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Like on a scale of one to ten, how plausible are Alcubierre drives in terms of our current understanding of physics? Plausible enough to be considered "hard sci-fi", or are they just another form of handwave like hyperspace?

This isn't asking about how the science behind the Alcubierre drive works (except as needed for other parts of an answer). Rather, it is about whether and how an Alcubierre drive can work in a hard-science level fiction work.

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    $\begingroup$ hard sci-fi, 100% possible theoretically, Even problems with energy can be managed by creating small pocket space, but I have my own reservation in it ever being practical. $\endgroup$ – Chinu Aug 1 '16 at 5:28
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    $\begingroup$ Hi Z.Schroeder. I'm afraid your original phrasing was an almost open invitation to opinion-based answers, so I tried to reword it in a way that retains what I believe was your intent, but does so while not as much seeking peoples' opinions. If you feel I changed your intent then by all means do feel free to edit further, but please do your best to allow us to objectively judge answers given on how well they answer the question. Every answer is equally valid and open-ended, hypothetical question are both specifically discouraged. $\endgroup$ – a CVn Aug 1 '16 at 11:52
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    $\begingroup$ @JaredSmith In all fairness, that doesn't really impact whether the technology itself is plausible; only its consequences. $\endgroup$ – a CVn Aug 1 '16 at 18:51
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    $\begingroup$ @MichaelKjörling FTL violates causality. If causality can be violated, our fundamental understanding of the universe is wrong. Since the OP framed the question in terms of sci-fi hardness, it needs to be addressed somehow. $\endgroup$ – Jared Smith Aug 1 '16 at 19:45
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    $\begingroup$ @Z.Schroeder As was pointed out also in a comment to a4android's answer, the effect on causality remains the same regardless of the mechanics of how FTL is achieved; all that is required is superluminal transmission speeds. Really, do read up on the tachyonic antitelephone. The Wikipedia article isn't that bad, and I wouldn't be surprised if it's touched on in physics textbooks also. See also searching for antitelephone on Physics SE. $\endgroup$ – a CVn Aug 1 '16 at 20:33
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The only real issue with Alcubierre drives is that they require negative matter to work. Negative matter (NOT antimatter, which is very real) is a hypothetical substance that generates negative gravity; i.e. that pushes matter away from it. In the "rubber sheet" analogy, where gravity is depicted as depressions in the fabric of space-time, negative matter creates "bumps" instead.

While mathematically possible under the geometry of special relativity, negative matter has never been observed, nor is there any place in the Standard Model of particle physics where it is expected to occur. So the big issue is, is negative matter possible? If so, an Alcubierre drive is simply a question of energy and engineering. If not, the drive is not possible.

Alcubierre drives are maybe around a 7.5 in sci-fi hardness - not a completely magical hand-wave but still requires an imaginary particle to work. They are more plausible than wormholes (which also require negative matter, as well as the need to 'tear' space-time) and less plausible than Dyson spheres (which require no imaginary physics, only scale and energy).

They are one of the least hand-wavey methods of FTL travel, so if your story needs FTL travel while remaining as plausible as possible an Alcubierre drive is probably the way to go.

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    $\begingroup$ The Alcubierre drive isn't "mathematically possible under the geometry of special relativity" it is a theoretical solution of general relativity. Apart from that, your answer is generally on the money. $\endgroup$ – a4android Aug 1 '16 at 6:30
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    $\begingroup$ Antimatter could be negative matter, but we haven't produced enough of it to find out. (I.e it goes boom before we have time to see which way it falls.) $\endgroup$ – wizzwizz4 Aug 1 '16 at 13:59
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    $\begingroup$ I would like to point out that negative density is possible - see the Casimir effect. Now of course the microscopic force created in a lab is far from a lump of negative matter you can stuff into an FTL drive, but the principle is there. I don't know if what we've observed qualifies as "matter", but if it does, then your claim of "negative matter has never been observed" is wrong. (IIRC they even made a nanoturbine powered by the Casimir effect, but I can't find any references now so it might have been speculation and I'm just misremembering.) $\endgroup$ – Boris Aug 1 '16 at 14:42
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    $\begingroup$ According to this Wikipedia article, the requirement isn't for "negative matter" but for negative energy density, which was thought to require exotic matter, but more recently it has been thought that a negative energy density can be produced without need of any exotic matter. $\endgroup$ – Anthony X Aug 1 '16 at 19:30
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    $\begingroup$ Doesn't negative matter potentially violate causality (because of the Alcubierre drive) making it logically impossible? $\endgroup$ – PyRulez Aug 1 '16 at 21:26
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The Alcubierre drive is theoretically plausible. It is a mathematically correct solution of the Einstein field equations. Its main problem is that the warped path through spacetime for its trajectory has to be set up before it travels. This makes it more like a FTL railway train than an independent vehicle like a spacecraft.

