# Alcubierre Drive without FTL?

Would the principles at work in the theoretical Alcubierre warp drive apply to a slower-than-light starship? I'm building a setting in which FTL communication and travel is impossible, (due to violations of causality, if nothing else) but would still like to have starships that can accelerate to relativistic speeds in a narratively convenient amount of time. Would the Alcubierre drive work or do I need something else entirely?

• Alcubierre drives don't violate causality. – Halfthawed May 11 '20 at 16:43
• Anything that allows a signal to get from point A to point B faster than light will allow causality violation, including Alcubierre drives. – Christopher James Huff May 11 '20 at 17:25
• @Halfthawed - The original Alcubierre solution didn't violate causality, it involved a single bubble moving in an otherwise asymptotically flat universe, but it's been shown that a solution involving two bubbles moving in different directions can violate causality in a way similar to the tachyonic antitelephone. This was shown in this paper and also on p. 124-129 of Time Travel and Warp Drives by Everett/Roman. – Hypnosifl May 11 '20 at 22:46
• +1 because scifi worldbuilding/storytelling embracing impossibility of FTL & consequences of relativity rather than papering over it to "recreate sea battles in space" is such an under-developed and under-appreciated niche. – R.. GitHub STOP HELPING ICE May 12 '20 at 1:02
• @Dragongeek en.m.wikipedia.org/wiki/… the Alcubierre warp bubbles could be used to construct closed time-like curves that violate causality. – BMF May 12 '20 at 1:19

## 6 Answers

The Alcubierre drive works by distorting space around a bubble: expanding space behind it and contracting space in front of it. It's a nice way to get faster-than-light travel without, well, technically traveling faster than light. But the basic mechanism behind it can, it works out, work at any speed. Nothing in the equations forbids sub-light speed travel.

The Alcubierre drive is one of a more general class of spacetimes (Natario 2001), and the metric describing how space curves around it is defined partly by an arbitrary choice of a function $$x_s(t)$$, from which we get the speed $$v_s$$ by differentiating:$$v_s=\frac{dx_s(t)}{dt}$$ While there are constraints on certain other parameters of the metric (such as the function defining the shape of the bubble), there's no mandate that $$v_s. We have some freedom there.

Unfortunately, even sub-light speed travel falls prey to one of the classic problems with Alcubierre's original idea: it still requires a negative energy density. The energy density inside the bubble, as seen by an outsider observer, is $$\rho\propto -v_s^2$$, and while this goes to $$0$$ as $$v_s$$ goes to $$0$$, it is still negative. It would be nice if traveling slower than light got us out of that little kink, but it doesn't.

However, traveling slower than light is an improvement in other ways. For example, particular horizons form (see Finazzi et al. 2009) when the drive reaches $$v_s=c$$; those horizons are absent at sub-light speeds. This enables communication (a term I use loosely here) between the bubble and the outside universe. The horizons have historically been regarded as another one of the practical problems with a classic Alcubierre drive, alongside the negative energy density.

All that said, yes, you can use an Alcubierre drive subluminally. Let me know how that goes for you.

• @Halfthawed By "absolute reference", do you mean an observer outside the bubble? – HDE 226868 May 11 '20 at 16:57
• So any drive system of the type (whether faster or slower than light) would require exotic matter that may or may not be physically possible to create? – Tim McClelland May 11 '20 at 17:14
• @TimMcClelland I believe so, yes. – HDE 226868 May 11 '20 at 17:36
• @Halfthawed I haven't been able to find a good enough source on null geodesics in the sub-light speed case to say much about that. – HDE 226868 May 11 '20 at 17:48
• @Halfthawed I thought Einstein addressed the issue of "absolute reference" in that little theory of his almost nobody has ever heard about? – Tom May 12 '20 at 13:03

Yes. The Alcubierre metric can produce arbitrary boosts, both above and below c. In fact, using it for fast sublight travel is more technologically plausible than the FTL travel for which it was initially investigated, because you can actually plausibly control a sublight Alcubierre bubble. FTL bubbles, as currently understood, have the unfortunate characteristics of not actually allowing any signals from inside the bubble to reach the boundary, so you can neither turn it on nor turn it off internally. Sublight warp bubbles, on the other hand, do not have that problem.

