The Union (it was the only name that would translate for so many species) has found a really nice super-weapon/life-form: intelligent grey goo. It's a massive swarm of drones each about 8 feet by 4 feet that do what grey goo does best, break down other things and make more of itself. Every drone in this swarm can communicate with each other instantaneously due to... science... and share one mind. This mind is intelligent, and it does what we want it too, in that, we say, seize control of planet x, and it can and will figure out how to do it and do it. So,

Why don't we use it against strong targets?

We use it to mop up the last vestiges of defeated enemies, but never use it in the main fighting. To clarify, the universe is a nasty place, with lots of powerful enemies.

Specifications: Weapons in this universe are very powerful, but have to break through the energy shields that anything important has protecting it. This means a part of a ship will seem totally undamaged until its shield goes down, when it is almost instantly destroyed. The drones have access to the best shields and weapons we can give them. There is no upper bound on the drones, but there is a lower one. Each drone has a weak computer that works with the others to form an intelligence, but if there are less than a hundred, it rapidly loses efficiency. However, there are Trillions of drones, and they can double every two minutes given sufficient supply. As such, the swarm usually has no care for it's individuals. We trust this swarm, we aren't worried about it going rogue.

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    $\begingroup$ One classic reason for a ace in the hole type weapon is the more you use a weapon the more likely someone will fund a way to counter or copy it. $\endgroup$
    – John
    May 28, 2021 at 1:46
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    $\begingroup$ Just to be sure: is the drone 8 by 4 feet, or each individual swarm? Grey Goo is used for a swarm of self replicating nanodrones. $\endgroup$
    – Trioxidane
    May 28, 2021 at 5:29
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    $\begingroup$ Asking just because it bothered me for no good reason: 8 feet by 4 feet by what? xD $\endgroup$
    – Qami
    May 28, 2021 at 12:30
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    $\begingroup$ Honor. You want to fight strong enemies on equal footing to prove your own strength and worth. Or any other cultural or religious reason. I dimly recall a science fiction book (Halo?) where a caste of aliens insisted in doing so. I don’t understand why my answer was removed. It’s at least as believable as any made up technical limitation. $\endgroup$
    – Michael
    May 28, 2021 at 20:15
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    $\begingroup$ "At first the {proximity} fuzes were only used in situations where they could not be captured by the Germans." (over water) $\endgroup$
    – Mazura
    May 28, 2021 at 23:46

12 Answers 12


Because we don't know about its past and the ordering system is not as straightforward as it seems.

Before anything, I'd like to say that these drones seem to function a lot like ants. They clearly function more like a superorganism, in the sense that they act more like the cells of an individual organism than as a group of individual entities, especially since the swarm is usually not worried about its members specifically, but rather a about the collective. In how they seem to attack, they seem a bit like army ants, in the sense that they work as a large group to take down things much bigger and more powerful (some army ants species are no joke, when they march, it's common to see other creatures, especially other invertebrates, running away).

As for why they'd be called "Grey goo", it probably means they can function as something that at least looks like a fluid or mass, especially if you don't look attentively,. This would tell me that they probably have the ability to come together and form basic structures using their own bodies, much like fire ants use their own bodies to form towers, bridges and rafts (yes, fire ants can build rafts with their own bodies to survive floods).

With that little commentary done, why wouldn't the union use them all the time if they seem so good at destroying and consuming enemy weapons and troops alike? Well there seem to be two good reasons for not doing so:

1: Ordering them is a hassle. You gave an example of a possible order:

we say, seize control of planet x, and it can and will figure out how to do it and do it.

Sounds straightforward, right? Except it totally isn't, because since you can't really control the swarm's every action without issuing proper orders, you can't be sure that the swarm will seek a solution that matches the union's interests.

In a speculative scenario: the swarm determines that the most efficient way to seize control of planet x is to neutralize its well developed military forces in an all out attack on several locations, which requires the swarm to grow in size in order to accomplish the task, and so it increases its numbers. Where did it get the resources? Well planet Y over there had a bunch of resources so it destroyed a good chunk of its cities and population. Planet x's military has now been subdued and the planet is under control (there were also some rebellions preventing complete subjugation at first, but the swarm quickly figured out that tearing their members apart publicly and using the planet's own weapons to nuke areas with high number of rebels was the most effective way to end the pesky interference). But hey, now the swarm commands the planet, which is a simple task for it now since it's increased its numbers fivefold. Hope you have a place to accommodate the new drones, since ordering the swarm to destroy them would be wasteful.

