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Alcubierre drive (wikipedia.org)
166 points by angrygoat on Aug 13, 2018 | hide | past | favorite | 125 comments

There was a time where relativists realized that instead of solving the Einstein equations subject to initial conditions coupled to some matter stress energy terms, they could instead fully specify what spacetime looks like, ram it through the Einstein equations and get a stress energy tensor on the right hand side.

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.

> 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.

As an aside, I'm loving this talk (the YouTube video). Thank you for sharing

> If we could ever produce such matter then we could start some spacetime engineering.

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? :)

[0] https://en.wikipedia.org/wiki/Void_(astronomy)

"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"

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.

"In what preferred reference frame does your "pollution" take place in?"

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."

"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.

The pollution would be internal to the civilization, not something lasting of import to outsiders. The ability to warp spacetime to the point of violating causality, time travel, might be a death nail for a civilization. Literal prescience could be dangerous. As such technology moved though a society it could be described as a pollution.

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.

> The ability to warp spacetime to the point of violating causality, time travel, might be a death nail for a civilization.

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.

"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"

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.

Pollution is undesirable on earth because it causes us harm from which we can't escaoe or reverse.

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.

Pollution, by definition, is something that causes harm:

"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.

Pollution is intimately tied to politics because most inhabited property is owned, which means it's regulated by some governmental systems having varying degrees of regulation and penalties for pollution. I have no idea why that has to be explained to you.

Being able to point towards a subset of pollution which is regulated does not necessitate the entirety of pollution as regulated/political. Pollution isn't defined by political fiat.

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.

You admit space is uninhabited. Your unnecessary and petulant copy/paste of the definition of pollution includes the words "poisonous" and "harmful". Which isn't possible to do to an uninhabited area. I have no idea why you keep forcing such a contrived idea, other than to draw in a political "bad humans, always polluting" message. Anyway I'm done with this silly conversation. Have fun

Much of our terrestrial pollution took place in "uninhabited areas" which decades later became inhabited. Your comment is reminiscent of 19th and early 20th century attitudes toward pollution.

It's true that it isn't a problem -- until it is.

I'll acknowledge that harm and poison require a reference for where the harm is done, but I don't think it requires human interaction necessarily.

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.

Putting "pollution" in quotes was to substitute it for "waste", or byproducts, or side effects, that may or may not be undesirable for some species somewhere and somewhen.

I just imagine that once you do work at such scales, and with such strange materials and results, then the "waste products" of that work would be just as weird.

Also, would black holes not be considered "broken space" themselves?

"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".

There’s no ‘nuclear holocaust’ term in the equations for atomic physics, but nonetheless it is an undesirable consequence of screwing around with nuclear weapons.

By violating conservation of energy are you referring to dark matter?

Dark energy. Generally speaking, energy is not conserved in general relativity: http://www.preposterousuniverse.com/blog/2010/02/22/energy-i....

According to General Relativity, energy on our Universe isn't conserved.

He could just be referring to the existence of matter at all to be fair. Violations of thermodynamics are required for anything to exist at all.

This is very similar to Strugatsky Brothers' Roadside Picnic[1], a book which inspired, among other things, Andrey Tarkovskiy's film Stalker and the S.T.A.L.K.E.R. video game franchise.

[1] https://en.wikipedia.org/wiki/Roadside_Picnic.

Thanks for the recommendation, I adore the Stalker film.

>Imagine the "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.

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.

> so that the more civilizations used FTL travel, the greater the energy requirements became.

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.

> Those who are the earliest to expand into the universe, would have access to the most resources

Eons ago, the universe was room temperature. I've always wondered how an intelligent species adapted to that time would perceive our universe.

Likely, he wouldn't. The window in which the universe was roughly room temperature is very narrow in astronomical terms, so even if life did form during that period, it probably would not have been able to form even animal life, much less intelligent life.

If everything was happening faster back then, and if we assume there were more interactions between everything and more kinds of interactions back then, then life and intelligence may have evolved faster too.

