[HN Gopher] Physical Warp Drives
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Physical Warp Drives
Author : phonebucket
Score : 103 points
Date : 2021-05-28 12:58 UTC (10 hours ago)
(HTM) web link (iopscience.iop.org)
(TXT) w3m dump (iopscience.iop.org)
| _Microft wrote:
| Am I correct that the interesting thing about subluminal warp
| drives is that they do not require working mass for propulsion?
| So if we had such a warp drive, we could put energy in and get
| motion out of it, without being subject to the tyranny of the
| rocket equation?
|
| It seems like this idea is not correct. The authors write in
| section 5.2: _" Warp drives, being inertially moving shells of
| normal or exotic material, do not have any natural way of
| changing their velocities. They are just like any other types of
| inertially moving objects. Similarly, just like for any other
| massive objects, achieving a certain velocity for a warp drive
| requires an externally applied force or, more practically, some
| form of propulsion."_, from
| https://arxiv.org/pdf/2102.06824.pdf#page=17
| jjk166 wrote:
| Just a causal reminder that solar sails, beamed propulsion, mag
| sails, photon rockets, etc are all examples of propulsion
| methods free from the tyranny of the rocket equation which
| require no onboard reaction mass. There is really no need for
| exotic physics if that's all you're looking to accomplish.
| azernik wrote:
| Yup.
|
| But also they're interesting as a theoretical construct for
| better understanding the whole idea of intentionally warping
| space.
| [deleted]
| pitaj wrote:
| Also the travelers are essentially in an inertial frame, so it
| acts kind of like "inertial dampeners" in Star Trek.
| zardo wrote:
| I don't understand why it would be called a warp _drive_ then.
| If it doesn 't cause movement it's not driving anything.
| akiselev wrote:
| The tyranny of the rocket equation isn't some fixed property of
| orbital mechanics, it's a limitation of our method for
| converting energy from chemical propellants to propulsion.
| Specific impulse for nuclear rockets designed decades ago (i.e.
| nuclear lightbulb) is in the thousands, which is enough to
| reach orbit in a single stage without dumping extraneous mass.
|
| The original Alcubierre drive required _the energy equivalent
| of the mass of Jupiter_ to create the warp field, with an
| impossible shortcut in the form of exotic particles with
| negative mass. The sun burns loses about 6x10^9 kg of mass a
| second which is the upper limit for the amount of energy a Type
| I civilization can extract in our solar system. Jupiter 's mass
| is roughly 2x10^27 kg, so assuming there are 4x10^11 stars in
| our galaxy and our star represents the average, a Type II
| civilization in our galaxy wouldn't have enough energy to
| produce a warp field. The drive doesn't just need a little bit
| of working mass to expel, it needs enough energy/mass to bend
| space time around it!
| ben_w wrote:
| Slight quibble, but the _original_ needed more negative mass
| than the universe, it was one of the subsequent improvements
| which reduced it to only Jupiter.
| akiselev wrote:
| Thank you for the correction! Though, I hope you don't mind
| that I quibble the quibbler: the original needed more
| regular mass than the universe - negative mass exotic
| particles were proposed as a more "practical" alternative,
| if they exist.
| ben_w wrote:
| Likewise, thank you -- what you say is new knowledge for
| me :)
| jameshart wrote:
| The rocket equation doesn't only apply to chemical energy
| propulsion - it applies to any propulsion system that
| conserves momentum and uses reaction mass - which nuclear
| drives do too.
|
| Chemical rockets are limited in how much energy they can put
| into the reaction mass (by the energy density of their fuel)
| and therefore the maximum exhaust velocity they can achieve -
| which is the input into the rocket equation
|
| But while nuclear rockets get away from the constraint of
| chemical energy density they still have to operate by putting
| energy into reaction mass and shooting it out the back to
| create a momentum change.
|
| And the rocket equation still applies to the reaction mass
| and its exhaust velocity.
|
| In fact in theory you can build a chemical rocket engine that
| burns a fuel for heat, then uses that heat to energize some
| other reaction mass that you eject - it's just that that is
| inherently less efficient than using the spent fuel as
| reaction mass since otherwise you're needlessly pumping
| momentum into spent fuel. Far better to throw the exhaust
| products away as reaction mass.
