How to Protect Against FTL Sneak Attacks

Background:

Following the development of faster-than-light technology, humanity has spread across the stars and encountered many kinds of alien life. While the galaxy is mostly peaceful, occasional wars do break out, and piracy is an issue.

The Tech:

All groups use an FTL drive based on the same basic principle: the drive creates a wormhole at the front of the ship and pulls it over to the rear of the ship, depositing it at the target location, with it seeming to an outside observer that the ship had simply disappeared from one place and reappeared in the next. In addition:

• The computer power (and therefore time) necessary to calculate a jump increases with range, up to a rough maximum of ~ 30 light-years
• Gravity wells also render the calculations more complicated, meaning that it takes progressively more power to jump the closer a ship is to a planet or other large body
• There is no warning ahead of time at the endpoint that a ship is about to jump in- it's just there
• Any material present at the endpoint is displaced to the origin point, preventing two objects occupying the same space

Situation:

I am an Admiral tasked with defending Earth from possible attack. While most avenues are covered, what really worries me is the possibility of a force sneak attacking by jumping in right behind my defenses, firing a few salvoes, and then jumping away before my forces can respond. Is there any way in which I can avoid or minimize the threat of this possibility?

• Please remember: Any FTL comm tech implies causality violations in your story. Answers to this question MUST take time travel fully into account. It does not matter what the FTL technology is. Full physics explanation is here for those unfamiliar with this requirement: askamathematician.com/2012/07/… I can provide more links for those with questions.
– SRM
Dec 19, 2016 at 3:04
• @SRM, only if you're in a relativistic universe. If you're in a Newtonian universe, FTL is just fine.
– Mark
Dec 19, 2016 at 4:34
• This question has a challenge I've seen often: "I've created the ultimate offensive weapon... now help me defend against it." When I see questions like these, I recommend one of two approaches. a) handwave in the ultimate defensive tool to counter it or b) Fill in the dirty details of how the weapon is implemented, and then leverage those details to balance it with a defensive weapon. If your implementation is flawless, you'll always find it difficult to counter. For example, are you sure there's no warning ahead of time? It's not possible that the hardware gives a few ripples beforehand? Dec 19, 2016 at 4:44
• @SRM Saying must is a bit strong, since neither Star Wars nor Star Trek make any attempt at accounting for such things. Dec 19, 2016 at 4:44
• This question has "reality-check" on its tags. That's the reality check. @kingledion Also, both Star Wars and Star Trek are "grandfathered" in... their story universes were created before sci-fi had a strong understanding of the full ramifications of relativity.
– SRM
Dec 19, 2016 at 6:45

Make it too dangerous to jump in

Suppose the inner solar system around your habitable planet has a series of minefields. These mines are placed in pseudo-random orbits which are changed on a weekly basis. The mines are only activated by a nearby wormhole opening.

This won't interrupt local commerce, since intra-system ships won't use wormholes. Commerce with other systems will have to use pre-approved jump points at scheduled times and designated places so they won't activate the mines.

The mines themselves should be designed to operate at low temps, and be made with low albedo, low emissivity materials, so they are hard to spot, even by a scout ship in the outer solar system. Moving the mines can be done externally, using mine tenders or tugs, so the mines themselves have no active power generation and negligible heat signatures.

• Except space is big etc. The numbers of mines needed would be astronomical or their destructive power would have to be colossal to take out pirate ships. Intrasystem travel could easy use wormholes too unless old-fashioned propulsion systems is cheaper and easy. Scheduled interstellar travel is an excellent idea. Tugs will have active power generation & non-negligible heat signatures. Mines was my first thought, but I abandoned it because they're either too dangerous or ineffective or both. Dec 19, 2016 at 8:25
• Since space is big, let's turn those "mines" into drones that activate when they detect any space-time deformations. We can already detect black holes colliding in a far-away galaxy, a wormhole in our solar system should be a breeze. Actually shooting down the intruders would be a tough challenge, though. Dec 19, 2016 at 10:17
• If it is dangerous to jump in, you may elect to just send in warheads anyway. The mines would just add to the yield you intended... Dec 19, 2016 at 14:25
• @a4android You don't need full coverage of mines, you just need enough in random places that the attacker doesn't want to risk instantly losing his fleet. Dec 19, 2016 at 14:51
• @JanDvorak It is possible the 'mines' could interfere with the wormhole itself. If the mines to disrupt the wormhole formation on the arrival side, anything trying to pass through that wormhole might be simply torn apart. That would be much more effective than explosives. I'm leaving how the mines actually work up to the OP and his selected technology level. Dec 19, 2016 at 14:57

The best defence is to use a variation of the drive to get rid of FTL sneak attackers. if the wormhole drive works by creating a wormhole at one point and it moves over the ship to allow the ship to jump.