With this brutal example, you can hopefully see the problems that come with leaving a hive-mind style swarm choose its approaches. The task was indeed achieved, but several people died and a third party was involved without having anything to do with it. The bigger the task the more you need to detail to the swarm what it must and mustn't do so that it doesn't only follow the orders it was given, but also that it's actions are in line with the Union's rules and interests. The bigger the task, the harder it is to predict potential problems that will hinder the swarm's objective, and the more likely that it will at some point face a problem that requires it to violate potential guidelines given before its mission. If all it has to do is small cleanup missions and dealing with leftover threats, it's a lot harder for things to take an unexpected turn, and easier to predict potential issues and instruct the swarm how to deal with them accordingly.

2- why are you trusting it? sure, the swarm composed of trillions of murderous drones ready to increase their numbers shows no sign of going rogue. Why would you completely trust them though? You didn't build them, you found them,and the majority of their abilities seem centered around one main purpose: to grow. Yes, it will obey any order you issue, but can you be sure it never had any intentions of its own at any point? Can you affirm that it will always interpret its orders according to the Uinion's wishes? Or that said orders can't be misinterpreted in any damaging way? Is there a way to ensure no rogue agent in the union can give the swarm orders that can result in the swarm taking actions that could harm the union itself? Or that could cause it to go back to doing what it did before the union got a hold of it, whatever it was? This once again begs the question on whether the system of ordering the swarm and trusting its choices is truly a reliable form of controlling this superweapon.

So tl;dr: why wouldn't the Union use it to deal with big threats? Because the union has little to no real means of reliably controlling the swarm's every decision on how to properly accomplish its orders, especially regarding big tasks with a large number of variables on what can happen; and because, no matter how you look at it, the swarm seems to have the primary purpose of expanding in size and power, meaning that the chances that it once had its own objectives can't be completely ruled out until everything about its past is known by the Union.

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    $\begingroup$ Wow, I had genuinely never considered the option of it interpreting orders in a harmful way despite that being the premise of dozens of movies. Thanks! $\endgroup$ May 28, 2021 at 17:39
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    $\begingroup$ And don't forget: When it's learning to fight enemies you consider difficult, the enemies are showing it strategies on how to fight you for evaluation. $\endgroup$
    – notovny
    May 28, 2021 at 21:32
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    $\begingroup$ This answer brings to mind the Stargate Atlantis version of the Replicators. They were built to fight an enemy, the Wraith, that feed on humans to survive. During the course of the show the Replicators decide that the best way to fight this enemy is to simply eliminate all humans in the galaxy, so that the Wraith have no more viable food sources and die out. Not exactly what the original creators had in mind. $\endgroup$
    – Kayndarr
    May 31, 2021 at 0:06


Grey goo in any of it's forms does what it does - destroys everything it comes in contact with. It makes more grey goo. Now grey goo is nice, but we all know what happened to king Midas when he wanted everything to be gold - the things he really valued were destroyed.

Grey goo as a WMD is only useful to destroy everything you point it at. But the Union doesn't want a universe full of grey goo. They want power plants, Dyson swarms, research facilities, and tech trees supporting factories that churn out the stuff the Union REALLY wants. Conquering the universe isn't a goal, it's a means to an end.

The Union might very well use these swarms against any parts of an enemy that are dispensable. What's another outmoded asteroid mine being used as a rebel base, after all? But if you're going to wipe out a civilization, you are going to wipe out the things you were hoping to conquer. You want to use more precision tools.

Then there's the concern over fairness. No, I don't think the Union is NICE. If the Union turns their grey goo weapon against a civilization capable of producing one themselves (i.e. one worth conquering) then the enemy will unleash a matching weapon against all the soft targets in the Union - like those previously conquered factories that are now supporting the Union juggernaut in its quest for wealth and power. The same math that allows the Union grey goo to do what it does allows the enemy grey goo to dissolve the Union (pun intended).