| akiselev wrote:
| Ah, my mistake - I thought the "tyranny of the rocket
| equation" was referring to the rocket equation's
| consequences on our attempts to reach orbit (i.e.
| multistage rockets), not the equation in general.
| gmartire wrote:
| Hey all, I'm one of the authors of the paper Introducing Physical
| Warp Drives and a Y Combinator alum (YC W'14). This video does a
| great job explaining our paper and Erik's as there a few
| misunderstandings in this forum https://youtu.be/PA66ah9b0U4. We
| all know each other and all have the warp bug as we now have a
| path; it's just going to be difficult. If you would like to join
| our warp drive cohort, please email us a bit about yourself and
| your ideas for warp at create at appliedphysics dot org. Our
| cohort is composed of PhDs and engineers from several STEM
| disciplines, we meet weekly and have more papers in the works,
| perhaps you can join the fun! https://thedebrief.org/new-warp-
| drive-model-requires-no-exot...
| thelastinuit wrote:
| I think applied a few months ago. But i never got a rejection,
| i assumed ghosting was the way to tell me: no.
| _Microft wrote:
| I do not understand how the proposed warp drive could help with
| moving a ship. It sounds more like a "time dilation device" to
| shorten the travel time for the crew than a setup to make
| moving through space actually easier.
|
| Would you mind explaining?
| nynx wrote:
| A conclusion of this paper is that a warp bubble that fits
| into the categories described requires external propulsion.
|
| I agree, the positive energy, spherically symmetric warp
| bubble isn't particularly interesting and certainly not a new
| effect.
| threesongs wrote:
| A conclusion of the paper is that any warp drive, including
| all the ones proposed in the literature, fits in the
| categories explained in the paper.
| lainga wrote:
| Do you have any thoughts on Lentz's comment that IPWD "made a
| rush to judgement" on his draft of Hyper-Fast []? Or has
| anything changed in your assessment between the draft and
| publication of his paper? (I did not see anything on his blog)
|
| [] https://eriklentzphd.blogspot.com/2021/04/review-and-
| respons...
| threesongs wrote:
| The version accepted in the journal clearly states that
| superluminal warp drives are physically unachievable because
| they violate the dominant energy condition. It means that
| superluminal drives require superluminal matter. This is in
| agreement with the conclusions in the IPWD paper. However,
| the version E.Lentz uploaded on arxiv or the press-release
| did not state this clearly.
| nynx wrote:
| What was your process for self-teaching yourself enough physics
| to work on a paper like this?
| jbdistaken wrote:
| https://arxiv.org/abs/2102.06824
|
| looks like CQG spell-checker added a subliminal typo in the
| abstract :)
| haydnv wrote:
| I think what's really interesting about this that most readers
| seem to overlook is the claims about _subluminal_ warp drive,
| which is practically realizable in a way that the science-
| fictional faster-than-light case is not
| detritus wrote:
| Sabine Hossenfelder mentioned this is one of her videos last
| year (around 2:30)
|
| https://www.youtube.com/watch?v=8VWLjhJBCp0
| SkyMarshal wrote:
| Fyi you can append the timestamp to YouTube links, like this:
|
| https://www.youtube.com/watch?v=8VWLjhJBCp0&t=2m30s
| detritus wrote:
| I know :)
|
| I just prefer not to presume when I link... .
| sosuke wrote:
| Baby steps! Every step is worth it. I know the math and science
| doesn't support me but I'm convinced FTL is possible and that
| the universe won't die a heat death.
|
| The universe heat death theory makes me think of comics where
| people extrapolate the data they have while lacking other
| critical data. https://xkcd.com/605/
| douglaswlance wrote:
| The heat death theory assumes life has zero impact on the
| universe.
| krapp wrote:
| Most of the universe is void. Most of the rest is whatever
| dark matter is. What we consider to be the entirety of
| material reality is a fraction of a fraction, and out of
| that, as far as we can tell, Earth has the only life that
| exists. There's probably more life out there, but if there
| is, as the Fermi Paradox points out, they're awfully quiet.