• Would you theoretically be able to use STL Alcubierre drives to visit other solar systems to build a network of FTL Alcubierre "Warp Gates" that accelerate and decelerate ships that pass through them? Just make sure to aim really precisely when you accelerate them to FTL speeds, because if you miss your target, you'll never stop going. – nick012000 May 12 '20 at 14:20
• @nick012000 That is a really neat concept, like the 'stepping stones' of Ring World. If it were done on a personal scale, instead of a 'spaceship' scale, it ramps down the power necessary considerably. Why take the entire spaceship, when all you want to do is move the person? – Justin Thyme the Second May 12 '20 at 15:12
• @nick012000 It's theoretically good enough for scifi! – Logan R. Kearsley May 12 '20 at 15:23
• If you've got the exotic matter and other tech necessary to create Alcubierre bubbles at will, just go another step and create some wormholes and make permanent links between stars. – Ross Presser Dec 29 '20 at 16:14

The Alcubierre metric is time independent, i.e. it assumes the drive has always existed in the past and will always exist in the future. It does not describe how the drive accelerates up to its cruising speed and it does not describe how the drive decelerates back to rest again.

The drive works by using a torus of exotic matter. As HDE 226868 describes in his answer this deforms spacetime in such a way that the drive moves at a constant velocity. Note that this does not require any energy to be supplied because as long as an object moves at constant speed, i.e. does not accelerate or decelerate, its kinetic energy is constant. However it does take energy to accelerate and it does take energy to slow down again. Your problem is working out how to supply the energy.

The Alcubierre metric doesn't help you here because as I mentioned it doesn't describe the acceleration or deceleration. To start the drive you would have to start with the exotic matter widely separated and you would have to bring it together to form the torus. As you did this the drive would start to accelerate. Then you'd have to separate the matter again i.e. dismantle the torus and take the pieces far away from each other. That would bring the drive to a halt.

And it's going to take energy to assemble the torus to start the drive, then it's going to take energy to dismantle the torus to stop the drive. This is your problem. Were do you get this energy? Alcubierre drives need huge masses - gigatonnes of mass. You have to figure out how to pull in this huge mass from a large separation and then push the mass back out to a large separation when you want to stop. It's not obvious that this is any easier than just accelerating a conventional drive in a conventional way. If you need superluminal speeds then you need something like the Alcubierre drive. For subluminal speeds it's not obvious that it offers any advantage.

Footnote: a quick note on the masses required. Alcubierre's original drive required exotic matter with the mass of Jupiter. Various modifications of the geometry have been made to reduce the mass required, and Harold White at NASA has suggested it could be reduced, though as far as I know he has not published a proof of this. I have seen the figure of 800kg in popular science articles but I cannot find this in any of White's publications so I don't know where this figure came from. His paper Warp Field Mechanics 101 is often cited but the figure does not appear in that paper.

Chris Van Den Broeck has suggested a radically different geometry described in his paper Alcubierre’s warp drive: Problems and prospects (behind a paywall I'm afraid) that could reduce the mass required to negligible amounts. However this requires densities that are so high as to be physically unreasonable i.e. 77 orders of magnitude more dense than the matter in a neutron star.

So at the moment there are no realistic proposals for reducing the mass of exotic matter required for the drive to a figure that could be reasonably handled by a spaceship.

For completeness I should point out that all the physicists I know (including myself) do not believe that exotic matter exists so the point is moot anyway.

• Comments are not for extended discussion; this conversation has been moved to chat. – Monty Wild May 12 '20 at 17:01

In the Odyssey One series early attempts to achieve a working Alcubierre drive failed to achieve a full warp field, but managed to create a field that distorted the mass of anything within. They used it to make ships "lighter" and therefore make existing propulsion methods far more effective. This allowed a large ship with a crew of around 300 to reach the heliopause in about 3 days. Anything within the field also did not feel the greater affects of physics. There is a passage that explains how the forces scaled in both directions.

Not sure if any of that is actually scientific or just pseudoscience, but it was a pretty great story mechanic. They use that particular technology to also make fighters a viable space combat tool as they can now move with more versatility, without killing the pilot from excessive G-forces. They also used it to make ridiculously effective kinetic missiles, basically the field reduced mass to accelerate the missile to relativistic speeds, then at the last moment the field would reverse, increasing mass to several times what it actually is. The resulting kinetic impact made nukes look like pee shooters.

Again, sounds cool, no idea if realistic.