Place a series of wormhole generator orbital stations around a planet you want to defend. When a sneak attacker appears, activate the wormhole generator and project a wormhole in the direction of the pirate vessel. Once the wormhole passes over the vessel it will jump to what may be a predetermined destination. The pirate ship will jump to somewhere far away, which can be up to twenty or thirty light years away.

The wormhole projector can be preprogrammed to make attackers to jump away to somewhere else. As an extra protection, the Admiral can station missile and laser batteries at the predetermined arrival point and immediately a pirate vessel arrives they can blast it to good old fashioned atomic dust.

• A really good destination is the center of the sun. Dec 19, 2016 at 3:40
• @WhatRoughBeast, not when "Any material present at the endpoint is displaced to the origin point". While you displaced the attacker into the star, you replaced it with large quantities of quite dense (150 g/cm³), very hot (15 million degrees Celsius) plasma. This stellar matter, which is no longer sufficiently large to maintain its form through gravity, will expand (effectively explode) outwards, probably destroying an area much larger than the area traversed by the wormhole. Doing this does make a good weapon (against the point outside the star). Dec 19, 2016 at 5:37
• A much better destination would be the center of a cold rock, such as a large asteroid. We get some free iron to mine, and the bad guy is encased in rock in a way that they may not be able to jump out of, depending on the details of the wormhole generator.
– user1975
Dec 19, 2016 at 5:39
• @Snowman Blowing up pirates has more colour and drama than slamming them into a rock! Also, bits of asteroid will appear at the departure point. This could be a space traffic hazard. Dec 19, 2016 at 8:29
• @Makyen That suggests an interesting alternate attack for the pirates, however. Equip an unmanned drone with an FTL drive, and have it precalculate two jumps -- one to the target area and the next to the centre of the nearest star. Result: near-instant stellar core firebomb at your original target location. (There would probably be some imprecision in targeting the second jump, due to calculation errors from not being at the origin of the jump when doing the calculation [doesn't matter; big target], and it would need a ton of power, but it seems feasible within the given rules.) Dec 20, 2016 at 5:29

Securing one's backstory is safe from temporal interference by FTL travel is quite difficult, especially if the universe has an unstable history (I.e. grandfather paradoxes are legal).

For a hard-science sci-fi space empire where paradoxes are legal, read "Singularity Sky" by Charles Stross. For a hard-science world where physics allows time but not grandfather paradoxes, read "In the Garden of Iden" (yes, I spelled that right) by Kage Baker. Pick your physics.

The only defense against FTL in a paradox violation cosmos is constant vigilance against every entity capable of FTL travel. Really really hard. You probably need some serious intergalactic UN-type entity or a technological elite that enforces FTL discipline on lesser species.

In a more rational universe, defense becomes more viable, but you still have limits. The enemy cannot just teleport into your history and conquer your people before they discover space flight. But they could fly into your planet's formative years and hide a giant bomb that doesn't go off for several million years (assuming they have the materials to build such a long-lasting device).

So after all of that, the best defense, in my opinion, is the same defense the USA and USSR used in the Cold War: mutually assured destruction. Every FTL-enabled political entity (planet or culture or species) is coerced by all the others to have a genocide bomb planted on its homeworld/main HQ that will be detonated if that entity abuses FTL during war to perform time travel. Or there are many variations of dead-man-switch to ensure détente. Pick your favorite.

After that, all the defenses discussed by other answers come into play.

