How powerful could a hard sci-fi civilization be?

[Sky Captain]

There is nothing a hard sci fi power could do against a base located in different star system in a time scales that matters. FTL jump capability gives too much tactical and strategic mobility advantage over a foe that is limited to sublight travel. Kinda like a modern fighter bombers fighting WWII era batleships. Sure, in theory battleship could destroy fighters with its cannons... if it could hit them. Hovever realistically it would be very unlikely a battleship manages to hit a fighter and highly likely a battleship gets killed by bombs and/or anti ship missiles.

[lPeregrine]

Also, you're overlooking just how long such a weapon would take to hit. Years for the closest targets to the hard setting, which is plenty of time to drop an asteroid in the way, watch the asteroid get hit, calculate how the asteroid has changed the path or the attack, then go pick up a new asteroid. Repeat daily until the weapon is not going to hit anything important with enough energy to overwhelm its shields.

As for the further targets, you're talking hundreds or thousands of years from firing the weapon till it hitting for the UFP. Tens of thousands for the Empire.

But over what range do the FTL sensors work against an simple kinetic projectile? Can they actually find the shot (or, worse, multiple shots fired on slightly different courses) and/or calculate its exact trajectory from far enough out to prepare the asteroid shield? You're talking about finding a very small target across interstellar distances, and it could be in a pretty huge volume of space at any given moment.

If they can't, it's easy to imagine a MAD scenario: the soft scifi universe can launch FTL hit and run attacks at will and there's little the hard scifi universe can do to stop it, but the soft scifi universe can't stop the hard scifi universe from launching an extermination campaign with relativistic planet killers. Sure, the delayed effect would make it come too late to have any effect on the immediate war, but as the leader of the soft scifi civilization would you launch the attack knowing that it means near-certain death to your entire civilization in a thousand years?

[Satori]

In a thousand years, couldn't you evacuate?

[Imperial528]

A soft sci-fi civilization could also use that time to figure out the trajectories of the projectiles, or most of them, just by looking at the orbits of likely launch centers. Heck, a hard sci fi civ could do the same thing, though they would not have the luxury of being able to get a close up look and possibly even take targeting data from the enemy.

[lPeregrine]

In a thousand years, couldn't you evacuate?

But where do you go? Every planet in your universe has been targeted, so all you can do is evacuate over and over again until there are no more planets left, and then you die.

A soft sci-fi civilization could also use that time to figure out the trajectories of the projectiles, or most of them, just by looking at the orbits of likely launch centers. Heck, a hard sci fi civ could do the same thing, though they would not have the luxury of being able to get a close up look and possibly even take targeting data from the enemy.

Maybe for a simple case, but let's say there are a million inhabited planets each launching multiple shots at each of a million target planets. You might know the trajectory from A to B, but do you know how fast the shot is moving and when it was fired at a particular planet so you can tell where on that trajectory it will be? Or let's make it even worse and give the planetkiller shots some engines so they can be fired on an indirect course that could be anywhere. You can't narrow that down using pure math and the location of the guns, you need sensors capable of spotting the actual projectile across interstellar distances.

[lPeregrine]

And note that even that isn't 100%. Remember, the target is moving (on the scale of hundreds or thousands of years), so if you don't know exactly when the shot was fired and how fast it's moving you won't know exactly where the target is going to be when the shot arrives. And without that knowledge you don't know the exact trajectory of the shot even on a purely ballistic course.

[Imperial528]

I understand this, but a soft sci civ has the advantage that if all else fails (enemy destroyed all intelligence related to the shots and the guns themselves, so you can only estimate based on which worlds had the industry to do it) you can spam probes everywhere and start sniffing long before the projectiles even get in your space.

I'd start with putting probes around their systems a few light years out from when the war started, and from when the war ended. That way, you'll eventually catch the light from firing the guns, allowing you to make an estimate of when and how fast the projectile was fired. Then, determine where each shot will be relatively, and start making and sending out ships to intercept them, pronto.