There's no reason to expect more kinds of interactions; the laws of physics were the same then as they are now. It also isn't clear that there would be more interactions. Galaxies hadn't formed, even if we stipulate that stars could have. So, the good: wherever a planet forms it should be warm enough for life (although it could still be too warm if it's close to its parent star). The bad: There are probably far fewer planets and ingredients for life and there is precious little time. For reference, for intelligent life to form during this period, it would have had to have done so at a rate of about 500:1 compared to the evolution of homo sapiens on Earth. Even life evolving in this narrow window would be difficult (~50:1 compared to on Earth).

That also sounds like a recipe for maintaining causality as in the faster you are trying to go to more you’ll expand space time maintaining the luminal speed limit from the point of view of an observer.

>That sounds like a recipe for some terrible monopolization by the first few civilizations that manage to achieve FTL:

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.)

> "evil empire/East India Trading Company but in Spaaaaace"

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.

> 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.

Decoding it to ROT18 makes some amusing changes to the numbers.

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.

Nice use of rot13.

Why thank you. It's common elsewhere in places that don't have rich comment markup supporting spoiler tags; it dawned on me recently that I didn't see it used much on HN for some reason.

And don't think for a minute that the first human company that gets that tech wouldn't do the exact same thing.

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.

This story is oft-repeated here but this paper provides convincing historical industrial evidence that it's a myth: https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2355673

Sounds like the Spacing Guild from Dune.

>Sounds like a monumentally hopeless reality.

Well... actual reality doesn't even old much hope for FTL.

hah but at least, we seem to have the luxury of exploring our neighborhood at our own pace, in any direction, without capitulating to alien overlords.

>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.

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.

Star Control II, one of the inspirations for Mass Effect, also had some very interesting cosmic-scale intrigue, between many different species interacting over eons.


You might enjoy John Scalzi's latest, The Collapsing Empire. Not quite the same, but it does feature fast "currents" in spacetime that are in the process of "drying up," leaving distant systems stranded by uncrossable gulfs.

I've read a scifi story with this exact plot, so it looks like you're not alone!

Yeah, it's driving me crazy that I can't remember the author or title. As I recall there are some older species desperately trying to keep younger worlds from using the technology because the problem is going to keep getting worse.

I want to say it's a Ted Chiang story, but I'm not sure why - probably because his is the last collection of short stories I recall reading - but none of the titles listed look familiar.

I don’t think so; as far as I know, I’ve only read his story that was adapted for Arrival.

Well, there was that time Earth was demolished to make room for a bypass. I'm not sure I recall all the details but I think the universe ended as a convoluted consequence?

Yep, that is exactly what I was thinking about. :)

In Accelerando, there was a similar plot device. The anisotropies in the CMB (or maybe just minor aberrations) were caused by waste heat from Type IV/V civilizations running gigantic computations (trying to probe/find vulns in the laws of physics, if I remember correctly).

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.

There’s some Star Trek episodes that touch on this, maybe in later TNG or DS9. Can’t recall exactly at present.

Edit: here we go: http://memory-alpha.wikia.com/wiki/Speed_limit

I missed your comment and made a similar comment without the attached episode. Thank you and the below comment on how they solved it. I assumed they just didn't care as it was the delta quadrant and the whole show is an emergency.

Preventing this damage without a speed limit was the function of Voyager's moving warp nacelles. I think they solved it without moving parts in later designs.

The Defiant class's warp nacelles are angled slightly toward the bow of the ship, at least in some parts of the canon. Some speculate this is for the same reason - a fixed-nacelle design that prevents this damage without a speed limit.

Tangential - comparison of warp coils in different Starfleet ships: https://www.youtube.com/watch?v=gXAoIGTQEBY

> Imagine the "pollution" caused by civilizations [...] Destabilizing solar systems or even entire galaxies, maybe even violating causality..

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.

One estimate of the total exotic matter required to form a bubble puts it around 10^20 galaxies worth.


Those insignificant civilizations don't need to worry about pollution, they need to worry about being gobbled up for mass-energy.

According to Wikipedia:

“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.”

If someone violates causality, who should be held responsible?


Despite the sword and sorcery veneer this is actually the premise of several of CJ Cherryh's works, for example : https://en.wikipedia.org/wiki/Exile's_Gate

Ha! Star Trek: TNG actually had an episode about that.