• I disagree that the causality violation applies in the case of instantaneous, or near instantaneous travel. It merely gives meaning to "simultaneous" in a situation where it would not otherwise have meaning. Dec 19, 2016 at 9:23
• While FTL travel might kind-of-imply the possibility of time travel, it does not mean that time travel technology is actually available. So whether causality violations actually happen depends on whether anyone has managed to develop the FTL drive into time machine yet.
– jpa
Dec 19, 2016 at 9:30
• TFL does not violate causality unless author wants it to Dec 19, 2016 at 10:19
• All wormholes use the same reference frame in which the mouths are simultanious. End of problem, and you have a good reason why it doesn’t work inside gravity wells. Dec 19, 2016 at 12:26
• "FTL weapons need a more pre-emptive solution." Too much so. Extremely high levels of safeguards would be necessary to prevent accidental wars or wars of any kind.This is MAD on steroids. Charlie Stross' Eschaton scenario makes sense. Dec 21, 2016 at 0:51

As a defense contractor, I would like to sell to the Earth Space navy a device call a gravity well projector. This projector uses the same wormhole technology used in FTL but rather than moving a ship, it creates a space-time warp that is static in space. Which would in effect create a wall against any FTL vessel, similar to a FTL vessel hitting a gravity well. If enough of such projector were use (purchased), it would be possible to create a fall all around earth space... leaving on well defined corridors to be used for military and commercial traffic.

I would also recommend the good Admiral consider purchasing our FTL enabled city ships.... which will always be motion. If humanity can be persuaded to live on city-ships, our population will be mobile. Able to seek new resources and be more resistant to attack. It is far harder to hit a moving target than a static one.

• Why couldn't they teleport in inside your wall? The question specs suggest that you would need gravity wells completely filling your near space to prevent a wormhole opening up. So many artificial gravity wells seems likely to be expensive to setup/maintain, but worse, seems like it might start interfereing with orbits of nearby planets/comets, pulling them in toward your inhabited areas. Does my interpretation of the question make sense?
– SRM
Dec 19, 2016 at 3:33

You can't

The reason is down to variants on all the examples that follow and the fact they present absolutely no risk to the party employing them.

• Any material present at the endpoint is displaced to the origin point, preventing two objects occupying the same space

The Wormhole Missile

This is the most conventional weapon in this list. It barely even needs a warhead, just a suitably large displacement. I target your capital(ship) and let it go. A partial hit is fine as it just transports half your capital(ship) to my location where it can be finished off. Any actual warhead in the missile can finish the job at the other end.

This drive is the ultimate toy for pirates

The same applies to your valuable cargo. I target the cargo ship, then use a drone or my last captured ship with a souped up drive and just dump it on your cargo carrier. Your cargo is now in my controlled space and you're too busy dealing with whatever entertainments I've put on the ship I sent.

To prevent this sort of thing you need some way to detect and prevent the wormholes from forming. See Schlock Mercenary and the Terraport Area Denial which is the prevention option for a very similar drive technology.

A few more thoughts on (ab)using this drive technology.

What happens if the field is shut off halfway through a transport?

If half the ship goes then it leads to far more interesting options. Most of these are based on the fact that the drive has to transport a defined volume of space and anything in it rather than specifically anything in contact with it. If you enforce the latter option you'll be leaving behind anyone who jumped at the wrong moment, or gyroscopes held in magnetic fields for example

Most of these can be happily planet based as they're not transporting the drive so you can use planetary grade power supplies.

The pod launcher

The drive runs for long enough to transport a cargo pod but the drive doesn't transport itself.

The pod exchanger

Similar to the pod launcher but actually swaps the pod with one left at a specific location at the other end.

The Come Hither or Pirate's Friend

A drive with a hacked field generator that transports an empty space rather than the ship. It simply transports a volume of empty space, gas cloud or otherwise and returns whatever was in the location it was transported to. This leads to

The Kidnapper

A much harder device to work with, but based on the pirate's friend it's able to extract the command staff off a ship by transporting a volume of air to their location and returning the person.

• I like the idea of the wormhole missile. I'd even go a step further and simply create one end of the wormhole inside the other ship and "beam" vital parts out of it. Simply displace the bridge of the capital ship to somewhere in space. Dec 19, 2016 at 14:40
• @AlexandervonWernherr, that sort of trick is as good as your targeting. If your targeting is really that good then you can go as far as the kidnapper and just pick off the crew before taking the ship intact. Dec 19, 2016 at 14:45
• This assumes that there aren't any nasty boundary effects to deal with at the edges of the wormhole. Though I assume if there are and the tidal effects tear a hole in the ship when you kidnap the command crew you won't care that much. Dec 20, 2016 at 8:28
• @JoeBloggs I don't care that much. I'm assuming I'll at least take a chunk of floor and maybe some ceiling, furniture, nearby crewmembers etc Dec 20, 2016 at 8:34
• @Separatrix Heck, just run a wormhole through their ship and let the spatial distortions tear them apart. Dec 20, 2016 at 8:38

So a problem is that instantly appearing in a system gives you tactical advantage. Your passive sensors immediately pick up everything in the entire system. Meanwhile you only show up in an expanding light speed sphere. So you can jump in, detect and shoot, jump out before retaliation. if you park your fleet in deep space it is cheap to jump there, and accurate. Then you execute a pattern where ships jump into the system and systematically destroy all defenses with zero chance to fight back and jump back to the fleet location where they can quickly compare data for the next jumps.