Even if all that fails, you do have the time to build massive space habitats to evacuate to. Or, depending on just how advanced your civilization is, change a planet's orbit so the projectile might miss.

[Eternal_Freedom]

I understand this, but a soft sci civ has the advantage that if all else fails (enemy destroyed all intelligence related to the shots and the guns themselves, so you can only estimate based on which worlds had the industry to do it) you can spam probes everywhere and start sniffing long before the projectiles even get in your space.

I'd start with putting probes around their systems a few light years out from when the war started, and from when the war ended. That way, you'll eventually catch the light from firing the guns, allowing you to make an estimate of when and how fast the projectile was fired. Then, determine where each shot will be relatively, and start making and sending out ships to intercept them, pronto.

Even if all that fails, you do have the time to build massive space habitats to evacuate to. Or, depending on just how advanced your civilization is, change a planet's orbit so the projectile might miss.

Catching the light from the guns firing assumes some incredibly good optics on your probes. Worse, since such weapons are likely to be coilguns or railguns or whatever firing in a vaccuum there won't be a distinctive flash of light. So you need even more poweful optics to be able to see an (optically) dim event from a distance of several light years? Veeeery unlikely nethinks.

[Srelex]

In the case of Trek, considering their sensors are able to pick up individual particles in vast areas of space...I really don't think picking up relativistic projectiles would be a problem for them.

In the case of Wars...I don't think there's as much detail on their own sensor capabilities, which tend to flip around a lot, but it's also possible, given the level of space traffic in their galaxy, that someone would end up passing such projectiles by and noticing them. This is of course assuming that said projectiles aren't gradually pushed off course thanks to impacts with minute space debris over the years.

[Eternal_Freedom]

In the case of Trek, considering their sensors are able to pick up individual particles in vast areas of space...I really don't think picking up relativistic projectiles would be a problem for them.

In the case of Wars...I don't think there's as much detail on their own sensor capabilities, which tend to flip around a lot, but it's also possible, given the level of space traffic in their galaxy, that someone would end up passing such projectiles by and noticing them. This is of course assuming that said projectiles aren't gradually pushed off course thanks to impacts with minute space debris over the years.

While that's all true I was addressing the "wait a few light years out to see the firing happen." point.

[Alerik the Fortunate]

An FTL capable society could probably also sent FTL probes to the home system of the aggressive STL society and observe the construction, aiming, and firing of the weapons directly without having to detect the individual projectiles from scratch. Though so far we've constrained the discussion to Type II civilizations. How would an FTL empire deal with a fully exploiting STL society that already occupies a substantial portion of the galaxy?

[Borgholio]

An FTL capable society could probably also sent FTL probes to the home system of the aggressive STL society and observe the construction, aiming, and firing of the weapons directly without having to detect the individual projectiles from scratch. Though so far we've constrained the discussion to Type II civilizations. How would an FTL empire deal with a fully exploiting STL society that already occupies a substantial portion of the galaxy?

Depends on the FTL empire you're referring to. A highly advanced one such as Star Wars would already occupy a substantial portion of the galaxy themselves, and have highly advanced tech on their side. A FTL empire such as in the movie Aliens would be no match. Even Babylon 5 and BSG would not be able to handle a large portion of the galaxy populated with planets discussed in the earlier posts in this thread. It'd take something large and resilient such as the Galactic Empire / Republic or the Borg.

[bilateralrope]

Also, you're overlooking just how long such a weapon would take to hit. Years for the closest targets to the hard setting, which is plenty of time to drop an asteroid in the way, watch the asteroid get hit, calculate how the asteroid has changed the path or the attack, then go pick up a new asteroid. Repeat daily until the weapon is not going to hit anything important with enough energy to overwhelm its shields.

As for the further targets, you're talking hundreds or thousands of years from firing the weapon till it hitting for the UFP. Tens of thousands for the Empire.

But over what range do the FTL sensors work against an simple kinetic projectile? Can they actually find the shot (or, worse, multiple shots fired on slightly different courses) and/or calculate its exact trajectory from far enough out to prepare the asteroid shield? You're talking about finding a very small target across interstellar distances, and it could be in a pretty huge volume of space at any given moment.