  • $\begingroup$ This also seems dangerous. What if someone lands there, gets some grey goo that's long dead, puts it in a box, ships it to a target location and then gives it just a bit of power to reanimate the goo again. Even if a kill switch is given, the enemy might now know the kill switch. If the goo is still alive, it could travel on the asteroid or the ship blasting off gives it speed, allowing it to be a projectile again... $\endgroup$
    – Trioxidane
    May 28, 2021 at 13:39
  • $\begingroup$ @Trioxidane True. The specific grey goo in this question is a little chunkier, but you're right, the basic principle is the same, just on a macro-scale. $\endgroup$
    – DWKraus
    May 28, 2021 at 13:43
  • $\begingroup$ It's not a problem that wouldn't be present at other targets now that I think about it. Backwaters would be better, as putting this ultimate weapon against a full population, the chances they could use it (unintended or not) against the union by flying at them with grey goo would skyrocket... $\endgroup$
    – Trioxidane
    May 28, 2021 at 13:48
  • $\begingroup$ The "Reanimate and return to sender" idea, wouldn't really work, due to the fact that the grey goo shares one mind, but that's a good thought. $\endgroup$ May 28, 2021 at 17:35


This swarm was used for that purpose, but strong targets found ways to disrupt communications between drones, preventing them from recognising each other. Each drone had to fall back to a basic pattern: Find material, build replica. So they wasted their time devouring each other.

Disruptors require testing and development. Keeping the swarm doing clean-up lets the Union ensure that nobody with the chance to test Mk-I disruptors survives to develop Mk-II disruptors that could suborn drones. Extra points if the Union can destroy/capture disruption tech on worlds they control.

The ability to fully seize a planet within two hours (1:40 to get 2 drones/sqm on the surface of Earth, starting from a single drone and doubling every 2min) is a massive tactical advantage. The swarm is an industrial backbone, and they can't risk losing it. So entire planets of drones beam a copy of the collective mind off, then cannibalise each other rather than risk capture. The last drone brings a nuke inside its shields.


The gravity of the situation presents danger.

NO seriously, you have trillions, capable of further doubling every couple minutes, and they are each the size of a rhinoceros, and mass about 3.5 tonnes?

Just how much gravity does a 3.5 trillion ton swarm present?

Or how about one that has replicated itself for just one hour, and now masses 52 times as much as the Earth's Moon?

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    $\begingroup$ Related $\endgroup$
    – DLosc
    May 28, 2021 at 14:29
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    $\begingroup$ Given the swarm builds itself from surrounding materials, presumably no danger; if you convert the top few feet of a planet to goo, it's still exactly the same mass, it's just goo now. It can't increase mass unless it's somehow capable of absorbing ridiculous amounts of energy and directly converting it to mass. $\endgroup$ May 29, 2021 at 2:42
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    $\begingroup$ @ShadowRanger but what of the next target? it will "inherit" the mass of all the previous conquered systems. $\endgroup$
    – PcMan
    May 29, 2021 at 6:15

We trust this swarm, we aren't worried about it going rogue.

Maybe you should be.

The 9000 series is the most reliable computer ever made. No 9000 computer has ever made a mistake or distorted information. We are all, by any practical definition of the words, foolproof and incapable of error.

-HAL 9000 on itself

Value drift

The Grey goo is intelligent, adaptive, and constantly learning. However, when it is attacking a large target, it is doing all of this unsupervised. Furthermore, once it has destroyed a large force, it will have a large force of roughly the same size. If you take over a planet with the grey goo you have destroyed all resistance, but now you have a different problem, turning off your grey goo.

This mission is too important for me to allow you to jeopardize it. -HAL 9000

After completing a large mission without someone supervising its learning, the grey goo may have come to some conclusions that make it no longer loyal to you. It attacks neighbors you have treaties with since it knows it can win and increase the number of conquered planets. It ignores your generals since it is smarter than them anyways. It modifies itself to do farming work and builds a planet with the same economic output as what you would get from conquering with it, but now it can't fight. Since you didn't monitor it, you might not know these changes have occurred until too late.

Training, testing, and validation sets.