[0] http://memory-alpha.wikia.com/wiki/Force_of_Nature_%28episod...

There's a plot point in one of the sequels to The Three Body Problem like this. Certain spacetime-warping propulsion technologies turn out to leave behind a large mark in spacetime which isn't directly harmful, but is easily detectable even at stellar distances. Because of the Dark Forest theory, such a mark is taken as a giveaway of a careless and technologically dangerous civilization, which causes other advanced civilizations to immediately try to kill the civilization that made the mark before it can advance further or launch its own attacks.

Take it up a level: maybe the entire universe is the result of this. The Big Bang was the result of flushing another universe’s waste processes.

Of course, there is a Star Trek episode (TNG) where some alien scientists discover this, but nobody wants to believe them including the Enterprise as the consequence is greatly restricted warp drive (warp 9 becomes only emergency). I think they mention it again once or twice. Maybe a super fan can attach the episode #.

Season 7 episode 9 (Force of Nature). I believe in Voyager they mentioned something about a new warp engine design that mitigated the problem.

I recall a bit of scifi, the title escapes me, where they placed an opaque cloud between our galaxy and Andromeda, to block any form of communication, so they could have FTL without violating causality.

What you describe is a teeny-tiny bit of a a spoiler. Erring on the side of cation, obfuscating my reply.

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.

My brother has (I think jokingly) suggested that the dark energy expansion of the universe is an indication of such pollution.

That sounds like some great sci fi, actually.

Ever since I played the awesome old RPG called Planet's Edge [0] and the better-known Star Control II, I've been taken to muse about cosmic coincidences.

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.

[0] https://en.wikipedia.org/wiki/Planet%27s_Edge

> indifference.. of the universe, and all the actors in it going about their own businesses and interacting only incidentally

That was pretty much the point of Rendezvous with Rama, until the sequels ruined it.

If you can engineer spacetime, there really isn't such a thing as a long-term consequence anymore.

Not even to those who can't engineer spacetime yet?

So the ant in the universe where shoes have just been invented and people stop hesitating to step on them?

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.

That's right. The engineers have eternity to achieve outcomes satisfying to all parties. And if they choose not to, well, that's an all-time consequence, not a long-term one.

That Michio Kaku episode[1] on Discovery where he said[2] that we might be able to harness the Casimir effect as a source of the negative energy needed for the Alcubierre drive probably didn't help.

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...

Even if your [3] doesn’t apply, I believe the Casimir effect doesn’t scale fast enough — even a cavity made from proton-sized parts will never exclude more vacuum energy than the mass energy of those protons: https://kitsunesoftware.wordpress.com/2017/04/20/can-a-casim...

> he gets contacted by all kinds of Star Trek fans and other people who believe they can produce the drive.

I would genuinely love to read some of the exchanges between Alcubierre and these people.

What makes you think he engages with them?

Some might be polite and somewhat knowledgeable (though most won't be either).

In the late 90s to early 2000s I was a teenager and communicated with various scientists and academics via email, some of which had something of a mentor role.

By those experiences I'd say someone like Alcubierre is more likely to engage young ducklings than, say, Michio Kaku.

make it so

No problem: https://www.youtube.com/watch?v=5q_z8BjiYng

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.

This is very true, it’s also worth mentioning that all ways of faster than light travel means travel back in time in some reference frame which in breaks causality.

Here's a good (IMHO) video showing how this works out, by looking Minkowski diagrams for different observers.


You don't travel faster than light with the Alcubierre Drive, you circumvent travel entirely. Spacetime does the traveling, not the ship.

Breaking causality is allowed by the laws of physics.

Breaking causality is allowed, though.

Is there nothing interesting within the constraint of a reasonable stress energy tensor?

Black holes and possibly topological defects.

> all the interesting spacetimes (such as the Alcubierre drive) require exotic matter

Does it have to be matter? Or could we coax the vacuum into behaving as if it were occupied by exotic matter?

Matter is just excitation in the vacuum. But excitation with positive energy. What is needed is excitation with negative energy.

.... But you make one warp drive....

Template for discussion:

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.

