Alcubierre drive
en.wikipedia.org
en.wikipedia.org
The Einstein equations became a recipe generator: if you want this spacetime, then you need this matter/energy.
Unfortunately, all the interesting spacetimes (such as the Alcubierre drive) require exotic matter, matter unlike anything we have ever seen. Other spacetimes people were interested in generating were stable wormholes. If we could ever produce such matter then we could start some spacetime engineerinng. Meanwhile, we are left with these oddities.
I heard that Alcubierre somewhat regrets having his name on this spacetime as he gets contacted by all kinds of Star Trek fans and other people who believe they can produce the drive. He has done all kinds of other quality research in General Relativity, but this is what he will be famous for.
This is untrue, it's really offensive to a person that's dedicated years of his life to this research, and it set a terrible tone for the rest of the comments. He gives talks all the time to groups that are interested in developing a "warp drive". He puts up photos of Star Trek in his talks! He even refers to his own work as potentially developing a "warp drive".
Look at one of his recent talks https://www.youtube.com/watch?v=5q_z8BjiYng and please stop spreading negative rumors about scientists.
Imagine the exotic "pollution" caused by civilizations which are advanced enough to tinker with the fabric of spacetime, but not advanced enough to understand (or care about) the long-term consequences of that tinkering.
Destabilizing solar systems or even entire galaxies, maybe even violating causality..
Imagine the home (and only) planets of younger, insignificant civilizations being caught up in those cataclysms, all traces of their existence wiped from cosmic history in what might amount to the equivalent of a traffic accident or "oil spill" of a greater civilization.
..maybe that has already happened and some of the leviathanic voids we've observed were a result of that? :)
That's fun science fiction, as others have pointed out, but in terms of real science, at the moment that's just a sort of subtle way of anthropomorphizing the universe and projecting human experience far, far beyond where it is applicable. Einstein's equations for spacetime evolution don't have anything that looks like "pollution" in them. There's no "stress term" for "space time that has been put through the wringer and is broken now". Black holes do not seem to leave "broken space" behind them or anything like that; indeed, "leaving broken space 'behind' them" would be an incredibly ill-defined term. In what preferred reference frame does your "pollution" take place in?
The only real concern is if there is some way to violate the conservation of energy, as real space does seem to do that at large scales at the moment. In that case someone might explode something, potentially large enough to make a mess of a galaxy or more. But we don't have even a hypothetical example of how to do that; just because the universe is violating energy conservation does not mean there's any conceivable organization of energy we can create that will do it, if the reason conservation is being violated is the injection of energy from some other effect or something.
Pollution would be any undesirable side effect and there are plenty to be imagined in these scenarios.
Your comment strikes me a bit like a chemist denying environmental pollution as it's all "just chemicals."
Chemicals admit of a form of pollution when they form themselves in ways we don't like. Spacetime doesn't have that sort of memory, excepting exceedingly weak gravity waves, for which it is basically impossible for those to ever be your biggest problem (if not simply impossible for those to really be a problem). There's no spacetime equivalent to dumping a nasty chemical somewhere, and a hundred years later that chemical is still there hurting people. Spacetime doesn't work like that.
You're basically anthropomorphizing, thinking that there just has to be a way for humans to screw this resource up because humans are just that icky and gross, because cynicism and fashionable misanthropy. There isn't. Not in the real math. There's nothing humans can do in the real math that we know that the universe hasn't already done a couple dozen orders of magnitude larger than we could ever dream of. If space was going to "rip" or something, it would have already. It doesn't. In fact, in the real Einstein field equations, space very, very, very aggressively doesn't rip or suffer any other such problems. Any attempt to introduce such defects results in those defects immediately (and often quite energetically) undoing themselves. That's why when you read about real wormholes, for instance, the story always ends with the wormhole collapsing and exploding before anything, even light, could possibly traverse it.
Real math may change, in which case this answer may change. However, I expect that by the correspondence principle, even the ultimate unified theory of everything is unlikely to have an outcome where suddenly all this stuff is possible. Whatever the ToE is, it's still going to reduce to General Relativity in the large scale.
Maybe such technology involves the creation of unnatural structures, something like stable wormholes on human scales. If those structures outlived their creators, areas of space containing them could be considered polluted, or at least dangerous to later civilizations.