Segei Krashnikov, another theoretician, was dubious about the effectiveness of the Alcubierre metric and he proposed his own solution the Krashnikov. A spaceship travelling to Alpha Centauri creates the warped spacetime path as it travels to Alpha Centauri at sublight speed. On its return journey the spaceship travels along the Krashnikov tube arriving home faster than if it had travelled at less than lightspeed. Effectively that's faster than lightspeed, but only for the return leg of its round trip.

This example is to illustrate the fact that while the Alcubierre drive is theoretically plausible and mathematically correct, not every scientist working in general relativity accepts the concept.

There is also the exotic matter issue, because it requires negative energy densities to keep open the warp bubble. There are problems about the possible of causality violation, because if Alcubierre drive vessels travel first one way and come back again this allows trip into its own past. OK, this can be regarded as a bonus. With FTL travel, you get time-travel as a free accessory. Still it's a worry for those who hold causality dear.

If a writer was using an Alcubierre drive for their FTL spaceships it is scientifically plausible and can be considered as almost a hard-science concept.

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    $\begingroup$ I thought the Alcubierre drive was one of the FTL solutions without time-travel, since it doesn't actually move faster than light at any point? - Being quicker than light at your destination doesn't mean you can go to your past - if the light had to take a longer journey. -> I can travel faster than light: I send a laser beam to the mirror on the moon so it comes back 3 steps in front of me - and while the light is travelling I do the three steps and are there faster than light, but without time-travel. And the Alcubierre is essentially that $\endgroup$ – Falco Aug 1 '16 at 13:39
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    $\begingroup$ @Falco Causality violations are an inevitable result of FTL travel, regardless of how it is achieved. There's a great discussion here: physicsguy.com/ftl/html/FTL_part4.html and see also en.wikipedia.org/wiki/Tachyonic_antitelephone It doesn't matter if you're "moving faster than light", only that you (or information) are arriving outside the light cone that defines normal causality. Substitute the tachyonic telephone messages with messengers travelling by Alcubierre-powered shuttle, and the paradox remains. $\endgroup$ – IMSoP Aug 1 '16 at 17:18
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    $\begingroup$ «Being quicker than light at your destination doesn't mean you can go to your past» yes, it does. See this answer. $\endgroup$ – JDługosz Aug 2 '16 at 10:59
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    $\begingroup$ @JDługosz But that's simply not true, since light is affected by gravity and bends/slows down around big gravity wells, you can create a real world scenario where a straight light-ray is slower at a destination than a longer way around the gravity well. This doesn't lead to time travel. $\endgroup$ – Falco Aug 2 '16 at 14:19
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    $\begingroup$ @Falco Right, that is a common suggestion for how to solve the problem, for instance some form of special "subspace frame of reference", in which everyone can agree that Alice's FTL drive arrived before Bob's departed. That is not, however, directly implied by Alcubierre's theory, based on what I've read about it. You can have 2 of {relativity, causality, FTL}. A "privileged frame of reference" resolves that by discarding (replacing) relativity, but Alcubierre was working entirely within relativity. $\endgroup$ – IMSoP Aug 3 '16 at 12:32
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There is a weak sense in which the Alcubierre geometry is a "solution" of general relativity. But anything is a "solution" of general relativity in this same weak sense. General relativity says that the matter distribution in a spacetime is related to the geometry of the spacetime in a certain way. You can take any spacetime geometry (as long as it's twice differentiable), plug it into that equation, get a matter distribution, and then say "if only we could make this matter distribution happen, we could make this spacetime geometry happen", with exactly the same plausibility as the Alcubierre geometry.

For example, suppose you want the Sun to suddenly disappear. I don't mean that it accelerates away, or blows up, or anything like that. It just disappears. Its gravitational field goes to zero, planets fly away in a straight line, etc.