Agreed. The pattern could range practically forever, with "B"s appearing for any "A":

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).

- --

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).

- --

[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

I'm not a physicist, but my understanding is that exotic matter isn't strictly required. They say that because what's needed is a sort of negative energy/mass to cause an anti-gravity effect. But even that isn't accurate. What's really required is a lower energy density than a vacuum. This can be achieved in some cases, for example, in the casimir effect. When you look at it that way, it opens up some possibility, that we could achieve this in some sort if fashion, at least on a small scale, right?

Can someone more knowledgeable than me weigh in on this?

the best counterargument for me is that remote galaxies run away from us with the speeds larger than c. So space expansion and the speed faster than c as a result is possible. Without exotic matter, etc. Now there is just an engineering task of implementing it as a practical space ship drive.

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.

> space expansion and the speed faster than c as a result is possible. Without exotic matter, etc.

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])

AsteronX is an awesome channel and goes into the mechanics of space travel [0] and then two videos on the Alcubierre-Froning Warp drive [1], [2].

[0]: https://www.youtube.com/channel/UC2kkCGRqZWaSIK3BmLC8vaw/vid...

[1]: https://www.youtube.com/watch?v=n38aP2tROBo

[2]: https://www.youtube.com/watch?v=M3fw2wQlWL4

Issac arthur has a fascinating video on the topic, that I recommend to anyone who's interested in the idea: https://youtu.be/yCCsmxGjEV0

He ties it in well with his Fermi paradox videos. If such a thing is possible, it makes the stakes of the Fermi paradox much higher. We aren't just wondering why we don't see other intelligent life in our galaxy, but why we don't see it in a much larger region of the Universe.

The frame shift drive in the ELITE video game series is based on this technology: https://www.reddit.com/r/EliteDangerous/comments/2nqtzp/fram...

For anyone interested in fiction based on ideas like this, check out the Three Body Problem series. I'll avoid talking about it further to keep from spoiling it.

Didn't they propose to elide the use of exotic matter with a Casimir vacuum.

EDIT: it says that in the wikipedia article too

It reminds me of Hyperion's Cantos' "Hawking drive"

how about we just figure out how to get as close as possible to 1C?

We already know how to get arbitrarily close to 1C: just keep adding energy to the system to produce enough thrust to overcome the additional mass of said energy's source and the relative mass increase from getting closer to 1C. It's just not practical and the laws of physics forbid it from ever being practical.

but if we keep trying there should be a spacefaring equivalent of Moores law?

what if instead of formula 1 we had races in orbit? this would push the boat out.

I doubt any metric in Formula 1 doubles every 2 years.

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.

No. Moore's law is about making things smaller for speedups.

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.

Nearly all racing formats have banned certain technologies.

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.

Without a preferential reference frame, that's a nonsensical question. You can define your velocity to be arbitrarily close to C just be defining in relation to which inertial reference frame.

I would point out that this is "nonsensically pedantic." Clearly the frames of note are the one you begin your acceleration in and the one you end your acceleration in.

The question isn't phrased that way, its "as close as possible", implying that there is some physical limitation that's passed by traveling 99.9% c vs 99.99% c.

That's rather a pointless nitpick. If we want to get from point A to point B, we can do it faster by travelling faster.

The reference frame of either the origin or destination is a perfectly good/useful reference frame to choose.

The more important question is with an Alcubierre drive, how do you decide which direction it was traveling. Since it is traveling faster than C, you can pick a reference frame where it goes frame A to B, one where it disappears from A and arrives at B instantly, and one where it goes from B to A.

Which of those gives you observables like a very nearly isotropic 2.725 K (and slowly cooling) blackbody spectrum, for example ?

The universe gives virtually everything in it some observables it seems silly to ignore:


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)

Of course you can easily define an inertial frame of reference that is not moving in relation to the CMB, a binary stare, or whatever else. You can easily define whether the journey in relation to those. The issue is you can change what most people would consider to be fundamental things about the journey by choosing a different inertial reference frame.

> what most people would consider to be fundamental

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]


> 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...

It would be way cool if one of these could be installed in an old VW Beetle. Totally excellent fuel economy!

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