It is safe to use it on small scales where you can monitor changes after the conflict, check for value drift, and then reset or keep the changes. You might actually want to do this constantly to increase the effectiveness of the weapon. However, doing this would likely require the shutdown of at least one robot to fully explore its net in a dormant state. Also, if a rebellion occurs, it is better for this to occur when they are the size of a city block, and not the size of a planet. By destroying the army after it is done you ensure you can never get steamrolled by your own weapon.

  • $\begingroup$ Not a fair comparison. HAL was working fine, until a bunch of idiots gave it an order that it was literally designed not to be able to follow. As someone stated so eloquently in the sequel, 2010: "HAL 9000 was programmed to always tell the truth -- and you ordered it to lie." $\endgroup$ May 29, 2021 at 15:48

Escalation and the Rules of War

Certain powerful alien species like to consider themselves universal police and step in when inferior species start to mess around with truly dangerous stuff like grey-goo, vacuum decay, or time travel. While limited use of these technologies is fine, they don't like to see it used as an opening salvo to a war, instead preferring that war be waged "honorably" for religious or social reasons.

They claim that if every war started out with one side deploying a superweapon--say gray-goo--the other side would be mandated to respond with their own superweapon thus escalating the conflict to the point where (although they don't like to admit it) the highly advanced police-aliens would be somewhat threatened.

As such, these dangerous technologies are allowed to be used as a utility or a tool to accomplish mundane tasks once the enemy is defeated and there's no chance of self-reinforcing escalation cycles.


Tonne-per-Tonne, Ships are Stronger.

In your universe, combat power increases exponentially with size. This is likely do to efficiencies in power or shield generation at scale.

Imagine 100 U of resources. When used for a single ship, it makes 100 U of destructive power, but when used with 100 ships, it makes 10 U of destructive power. The difference is so overwhelming, a large ship can fight swarms of goo in relatively inefficient ways, which leave them inert and unable to reassemble (e.g. atomizing, gravity wells, EMPs) and still win.

Fielding a navy entirely of Grey Goo is wasteful especially if FTL is dependent on weight, so you want to maximize effectiveness-per-tonne.

So why field Grey Goo at all?

Goo Gets Stronger, the Longer the Fight

Goo has one major tactical advantage as the fight prolongs: wreckage. A ruined ship (on either side) can be converted into goo, weaponizing an otherwise useless hunk of metal. As the battle prolongs, the tonnage of ships will decrease and the tonnage of goo will increase, until a tipping point is reached and the goo is more powerful than all remaining enemy ships. Then it is unleashed on the enemy, mopping up their remaining forces, without damaging capital ships.

  • $\begingroup$ Can the goo still utilise ships that've been completely vapourised? $\endgroup$
    – Vikki
    Jun 3, 2021 at 0:09

Nukes can stop them.

Targeted nuclear strikes can destroy them before they can mass replicate, and any experienced enemy has either nukes or something similar. Using grey goo tends to be ineffective and ruins biospheres.

As such, it isn't used until all nuclear powers have been defeated.

Once the grey goo has reached enough mass it can produce large shields to protect itself, but it's vulnerable until it reaches a critical mass.

The initial grey goo can't be used from space

While generic grey goo is cheap to produce, grey goo is not tough enough to easily handle being fired from orbit. It's a lot cheaper if there's already an outpost so that the grey goo can reproduce itself explosively from there, with access to whatever rare materials and ores it needs to grow quickly.


Grey Goo Needs Matter

Strong opponents will construct shield generators capable of protecting virtually all mass within their area of influence. In a universe full of super-weapons, it would be folly not to. A stray shot - perhaps fired decades or centuries ago - is liable to destroy all kinds of valuable things if they aren't shielded: ships, Dyson Swarms, and planets are all vulnerable in this universe.

A Dead Ship is Matter

But after you've pummeled your enemy to the point of defeat, there will be lots of un-shielded mass. Battles will generate debris fields, and leave moons and shipyards unprotected. The Grey Goo can go to town with the mass you have generated, and overwhelm whatever defenses are left.

It's a natural "mop up" force in the universe you describe.


You might trust the swarm when it is mopping up a defeated enemy, but an enemy still able to resist has a variety of weapons or tricks that can deal with it.


