I've been thinking of ways that an FTL-capable galaxy would prevent weaponized FTL missiles.

Ignoring causality violations and paradoxes, I'm looking for a way to keep something based on an Alcubierre drive from being launched at a planet or other ship.

I didn't want to simply say "computers don't allow it" because a computer can be reprogrammed. I wanted there to be something intentional in the design of this drive that prevents it from being used as an FTL battering ram. Something that there was no workaround for because it's simply how it functions.

One idea came in the form of the magnetic fields produced by planets. Starships in this setting already use artificially produced magnetic fields to keep cosmic radiation out. Perhaps this field can also be the deterrent against an FTL object?

Is there a plausible way for a more powerful magnetic field (like that of a planet or a significantly larger ship/station) to tear an FTL traveling object apart in such a way that it does not impact its target, but instead harmlessly scatters away from/around it?

This would at least make FTL missiles impractical since they would have to be larger than their target or have to have equipment to generate a much more powerful magnetic field than its target.

  • $\begingroup$ the way alcubierre drive works is by manipulating the curvature of spacetime around it with the help of negative mass, it compress the region of space between the destination and itself while at the same time expand the region of space behind it. Magnetic field is utterly useless against it and your best bet is the MAD doctrine or inflate the cost of negative matter aggressively ;D $\endgroup$
    – user6760
    Nov 13, 2017 at 3:42
  • $\begingroup$ Well said, @user6760, a good summary of the Alcubierre warp. Although inflating the cost of negative matter would make warp travel uneconomic. A version of the MAD doctrine makes sense in its own crazy way. Your comment should have been an answer. Why waste it here? A quick cut & paste, add more details, and it's a goer. $\endgroup$
    – a4android
    Nov 13, 2017 at 5:24
  • $\begingroup$ So I'm not great at physics but does an object which travels by compressing space even have a momentum in normal space? I mean, it could still carry a detonator and cause damage that way but maybe not by impacting something at a high speed? $\endgroup$ Nov 13, 2017 at 8:03
  • $\begingroup$ @RealSubtle no, it does not. Though the drive itself could be more distructive than such a payload! $\endgroup$
    – JDługosz
    Nov 13, 2017 at 22:10
  • $\begingroup$ See also worldbuilding.stackexchange.com/q/82619 A missile is just a special case of a space craft. $\endgroup$
    – JDługosz
    Nov 14, 2017 at 17:52

8 Answers 8


There is actually no problem here. The Alcubierre Drive works by moving the space already situated around the space craft. Nothing actually locally moves FTL. The missile moving under the Alcubierre Drive principle would pass through it's target, due to the fact that the drive warps space around it.
If your target is a planet, and the missile is approaching its surface, the drive would warp the space containing the surface (rock, buildings, trees, etc) around the local field of space which contains the drive.
People on the ground or in those buildings would not directly see anything.
In short, the missile would only interact with things within its local spatial distortion, and not the undistorted outside universe. There would be no impact.

  • 1
    $\begingroup$ Huh... Hadn't thought about that. $\endgroup$
    – Arvex
    Nov 25, 2017 at 15:23

Postulate another “exotic matter” that interferes with the warping. Deploy this around your planets for easy, ubiquitous, defense.

Instead of magnetic fields, look at gravitational fields. It is a common trope that warp drive simply can’t work near a gravity well. (As explained in this answer that may be generally true for any postulated FTL mechanism in order to disallow causality violations.) So, your weapons pop out into real-space (say) beyond the orbit of Neptune.