It's easy to do this. You just take a spacetime metric with the Sun in it (the Schwarzschild interior and exterior solutions stitched together), and a spacetime metric without the Sun (Minkowski space), interpolate between them with a smooth (or at least twice differentiable) function of time, and plug that into the Einstein equation. The result will not make much sense: you'll find that the Sun's mass flows out to infinity through a region with zero mass density. Exotic matter to the rescue! By introducing negative-mass matter, you can counter the mass of the outflowing matter and make the total mass density zero in the region where general relativity says it has to be zero.

It's important to understand that this matter isn't "exotic" just in having negative mass. It's exotic in that it doesn't follow any physical laws. It just shows up out of nowhere (literally, out of vacuum) during the disappearance of the sun, then disappears into nowhere. It isn't even subject to cause and effect, much less any more specific physical theory.

The same is true of the exotic matter in Alcubierre's spacetime. It is not a "warp drive", because that would imply that the exotic matter could come from the spaceship that's going to ride the bubble. It can't, because the exotic matter on the outside of the bubble is spacelike. That means that either it travels locally outside the light cone, or it arises independently everywhere along the path. The first case would make the solution uninteresting, since if you can travel outside the light cone then you don't need a general-relativistic warp drive to circumvent light speed. So the exotic matter can't come from the ship. In Alcubierre's geometry, it appears miraculously out of vacuum just before the ship arrives, then disappears into vacuum after it leaves. General relativity is perfectly fine with this. As soon as you add any additional physical laws, it's ruled out.

People have speculated about exotic matter guns that could pre-distribute exotic matter with the right properties. You could likewise speculate about a Sun-vanishing gun. The argument in both cases is "Well, based on everything that we think we know about the world, this is impossible. But if you ignore some of that, the remaining premises aren't strong enough to prove it's impossible any more. So maybe it's possible!" Yes, maybe. All science is subject to revision. But based on everything we know right now, the Alcubierre drive is as scientific as time travel. (In fact, it could be used for time travel.)

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The Alcubierre Drive is indeed possible from a scientific point of view. Exotic matter is not hypothetical, it has being observed, but is just very difficult to produce and it is almost impossible for us, right now at least, to make it in amounts large enough. Also, Alcubierre proposed that Casimir vacuum might work instead of exotic matter, for that matter (no pun intended). Whether it violates causality or not that was debated somewhere else.

To me, the real issue is more about practicality. The Alcubierre Drive generates a lot of practical problems, apart from costs. For example:

  • Maybe no living being can survive inside the bubble do to the extreme amount of heat and Hawkings radiation. Of course this does not precludes for sending intelligent robots to explore the cosmos instead which is a pretty good idea for a sci-fi setting. -The planet-destroyer shock wave of particles at the arrival.
  • How to maneuver and stop the vessel if the people inside can’t interact with the universe outside the bubble. Etc.

But any talented sci-fi writer can find clever ways to overcome these issues.

Another thing that most writers overlook is that most of the problems I mentioned do not happen in subluminal speeds, and that makes the Alcubierre Drive interesting for a non-FTL setting becoming the by far fastest way to travel, for example, inside the Solar System.

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Alcubierre drive is a sound way, under the current laws of physics, to explain a valid way of travel faster-than-light and the only way (unless wormholes are ever observed) to “break” the speed of light. However, and this is a big however, what makes AD improbable or implausible for our current civilization is how to make it work. For it we would need whether exotic matter with negative mass (which still hypothetical) or harnessing somehow the Casimir effect which causes negative mass. Both things are not impossible, but we still don’t know how to do either yet. So, maybe tomorrow someone would find the way or maybe never.

So, if for example you’re working on science fiction book and wanted it to be hard sci-fi, the “hardish” way to make FTL is with the AD, but for a lot of people even in that case is then no longer “hard” sci-fi (but this is kind of subjective).

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I consider to be a very high probability that we will be able to make such a thing. Unfortunately there is a good chance it wouldn't be useable as a form of travel due to some of the possible secondary effects like having to be on rails and carrying high energy particles with which would end up just making it a very expensive and time consuming way to shoot a planet with the worlds largest most powerful 1 target cannon...