The only defense here is constantly jumping defenses. Even then, your rules makes it impossible because the high value planetary targets cost more energy to jump around near.

Other posters have postulated some magic devices to prevent the jump in or make it harder etc. A simpler idea is that jumping causes disruption in systems and leaves you vulnerable for a few seconds or minutes which gives defenders a chance to defend. You can scale it with gravity just like your jump cost.

The massive amounts of energy required to create the wormhole bubble is released upon arrival as a massive EMP.

It is possible to shield satellites and defensive structures around the planet given that they tend to be far away (due to the nature of space). Earth's magnetic field would also protect it from any effects.

However, since the warp bubble surrounds the entire craft, no system can stay online after such a blast. This also means that you cannot have any charge left in your capacitors, else they might blow. Thus you arrive with no power and no backup charge - easy pickings for any defence force.

Your ship would require some time before it comes back online, and a lot of time before it can recharge it's capacitor banks in order to jump away again. Hence guerrilla tactics are impossible.

This very much sounds like a situation in which M.A.D. applies without any extra special considerations. Just make sure that it's understood that any enemy that uses such tactics against you is going to find the same tactics used against them, either by you or your allies. Build and maintain an arsenal of FTL enabled nukes, preprogram the co-ordinates of various enemy home worlds,send them out into the void of space so that nobody knows where they are, then sit back and watch an interstellar Cold War galactic peace unfold.

This doesn't require any further stretching of the laws of physics: it's just politics and sociology from this point on. You can expect combat between large nations to split into more codified 'official' conflicts and plausibly deniable attacks that no nation is willing to own up to, but which aren't large enough to cause a major diplomatic incident. When an outsider group emerges that threatens the stability of this system you can expect all the nations to work together to neutralise the threat, fearing that once the cycle of destruction begins nobody will be able to stop it, and every world will end up devastated.

Of course there will be objections, but those people are just luddites who can't see the benefits of having galaxy-wide destruction looming over their heads at all times.

• If you have more than one enemy, how do you know which of them just exchanged a piece of a star with your capital? Dec 19, 2016 at 23:45
• You don't, you just assume it was anyone who isn't your ally and make sure that everybody knows that's what you'll do. If the People's Republic of Lesser Nowhere drops a planetoid on your home world then you drop the molten remnants of your homeworld on the Glorious Kingdom of Greater Nowhere as well as Lesser Nowhere and Terra. That principle gives Greater Nowhere a very good incentive to make sure everyone else stays in line, like (for example) making sure they know that if Greater Nowhere is attacked then they will drop rocks on Terra and bash the two Centauris together... And so on. Dec 20, 2016 at 8:26
• The question says "piracy is an issue". Pirates tend to be Non-State actors, so a MAD strategy may be less effective. However, your argument that politics & sociology may be a better defence than technological fixes makes really good sense. Dec 20, 2016 at 12:09
• @a4android As a pirate I'm going to avoid any action that might reduce interstellar trade hubs to molten balls of slag. It's bad for business. Dec 20, 2016 at 12:11
• Very wise too. We need more space pirates like you. Dec 20, 2016 at 12:50

It’s not just a matter of computing power: Using the approach that all wormholes give transit that’s considered simultaneous in the same special reference frame, you will naturally have problems making a jump within a gravity well. Due to GR, the wormhole mouth will not experience time at the “correct” rate and this causes severe limitations in the size of body that can be transferred. You can use GR to drive a useful set of restrictions for your story, and impose extra logistical issues that prevents too abrupt turns of a plot.

So, you will only have ships arriving far from the planet. Small durable message capsules can arrive as close to the sun as the planet is, but still some millions of miles from the planet.

A battleship would have to appear well past the orbit of Neptune. The problem is that it could appear from any direction, and there is a huge area to watch. Ships will be stealthed. Defense will concentrate on detection as early as possible.

