[HN Gopher] Physical Warp Drives (2021)
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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.
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