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    $\begingroup$ Why do you think that there is a "very high probability that we will be able to make such a thing"? $\endgroup$ – Philipp Aug 1 '16 at 8:12
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    $\begingroup$ This reads less as an answer and more as an opinion. $\endgroup$ – Trotski94 Aug 1 '16 at 10:18
  • $\begingroup$ Stack Exchange is confusing for a beginner. What I fail to understand is how you can be a beginner if you have 738 reputation, because that's: 73 answer upvotes, one question upvote and an approved edit; or 147 question upvotes and an approved edit. $\endgroup$ – wizzwizz4 Aug 1 '16 at 14:03
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    $\begingroup$ @Durakken Ok, but by saying "I consider to be a very high probability that we will be able to make such a thing" without at least any relevant credentials or reliable sources to back it up makes this answer as useful as if i asked my Nan the same question and took her response. $\endgroup$ – Trotski94 Aug 1 '16 at 14:16
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    $\begingroup$ You have already included, in this comment thread, details that would improve your answer: that experiments are already being conducted; that NASA has a project related to it; that the maths appears to be sound. Expand each of those into a sentence, and you have an answer. Back them up with some clues for where to find further reading, and you have a good answer. As currently written, there's no clue that you consider it likely based on evidence, or based on a dream you had once where you were captain of the USS Enterprise. ;) $\endgroup$ – IMSoP Aug 1 '16 at 18:20
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It depends on your definition of "plausible". I would take it to mean anything not disproven by established fact or available evidence. There is no need for a working demonstration test article to exist just to make a claim for plausibility.

Consider a few established realities of our modern world:

  • jet airliners
  • nuclear powered submarines
  • humans have walked on the Moon
  • smartphones and all the wonderful tricks they can do
  • GPS
  • CGI movie effects

and how plausible any of that would have been to a Jules Verne era audience.

Consider also that NASA has taken the Alcubierre drive seriously enough to fund related projects at their Advanced Propulsion Physics Laboratory.

On the other hand, Star Trek style artificial gravity, superluminal ("subspace") communication, force fields, phasers and Vulcan telepathy are all far less plausible. Nothing in our current understanding of physics permits the existence of any of them. Although teleportation (a "transporter" in Trek-speak) is theoretically possible and even demonstrated to a limited degree at a quantum mechanical level, it is nevertheless implausible on a human scale because of all the difficulties which arise. Without a whole new understanding of physics, none of this belongs in hard sci-fi.

What may have been seen as totally implausible a century and a half ago is now part of our everyday world. Who could have predicted the wireless revolution or the internet at a time when there was no such thing as radio, telephone, or anything more computationally sophisticated than a mechanical adding machine?

Even if it turns out that an Alcubierre drive is impossible to build/operate, it has passed the threshold of plausibility not just in hard sci-fi but even respectable science because:

  • the laws of physics don't forbid it
  • establishment funding has been applied toward its investigation
  • related experiments have produced some interesting results, even if inconclusive
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  • $\begingroup$ To be fair, everything you listed is possible, it just depends on how many caveats you want to give it. Quantum teleportation is possible, as you said, but creates an absolutely ridiculous number of problems when scaled up. Artificial gravity can be created, but it's more of an illusion of gravity created by centrifugal force. With some incredible but still plausible advances in energy generation and miniaturization, phasers could be feasible in the far future. And telepathy only requires a few cybernetic implants that allow you to communicate wirelessly with other people's implants. $\endgroup$ – Z.Schroeder Aug 1 '16 at 21:18
  • $\begingroup$ @Z.Schroeder I was referring to devices/phenomena as depicted - artificial gravity handwaved as "gravity plating" in later shows; handheld weapons capable of emitting an adjustable energy beam which can stun, kill or vaporize according to a setting; telepathy by means of some natural ability (not artificial implant) between a "telepath" and an otherwise un-endowed subject. $\endgroup$ – Anthony X Aug 1 '16 at 21:56
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    $\begingroup$ There are two sorts of plausible. "Could you explain how it works (fundamentals) to a physicist" and "Could you explain how to make one to an engineer". The physics of a jet engine would have been pretty clear to Newton. The physics of anything nuclear or electronic would not have been. Travelling to the moon and walking on it would not have been physically implausible to Newton, but the engineering would be quite another matter! $\endgroup$ – nigel222 Aug 2 '16 at 13:45

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