If they can't, it's easy to imagine a MAD scenario: the soft scifi universe can launch FTL hit and run attacks at will and there's little the hard scifi universe can do to stop it, but the soft scifi universe can't stop the hard scifi universe from launching an extermination campaign with relativistic planet killers. Sure, the delayed effect would make it come too late to have any effect on the immediate war, but as the leader of the soft scifi civilization would you launch the attack knowing that it means near-certain death to your entire civilization in a thousand years?

Interesting how you talk about the limitations of FTL sensors (a legitimate point), but assume that the hard civilization is able to magically locate all the inhabited planets of their enemy and aim their weapons accurately enough to hit those planets. How is the hard civilization getting the location of the planets ?
How are they aiming their guns accurately enough to hit them ?

Lets take the Federation as an example. Memory Alpha says it's spread over 8,000 light years. It's only existed for a few hundred, meaning it's expanded faster than the speed of light.

Identifying Federation planets is really important. If they just decide to target every earth-like planet they can hit, they are going to piss off one of Trek's godlike entities.

[lPeregrine]

Interesting how you talk about the limitations of FTL sensors (a legitimate point), but assume that the hard civilization is able to magically locate all the inhabited planets of their enemy and aim their weapons accurately enough to hit those planets. How is the hard civilization getting the location of the planets ?

The same way that we identify stars with planets in the real world, except (presumably) with much better speed and accuracy thanks to superior technology. And then you destroy anything with a planet. After all, even if it wasn't inhabited when the light you're looking at left the planet it could be inhabited now, or could be inhabited by the time your planetkiller arrives. Rocks are cheap, so you might as well just destroy everything and be sure.

How are they aiming their guns accurately enough to hit them ?

First of all it's not exactly a complicated thing to figure out, orbital mechanics is a well understood field (and presumably would be even more so for an advanced civilization) so all you need to do is use a mathematical model to tell you where your target is going to be. And then you add engines to your planetkiller to refine its precise trajectory shortly before impact. Ideally you'd also deploy a cloud of submunitions, each with their own engines, to wipe out any space stations/inhabited asteroids/etc while the main shot goes for the planet.

Identifying Federation planets is really important. If they just decide to target every earth-like planet they can hit, they are going to piss off one of Trek's godlike entities.

Except that once you appeal to those godlike entities it's a concession of defeat. The question is hard scifi civilization vs. soft scifi civilization, not hard scifi civilization vs. god.

(And of course that's assuming that god even decides to help, which seems to be a rather significant assumption.)

[Imperial528]

Catching the light from the guns firing assumes some incredibly good optics on your probes. Worse, since such weapons are likely to be coilguns or railguns or whatever firing in a vaccuum there won't be a distinctive flash of light. So you need even more poweful optics to be able to see an (optically) dim event from a distance of several light years? Veeeery unlikely nethinks.

If they're firing at very low speeds, yeah. But to actually threaten an FTL civilization they'd have to at least fire these things at 0.25c or so. They're also going to have to be rather large projectiles, and likely carry course correction equipment, so that they don't go through a dust cloud and get slowed down as would a swarm of much smaller projectiles.

They gun itself is going to be massive, and the firing will give off a massive heat signature to match. Let's be a bit conservative with the mass end and say that they're firing 1 ton masses from each gun at 0.8c (impact energy of 11 GT, damaging but not world-ending) and that they can accelerate these at rates of 10g. This requires a gun of 1961 AU in length, and one that can radiate at each point the waste energy from firing. So this thing is pretty damn big. Now, let's say that they used superconductors and somehow, the gun is 99% efficient. To accelerate the projectile, it needs to consume about 98989 watts, 98000 of which are used to accelerate the projectile, leaving the remaining 989 in the gun as energy to radiate off, which we'll also assume it is 99% efficient at.
So then the gun will be radiating about 980 watts, and storing the remaining nine as heat. Over the time it takes to fire (28.3 days) this will build up in the end so that the gun has radiated off about 2.39 gigajoules, and is storing about 24.22 megajoules, which it will take seven hours to radiate off, assuming it can radiate at the same rate it did when firing.