• Edited your answer with "abrupt turns of a plot", "turns" is my guess, if that isn't right, re-edit and make good. Dec 20, 2016 at 11:58
• The time differences will be due to differences in gravitational time dilation between points of departure and arrival. This will be miniscule (in the same reference frame). Battleships arriving beyond Neptune orbit (very Niven) will take a week at one gee to reach Earth. This hardly the stuff of a FTL sneak attack. I assumed the pirates can arrive within tenths of a light-second of an Earth-mass planet. That allows FTL sneak attacks. Dec 20, 2016 at 12:05
• @a4android see english.stackexchange.com/questions/364446/… Dec 20, 2016 at 13:50
• I see. You did think carefully before you wrote the phrase. However, it feels to me that abrupt should be attached to something, not be of a thing. Vide dictionary.com/browse/abrupt The common construction adjective of a noun surprises me that it's common, but I may have never labelled it as such. My reaction is that this phrase feels as if something is missing. That may be my problem. Finding the concise expression in writing of our thoughts is never easy. Dec 21, 2016 at 0:43
• I don’t see a difference between “abrupt descent” and “abrupt plot”. Dec 21, 2016 at 1:59

Place artificial gravity wells on strategic points. This technology may be similar to the Gravity Well Projector in Star Wars, with a significant difference because of your physics. The SW-GWP interrupts the hyperdrive in Star Wars vessels, forcing them to drop to normal space.

Your technology will work slightly different, it'll alter the location where the wormhole will pop up, forcing the ship going through the wormhole to end up some AEs away.

On the other hand, opening a wormhole to fly through is very energy consuming, so it might not be the best idea for the attacker to show up to close to the target, because the flight used up 95% of the ships power, so it takes time to start up weaponry and shields. This is similar to Sun Tzus "Art of War" Rule to be first on the battlefield, so you can recharge and prepare for battle.

• It really depends on what happens when there's an unexpected gravity well at the far end. Crushed ship? Good. Displaced halfway across the galaxy? Good. Displaced a few dozen meters, but otherwise fine? Won't work. So using this method will also have effects on what "high risk manuevers" can be pulled off when you're short on time or information to calculate the jump. (Which is fine, just something the creator should be aware of). Mar 3, 2017 at 22:02

You use the FTL drives as weapons. The tech definition doesn't mention rules about the edges of the wormhole endpoint surfaces, so the wormhole generators can be used as powerful weapons by swapping a partial volume of an enemy ship with some other volume, be it empty space very nearby, a different volume of the same ship, or some volume of another ship in the same fleet.

This assumes that the "originating" wormhole endpoint can be generated entirely outside the generator. (The "destination" wormhole endpoint obviously can be, so it's reasonable to assume the "originating" end could too. There may be no reason to consider the two endpoints as ordered; they are simply swapping. Many other answers assume a generator at some "external third point" can cause a wormhole between two points.)

Because the distance between the wormhole endpoints is minimized, computation time is minimized, and it becomes a footrace between attackers' computers which possess the element of surprise but need to compute a jump of at least some distance in order to get away, and defense computers which must react to the attackers but only need to compute very small jumps - possibly smaller than the length of an attacking ship. (Bonus points if defensive scanners can typically identify the drive core and the bridge of all attacking vessels very quickly, the two preferred endpoints for a wormhole swap. If swap volume shapes can be tailored, defense may simply opt for shuffling numerous thin slices of attacking ships along their major axis like a deck of cards.)

(Potential) spoiler for Vernor Vinge's book "The Witling", a note about prior similar art:

Swapping volumes (instantaneously) is a key part of Vernor Vinge's "The Witling"; they called it a few verbs ending in "-eng", depending on whether they were using it for travel, slinging missiles, or scrambling opponents' brains. In that universe, the generators were the minds of most native humanoids on a certain planet, and both endpoints needed to be locations the generating person had witnessed personally at some point in his or her life. Any person capable of performing these acts would automatically protect himself/herself from being bodily scrambled - at least, while conscious. Difficulty of performing the swap of two volumes scaled with the volume swapped but not with distance. Velocity was conserved through the swap; that's useful for slinging missiles, caused them some trouble traveling rapidly about the planet, and could be a major concern for spacecraft, e.g. wormholing from the Western Spiral Arm to the Eastern Spiral Arm within the same galaxy.

