Moon on the opposite side of the sun. Can one zip this warp bubble safely past a 1 solar mass star at a distance of 1 au? Ten au? Can the warping region or passenger region be adjusted to allow for close approaches/operation within a star system? All questions for some possible future paper. Apart from stars, we inhabit a region of space which is dotted with things you can smack into (from the very solid like rocks and iron to the very wispy, stuff will still tend to slow you down when you hit it; deceleration and acceleration are the same issue). Near luminous enough stars, radiation pressure might also induce non-negligible accelerations. Is any of that relevant? If relevant, is it "fixable"? How? Alter the shape of the warping region, or the passenger region, or both? Questions for some possible future paper. So, even if one accepts for the sake of argument that the maths are flawless and further one just assumes the existence of controllable negative energy densities, this paper's maths are designed for a spacetime which overall isn't very much like what we have within a few hundred lightyears of here. Maybe some future paper will attempt to show that a warp bubble can work in more realistic spacetimes than almost-everywhere-empty-of-matter-and-feeling-no-gravitation setting of this paper. ("Almost-everywhere" means that there is matter and curvature in the warping region, and maybe the passenger region, but nowhere else, not even a destination star a dozen light-years away).
naasking 4y ago
Two different groups tried this move around the same time, one proved that subliminal drives using only positive mass were possible, the other proved the contrary. Such is the difficulty of GR field equations that it's not yet clear who's correct.

NegativeLatency 4y ago

I hope that some day there’s a paper like this to look back on like we do with Lovelace and Babbages stuff (foundational for something that seemed like magic at the time) Would be cool if it happened during my lifetime though.

23skidoo 4y ago

> and allows for new metrics without the most serious issues present in the Alcubierre solution. ...like violating causality?

geuis 4y ago
Subliminal, which is mentioned multiple times.
- texaslonghorn5 4y ago
  ^ this, and I'm guessing it was autocorrected (because that's what almost happened to me) but the term is subluminal
  - deleted 4y ago
    [deleted]
usrbinbash 4y ago
In case someone wants to read a really good explanation about this; http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti... Generally speaking, all three of FTL (travel/communication), causality and relativity cannot all be true at the same time. If you can FTL-travel/communicate (they are the same really), then you can come up with a scenario in which an observer can see an event happening, and then see the cause of that event after it happened. Extending that logic, if the observer can also move at superluminal speeds, he could prevent the cause of the event after seeing the event happen, leading to a paradox.
- codethief 4y ago
  > Extending that logic, if the observer can also move at superluminal speeds, he could prevent the cause of the event after seeing the event happen, leading to a paradox This is a guess, i.e. one possible outcome physicists are considering. People have proposed alternative outcomes of FTL like the (in my opinion much more sensible) Novikov consistency principle, which roughly proposes that spacetime and the entities it contains (e.g. an observer's wordline) should be looked upon as a whole, in the sense that they need to be self-consistent. Spacetime is not time-dependent and does not evolve, so it does not make much sense to say "something something leads to a [spacetime] paradox".

puffoflogic 4y ago

Apropos of nothing, in 1903 the NYT confidently declared airplanes would take ten million years to develop.

cwillu 4y ago
It's the difference between scaleable quantum computing, and hypercomputation: one appears to be a matter of engineering, which may turn out to be completely impractical/require a million years/whatever. The other is ruled out by our understanding of the universe. Pre-Wright, flight was the former, not the latter.
- hnews_account_1 4y ago
  Our understanding of gravity isn’t complete. General relativity looks relatively unbreakable but the smartest minds agree non particle gravity just doesn’t make sense in an otherwise QFT universe so we’re missing something deeper. Whether that will provide a way to travel FTL in space time or outside it, we don’t know. But the science is not as settled as it seems even if mere mortals cannot think up a theory to beat the truly incredible power of GR.
  - puffoflogic 4y ago
    No no you don't understand. Science was wrong and incomplete in the past but now (as everyone learns in school) The Science has been completed. What we know is infallible and what we don't know is unknowable.

cletus 4y ago

The lesson here is an idea, no matter how implausible, can have legs as long as people want it to be true. That particular idea here is FTL travel (or communication). Nearly all of these ideas spring from a basic inability to understand the domain of a function. The domain is the set of values for which the function is defined. Any other values are undefined f(x) = 1/x for example has a domain of anything but 0. So what happens is that people plug negative values into quantities like mass without any basis for what negative mass is and then use that as a basis for [insert FTL system here]. Why stop there? Why not use imaginary masses? There is no negative mass. There is no negative energy. The domain of velocity is [0,c]. Even if you ignore that, no one goes far enough to calculate the energy requirements of, for example, "folding space" or creating a wormhole. If you do the math, it turns out you need to convert a significant portion of a stellar mass into energy. The future isn't Star Trek nor Star Wars no matter how much you want it to be.

