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In the story I'm currently working on an alien race holds the monopoly on FTL travel. They're using a modified Alcubierre drive system, powered by Negative Mass. Humanity stumbles on a source of Negative Mass, and with assistance manages to bootstrap an FTL drive for themselves.

The aliens have always jealously guarded their monopoly (picture a society based on similar lines to the British East India Company), so inevitably there will be a battle. The aliens have traditionally relied on their speed to take them out of harms way, as they're merchants at heart rather than warriors. They have always relied on Economic warfare, as opposed to combat, though when the need arises they have no problem hiring mercenaries to do their dirty work.

At the end of the tale we have a human FTL ship chasing down the alien vessels. My question is this...exactly how would they do that? What kind of weapons and tactics actually work at FTL speeds? Lasers and other energy weapons are limited to light speed, and the Alcubierre drive is too massive for missiles. Not only that, their supply of Negative Mass is limited, so what are their options? Outside of ramming the other ship, that is, I'd like to avoid a kamikaze approach. Boarding actions are possible, but a suboptimal choice.

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  • $\begingroup$ Basic problem is that with FTL there is hardly any casuality, and you cannot know where the ship is / will be when you shoot. $\endgroup$
    – Mołot
    Jan 11, 2019 at 12:49
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    $\begingroup$ "science-based" is hard to combine with FTL since, if you play it straight, FTL also means time travel.... which opens a whole other can of worms re:causality. $\endgroup$
    – Murphy
    Jan 11, 2019 at 13:46
  • $\begingroup$ Yeah, lets avoid the whole causality issue if we can. That's a whole different story. :) $\endgroup$
    – Hewholooksskyward
    Jan 11, 2019 at 13:52
  • $\begingroup$ Perhaps add this link for some basic information, as many people seem to stumble over the FTL part even though the alcubierre is an APPARENT FTL drive. It makes the travel time shorter and potentially you arrive before light outside of your alcubierre field but does not make you faster than the actual light. Because of this, actual time dialation until the time moves backwards when exceeding the lightspeed isnt possible with the drive. en.m.wikipedia.org/wiki/Alcubierre_drive $\endgroup$
    – Demigan
    Jan 11, 2019 at 14:00
  • $\begingroup$ Deleted previous comment where I mention "hang something in their path and watch the fireworks". Since the ship inside the bubble does not need to have a great velocity, if any at all, anything in front would either start traveling with the ship as it enters the bubble or suffer from whatever happens if you are being contracted by an alcubierre drive. $\endgroup$
    – Demigan
    Jan 11, 2019 at 16:03

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My understanding of the Alcubierre Drive is that it maintains a "warp bubble" of real space that our ship safely sits in whilst it contracts space-time in front, and expands space-time behind.

Assuming two separate Alcubierre warp bubbles don't catastrophically interfere with each other (I have no idea if they do, and I don't know if anyone does have any idea either way), then that would allow two FTL ships to move side by side and merge their warp bubbles to make a peanut-shaped zone of real-space, surrounded by expanding/contracting space time.

In this bubble, standard ship weapons could be employed, be they missiles, lasers or kinetics and they would be carried in this warp bubble along with the ships. Naturally the defending ship will want to split the peanut back into two bubbles so as to prevent getting shot, so you'll be able to have plenty of mind games between the pilots as they try and predict one another's actions as they use their ship's FTL manoeuvrability to best effect.

Projectiles hitting the edge of a warp bubble will also have a really bad day, possibly ranging from instant vaporisation as their atoms are sprinkled across Light Years of space, to (if they're lucky) being shunted into the empty void between stars. I would thus not recommend launching manned fighters or boarding parties when in FTL combat, unless you want to then write the story of some lone snubfighter pilot stuck in the expanse.

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  • $\begingroup$ I'll upvote if you can explain why projectiles would be destroyed upon entering the field? As for what happens if the fields collide, since the one in front is making the space expand behind him but the one behind is making the space in front contract, the netto result would be zero space distortion of the fields and likely cause them to stop working. $\endgroup$
    – Demigan
    Jan 11, 2019 at 15:59
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    $\begingroup$ @Demigan The edges of the field have extremely curved spacetime, and therefore really massive tidal forces. $\endgroup$
    – David Thornley
    Jan 11, 2019 at 17:27
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    $\begingroup$ @Demigan. David has the right of it. Contorting space time is not my forte, but one can imagine there will be huge amounts of stress across any object only partially within the bubble, causing the item to split at the bubble's edge. Thus if an object is leaving the bubble, each layer of atoms is shaved off in turn and deposited along the route. Perhaps this effect could be used intentionally as a bread crumb trail? $\endgroup$
    – Kyyshak
    Jan 11, 2019 at 18:02
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Tachyonic beam

Tachyons are ( so far sci-fi only) particles that travel faster than light. You can weaponize them.

Messing with FTL involves time travel, so you may end up hitting someone in the past. Don't shoot anywhere close to Earth or you may suffer from the grandpa paradox.

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    $\begingroup$ The existance of an alcubierre drive does not automatically make tachyons possible. Right? Also as Dutch said, it would likely be a particle beam. $\endgroup$
    – Demigan
    Jan 11, 2019 at 11:01
  • $\begingroup$ @Demigan no, but there is a science-fiction tag in the question. $\endgroup$
    – The Square-Cube Law
    Jan 11, 2019 at 11:37
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There's not much.

It partially depends on how quickly they are going. For instance, suppose they are expected to travel 1 minute at 2c. It might be reasonable to aim at their projected location with a laser and wait a minute (though modern lasers spread out too quickly for that to be reasonable). If they are immobile, you can warp to them and then fire.

But that's more or less moot without a way of detecting where they actually are. At superluminal speeds, ship leave echoes wherever they go (both light and gravitational). It's imperative that ships first have the capability of detecting ships moving FTL more quickly than light can travel, or the only way to fight a ship in FTL is to follow it and shoot it when it stops.

So assuming that your ships are equipped with a tachyon detector or the like, you have two options that I can see:

  1. Superluminal weapons (which you say you prefer not to use)
  2. Getting in front of your target

To get in front you need to be faster or smarter. Take the kessel run in 12 parsecs or something. Once you're in front, anything works - you just need to aim at where they're going to be. I will also note that superluminal weapons don't actually solve the problem. In order for them to work, you have to have a faster drive than your target anyway.

In space, you can't force someone to fight

In the rules for the board game Federation and Empire, there's a section on the decisions they made when designing it. They make the following point (paraphrased):

In space, you can't force your enemy to fight. There are no coastlines where you can pin them against. Most of the time, they'll fight a little and then just leave. If you want them to fight, you have to threaten something important to them.

I'll add that because the ship in front doesn't have to have superluminal weapons to hit, they have the advantage.

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If the human ship chasing them is capable of overtaking the chased alien scums ship all they really need to do is to open an airlock and drop a bucket full of bolts\nuts\etc outside and let the alien scum ship run into it, the FTL speed they are going will mean that even the smallest of mass will be enough to blow holes clean through the ship and destroy it.

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    $\begingroup$ Since the ship in an alcubierre drive doesnt actually need to have a velocity, the nuts and bolts would enter the field and then slooowly glide passed the ship (assuming the other ship has a similar velocity) $\endgroup$
    – Demigan
    Jan 11, 2019 at 16:01
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would you not be able to fire a weapon in front of them attempting to predict their location at time of impact?

Because they also wouldn't be able to see what is in front of them unless they have FTL sensors too but then a laser is undetectable until impact

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    $\begingroup$ Please use some punctuation. Sentences and such. $\endgroup$
    – Mołot
    Jan 11, 2019 at 13:30

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