Douglas Adams did some thinking on this for Hitchhicker's, that time travel caused cultures to stagnate and eventually degrade for lack of motivation.
death knell
I know you probably know this, and I don't mean any disrespect. I'm writing this for other people who don't speak English natively (I'm also not saying you're not a native speaker, just that I'm not). I don't want anyone to go away with the wrong idiom.
I said nothing of the sort. You are, perhaps, confusing me with another poster.
"There's no spacetime equivalent"
Nonsense. The obvious example is emitted radiation, though any type of energy, matter or field byproduct might fit the bill.
The idea of "polluting" spacetime makes no more sense than humans "polluting" the natural organization of air molecules by displacing them by existing, or polluting the natural distribution of entropy by making decisions.
It's just bringing politics into a place where it doesn't belong.
"the presence in or introduction into the environment of a substance or thing that has harmful or poisonous effects."
I have no idea why you have this idea that polluting is some politicized term.
Your argument also doesn't hold considering that in the case we're talking about right now, the vast majority of spacetime is not inhabited nor is it regulated by anybody. Yet we're still able to talk about pollution because pollution in no way is tied towards a politicization of the conversation.
One could even go so far as to say you've polluted the conversation by including this "but you're making it political" distraction.
It's true that it isn't a problem -- until it is.
We are able talk about harm being done to insect populations or coastlines or any other inanimate objects. Again, humans, and their politics, aren't necessary.
I don't find my quoting of the definition as being petulant. I was wanting to frame the conversation around a shared definition of the word rather than asserting requirements.
Also, would black holes not be considered "broken space" themselves?
Even that is really an anthropomorphization. They don't "spread". They don't "damage" the space around them. They don't leave behind anything as they pass through space. They aren't having the effects that people imagine when they think about "pollution".
I once had, and never used, an idea that the consequence of using such technology was to accelerate dark energy, so that the more civilizations used FTL travel, the greater the energy requirements became. As if seafarers had to deal with the oceans expanding just a bit every time they were crossed.
That sounds like a recipe for some terrible monopolization by the first few civilizations that manage to achieve FTL:
Those who are the earliest to expand into the universe, would have access to the most resources, so they would be the only ones left who can afford to use FTL anymore, while younger civilizations may never have a chance to get out of their home stars.
Eons ago, the universe was room temperature. I've always wondered how an intelligent species adapted to that time would perceive our universe.
Yeah, that's where the "evil empire/East India Trading Company but in Spaaaaace" was going to come from. I wanted to use hyperspace/FTL as a vague metaphor for a scarce but crucial resource like fossil fuels (or whale oil, back in the day.)
I think it would be much more sinister.
The FTL controllers would be seen as gods by species who have yet to leave their homeworld. Every civilization within their sphere of influence would be either have to be their allies, thralls, unable to move around in the galaxy unless they get piggyback rides by the FTL barons, or engulfed and extinguished by them.
Imagine if we in the real world found out that there was no way to explore the universe we've been observing for centuries, unless we allied with some alien species that does not, can not, have our best interests on their minds.
Interstellar travel would require resources, and resources would require interstellar travel..
To go against the FTL species you would need to control at least as many planets as they do, and to control that many planets you would need FTL travel..
Sounds like a monumentally hopeless reality.
In terms of darkness and hopelessness, this is getting close to Cixin Liu's The Three-Body Problem trilogy.
A rot13 spoiler follows.
V xabj V fgvyy unir cnatf bs rkvfgragvny qernq nsgre gur frpbaq be guveq obbx vagebqhprq gur pbaprcg bs qvzrafvbany erqhpgvba jrncbaf. Onfvpnyyl, n fhcrejrncba gung fgnegf n furrg bs 2Q fcnpr rkcnaqvat ng yvtug fcrrq, juvpu erqhprf rirelguvat vg gbhpurf, vapyhqvat fcnpr vgfrys, gb 2Q. Vg'f bar bs znal JZQf ninvynoyr gb cbjreshy fcrpvrf. Abj gur qnex guvat vf, vg pna'g or fgbccrq. Bapr hayrnfurq, vg'yy xrrc rkcnaqvat ng yvtugfcrrq, qr-qvzrafvbavat fcnpr sberire. Ohg fcrrq bs yvtug vf fb fybj, gung lbh pna qebc vg ng lbhe rarzl'f ubzr flfgrz naq abg pner nobhg vg sbe zvyyravnf.