tsimionescu 4y ago
The article here actually presents a warp drive that doesn't require negative mass/energy, but also doesn't move at speeds greater than c. It's interesting because it seems to be permitted by all of the laws of GR and also build-able from known materials, and it could represent a new mode of propulsion. It's also important to remember that space has expanded far faster than the speed of light in the past - c is only a limit for the speed that matter or energy can move at, it's not a limit for how space-time can change shape. We know this because the observable universe is ~93 billion light-years in size, but only ~13 billion years old - so it's size has grown faster than c. I'm with you on the idea that none of this means practical controlled faster-than-light travel will ever exist, though. Jury's still out on slower-than-light folding-space drives, though.
- cletus 4y ago
  Doesn't this warp drive depend on negative energy density (whatever that means) or has the fundamental flaw from out-of-domain values shifted in this version to something else?
  - 4y ago

Loquebantur 4y ago

Notably, energy conservation is a local property of spacetime. Meaning, if you can manipulate the metric, you should be able to create (or vanish) energy. Creating the warp-bubble clearly takes energy dependent only on its volume, not the mass contained therein. Here, with a subluminal warp drive, you could hoist matter out of a gravity well and let it fall down afterwards.

pdonis 4y ago
> if you can manipulate the metric, you should be able to create (or vanish) energy. No, you can't. That would violate local energy conservation. The fact that energy conservation is a local property just means that there is no well-defined global energy in a general spacetime (though there is one in some special classes of spacetimes). It does not mean that stress-energy can be created or destroyed. > with a subluminal warp drive, you could hoist matter out of a gravity well and let it fall down afterwards. But the hoisting process would require you to expend at least as much energy as you would be able to collect from the falling down afterwards.

api 4y ago

> We show that a class of subluminal, spherically symmetric warp drive spacetimes, at least in principle, can be constructed based on the physical principles known to humanity today This is awesome. Ever since I read about the Alcubierre paper and it’s followups it seemed obvious to me that while there are huge basic physicality issues with FTL those same issues don’t exist for a near or at light speed warp drive. Such a drive would not necessarily reduce the energy requirements of travel near light speed, but I wonder if it would eliminate some of the other huge problems. Things like a single dust particle annihilating you or blue shift turning all incident radiation into a gamma ray laser aimed at the front of your ship. Would it have any impact on time dilation? If you approach the speed of light conventionally you can experience the trip subjectively as near instantaneous due to time dilation. Is that still true if you are in a warp bubble? Light speed gets you to the centauri system in about 4.5 years Earth time. With time dilation the main cruise phase of the flight would seem instantaneous on board the spacecraft. (It would be years on Earth of course.)

euroderf 4y ago
> Light speed gets you to the centauri system in about 4.5 years Earth time. With time dilation the main cruise phase of the flight would seem instantaneous on board the spacecraft. (It would be years on Earth of course.) Oh? But if there is no ether, and no absolute dead reckoning for motion in the vacuum, then for all we know, the Earth system is moving at 0.8c in one direction and the spacecraft would be moving at 0.2c in the opposite direction. Or something like that.

dsp_person 4y ago

Subluminal warp drive only needing positive energy seems like cool method to get around conservation of momentum (remember EmDrive?). I wonder if it could be used at very small scales with lower energies for some applications. Another interesting reactionless drive within possibility: > "Swimming in spacetime" is a general relativistic effect, where an extended body can change its position by using cyclic deformations in shape to exploit the curvature of space, such as due to a gravitational field. https://en.wikipedia.org/wiki/Reactionless_drive#Movement_without_thrust https://en.wikipedia.org/wiki/Reactionless_drive#Movement_wi...

pdonis 4y ago
> Subluminal warp drive only needing positive energy seems like cool method to get around conservation of momentum No, it isn't. You can't "get around" conservation of momentum. > Swimming in spacetime Still doesn't allow you to violate conservation of momentum. The article you link to notes this.
- naasking 4y ago
  Yes, if I recall correctly you get displacement but without any momentum.

analog31 4y ago

Analogous to the Fermi Paradox, if warp drive / time travel are possible, why have I not been visited by someone trying to sell me one?

api 4y ago
If either is possible it nearly proves that we must be very early for intelligence in the universe. A universe with intelligent life and FTL turns into a party pretty quick as soon as someone invents it. If we even proved it was possible at table top scale I’d be about 99% convinced we are first in at a bare minimum our galaxy. I would also argue that we should scale it up and get a presence out there fast in case someone less nice than us is also inventing one right now. (And we aren’t all that nice.)
- kloch 4y ago
  > we must be very early for intelligence in the universe. Not under the Zoo hypothesis: https://en.wikipedia.org/wiki/Zoo_hypothesis https://en.wikipedia.org/wiki/Zoo_hypothesis
  - yellowapple 4y ago
    So basically Earth is the Milky Way's version of the Sentinel Islands. Makes me wonder if UFO sightings are from misguided extraterrestrial missionaries trying to teach us about their Lord and Savior Behemecoatyl.
- analog31 4y ago
  My thinking is that if time travel / warp drive is possible, then words like "early" and "first" have no meaning. After all, if humans invented it, I would 100% expect someone to be abusing me with it right now. People would steal from the past to pay for the future.
- captainmuon 4y ago
  You can always play with the factors. Maybe FTL is possible, but requires a sizable fraction of the planet's resources. Or you can eventually reach another planet, but terraforming it and building the technology for creating more FTL ships will take millions of years. Basically: FTL is fast but pointless, because getting to FTL takes longer than getting to your destination. Maybe space is a bit more emptier than you think, and FTL is a bit slower. Or maybe it is busy, but we haven't waited long enough. I mean think of a native American in the 1400s. If transoceanic travel is possible, why haven't we seen visitors from other continents yet? Finally, it is still entirely possible that "they" are already there, and there is a cover up. I don't believe it (Occam's razor), but if FTL turns out to be theoretically possible, than I think I will have to update my beliefs :-)