If the defense can use the FTL drive tech as defensive weapons, then attackers could use them as well. Are the gravity-well rules enough to prevent this? Is conservation of momentum a concern - i.e. attackers warp in cold rocks moving at high relative velocity and don't need to plan for an escape?

Your solution is that your and then jumping away before my forces can respond is not possible.

It's reasonable to assume/state that you cannot calculate your jump beforehand, i.e. you can only do the calculation at your place of origin for the next jump (local gravity and such need to be taken into account).

If there is a minimum 'base' time for the calculation + as you say additional time necessary to calculate a jump increasing with range, the enemy is vulnerable for at least the base time. With that base time they can only make a very small jump (make something up), or they have to take their chances staying longer in order to make the calculations to get away a safe distance.

You can work this out further assuming that your defense is faced outward from earth. Appearing in front of your guns would be stupid. So you have a second line of defense (can be smaller, can be earth based) in lower orbit than your main defenses.

What this cannot prevent against is suicide missions, but assuming that the enemy comes from far away and has to haul along all his material, while your resources are only one earth orbit away, you will win a war of attrition.

Hm, I've thought about this before. My solution for this always has been to assume we also have the ability to create temporal-warp fields.

Using that technology you could create a planet-wide shield, and the space between the outer shield and the upper most part the planet could be a temporal-warp field. You could manipulate this shield to let air-traffic in and out; closed otherwise. Now if someone makes a jump within that space they would be caught in the time warp, in which case you could then disable the field around the attacking ship. By then you could carry out a typical attack plan, leave, disable the warp field where the intruder ship stands let the ballistics take effect....or skip all that and simply create time-warp shears around the attacking ship so once they attempt to jump again its caught in the time-field and ripped apart.

Speculation of course. The only problem is if they attach a FTL device to some 'super-nuke' or black-hole bomb get it past into your atmosphere.

The computer power (and therefore time) necessary to calculate a jump increases with range, up to a rough maximum of ~ 30 light-years

Here's the tricky thing. If its faster, and you're nearer, the defender has the advantage of being able to deploy units in reaction, and since its essentially instantaneous...

Have a rapid reaction force. Jump into the projected path of the projectiles, hit them with point defence or decoys, jump them out. heck, you could jump an asteroid into the way, let it take the damage and jump it out.

Lets munch this out a little more. Since you're fighting in friendly territory, have networked or predetermined jump sequences for the tour of each of your defensive craft. Have them parked near a big space station which sends them jump instructions calculated on a very bigass computer, so your reaction time is shorter.

In a sense, don't bother about stopping the attackers. Make any attack toothless.

Pre-calculating jumps makes things even more fun for the defender since they only need short range jumps inside a known gravity well. They don't need to flee, and they basically just need to menuver.

If you have anything more than a hit and run, these tactics get even deadlier. Jumping from pre-determined jump point to jump point. Jumping an enemy ship into the line of fire of a missle they don't even see.

I will address this with a couple of points.

1.) There is no concept of direction in space, and ships would be designed and armed accordingly, likely with automated turrets covering the ship relatively evenly, with larger than usual concentrations covering sensative sections of the hull.

2.) Standard fleet formations would adapt to negate the risk of this threat. For example, fleets would be arranged in hexagonal-donut formations or circular formations, preferably with multiple rings, when not moving or patrolling. This way, no matter where the enemy jumps in, a significant number of guns will always be facing them.

3.) The risk of being "caught from behind" is negated completely if you opt for spherical ships evenly covered with turrets, and with all vital components being in the "core" of said sphere. Thrusters would be scattered around the surface as well, for sublight movement.

4.) Perhaps both the main weaponry of your advisary uses crap-loads of energy, which means certain things have to be taken offline upon engagement - including FTL.

5.) Your fleets could alternatively conpletely disperse evenly into your territory IF they could both cloak themselves in some way, AND detect enemy ships.

6.) You have some sort of "secret weapon", be that a simple technological edge, ancient "precursor" technology reverse-engineered, etc.

7.) Another "fleet formation" idea: Arrange your fleets in overlapping arcs so as to extend your field of fire as far as possible, and perhaps, if you are lucky and they are not, catch them in a major crossfire.

8.) Have some sort of targetted "jump jammer" that creates large gravity wells as soon as the enemy jump in. These would probably use a lot of power, and a plot point could be that they need an entire ship's worth of power to work - meaning that entire ships have to take their weaponry offline to devote their entire power to this task, and making the defense of such ships of paramount importance.