Nsgre gung, sbyybjf gur dhrfgvba. Unir lbh rire jbaqrerq jul jr yvir va 3Q fcnpr naq abg, fnl, 12Q? Gung'f jul.
I think I have kinda grown out of such existential dread, once I started having a sort of intuition that the Universe is too inconceivably large to be uniformly altered, let alone "damaged", by any single event or entity (which also makes me cringe at sci-fi which throws the word "universe" around so lightly.)
More importantly, I started thinking of the "backdrop" of Reality itself to not be Nothingness, but Everythingness.
That is, the blackness we see out there, must not be the default state. I imagine a flood of "white" behind the void, and the reality we perceive is just a heavily-filtered slice of it.
What led me to this way of looking at things was a comment on HN or Reddit, that talked about how, in certain axioms of mathematics, you can have a null/empty set but it's impossible to have a set which contains everything including itself. Whereas in another group of rules, it's impossible to have an empty set because everything else is arrived at by "subtracting" from the "infinite" set!
Maybe someone here knows better of what I'm referring to.
Wilbur and Orville Wright did precisely that for flight until the US govt dismissed the patents due to WWI. The patents, and their insane lawsuits, were poisoning the whole US on flight. Europe grew by leaps and bounds, and had all sorts of aircraft.
Well... actual reality doesn't even old much hope for FTL.
I believe one of the aborted ending ideas (and dangling plot-threads) for the Mass Effect series was something like this.
Like use of "Element Zero" (the exotic matter unobtanium that powers everything, including FTL travel) releases some kind of "dark energy" that is bad for vaguely defined reasons; one of which being that it destabilizes stellar fusion and makes stars go nova prematurely.
So the "Reapers," or big-bads of the series, were an ancient civilization that set up a recurrent process of culling nascent advanced civilizations to prevent them from growing and using such technology to problematic levels.
They ultimately scrapped it and went with an "AI and Man can't coexist" angle instead.
I thought that was a cool idea: that something as studied, ubiquitous and generally accepted as a natural phenomenon as the CMB is just a landfill, we just don't know what we're looking at.
Edit: here we go: http://memory-alpha.wikia.com/wiki/Speed_limit
Tangential - comparison of warp coils in different Starfleet ships: https://www.youtube.com/watch?v=gXAoIGTQEBY
If you are interested in such a plotline you should read the "The Three-Body Problem" triology by Liu Cixin.
Without going to much into detail: This topic pops up several times in the third (and perhaps already in the second) book of the series.
https://arxiv.org/pdf/gr-qc/9702026.pdf
Those insignificant civilizations don't need to worry about pollution, they need to worry about being gobbled up for mass-energy.
“In 2012, physicist Harold White and collaborators announced that modifying the geometry of exotic matter could reduce the mass–energy requirements for a macroscopic space ship from the equivalent of the planet Jupiter to that of the Voyager 1 spacecraft (~700 kg) or less, and stated their intent to perform small-scale experiments in constructing warp fields. White proposed changing the shape of the warp bubble from a sphere to a torus. Furthermore, if the intensity of the space warp can be oscillated over time, the energy required is reduced even more.”
[0] http://memory-alpha.wikia.com/wiki/Force_of_Nature_%28episod...
The book you're talking about was by Alastair Reynolds. It's the third book in the "Other" section here: https://en.wikipedia.org/wiki/Alastair_Reynolds#Novels.
Popular sci-fi like Star Wars or Star Trek etc. don't really touch on those possibilities in a convincing way.
As in, very few fiction truly conveys the ..indifference.. of the universe, and all the actors in it going about their own businesses and interacting only incidentally, even if to monumental consequences to each other, but without a lot of conscious intent or an overarching plan.
Kind of like how most of Earth's history has played out, even if it may all seem interconnected and purposeful due to the much more relatable scales involved.
That was pretty much the point of Rendezvous with Rama, until the sequels ruined it.