  • $\begingroup$ Your postulated "exotic matter" is a really nifty idea. Not sure if the gravity well trope can be applied to Alcubierre warp drives. Although if one postulates that this version of an Alcubierre warp requires flat space for its safe operation, then gravity well could interfere destructively with the drive. Plus one for the good ideas. $\endgroup$
    – a4android
    Nov 13, 2017 at 5:13
  • $\begingroup$ @a4android I added a note to justify it. $\endgroup$
    – JDługosz
    Nov 13, 2017 at 5:32
  • $\begingroup$ This trope works fairly in well in the current online game Elite Dangerous, where the max FTL speed is dependent on distance from mass. The compression ratio achievable depends on how gravitationally empty your surrounds are. On the small scale, when in an asteroid belt (or even near a large spaceship) the drive can't calculate the space compression equations appropriately until you fly around 2 km away from the source of mass and so FTL can't start. On the large scale it slows ships automatically as they approach planets and stars. In your universe, maybe Gravitron density disrupts the drive. $\endgroup$ Nov 13, 2017 at 6:51
  • $\begingroup$ The added note is an improvement especially since it works in the general case & not just the specific case of the Alcubierre warp. I realize my comment assumes people will know gravitation equals curved space to justify my postulated safe operating in flat space & failure in the curved space of gravity wells. I hope I haven't assumed too much. $\endgroup$
    – a4android
    Nov 13, 2017 at 8:21

The Alcubierre drive is a special type of theoretical FTL engine. The physics stack has a tag for Alcubierre drive and 161 questions! https://physics.stackexchange.com/search?q=alcubierre A drive comes with specific dangerous downsides: 2 I can think of. To work the A drive requires a specific form of exotic matter: negative mass. Negative mass exists on solid theoretical grounds and so tweaking its properties is probably disallowed.

alcubierre drive image http://www.untold-universe.org/2017/10/nasa-admits-alcubierre-drive-initiative.html

That hole in front of the ship is supposed to be where space is crushed very small. The red hump is where space is created.

Here is a non-danger of the A drive: whacking into stuff with a huge amount of kinetic energy. It achieves FTL travel because the ship does not actually move, but instead it moves space around it. It does not have any kinetic energy because it is not moving. That is how it does not violate the rule about FTL. Space is crushed to a singularity ahead of the drive and in effect "created" behind the ship. Once the drive is turned off, as I understand it the ship is sitting there. This is unlike some rock which comes hurtling in at the speed of light or more or less and whacks into something with tremendous kinetic energy.

Here are 2 dangers. 1: Warping space (crushing it to a point) warps matter in it. That is bad for the matter (like a planet or a ship) because it is crushed into a singularity ahead of the ship. So your ship would destroy whatever it drove through. But some people think the nature of the drive means it would not interact at all with matter outside the field.


This 2-dimensional representation of spacetime shows that it would exclude interaction with any baryonic matter between outside and inside of the ripple while such drive would be able to warp spacetime around it, but at the same time does not prohibit normal interaction with matter inside it. This means that, unless you've created a ripple large enough to include in it an object already on collision course with your spaceship before you turned your Alcubierre drive on, or turned your drive off at an inconvenient place, you wouldn't have to fear collision. Well, that's to my understanding at least, it's not an easy idea to wrap one's head around.

  1. Turning off the drive releases the negative matter singularity ahead of it, and any matter accumulated in the crushed space there explodes with huge amounts of hard radiation.


Hitch-hiking matter becomes planet-killer. The problem is that the Alcubierre Drive spaceship is going to encounter matter during its trip: space is only nearly empty, not completely empty. Matter traveling towards the ship, the paper says, will become "time locked" with the ship. When the ship decelerates, these hitch-hikers are released from the bubble emitting huge amounts of energy as gamma rays and high-energy particles.

But if this drive fails to interact with any matter in its path as posited by Tildalwave above, this would not be a problem either: the singularity would warp only the empty space in the vicinity when it started.

If you make your Alcubierre drive fail to interact with any normal matter when it is on, it cannot be weaponized. A moving ship in the warp moves through whatever it encounters. When the ship stops and turns off the compressed singularity in front of it, nothing happens. This drive could be used to warp a sack of dog poo (or a bomb) inside another ship and then materialize inside it by turning off the drive, but this is not a threat on the order of light-speed impactors.

ADDENDUM Here are two reasons to make it unlikely there will be warp bombs.

1: When bomb goes off you lose your A drive and the exotic matter. This is a lot more expensive than rocket fuel.