cletus 4y ago

This is an abstract of an article behind a paywall so we can't see this details but it is from 2021 and I managed to find an article discussing the concept [1] > In this case negative energy is not required — rather it’s gravity itself which bends spacetime and gives rise to time dilation. In essence the gravitational field makes the passage of time within the passenger area much slower than the passage of time outside of it. A few minutes for them may be thousands of years worth of space travel, yet to leverage this powerful effect would take enormous amounts of gravity. The paper calls for the compression of an entire planet down to just a thin shell surrounding the passengers. The introduction of a physical warp drive model has changed the question from “Does something like negative energy even exist?” to “How do we compress the mass of a planet down to the size of a spaceship?” And I'll pull out the most apropos sentence: > The paper calls for the compression of an entire planet down to just a thin shell surrounding the passengers Yes, no negative energy or negative energy mass, just an entire planet compressed into a thin shell that somehow doesn't further collapse. So even if this were possible, how would you accelerate or maneuver such a massive object? I've seen so many of these proposals over the years. It's all nuclear-grade hopium with a massive flaw or huge, highly theoretical unknown. Always. [1]: https://medium.com/predict/the-first-physical-warp-drive-models-921055800764 https://medium.com/predict/the-first-physical-warp-drive-mod...

kaashif 4y ago
> I've seen so many of these proposals over the years. It's all nuclear-grade hopium with a massive flaw or huge, highly theoretical unknown. Always. True, the planet shell thing is nuclear grade hopium. But negative energy is big bang level hopium. This is a big downgrade.
Steuard 4y ago
It's an article about theoretical physics published in the 21st century, so it would be very surprising if it had not been posted on the arXiv before being published in a journal. And sure enough: https://arxiv.org/abs/2102.06824 https://arxiv.org/abs/2102.06824

deleted 4y ago

[deleted]

hobz22 4y ago

Will. It. Blend?

captainmuon 4y ago

I always find these kind of papers interesting. It seems like they found a more general "warp drive" metric. But after skimming it, I don't understand how they want to create it in reality. The best shot I think we have at metric engineering - unless you want to build something crazy like moving spheres and rings of ultra dense matter - is to look into materials with interesting spin-gravity coupling. We know that rotating bodies have additional gravitational interactions. The effect is tiny, but measurable in experiments (e.g. Gravity Probe B). We also know that atomic spin is not just an abstract quantum number but actual angular momentum. So there should be some kind of spin-gravity interaction, even though it would be incredibly weak. But even though the absolute force is small, if we manage to make it time dependent we could amplify the effect. Just like a changing magetic field creates an electric field, a changing "gravitomagnetic" field would create a gravitational field. This is basically the insight that took us from compasses and static electricity to the whole of electrical engineering. I am not sure such materials could even exist. And even though I have a PhD in Physics, I have no idea how one would approach this theoretically. The treatment of particle spin seems to be very rare in GR, and then you would have to marry this to solid state physics and somehow calculate how the situation changes when the spin density changes and so on... The furthest I got was to open a thread on stackexchange about it :-) but who knows, maybe someday somebody looks into this stuff: https://physics.stackexchange.com/questions/46099/materials-with-different-gravitomagnetic-permeability https://physics.stackexchange.com/questions/46099/materials-...

t8sr 4y ago
I was with you until the analogy to electromagnetism and time dependency. I admittedly don't have a PhD in physics (undergrad astrophysics), so I possibly won't understand your answer, but isn't gravitomagnetism more of a useful fiction than anything people actually believe models reality? I mean the equations look similar, but there's no physical justification: gravity is not a field in the QFT sense. I guess I'm asking why you'd expect there to be an analog to the interaction between electrical and magnetic fields, when the nature of the thing is so different?
- zachf 4y ago

AdamJacobMuller 4y ago

Alcubierre warp drive are a major plot two of my favorite hard sci-fi books, Flood[1] and Ark[2] by Stephen Baxter[2]. Really excellent books. 1 https://en.wikipedia.org/wiki/Flood_(Baxter_novel) https://en.wikipedia.org/wiki/Flood_(Baxter_novel) 2 https://en.wikipedia.org/wiki/Ark_(novel) https://en.wikipedia.org/wiki/Ark_(novel) 3 https://en.wikipedia.org/wiki/Stephen_Baxter_(author) https://en.wikipedia.org/wiki/Stephen_Baxter_(author)

nynx 4y ago
I love Baxter’s books but haven’t read those, thank you!