Unless they build it in deep space, you are correct, it won't shine very brightly, at least not as brightly as we want it to. But it's also going to be almost five times the diameter of the solar system in length. At this point, the soft sci-fi civilization could show up and destroy it on the day it was finished, since it would either take so long to build that they'd have reached it by then, or if they can build it faster, they will probably notice something fishy. Especially if they're building hundreds of thousands of these things.

Hell, they could even be huge dicks and destroy it while it was firing.

Now, let's assume they can accelerate it at 1000g. Much better, right? Well, kind of. Now it only takes a 20 AU gun 6.7 hours to fire the projectile. However, it also radiates 98 kilowatts that whole time, and then spends four minutes radiating off the build up heat at the end.

Actually, after writing all that up, I'm pretty sure it proves my original idea to be incorrect. Though I suppose if they used a star's gravity as a lens... (Is that cheating? Soft sci-fi using hard sci-fi techniques?)

Back to the math for now.

Either way, right now I'm going to say that their best bet is to watch for all the indications of the hard-sci people actually building the guns, and then go from there, since the building of the guns should be a lot more noticeable than the firing, at this rate. Though if they built the two guns described in this post, I do hope that they paint them black and put them far out from the system's star, because then you have essentially built a very long reflective object that just so happens to emit WMDs.

[Jub]

Catching the light from the guns firing assumes some incredibly good optics on your probes. Worse, since such weapons are likely to be coilguns or railguns or whatever firing in a vaccuum there won't be a distinctive flash of light. So you need even more poweful optics to be able to see an (optically) dim event from a distance of several light years? Veeeery unlikely nethinks.

If they're firing at very low speeds, yeah. But to actually threaten an FTL civilization they'd have to at least fire these things at 0.25c or so. They're also going to have to be rather large projectiles, and likely carry course correction equipment, so that they don't go through a dust cloud and get slowed down as would a swarm of much smaller projectiles.

They gun itself is going to be massive, and the firing will give off a massive heat signature to match. Let's be a bit conservative with the mass end and say that they're firing 1 ton masses from each gun at 0.8c (impact energy of 11 GT, damaging but not world-ending) and that they can accelerate these at rates of 10g. This requires a gun of 1961 AU in length, and one that can radiate at each point the waste energy from firing. So this thing is pretty damn big. Now, let's say that they used superconductors and somehow, the gun is 99% efficient. To accelerate the projectile, it needs to consume about 98989 watts, 98000 of which are used to accelerate the projectile, leaving the remaining 989 in the gun as energy to radiate off, which we'll also assume it is 99% efficient at.
So then the gun will be radiating about 980 watts, and storing the remaining nine as heat. Over the time it takes to fire (28.3 days) this will build up in the end so that the gun has radiated off about 2.39 gigajoules, and is storing about 24.22 megajoules, which it will take seven hours to radiate off, assuming it can radiate at the same rate it did when firing.

Unless they build it in deep space, you are correct, it won't shine very brightly, at least not as brightly as we want it to. But it's also going to be almost five times the diameter of the solar system in length. At this point, the soft sci-fi civilization could show up and destroy it on the day it was finished, since it would either take so long to build that they'd have reached it by then, or if they can build it faster, they will probably notice something fishy. Especially if they're building hundreds of thousands of these things.

Hell, they could even be huge dicks and destroy it while it was firing.

Now, let's assume they can accelerate it at 1000g. Much better, right? Well, kind of. Now it only takes a 20 AU gun 6.7 hours to fire the projectile. However, it also radiates 98 kilowatts that whole time, and then spends four minutes radiating off the build up heat at the end.

Actually, after writing all that up, I'm pretty sure it proves my original idea to be incorrect. Though I suppose if they used a star's gravity as a lens... (Is that cheating? Soft sci-fi using hard sci-fi techniques?)

Back to the math for now.