2: It is hard to steer. When you are in the A drive you cannot see outside the bubble. If you have the "speed" of the object inside the drive vary according to unpredictable factors one could have these ships emerge from the warp bubble within a fairly large possible area. Cargo and personnel ships will want to emerge in large empty spaces (possibly cleared in advance of their arrival) to minimize the chance of appearing in a solid object. Your A-drive warp bomb might emerge from warp several thousand km away from its target.

  • $\begingroup$ Warping past defenses can be a huge issue, but not the huge issue that traditional FTL or high-fractional C impacts would be. $\endgroup$
    – Andon
    Nov 14, 2017 at 4:23
  • $\begingroup$ I forgot about the crushed space and its destructive release. Maybe I should go back to the drawing board about which FTL theory I should use as a foundation... $\endgroup$
    – Arvex
    Nov 14, 2017 at 13:00
  • $\begingroup$ @Will If you can destroy a large fixed (planetary or orbital) installation producing or storing exotic mater/functional drives, it will be cost-effective: the opponent will necessarily take more damage than your small ship loss. Similar reasoning can be made for other vital facilities or decapitation strikes. Then there are the covert ops/terrorist/criminal case of stolen ships. Even if it is not steerable, it can still hit fixed targets. So warp bombs will be usable. $\endgroup$
    – Eth
    Nov 14, 2017 at 18:17
  • $\begingroup$ @Eth: remember a warp bomb is not like a rocket that will impact a fixed target in front of it. If this kind of A-drive space warp bubble does not interact with normal matter you will not be able to see it and it will not be able to see anything. You have to tell it when to pop back into normal space based on elapsed time and direction. It might not have reached the fixed target or it might be well past it when the A drive shuts off. I suppose if it popped into space and was already past the target you could turn the drive back on and back up. Beep...beep....beep... $\endgroup$
    – Willk
    Nov 14, 2017 at 19:26

As many others before have commented, Alcubierre's drive requires negative mass.

The Woodward/Mach effect (https://en.wikipedia.org/wiki/Woodward_effect) is the only technique I know of that provides a roadmap towards negative mass. The experimental concepts use extremely high frequency electromagnetic components (capacitors and inductors) to achieve the Woodward/Mach effect.

Your negative mass emitters would have to be exposed.

Since they are EM, and exposed. Yes, you could set up a magnetic field that interfered with the negative mass emitters, and thereby interfere indirectly with the Alcubierre field they are producing.


I'm going to suggest that it just can't be done. Here's why...

Assumption: I'm assuming here that FTL drive accelerates the mass to high speed, and that turning off the drive doesn't drop it down to sane velocities.

First, it doesn't matter if the star's gravitational well stops FTL drive from working well away from the planet. Let's imagine that FTL engine conks out around the orbit of Pluto [1]. Enemy would start the acceleration further away, and let the engine drop away as you approach the fail-point. So projectile is now FTL and ballistic, and -- now this is key -- you can't intercept it. It is traveling faster than the light and/or gravity waves signaling its approach. In fact, if you videotaped the whole episode, the tape would show the projectile moving backward away from the planet!

Second, electromagnets in orbit wouldn't help either. Why? Because the projectile doesn't have to be made of ferrous metals. At the speeds it's going, you may as well carve it out of stone, or wood, or anything you like.

My best suggestion for defense would be to have a layered defense of "shoals" of sand and of mines orbiting the planet in such a way that the safe path through requires maneuvering (ie a ballistic shot could not get through). I doubt this would be foolproof, as you'd need a tremendous thickness of orbital junk to make it work, and at some point you'll start putting your own planet into shade...

[1] For purposes of this or any discussion, Pluto is a planet

  • $\begingroup$ Alcubierre warp drives don't accelerate objects with mass to high speed. A warp missile could approach at FTL speed & be unseen. However, if the missile was accelerated to near-lightspeed during its FTL mode, once it dropped below lightspeed its approach would leave little time to respond. Shoals of mines is a promising idea. $\endgroup$
    – a4android
    Nov 13, 2017 at 5:18

Analogy: How do you prevent someone with an axe from popping your balloon?