9.) If the combat involves projectile weapons or missiles, and if the majority of weapons are "guided" or "heat-seeking", you could fire a random salvo of your own missiles to give the enemy projectiles something else to hit.

10.) Primary weapons - the kind that can easily destroy entire ships in a single shot - cannot function in close-ranged combat. Meaning, they could probably do more damage in a direct confrontation, than a sneak attack.

You haven't mentioned anything about weapons in your universe, and I think that matters.

Use lasers... Friggin' Lasers!

So here's a solution: big, powerful laser weapons (either built on the ground or on large enough space stations). This kind of weapon needs to be big enough (or have power requirements large enough) to be a poor fit for any (combat) ship*. The targeting lenses also need to be agile, to quickly track targets in the weapons range.

Since such lasers don't use projectiles and can hit the target almost instantly, then while a suicide FTL attack is still possible, it's just that - a suicide attack. Anything that appears suddenly within the range of the lasers can likely get targeted, fired upon and blown up before they get a chance to jump away (I'm assuming increased calculation times due to planetary gravity).

*- that's not strictly speaking a requirement. However if ships can fire projectiles at best, then said lasers can also defend against those projectiles. If ships can also be outfitted with potent lasers then their attack COULD potentially be organized well enough to fire on the defending lasers before they have a chance to respond... but this assumes that:

a) defensive laser positions are all known
b) despite just warping in attackers can fire instantly; didn't they just spend a lot of energy opening a mini worm-hole? ;) (I assume lasers need some power to fire, unlike missiles or some such)
c) the attacking fleet has enough lasers to attack each defensive laser and destroy / cripple it

• A good answer, but: The bad guys can use lasers too, and they have a tiny speed advantage because they can see the defensive emplacements and fire before the defensive emplacements can see them... Dec 20, 2016 at 11:40
• @JoeBloggs Can they fire on ALL the defensive lasers? If so, then that's probably a big enough fleet to be a danger even without using FLT trickery. Otherwise, can the smaller (I assume) lasers dispatch of the larger lasers that quickly? Finally, how can the attackers know of all the defensive lasers? I'd assume some (if not a lot) could very well be camouflaged and / or hidden in some way. In any case, what your write about I tried to tackle in the "asterisk" in my answer. Dec 20, 2016 at 12:52
• The issue is that the light from the arrival of these ships will take time to reach the defensive emplacements, whereas the ships themselves will already have accurate info on where the defensive emplacements are. They can use the extra milliseconds that gives them to aim, fire, and jump away. As long as the damage can't be repaired instantly they can rinse and repeat that procedure as much as they want, barring (as you noted in your answer) any energy/cooldown requirements. Dec 20, 2016 at 13:43
• @JoeBloggs Did I miss something in the original question? I assume that, once the ships arrive, they still need some time to jump away - more than they might have needed to jump in due to the gravity of the planet. That should be ample time for the defenders to barrage the fleet with deadly, concentrated photons at least to some degree. Dec 20, 2016 at 14:22
• The original question doesn't say anything about how long that time is. As long as the attackers park themselves a suitable number of light seconds away from your defensive barrier they can appear, launch their own barrage of photons and leave before your lasers have chance to even see them, much less fire back. That's not an issue with physical projectiles because they're so much slower than light, but if both sides have powerful lasers then the FTL attackers had an overwhelming advantage. Dec 20, 2016 at 14:36

This isn't an answer that would help a particular admiral, but it presents a sociological solution: after the first two or three times we're hit by this kind attack force, we realize that remaining in large, centralized populations allows this attack to pose a genuine existential threat to said populations. The solution: a deliberate diaspora, using the same wormhole technology to keep people and resources (and culture) flowing between colonies of only a few hundred people each.

All of this assumes, of course, that the amount of energy required to make these jumps between points in low curvature spacetime is very easily available and we don't have to seriously worry about running low on energy reserves. If we do... Well, this might be just what prevents serious attacks like those described from being a problem.

The diaspora solution is also troubled by the fact that space is really big, and important resources are all clumped together in solar systems; the need for resupply runs, long term mining operations, or large agricultural projects would provide points of vulnerability. Smaller colonies would potentially be more vulnerable to a small number of heavily armed ships lying in wait. But I think this might be surmountable.