Either way, right now I'm going to say that their best bet is to watch for all the indications of the hard-sci people actually building the guns, and then go from there, since the building of the guns should be a lot more noticeable than the firing, at this rate. Though if they built the two guns described in this post, I do hope that they paint them black and put them far out from the system's star, because then you have essentially built a very long reflective object that just so happens to emit WMDs.

Why are you presuming that these guns don't already exist before the other force invades? Why are you expecting that the soft-scifi side will find them if they are built?

[bilateralrope]

Interesting how you talk about the limitations of FTL sensors (a legitimate point), but assume that the hard civilization is able to magically locate all the inhabited planets of their enemy and aim their weapons accurately enough to hit those planets. How is the hard civilization getting the location of the planets ?

The same way that we identify stars with planets in the real world, except (presumably) with much better speed and accuracy thanks to superior technology. And then you destroy anything with a planet. After all, even if it wasn't inhabited when the light you're looking at left the planet it could be inhabited now, or could be inhabited by the time your planetkiller arrives. Rocks are cheap, so you might as well just destroy everything and be sure.

There are always going to be limits on the detection range of that technology. What are these limits ?

What is the maximum range at which they can detect an earth sized planet ?

How are they aiming their guns accurately enough to hit them ?

First of all it's not exactly a complicated thing to figure out, orbital mechanics is a well understood field (and presumably would be even more so for an advanced civilization) so all you need to do is use a mathematical model to tell you where your target is going to be. And then you add engines to your planetkiller to refine its precise trajectory shortly before impact. Ideally you'd also deploy a cloud of submunitions, each with their own engines, to wipe out any space stations/inhabited asteroids/etc while the main shot goes for the planet.

What I'm asking is how accurately can the guns be aimed. To the nearest 100th of a degree, or nearest 1000th, etc. ?
(pulling the numbers out of my ass for now. Get me an exact number, then I'll do the math)

If you need to aim something to within 1,000,000th of a degree, but the best you can do is 10,000th of a degree, you aren't likely to hit it. So the accuracy of their weapons does determine the range at which they can hit. Some form of guidance on the projectile does make things easier, unless the target planet has enough warning to try and fool its sensors.

Using Imperial528's numbers, I doubt a 20AU gun is going to be quick to aim. Which means it's rate of fire is going to be bloody low.

It also makes me wounder how much reinforcement it's going to need to stop the gun from collapsing under its own weight.

Identifying Federation planets is really important. If they just decide to target every earth-like planet they can hit, they are going to piss off one of Trek's godlike entities.

Except that once you appeal to those godlike entities it's a concession of defeat. The question is hard scifi civilization vs. soft scifi civilization, not hard scifi civilization vs. god.

(And of course that's assuming that god even decides to help, which seems to be a rather significant assumption.)

Normally those godlike entities wouldn't interfere in a VS because the war doesn't directly affect them. But when you have one side using a strategy of "shoot everything that might belong to our enemy" I can't see how you can avoid talking about the reactions of those neutral parties who get hit.

[lPeregrine]

Unless they build it in deep space, you are correct, it won't shine very brightly, at least not as brightly as we want it to. But it's also going to be almost five times the diameter of the solar system in length. At this point, the soft sci-fi civilization could show up and destroy it on the day it was finished, since it would either take so long to build that they'd have reached it by then, or if they can build it faster, they will probably notice something fishy. Especially if they're building hundreds of thousands of these things.

That's probably a bad assumption. Relativistic planet killers are a nice thing to have in a hard scifi universe, so it's not difficult to imagine that any system of any real importance would have one available for the deterrence factor. It's just like how every major power in the real world keeps a stockpile of nuclear weapons even though they never want to use them. It's the ultimate insurance policy that you'll never be invaded and destroyed.

So, what would actually happen is that the weapons are already in place, and the first invasion attempts from the soft scifi civilization would be met with a "go away or we exterminate your civilization" warning once it became obvious that there was no other effective way to fight back against FTL battleships. At that point it's suddenly a race to see if the soft scifi civilization can invade and destroy every populated system before the light-speed message to open fire arrives. And given the potential size of a fully developed hard scifi civilization this is not exactly an easy task.