Given the energy used in Alcubierre drive (mass of jupiter, last time I looked...) I suspect there are going to be rules about how close you can park them to an inhabited star.

But destroying a planet is so easy: Fractional c rocks.

Right now we are at the edge of detecting stuff moving at orbital speed that may impact the Earth. We're a long ways away from shifting anything significant away. And that's working with objects that are moving at 20-70 km/sec. Send a rock the size of a cruise ship at .1c, and you have a really really no good awful bad day.

So if you have FTL

  • You either need some degree of handwavium that substantially reduces the energy required
  • You need to invent a bunch of science in related fields as auxilary explanations.


  • FTL is done between gates constructed at the edge of the system. They can send you one way to a nearby star, but you have to build a gate to come home.

  • FTL is expensive -- really expensive. An FTL ship is equivalent to the Gross Global Product of several decades. You don't waste these knocking your neighbor on the head.

  • You have an Eridani Convention (Weber's Honorverse) that you don't bombard the surface, and that you surrender once the orbital defences are defeated. Total destruction of the home planet for anyone who defies the convention.

  • FTL takes no energy. You disappear into {sub|warp|hyper}space where distance is much easier to cover, and reappear at the other end. You can choose if travel is instantaneous or not. If you want to add some restrictions: There is a limit to how close to the star you can be. (Niven's Known Space, Weber's Honorverse, Piper's Federation stories) Or there are special places that permit fast travel. (Barayar wormholes; Honorverse wormholes; Pournelle's Alderson transfer points.)

  • Coming out of FTL leaves you at rest relative to the local gravitational field, and with no velocity relative to the nearest significant body. (Potentially this leaves putting a big object in the way of an orbiting planet, but you have to appear at a distance that makes the star the most significant body, and now you are only dealing with orbital velocities.

Overall, I suggest that you avoid the Alcubierre drive. As present science, it's too subject to change, and you may date yourself before publishing.


There is something similar in Howard Tayler's Schlock Mercenary webcomic, called a Teraport Area Denial.

It is basically a high-frequency random gravity "noise" that has no effects on anything except "teraport", which gets disrupted in such a way that traveling objects are destroyed (or would be, except that the transmitter can sense a no-lock condition and abort).

You might imagine the same thing working against Alcubierre drives, the high-frequency waveform disrupting the Alcubierre bubble in such a way that traveling objects become unstable. This in turn would result in the bubble collapsing, and its content (plus the "front wave" pushed by the drive) being converted into energy.

Or the Alcubierre drive cannot abide another Alcubierre drive nearer than a certain (significant: many millions of kilometers) distance, or again it collapses. This means that only designated areas can be routinely used for arrival and departure, and Alcubierre travel becomes risky. But surround a system with Alcubierre FTL satellites, and the system is secured.

So your denial system would be composed of a grid of satellites far away from the Sun, each containing the equivalent of a very small oscillating black hole pair (once set in motion, the satellite only supplies the rotational energy that's being radiated as gravity waves) or something similar to a static Alcubierre drive, and a very large reflector plus thermal sump, designed to survive the multi-gigaton explosions of an Alcubierre projectile stream long enough for more satellites to be deployed (because an obvious strategy would be to send in N small and massive tracking shots, to burn a hole in the denial system, followed by the larger true impactor).

This allows for a defense that can be as perfect or as imperfect as you need - also, a balance could be sought between nearness to the defended system, satellite density, black hole mass.

In the Alcubierre satellite model, the position and trajectory of each satellite would be a closely guarded secret.


A FTL drive could bend space to allow the ship to bypass the speed of light limit without having to break it.

Being able to bend space without ripping yourself to pieces could reply on remaining away from large gravitational fields such as planets and stars.

Any ship would fly out using conventional drives well away from large gravitational fields before engaging the FTL drive and would stop well away from the destination star and fly in normally.


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