|
| I think it's fair to assume that life will have practically
| zero impact on the universe. Even if the universe was
| teeming with it, it would be of such little significance at
| scale that it might as well be a rounding error in reality.
| Teever wrote:
| There is merit to your thought process but it could just
| as easily be that life is the spark that sets off an
| explosion that irrevecobly changes the universe.
|
| Think of a virus. A handful of specially shaped molecules
| has permanently changed human society.
| nynx wrote:
| There is another paper on warp drives that came out around the
| same time, _Breaking the Warp Barrier: Hyper-Fast Solitons in
| Einstein-Maxwell-Plasma Theory_ [0].
|
| [0]: https://arxiv.org/abs/2006.07125
| lainga wrote:
| Wow, is that paper saying what I think it is? Previous papers
| only found a negative energy requirement (pg. 12) because they
| only considered interior spaces which could reduce down to a
| point-like region? That's an extraordinary claim, and, I hate
| to use the phrase, but "big if true".
| threesongs wrote:
| It is an important paper. However, it is also important to
| realise that superluminal solutions introduced there are
| still prohibited by the laws of physics. Making such
| superluminal drives is at least as hard as making
| superluminal matter, which we believe requires infinite
| amount of energy. In physics, they say that such spacetimes
| violate the dominant energy condition. The issue is
| relatively clearly stated in E.Lentz's paper accepted in the
| journal, but not in his arxiv paper or the press-release.
| nynx wrote:
| Indeed it is. There are still significant (in the case of the
| required mass-energy, massive) issues.
|
| The writer has a blog [0], which is worth reading if you're
| interested in next steps for the theory.
|
| [0]: https://eriklentzphd.blogspot.com/
| Gravityloss wrote:
| > Once the energy requirement is lowered, the space-time
| signatures of positive-energy solitons may be studied in a
| laboratory setting using existing or novel methods. ... The
| highly magnetized energetic and diffuse atmospheric plasma
| of magnetars may also bea natural place to look for
| signatures of positive-energy soliton geometries even prior
| to advances in energy reduction.
| yosito wrote:
| > 10% the mass of the sun
|
| Where could we source so much mass? That is far more than the
| mass of the earth, and surely altering the mass of the sun by 10%
| would have catastrophic effects.
|
| > physically possible
|
| Does an element exist that is dense enough to compress 10% of the
| mass of the sun into a 620m sphere? If not, how can the claim
| that this is physically possible be supported?
| ithkuil wrote:
| According to Wikipedia "The entire mass of the Earth at neutron
| star density would fit into a sphere of 305 m in diameter"
|
| So I assume 610m in diameter would be 8 earth masses?
|
| That's still 4000 times less than 10% of the mass of the sun
| mdorazio wrote:
| I've only gotten a few pages into this, but it looks like their
| general solutions still require negative energy? Regardless, it's
| been very interesting to see the development of warp drive theory
| over the last ~20 years.
| guepe wrote:
| The abstract claims 2 orders of magnitude less "negative
| energy" than Ablucierre Drive. So... it's an improvement ?
| deusum wrote:
| 100/-0 < 1/-0 ?
| WJW wrote:
| No, more like only a Jupiter worth of negative mass instead
| of a few solar masses.
| jcadam wrote:
| Ah, so even in theory, we could use such a drive exactly
| once.
| garmaine wrote:
| It's positive energy, not negative.
| fnord77 wrote:
| I thought the Alcubierre effect was theoretically possible by
| getting near a rapidly rotating neutron star?
| ben_w wrote:
| There isn't anything with that name, AFAICT. Did you mean
| "frame dragging"?
| jonplackett wrote:
| I think they mean Alcubierre Drive:
| https://en.m.wikipedia.org/wiki/Alcubierre_drive
| ben_w wrote:
| While the name is similar, the drive has nothing to do with
| spinning neutron stars
| imdoor wrote:
| Does an Alcubierre warp drive violate causality? From my
| understanding, it's no different than _regular_ FTL travel, which
| implies time travel, which implies violation of causality.