[lPeregrine]

There are always going to be limits on the detection range of that technology. What are these limits ?

What is the maximum range at which they can detect an earth sized planet ?

What IS the theoretical limit on those? We're talking about a fully-developed hard scifi civilization here, so we can assume that technology will allow detection of planets at the theoretical limit.

Some form of guidance on the projectile does make things easier, unless the target planet has enough warning to try and fool its sensors.

Exactly. Why throw a lump of rock when you can throw a lump of rock with engines and a load of guided submunitions that will be deployed shortly before impact to ensure you also get any ships/stations/etc in the system?

Normally those godlike entities wouldn't interfere in a VS because the war doesn't directly affect them. But when you have one side using a strategy of "shoot everything that might belong to our enemy" I can't see how you can avoid talking about the reactions of those neutral parties who get hit.

Because:

1) Those "gods" have their own agenda and it's not "help the lesser races". You might as well assume that the "gods" like the hard scifi civilization better and help them win the war and get rid of that annoying Federation.

2) It's the ultimate concession of defeat. You're no longer talking about hard vs. soft, you're talking about hard vs. omnipotent being. Obviously an omnipotent being wins, but that's not a very interesting question to ask.

[Eternal_Freedom]

Would it even be possible to build a 20AU coilgun? That would surely require a stupendous amount of mass. Let's be simple and assume it's a 20 AU long cylinder with a 100m outer diameter and a 50m barrel diameter:

Cross section area is 9(pi x 50^2)-(pi x 25^2)) which is 5890 square metres.
20 AU is 1.496x10^11 times 20 which is 2.992x10^12 metres.
Total coilgun volume is 1.76x10^16 cubic metres.

Density of iron (which is, I suspect, the only material present in the solar system in enough quantites) is 2.7 tonnes per cubic metre, so the total mass is 4.8x10^16 tonnes.

Now obviously that's not an exact answer, I've made a few assumptions, but I think it's a reasonable order of magnitude estimate to say a 20 AU coilgun would need 10^16 tonnes of material. Which is equivalent to Mar's moon Phobos, an asteroid 27x22x18 km. So, possible, but still a huge amount of effort.

And that is for the 1000g acceleration coilgun, which strikes me as an unrealistically high figure.

EDIT: I'll change the part about using Iron because its the only material sufficiently abundant. THere's plenty of aluminium and titantium kicking about, but I suspect a lot of that would get used in solar colelctors, habitats and spacecraft. When allocating resources in such a setting, I suspect "build honking great space gun bigger than Betelguese" is low on the priority list.

[lPeregrine]

So, possible, but still a huge amount of effort.

But you also have a huge amount of time, and it's a pretty nice weapon for ensuring that your neighbors don't decide they'd really like to have your planet (or just kill you for worshiping the wrong god). It's just like how we spend huge amounts of effort on nuclear weapons even when there are much "better" things to spend it on. Plus, it's kind of a given that we're talking about a civilization that has made military power a relatively high priority, since "soft scifi military vs. hard scifi peaceful civilization that gave up war ten thousand years ago" isn't a very interesting subject.

Also, it's not entirely wasted effort since you've now built a huge and sturdy structure. Why not start building habitats/mining platforms/whatever on the gun? Or, to prove the "any interesting propulsion system is also a weapon" rule why not use it to launch spacecraft between systems (with a matching "gun" at the end to slow it down)?

[Imperial528]

I imagine that to build any such coil gun it would need to be far out in space, far away enough that gravity won't cause it to fall apart via orbital differences. Even then, the only physical connections would probably just be cabling and such. You'd need to use electromagnetic fields to stabilize it when firing, since it would essentially be a collection of accelerator rings. You could perhaps do it by making a whole bunch of them in orbit and firing whenever they all line up. Or try using a cyclotron-like or ring shaped accelerator instead of a linear one. Though that presents a whole mess of other issues, like how to hold that together.