The scale between ant and human is large, but probably still smaller than the ratio power-wise between the civilizations with and without exotic matter. Might as well just call one gods and the other mortals.
There are more realistic explanations[3] for the Casimir effect however, so no Alcubierre drive for us anytime soon.
[1]: Think it was this show https://en.wikipedia.org/wiki/Sci_Fi_Science:_Physics_of_the...
[2]: To be fair, Alcubierre suggested it himself first
[3]: https://en.wikipedia.org/wiki/Casimir_effect#Relativistic_va...
I would genuinely love to read some of the exchanges between Alcubierre and these people.
By those experiences I'd say someone like Alcubierre is more likely to engage young ducklings than, say, Michio Kaku.
This is one example of many. The original poster was just spreading FUD and saying nasty things about Alcubierre as a researcher. He gives talks like this all the time, he refers to his own work as a warp drive, he quotes and shows off Star Trek, etc. This isn't an issue of crazies trying to steal someone's research.
Does it have to be matter? Or could we coax the vacuum into behaving as if it were occupied by exotic matter?
A: It requires hypothetical exotic matter. It's just theoretical thought experiment.
B: counterargument: The existence of exotic matter is not theoretically ruled out, so it could be real.
A: That's not a good argument. Something completely hypothetical invented so that it allows what you want to happen not ruled out is not good argument for relevance.
B: platitude counterargument: "If an elderly but distinguished scientist says that something is possible, he is almost certainly right; but if he says that it is impossible, he is very probably wrong." - Arthur C. Clarke. MOT.
A: It's a static solution; the warp bubble is eternal. There is no recipe for turning it on or off.
A: It's a vacuum solution; there is no "ship" inside, and no matter outside. It is everywhere exactly flat except for the compact region of spacetime in the bubble wall. The only way to decide whether it is even meaningfully moving in the sort of way intended is to add something else to the spacetime. One could make a more complicated vacuum solution with a qualitatively similar compact region, or "stitch" Alcubierre's solution into some other solution in a region where the latter is sufficiently flat to use an Israel-Darmois[1] approach, or add matter.
A: Adding matter is hard. Practically anything either breaks the metric or alternatively requires releases from well-tested constraints on the eigenvalues and eigenvectors of the stress-energy tensor. There is really no point in rehashing Lobo & Visser [2] here, although it would be a decent public service if a good science communicator explained its results.
At least Alcubierre's original paper[3] is short and easily digestible. Everyone should read from the paragraph before eq. (18) to the paragraph following eq. (19).
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Ultimately though "B"s are slogans, and matching them with pithier "A" slogans won't make "B"s go away (cf. young Earth creationists and anti-vaxxer). Perhaps it's more fruitful to make it easy for people caught in the middle to distinguish between a sloganeer with no idea of the content of the physical theories involved, and someone who at least knows the standard theory and how to use it before attempting to depart from it (thus excluding e.g. Sonny White).
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[1] Misner, Thorne, & Wheeler, §21.13.
[2] Lobo & Visser (!) https://doi.org/10.1088/0264-9381/21/24/011 https://arxiv.org/abs/gr-qc/0406083
[3] Alcubierre https://doi.org/10.1088/0264-9381/11/5/001 https://arxiv.org/abs/gr-qc/0009013
Can someone more knowledgeable than me weigh in on this?
The arguments against FTL are based on fixed spacetime frames of references. Which is obviously not-applicable to the situation where the spacetime is what you actively manipulate. Spacetime manipulating approaches don't violate SR as nothing is moving faster than c wrt. local space. The same like with those remote galaxies - while running faster than c from us they don't move faster than c wrt. their local space.
Except that in order to get the observables right, dark energy is required to keep the galaxy clusters running away from each other faster over time. It's a heavy price that no physical cosmologist is really happy about, but the concordance cosmology gets its name from how it is explicitly constructed to match ("concord with") observation (and it is consequently updated from time to time).
It is not really surprising that some energy is required to keep the warp bubble from dissipating away from its compact region of spacetime, particularly since the Alcubierre model is a vacuum spacetime, so there's no mass in the middle to keep the bubble local to it (or source it, or however you want to look at it). He says so at the end of his paper[1]. It's important to remember that there is no observational evidence supporting Alcubierre's metric; it concords with nothing. However, one can still attempt to answer the questions he himself raised, and Lobo & Visser do just that in their 2004 paper[2], where they note that the geometry of the Alcubierre metric requires an even-more-unusual-than-dark-energy stabilizing term even at arbitrarily low warp-bubble velocities.
