[HN Gopher] Physical Warp Drives (2021) ___________________________________________________________________ Physical Warp Drives (2021) Author : JPLeRouzic Score : 50 points Date : 2022-11-05 17:02 UTC (5 hours ago) (HTM) web link (iopscience.iop.org) (TXT) w3m dump (iopscience.iop.org) | JPLeRouzic wrote: | _" 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 | [deleted] | bilsbie wrote: | Is this for real? What are we waiting for? | rolenthedeep wrote: | 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 wrote: | I was under the impression this version doesn't need exotic | matter? | ghkbrew wrote: | 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 | ben_w wrote: | 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 wrote: | 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. | sph wrote: | A way to create negative energy / mass. Until then, an | Alcubierre drive remains just a theory | ben_w wrote: | "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. | Loquebantur wrote: | 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. | 23skidoo wrote: | > and allows for new metrics without the most serious issues | present in the Alcubierre solution. | | ...like violating causality? | usrbinbash wrote: | In case someone wants to read a really good explanation about | this; | | 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 wrote: | > 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 ". | geuis wrote: | Subliminal, which is mentioned multiple times. | texaslonghorn5 wrote: | ^ this, and I'm guessing it was autocorrected (because that's | what almost happened to me) but the term is subluminal | [deleted] | tsimionescu wrote: | Space-time can expand at faster-than-light speeds - this is | known for sure, since we live in a 93-billion light-years wide | universe that's only 13 billion years old. | | As far as I understand, the current conjecture is that an | Alcubierre drive could move at faster than light speeds (if | negative mass/energy to build it existed), but that it it tried | to move to its own past, it would either destroy itself because | of some conjectured quantum gravity phenomenon - this is called | the "chronology protection conjecture" and Alcubierre himself | talked about it: | | > The conjecture has not been proven (it wouldn't be a | conjecture if it had), but there are good arguments in its | favor based on quantum field theory. The conjecture does not | prohibit faster-than-light travel. It just states that if a | method to travel faster than light exists, and one tries to use | it to build a time machine, something will go wrong: the energy | accumulated will explode, or it will create a black hole. | | [0] | https://web.archive.org/web/20160318223348/http://ccrg.rit.e... | (last three slides touch on this area) | captainmuon wrote: | 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-... | cletus wrote: | 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- | mod... | api wrote: | > 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.) | dsp_person wrote: | 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_wi... | cletus wrote: | 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 wrote: | 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 wrote: | 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? | tsimionescu wrote: | The article claims not: | | > 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. | | I'm no physicist to be able to evaluate the claim more | deeply, but it is a published peer-reviewed article. | | The more famous Alcubierre drive, the one that _can_ go | faster than light, does require negative energy density. | [deleted] | hobz22 wrote: | Will. It. Blend? | analog31 wrote: | 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? | vbezhenar wrote: | Did you try to sell something to ants? | api wrote: | 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.) | captainmuon wrote: | 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 :-) | vbezhenar wrote: | It's highly unlikely that we're first or even early. So any | reasoning that leads to this conclusion is highly likely to | be wrong somewhere. | kloch wrote: | > we must be very early for intelligence in the universe. | | Not under the Zoo hypothesis: | https://en.wikipedia.org/wiki/Zoo_hypothesis | analog31 wrote: | 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. | puffoflogic wrote: | Apropos of nothing, in 1903 the NYT confidently declared | airplanes would take ten million years to develop. | cwillu wrote: | 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 wrote: | 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. | NegativeLatency wrote: | 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. ___________________________________________________________________ (page generated 2022-11-05 23:00 UTC)