• I have not read that series. It sounds interesting. Does the setting allow for FTL communications, or does it strictly obey the light speed limit? – Tim McClelland May 11 '20 at 17:10
• Well the premise kind of starts with finding a workaround to an alternative method of FTL travel. Basically they find a way to teleport through space (via sudoscience tachyon drive) instead of traveling via warp drive. They then almost immediately stumble into an interstellar war. The aliens have FTL communications, but humans not having it becomes a main issue for the 2nd and 3rd books as we then develop a crude warp drive, but can't navigate properly. Struggling with communication times and waiting for sensor data is actually a pretty big tension builder through out. – TitaniumTurtle May 11 '20 at 17:40
• The kinetic missile application would not be physically realistic, since it's creating energy out of nowhere. It would be trivial to turn such a drive into a free energy machine. An actual Alcubierre drive wouldn't accelerate the craft (though creating the bubble itself might require amounts of energy that make thermonuclear weapons look like party poppers, and that energy might be released when the drive is turned off). – Christopher James Huff May 11 '20 at 17:49
• @TitaniumTurtle - the series sounds fascinating, and I have added it to my to-read list. But I am compelled to point out: "pseudoscience". sudoscience sounds like you're ordering linux to do something. :) – jdunlop May 11 '20 at 18:25
• @jdunlop HAHA, I grew up in a programmer's household and only ever heard pseudoscience in discussion, not written. That spelling just made since I always thought of it as "science does what I want" haha. – TitaniumTurtle May 11 '20 at 19:19

Yes, the Alcubierre warp drive would work for you in a sci-fi setting. There is currently no absolute irrefutable evidence that it is possible, but nor is there any irrefutable evidence that it is not possible.

Here are a number of references to solid current research on the feasibility of Alcubierre warp drives, be they FTL or subliminal. None of the articles absolutely requires the drive be FTL, they just assume the best use of it would be for FTL. I think the idea of using the drive for subluminal use is one of the unexplored areas of Sci-fi. It eliminates the need for the author to consider relativistic time/distance distortion and dilation effects on humans. As a plot device it is a neat way to hand wave them away. If you are 30 years old on the spaceship, your twin is 30 years old on earth, and your other twin is 30 years old on another ship.

Most authors who consider time dilation just assume that the human mind and biological processes can handle the fact that the mind and body is supposed to be younger than its twin back on earth. By using this plot device this way, you eliminate the problem of your future grandson being older than you.

In other space operas, crews go on long journeys at 0.3 c and come back home to ... their wife and children who have aged at the same rate. Say what? Or they have ship mates who were at the academy at the same time, same age, decades ago, but went on different deployments for differing time periods at differing variations of c, yet they are still exactly the same age. In point of fact, such a space/time bubble drive is almost always assumed in these space operas, without explanation, as a given. The OP is just giving it a credible name.

Standing room only crowd engages in student’s warp drive theory speech

Agnew recently provided a standing room only crowd at the American Institute of Aeronautics and Astronautics (AIAA) Propulsion and Energy Forum in Indianapolis, Ind. with an overview of the state of the art of warp drive theory and the feasibility of a future working system, based on a paper he wrote. Agnew’s advisor is Dr. Jason Cassibry, an associate professor of mechanical and aerospace engineering.

https://www.uah.edu/news/news/standing-room-only-crowd-engages-in-student-s-warp-drive-theory-speech

Scientists Are Starting to Take Warp Drives Seriously, Especially One Specific Concept

As part of a session titled "The Future of Nuclear and Breakthrough Propulsion", Agnew shared the results of a study he conducted titled "An Examination of Warp Theory and Technology to Determine the State of the Art and Feasibility".

As Agnew explained to a packed house, the theory behind a warp propulsion system is relatively simple.

https://www.sciencealert.com/how-feasible-is-a-warp-drive-here-s-the-science

Finally, an overview of the warp field interferometer test bed being implemented in the Advanced Propulsion Physics Laboratory: Eagleworks (APPL:E) at the Johnson Space Center will be detailed. While warp field mechanics has not had a “Chicago Pile” moment, the tools necessary to detect a modest instance of the phenomenon are near at hand

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110015936.pdf

An Examination of Warp Theory and Technology to Determine the State of the Art and Feasibility

https://arc.aiaa.org/doi/10.2514/6.2019-4288 This one is behind a pay wall, unfortunately, and can not be quoted.

Last year, Sonny White revealed a new design (pictured top) for the Alcubierre drive that reduces the energy requirement from the total mass-energy of a planet the size of Jupiter, down to the mass-energy of Voyager-1 (700 kilograms).

https://www.extremetech.com/extreme/164326-nasa-discusses-its-warp-drive-research-prepares-to-create-a-warp-bubble-in-the-lab

Yes, these are pop-sci sources, but the really good stuff is all behind paywalls. The only way for the general public to get a hint of what is in them is through these pop-sci publications.

But the synopsis of my answer is that there is credible scientific research being conducted on the drive, and this new evidence really challenges a lot of the old 'can't be done' arguments. The 'exotic matter' requirement and 'impossibly huge energy requirements' or even the requirement for 'negative energy densities' are not, in fact, supported as anything more than 'engineering challenges' to be overcome by further scientific research.

So science says you are clear to go. No insurmountable problems. A good, solid, scientifically supportable plot device.