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[1] https://doi.org/10.1088/0264-9381/11/5/001 https://arxiv.org/abs/gr-qc/0009013 (short, not especially difficult, fewer than 20 equations, 4 refs) See the text bracketing eqns (18) & (19)
[2] https://doi.org/10.1088/0264-9381/21/24/011 https://arxiv.org/abs/gr-qc/0406083 (twice as long, but mostly developed through more than 120 equations, 38 refs incl. [1])
[0]: https://www.youtube.com/channel/UC2kkCGRqZWaSIK3BmLC8vaw/vid...
EDIT: it says that in the wikipedia article too
what if instead of formula 1 we had races in orbit? this would push the boat out.
To get something like Moore’s law you need a breakthrough that opens up possibilities for many orders of magnitude in improvements. Baring new physics there’s just nothing like that available for rocketry.
We can't shrink Relativistic Physics like that to lower the energy required to reach 1c (infinite). Of course, it is possible that there is a better standard model of physics which does a better job of explaining things and which does say we can do so.
Another space race or space F1 would definitely make our engineering better and more reliable, but at massive cost and we still won't get FTL.
For instance, swimming banned LZR Racer swimsuits as "technology doping". Formula 1 has banned a variety of technologies, some just for increasing risk to spectators. America's Cup banned pedal power and cyborgs. Most human-powered races ban performance enhancing drugs.
I could easily see space racing banning negative mass.
The reference frame of either the origin or destination is a perfectly good/useful reference frame to choose.
The universe gives virtually everything in it some observables it seems silly to ignore:
https://en.wikipedia.org/wiki/Scale_factor_(cosmology)
just because the vast majority of observers aren't strictly Eulerian and thus there are lots of little details, like a dipole anisotropy in the CMB, or small deviations from a perfect blackbody spectrum. For A and B separated by non-cosmological distances, there are plenty of other local clocks available; around here one might use the orbital period of the Hulse-Taylor binary, for instance.
Of course the Alcubierre metric doesn't use the scale factor, since it is an everywhere-flat spacetime (i.e., not expanding) except in the compact region of the warp bubble's walls, and the metric does not admit a varying scale factor, and it is a vacuum solution so there are no CMB photons, binary pulsars, or any other matter -- not even a spaceship.
Making the Alcubierre metric even slightly more realistic exposes problems [1] which don't vanish when you make a reasonable (or any) choice of frame of reference.
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[1] Lobo & Visser (2004) https://doi.org/10.1088/0264-9381/21/24/011 https://arxiv.org/abs/gr-qc/0406083 (note that the problems do not depend on superluminality)
Most people should then have it explained to them that frame-dependent quantities are not fundamental.
This is something that arises even in their first exposure to Newtonian physics: kinetic energy is just such a frame-dependent quantity.
Generally covariant and invariant quantities are candidates for fundamental quantities.
The stress-energy tensor is, for example, generally covariant. However, one tends to want to slice it up into energy, pressure, stress, and so forth, and the fluxes of those in various directions. Those "sliced" quantities are frame-dependent.
Let's make the ship extremely tiny and have it represented as a contributor to the stress-energy tensor T at some point on the manifold, p \in M. The total of T at p is invariant. We're interested in the content of T at p that represents the ship, practically all of which will be in T_{00} when we write down exactly what axis 0 (the timelike one, unlike the spacelike 1, 2, and 3 axes) is.
Now let's move the ship. We want to shift the relevant covariant content of T at p \in M to p' \in M using parallel transport. With the mild assumption that locally at every point \in M there is a tiny patch of 4d Lorentzian spacetime, we can then talk about any path we like between p and p' as being everywhere timelike, everywhere lightlike, or neither. We still don't need coordinates; this is just a feature of a Lorentzian (sub-)manifold[1].