The swarm can communicate instantly. This is a weakness if you can send signals too as if you are part of the swarm. Maybe you captured drones and manipulated them or you simply use the fact that it's not really possible to determine who or what is signalling. Since other drones will have to verify each other's signals all the time and you cannot allocate all memory to verifying every member of he swarm you are forced to let only a portion verify other drones. You introduce drones that are "verified" by other drones you own. Now you can do things to the swarm like attempting to alter it's parameters, feed false information or even say "hey there seem to be some imposters between us and I think it's this section of drones", after which an internal war breaks out.

Another example:

The swarm requires energy to function. Normally they get the energy out of the enemies they devour, but if those same enemies resist then suddenly the power requirements to devour and convert them go sky-high. If it takes a thousand drones worth of materials and energy just to defeat and convert one enemy it becomes an inefficient task.


You cannot let your secret weapon fall into enemy hands

(This answer works generically for super-weapons of any type, not just Grey Goo.)

The Union has this devastatingly powerful military technology and their enemies do not. This weapon has several key advantages over the super-weapons of other factions, such that the other factions would desire to have the Grey Goo, if they knew it existed.

Deploying the Grey Goo at the beginning of a fight against a powerful enemy carries numerous risks. Firstly, by not yet having control over the area, the enemy is able to freely send transmissions to other bases, spilling your secret. Secondly, the Union is not guaranteed to win this engagement. If they lose, the Grey Goo is likely to be captured and reverse engineered. This would spell the end of the Union's strategic advantage.

However, by waiting for the enemy's defences to be broken and using the Grey Goo only for the final push, the Union ensures two things. Firstly, they have sufficient area control to block transmissions from being sent out (using whatever Sci-Fi transmission blocking tech works for your setting). Secondly, they are confident in their eventual victory, so there is virtually no risk of the Grey Goo being captured.

Why deploy the Grey Goo at all? Because mopping up those last vestiges of the enemy in their bunkers is costly and dangerous to do by other means, due to a tendency to make last stands and hide in fortifications and lay desperate traps, but Grey Goo works very well in these scenarios.

There may come a time in the future when the Union's backs are against the wall and they need to pull out all the stops to win a battle or a war. Only then will the Union openly deploy the Grey Goo and hope that it wins them the war fast enough that this enemy will not be able to reverse engineer it. However, by doing this the next enemy will know about Grey Goo and will become even more dangerous, so the Union will only do this if they are desperate.

This theme of being reserved in the use of advanced weapons is common in real life. Proximity fuses in World War II "were only used in situations where they could not be captured by the Germans", in this case over water (credit to Mazura for identifying this in a comment on the question). The nuclear bomb was developed in utmost secrecy in the Manhattan Project and was only deployed at the very end of the war when the Americans were confident that it would push the Japanese to surrender.

This theme appears in science fiction as well. E.E. Doc Smith's Lensman series contains galactic superpowers with weapons of similar scale to what you describe (plus or minus a few orders of magnitude, depending on whether you are at the start or end of the series). On numerous occasions a faction develops a potent new military technology, but is then extremely careful about when they deploy it because they know that the enemy faction will attempt to replicate and exceed it. They only deploy their new superweapons when they either can guarantee that no enemy witnesses will survive or send any transmissions, or that they desperately need to overcome the enemy right now in order to gain a long-term strategic advantage.

However, there is a BIG CAVEAT to this strategy of secrecy: It won't work forever. It is inevitable that, at some point, the enemy factions will learn about the Grey Goo. This might be due to them finally witnessing it in a deployment. Or it might be due to spies stealing the technology. If the timescale is measured in years you should be fine. If the timescale is measured in millennia you'll have plausibility issues. Because of this, the Union should be actively researching a bigger and better super-weapon for when their Grey Goo stops being novel.


Larger masses of grey-goo become susceptible to takeover of control

Imagine the grey goo being some kind of susceptible to telepathic control or hacking once a certain mass is reached. If your enemies are capable in that area, you might have your super weapons wrestled from your hands and turned against you.

So better just use it to wipe the floor with the inferior ranks of the enemy and leave an impression after the decision has been reached already.


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