This is a question near and dear to me. I have posited using such a drive for my generation ship, for several general reasons.

First, we really do not know what effects relativistic speeds will have on life in general, and humans in particular. Life on earth formed specifically to survive in our relativistic framework, biorhythms and circadian rhythms, gestation periods, and such for instance. We do know that life on the ISS, just a minor speed increase over earth, produces significant biological changes that can not be attributed to anything in particular, and could indeed turn out to be 'relativistic in nature'. The human body somehow does not seem to like 'aging' at a different rate than its twin would on earth. It just KNOWS it should not be that young. This possibility has, as much as I can determine, been ignored in the sci-fi world. But the human body operating in a 0.3c frame of reference to that which it was designed for? How firmly is 'life' and mechanisms like the 'soul' tied back to 'home' (our relativistic framework)? Will life always see itself from THIS particular reference? The space/time bubble removes any need for any conjecture. The body ages at the rate it was designed to, relative to every other human body back on earth.

Second, the drive needs no reaction mass. As long as one posits a large enough energy source, it becomes feasible to travel very long distances at high delta-v.

Third, it removes any restrictions on mass size of the generation ship. Reaction mass has to be proportionate to the mass of the ship, and is particularly useless to the final objective. No matter how efficiently or at what speed it is ejected, reaction mass ultimately restricts the size of the ship, and the energy requirements always have to accommodate the original fully-loaded reaction mass at the starting point. None of the reaction mass has any purpose or function at the end of the journey, a complete waste of room and energy. The final size of the ship using this drive is based on the amount of energy that it can produce, not on how much reaction mass it carries.

Fourth, the mass of the ship does not decrease over the duration of the trip, except for the loss of mass in its conversion directly to energy. Thus, the gravity of the ship does not change. A useful quality if you use natural gravity to augment artificial gravity. (Antimatter generators, for instance, use up antimatter. Fusion and fission, I understand, do not result in exactly the same amount of mass after as before.)

Fifth, people age at the same rate as their relatives back on earth. No problems with coming back to marry your great-great-great niece. If you believe that souls can somehow connect over great distances, this might be important psychologically.

Sixth, the statements requiring absolutely impossible amounts of energy all relate to traveling faster than c, requiring a 'negative energy density', a requirement not necessary for speeds below c as far as I can tell.

Seventh, Large enough generation ships also allow for large colliders to be placed on board. A neat trick if you need specialized particles.

So, in this vein, I have researched the drive extensively (not the pop-sci stuff, which is generally gobbledegook really spooky stuff, but the real scientific postulations) based on the original formulation and subsequent restatements, and I have found nothing in the equations regarding the operation or generation of the drive itself with 'c' in it (the equations with c in them are all about requirements to travel at or faster than c, and thus the need for c as the target speed, but not in the equations for generating the bubble itself), and I have found no equations with 'c' in them that could in any way impact on the drive. The one caveat is that, traveling exactly at c there may be some problem with infinite mass and energy at he boundary layer of the bubble. A 'divide by zero' error, perhaps, as infinity minus infinity is zero, or a multiplication by infinity. Any variation of e=m****c^2 really gets complicated if either e or m are infinite.

• As far as your second paragraph goes, assuming special/general relativity are roughly correct then there can be no physiological effects resulting from relativistic travel. Every observer is able to claim to be at rest, indeed every observer is at rest in their own frame of reference. – or1426 May 12 '20 at 9:34
• What or1426 said, and the Alcubierre drive doesn't somehow avoid time dilation in the first place. And the energy requirements are still enormous for sublight speeds. And the gravity of the ship is utterly irrelevant unless you're warping planets around. And what do particle colliders have to do with anything? – Christopher James Huff May 12 '20 at 12:09
• Yes, starship designs that are basically giant tanks of reaction mass with a tiny crew compartment attached somewhere aren't that appealing, even if they might be more realistic. – Tim McClelland May 12 '20 at 17:07
• @or1426 The problem is the psychological mind conflict. You are SUPPOSED to be as old as your twin brother. Everything about your existence, about human history, culture, biology, says that is so. Even you body says that is so. Can lie REALLY handle not being as old as the biological twin back on earth? Lots of evidence says not, no matter how loudly 'relativity' proclaims other wise. – Justin Thyme the Second May 13 '20 at 3:19
• @ Christopher James Huff So you calculate the gravity of your ship, that will provide a given gravitational pull, in place of artificial gravity, sufficient to 'hold you down' at the beginning of your trip. You use half of your mass as reactionary mass. Now suddenly you are floating. Gravity DOES matter, when you use up most of your mass as reactionary mass. It also matters for inertial dampening on those sudden turns. – Justin Thyme the Second May 13 '20 at 3:53