In perfectly flat spacetime, we have the advantage that there is exactly one everywhere-lightlike path from p to p'. However, that is often not the case when there is real spacetime curvature; and the curvature in the walls of the Alcubierre warp bubble cannot be ignored in this regard. We also have to consider curvature if p and p' are at cosmological distances, even in the non-Alcubierre/always-sublight case.
Coordinates now become useful, and here we will typically want to take a 4d Lorentzian spacetime and slice it into 3d spacelike hypersurfaces arranged by some time coordinate t_{past} < t_{0} < t_{future}. We sprinkle matter[+] on such a surface and use the initial values formulation[2] and covariant laws for the matter content, and predict how it evolves from one time coordinate to the next. In the case of a space ship, we are hoping to evolve to a final values surface, with matter sprinkled differently at some t', t < t'. It turns out that we can do this for matter that is not constrained to an always-timelike (i.e., subluminal) path -- with some mild assumptions[3] one can in principle make concrete predictions of the behaviour of e.g. a sometimes-FTL spaceship. However, what happens is generally very far from intuitive, and so should be determined by actually grinding out each infinitesimally-short-duration hypersurface from initial to final, solving each surface numerically.
Alcubierre, as it happens, has written a textbook about numerical relativity. [4]
So,
> you can change what most people would consider to be fundamental things about the journey
arises here in the choice of a slicing and a set of initial values. In effect, one is choosing an axis, calling it time, labelling it with tiny markings, and choosing a way to reflect "all space everywhere" at that particular time. One has enormous freedom with each of these choices, but some choices would be ridiculously useless to make, and others are seemingly quite sensible. However, note that our choices of initial and final values have ship @ p and ship at p' with p' at a later time coordinate. There is no ambiguity about which came first, and so no opportunity for either case in : "where it disappears from A and arrives at B instantly, and one where it goes from B to A.", where A is p and B is p'.
More rigorously, we would not specify the the slice in which ship is at B at all. We would instead specify all the content of a slice in which ship is at A, and then evolve slice by slice seeing what happens ("case A"). Or alternatively, we could specify all the content of a slice in which ship is at B, and then evolve slice by slice seeing what happens ("case B"). The time-symmetry of the physical laws of matter let us march in either direction from one slice to the next. But for a one-way journey, in "case A" we have only one slice in which we are closer to "case B", and vice-versa. If we evolve backwards from "case A", ship never reaches B. If we evolve forwards from "case B", ship never reaches A. It is only forwards from "case A" or backwards from "case B" that we recover the journey.
However, with realistically specified values surfaces, macroscopic thermodynamics come into play. One direction will have an overall increase in entropy (everywhere, from one slice to the next), the other direction will have an overall decrease in entropy. This difference sharpens as one makes the values surfaces more realistic, and as one applies finer and finer scale laws to the matter so specified. Observing B->A also means seeing eggs unscramble in frying pans at A and martinis being stirred apart into gin and vermouth at B, and many similar things everywhere one looks.
Is this fundamental? No, it's the result of having made a set of choices. However, the arrow of time is important to our experiences, and imagining aliens that experience a backwards arrow of time and interact with us and the things we see marching forward, leads to the latter being washed out by the difference in degrees of freedom (there are a lot more of them when you scramble than when you unscramble) -- by interaction with "our" matter, their arrow of time would be "corrected".
Sean Carroll had a series of blog entries on this topic some years ago [5].
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[+] Here this means the contributions to the stress-energy tensor; we may also need to supply values the left-hand-side of the Einstein Field Equations too, in cases where there is non-negligible gravitational radiation
[1] https://en.wikipedia.org/wiki/Causal_structure#Curves
[2] https://en.wikipedia.org/wiki/Initial_value_formulation_(gen...
[3] Geroch, R. AMS/IP Stud.Adv.Math. 49 (2011) 59-70 ("New Developments in Lorentzian Geometry", held in November 2009 in Berlin, DE), https://arxiv.org/abs/1005.1614 top of p.8 in preprint ("Initial-Value Formulation" subsection).
[4] https://global.oup.com/academic/product/introduction-to-31-n... (and cf. https://arxiv.org/abs/gr-qc/0412019)
[5] http://www.preposterousuniverse.com/blog/2004/10/27/the-arro... and later a series starting at http://www.preposterousuniverse.com/blog/2008/03/31/incompat...