|
| To me that would indicate that an Alcubierre warp drive still
| shouldn't be possible despite the negative energy requirement
| being lifted.
| vbezhenar wrote:
| Why do you think that violation of causality is not possible?
| threesongs wrote:
| Violation of causality is equivalent to travelling back in
| time. Lots of problems there, and no good model for such
| things.
| imdoor wrote:
| I don't know enough to think that violation of causality is
| not possible. It just just seems very unlikely. I can't
| imagine how the world would look like if it wasn't the case
| because of the paradoxes it would imply.
|
| You seem to imply that you think differently. Care to
| elaborate?
| vbezhenar wrote:
| I have no idea either, but I'm not aware of any fundamental
| reasons that violation of causality is forbidden.
|
| And imagining world with all those spooky quantum effects
| is already almost impossible, at least for me :)
| randallsquared wrote:
| Not the GP, but both the many worlds interpretation and
| superdeterminism would avoid (the issues of) causality
| violation, would they not, even in the presence of time
| travel?
| pugworthy wrote:
| ...regular FTL travel...
|
| It's kind of exciting that we see phrases like this now. I mean
| it wasn't that long ago that the idea of a 9 story first stage
| rocket coming back to land (and fly again) was fantasy.
| shepardrtc wrote:
| No, for the original Alcubierre, its warping spacetime around
| the information and moving that pocket. The ship or information
| or whatever is still traveling inside the pocket at its normal
| speed. So while it could be virtually instant (ignoring energy
| requirements), it wouldn't be time travel. However, I believe
| you could arrive to your destination and look back and see
| yourself before you warped - and maybe as you traveled - as the
| light catches up to you. At least that's my understanding.
| Filligree wrote:
| No, every FTL construction can be used to violate causality.
| There's no way at all around that. Fundamentally it's a
| geometric problem -- relativity has a hard barrier at light
| speed, but if you can get past that, there's nothing at all
| special about infinite speed. Is the same sort of 4-momentum
| as standing still, and you can accelerate in any direction
| from there.
|
| The warp drive in the article is subliminal, though, so it
| doesn't have that problem.
| ben_w wrote:
| That's something I've been meaning to ask about:
|
| Does FTL _really_ inevitably mean that, or is that a
| consequence of saying "no preferred frame of reference" and
| therefore having e.g. "100c" meaning different things to
| observers in relative motion?
| bloopernova wrote:
| I've always had serious trouble understanding why FTL
| travel will break causality.
|
| Do you know of any good layman-level explanations?
|
| Like, if I FTL from point a to point b, 10 light years
| apart, in my super duper warp vessel. It takes me, for the
| sake of argument, 10 minutes to make that journey. Now say
| I set off a big comms laser at point a, sending a message
| to point b, before I left. I don't see that laser until 10
| years later.
|
| What am I missing? I know I'm missing something, but that
| seems straightforward to me. It's weird to butt up against
| that seemingly incomprehensible.
| weavejester wrote:
| Suppose you set off in a spaceship at 80% of lightspeed,
| or 0.8c, travelling away from Earth. At this speed,
| according to relativity, time is slowed to 60% of it's
| 'usual' value. So for every 10 hours that pass on Earth,
| only 6 will appear to pass on the spaceship.
|
| However, this is only true from the perspective of
| someone on Earth. From the point of view of someone on
| the spaceship, the opposite is true. From their
| perspective, the spaceship is stationary, and Earth is
| travelling away from it at 0.8c. Therefore, for every 10
| hours that pass on the spaceship, only 6 will appear to
| pass on Earth.
|
| Suppose there was a way of instantaneously communicating
| between the two. On Earth, 10 hours into the mission,
| mission control sends a message to the spaceship. Because
| of time dilation, the spaceship receives the message only
| 6 hours into the mission, from their perspective. The
| spaceship then sends a message back, and due to the same
| time dilation effect, the message arrives on Earth 3
| hours and 36 minutes into the mission (60% of 6 hours).
| In other words, the reply from the spaceship will arrive
| 6 hours and 24 minutes _before_ mission control sends the
| original message.
| Sanguinaire wrote:
| I'm in the same boat as the person you replied to;
| breaking causality never made sense to me.