If they could somehow accelerate it at 1,000,000g, then it would only be 3 gigameters long. Or ten light-seconds. Which is still massive. And it would heat up like all hell breaking loose.

Also, you don't want to put habitats on these things. I wouldn't want to be near one of these things, god forbid living on one. If that projectile does not align with the ring you are on, dead instantly. If the radiators fail you burn up. Heck, arrange things the wrong way in your room when it fires, and you could be impaled by you cutlery set as the electromagnets power on.

And with the amount of effort put into this gun, you can build massive laser arrays that can keep you safe from any invader, or even other relativistic weapons, and detection systems to actually see such weapons being fired.

[lPeregrine]

And with the amount of effort put into this gun, you can build massive laser arrays that can keep you safe from any invader, or even other relativistic weapons, and detection systems to actually see such weapons being fired.

But can the laser array retaliate across interstellar distances? Without that ability all you can do is sit behind your wall and hope your enemy doesn't find a way through, which isn't usually the smartest strategy.


Edit: to clarify a bit: the MAD factor may not necessarily be a giant coilgun, that's just one of the more useful forms since it can also act as a transportation system when you don't need to use it as a planet killer. But in a hard scifi civilization where war still exists relativistic planet killers are just too useful to abandon, even if they're just a bunch of rocks with engines sitting in orbit waiting for the "fire" order.

[Imperial528]

And with the amount of effort put into this gun, you can build massive laser arrays that can keep you safe from any invader, or even other relativistic weapons, and detection systems to actually see such weapons being fired.

But can the laser array retaliate across interstellar distances? Without that ability all you can do is sit behind your wall and hope your enemy doesn't find a way through, which isn't usually the smartest strategy.

Actually, yes. You can use it to fire RKVs by attaching laser sails to rocks. Or by sending warships by the same method.

Edit: to clarify a bit: the MAD factor may not necessarily be a giant coilgun, that's just one of the more useful forms since it can also act as a transportation system when you don't need to use it as a planet killer. But in a hard scifi civilization where war still exists relativistic planet killers are just too useful to abandon, even if they're just a bunch of rocks with engines sitting in orbit waiting for the "fire" order.

Yeah, but the problem with those is that they will take much, much longer to get up to speed. And since they have engines, they will have nice little thermal signatures and fuel trails that can be tracked, as we've seen soft-sci civs do in the past. Laser-sail driven rocks have the same acceleration issue, but I suppose what you can do is make the projectile large enough that instead of a sail, you just ablate the surface off. Heck, with a big enough array, you could try and shoot planets at people. Still, it will be very easy to see. Of course, if you spent all of your time building a giant coil-gun instead of a laser array with detection systems, you won't see it coming until it is too late to build your own array. Maybe.

[Sea Skimmer]

Just musing, its kind of silly how a hard sci fi power can only communicate at the speed of light... so, how many years until the whole hard sci fi power even knows its at war? That would get... pesky real quickly. Even if we assume they all 'know' the war started out of hand, and word doesn't come from a central point, that still means all communication and coordination of action is on a scale of years, minimal, and ten of thousand of years to exchange a single two way communication exchange in a small part of one galaxy. Just located the enemy galaxy and want everyone to bombard it? Okay your orders will only take ~60,000 years to reach an entire galaxy.

The wormhole concept mentioned before basically falls prey to the same issue. Even if you could make wormholes, if you need the energy of a galaxy to build them its going to take tens of thousands of years to concentrate that energy at a given point. Also such stable mass transmitting wormholes may not be possible at all.

And with the amount of effort put into this gun, you can build massive laser arrays that can keep you safe from any invader, or even other relativistic weapons, and detection systems to actually see such weapons being fired.

Or use a bunch of big Star Wars ships to start parking large ingots of metal or rocks in the path and building planetary shields on targeted planets. The idea that someone like the Empire wouldn't notice the attacks is pretty absurd, since even if they could not detect the attacks directly, they'd certainly observe evidence of them being mounted as they inspect/attack various hard sci fi worlds. They would then have absurd amounts of time to respond.