|
| In the case of your explanation, what sticks out to me is
| the "Suppose their was a way of instantaneously
| communicating" part - it seems more intuitive to me that
| the warp bubble would not allow any communication across
| the threshold, effectively becoming a pocket universe.
| weavejester wrote:
| Instantaneous communication makes the numbers easier
| because you don't have to account for travel time, but
| causality can be broken with any superluminal form of
| communication.
|
| If you have a ship with warp speed, then you have
| superluminal communication, because you can just carry a
| message on board. Even if you can't communicate inside
| the warp bubble, as long as you can exit the bubble at
| some point, then you can travel via warp, pop out, and
| transmit your message conventionally.
|
| In my earlier example, if the ship and mission control
| had messenger drones capable of travelling many times
| faster than light, then the ship's response drone could
| arrive on Earth before mission control's messenger drone
| was launched.
| [deleted]
| NoGravitas wrote:
| The ship you're in isn't what's traveling FTL, though;
| it's going at normal relativistic speeds. But say both
| you and Earth have an ansible (a faster-than-light
| communicator); then you get the problem in the previous
| comment.
| uhhhhhhhhhhhhhh wrote:
| Here here! Antisuperluminalists evolved from those who
| said we'd never fly.
| NoGravitas wrote:
| This is the best explanation I've been able to find:
|
| http://www.physicsmatt.com/blog/2016/8/25/why-ftl-
| implies-ti...
| xucheng wrote:
| https://youtu.be/HUMGc8hEkpc
| spoiler wrote:
| Oh wow, this was fascinating! I was vaguely made aware of
| some of these concepts by a friend who studies quantum
| physics, but on he's never been able to explain it so
| "simply" (that's not doing the explanation any justice,
| it's not simple and some of it still went over my head,
| but "it clicked").
|
| I'm eager to check out more of their channel!
| imdoor wrote:
| Ah, i somehow read the _subluminal_ as _superluminal_ in
| the abstract and got all excited. But the subluminal
| restriction makes the paper 's findings make more sense.
| [deleted]
| mabbo wrote:
| > I believe you could arrive to your destination and look
| back and see yourself before you warped - and maybe as you
| traveled - as the light catches up to you
|
| That _does_ sound like traveling faster than light, but I
| think there 's a reasonable view that makes it not.
|
| Suppose you traveled at some very fast speed around a black
| hole. It bends space time in such a way that you might, at
| some point, be able to collect photons that could be
| reconstructed into an image of you earlier in time.
|
| In short, when space time is flexible and bending, there are
| paths in which you can see your previous self. An alcubierre
| drive is a means to bend space time.
| pdonis wrote:
| _> Does an Alcubierre warp drive violate causality?_
|
| The original Alcubierre metric does not, but that's because it
| only describes a single "warp bubble" that travels in one
| direction, never starting or stopping and never turning around.
|
| As soon as you have more than one "warp bubble", or you let a
| single "warp bubble" turn around, then you will have closed
| timelike curves, which violate causality.
| aidenn0 wrote:
| What makes something a "warp drive?" Per TFA this is subliminal,
| so it's clearly not "superliminal."
| pranade wrote:
| The subluminal application is particularly interesting, because
| it seems almost feasible. Is there a practical path to meeting
| these mass/energy requirements though?
| andruc wrote:
| Merely an engineering problem!
| _Microft wrote:
| Arxiv: https://arxiv.org/abs/2102.06824
| mabbo wrote:
| Anyone selling "warp drives" that do not go faster than light
| kind of missed the point of why people are excited about warp
| drives.
|
| And if the video @gmartire just posted is explaining the paper
| correctly, this warp drive also doesn't accelerate anything. You
| have to figure out how to speed up on your own.
|
| I don't really see why anyone would bother doing any of these
| things. To dilate time better? I'd rather invest the same amount
| of mass towards a larger engine/fuel so that I can go faster,
| dilating time further and getting me there sooner.
| threesongs wrote:
| IMO the paper clarifies the misconceptions about all warp
| drives. They all need propulsion, for example, to be consistent
| with physics.
| feynmanzhou1 wrote:
| cool
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