I am in a timeline which would be tens of thousands of years advanced from yours, and humans have colonized a significant portion of the stellar neighborhood, multiple thousand light-years across (Last I checked - a century maybe?). Sadly, the laws of physics you devised, on the grand scale, seem to mostly (with a few exceptions) have held up (importantly - relativity). Many of you can probably extrapolate our technology (Ignore the timeline communicator - It was given to me by the Great God of Plot and will be taken away by Him after I have received my answer). Probably the greatest invention is fusion energy and torchships - capable of supporting constant 1g (or more upto Plot) acceleration for an extended period of time, without being a weapon of mass destruction from the heat (due to Plot. One of the few deviations from your world physics. Thermodynamics turned out to be somewhat forgiving! It is still destructive due to its kinetic energy.)

I have reached my destination, System X. I was sent here to build a semi-colony semi-military-base, and am the grand authority here. This system is particularly close to routes fleets of opposing factions would need to take to into allied space. I have a few million colonists, and a drone fleet that was sent at greater accelerations and arrived a few years before. It has mined up a lot of the resources we would need to begin colonization in luxury. They even have a Dyson swarm set up that can extract as much energy as I will need. This system is ours for all intents and purposes.

The test target is an enemy fleet that will - at its closest point - pass within a few light-years of this system. This will happen in a bit more than a century. We know the ship design and know they have accelerated to maximum velocity - They don't have enough fuel for significant deviations from this course. Thus, I have a few decades to build my weapon.

What I have in mind is a huge battery of railguns - as many as Plot demands! They will be capable of accelerating a projectile of mass Plot to around 0.2c max (if that is insufficient, it will be accelerated to Plot). Using my massive fleet of drones, and some calculations about the rate of material gathering, I believe this can be done in the required timespan.

But is it viable? Can using thousands of railguns to attack this fleet at interstellar distances work? I must send a report of my plans to my superiors in a few hours (technically, years), so I must be certain my plan will work. It will be really bad for me if they calculate it will not - and will thus have wasted more then a decade by the time their new orders get here. So I want to be sure this will be able to significantly damage any fleet which passes within a reasonable distance of this star system.

Additionally, will they or a nearby star system be able to tell exactly what I am constructing? Not that they could do much about it - this fleet is doomed either way (even if it changes course, it won't have the fuel to stop). But having the element of surprise for the century or so before this fleet is destroyed and their information reaches the enemies would be very useful, especially if our enemies decide to launch other fleets during that time.

As a bonus, there is a rather pesky system a few parsecs away whose continuous existence and defiance is a thorn in the Empire's side. Can I use these railguns to attack them? Perhaps I will be able to destroy their off planet installations. Maybe I'll even be able to bombard their planets.

  • $\begingroup$ I seem to be unable to add a word before the first word of the post. Is it a bug or have I hit some word limit? $\endgroup$ Mar 18, 2021 at 18:26
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    $\begingroup$ @RibhuHooja it might just be a general stackexchange filter to remove a common redundant question start word. $\endgroup$ Mar 18, 2021 at 18:30
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    $\begingroup$ the simple answer here is "No" ! $\endgroup$
    – Fattie
    Mar 19, 2021 at 14:57
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    $\begingroup$ So you do realize that these shots would take tens, hundreds or even thousands of years to reach their targets, right? On an unrelated note, where exactly is your throneworld located tens of thousands of years in the future? We just need to know where not to test this and we “promise” not to use live rounds in that direction. $\endgroup$ Mar 19, 2021 at 22:58
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    $\begingroup$ @RBarryYoung A few light-years away at 0.2c would take, by definition, tens of years at most. $\endgroup$
    – reirab
    Mar 21, 2021 at 1:23

9 Answers 9


They even have a Dyson swarm set up that can extract as much energy as I will need

You should be building a Nicoll-Dyson beam. If you weren't aware of these engineering marvels, it involves constructing a phased laser array on the outside of each element of your Dyson swarm. In the limit, this results in a laser weapon that can turn the output of your star into a beam of destruction that can reach out to stupendous distances with astonishing power... a full swarm with a 1AU diameter with a diffraction limited phased array emitting 500nm light can render planets uninhabitable 100s of lightyears away, and should be able to project a huge zone of deadly power at something mere lightyears away.

This isn't to say that building your railguns is pointless as such, but their power and effectiveness is unlikely to match that of even a partial star-sized laser array. If you've got stuff to spare that isn't useful for expanding the Dyson swarm, knock yourself out.

Really though, the laser can reach further in the same period of time, striking the fleet further away, inflicting early damage and casualties. It can also apply damage for a longer duration, maximising the chances of you destroying everything flying towards you. As the beam propagates at lightspeed, there's no practical way for the fleet to realise they're under attack until things start exploding, unlike railgun projectiles which might be detectable by various means that I won't go into here (but feel free to ask a separate question on that).

The biggest problem you may face is if the fleet is a) watching you and b) has larger fuel and propellant reserves than you thought, and is able to tweak their course slightly. It'll take years for you to notice and respond, but as they have to keep playing this game for at least a century they'll eventually run out of juice and you can waste them at your leisure. They might even send you a surrender message, and dealing with that is definitely a job for a separate question.

You could, of course, construct vast zone plates and blow them towards the enemy fleet like solar sails, and put enough intelligence in them to appropriately direct and focus your death beams if you happen to be a bit off. So many different possibilities!

Once you've reduced your foes to a thin cloud of expanding ionised gas, you can still use your ND beams for peaceful purposes including terraforming, transportation (both local and interstellar) and reminding your neighbours that if they like the planets they live on or the stars they live around that they'd better not try any funny business.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Mar 21, 2021 at 14:44

The problem with an unguided projectile at interstellar scale isn't accelerating it, it's hitting anything at the other end.

Here's the scale of the problem. Assume that your railgun has perfect bench accuracy: that is, if you fire it twice at the same point, it will hit the exact same point in space, every time. Assume that it's 1km long, but despite that length, its muzzle end can be oriented with nanometer precision (and its back end will remain perfectly stationary as you orient it).

Even with this ludicrously well-calibrated gun, when you're aiming at a target 1 LY away, the effect of each nanometer of traverse on the end of the barrel is to move the potential point of impact about 10km. If your opponent's ships are significantly smaller than this, your gun still isn't accurate enough to address them.

And this is assuming that pointing the gun is the only aspect of accuracy, which it isn't. If there's anything at all even slightly off about the gun - mass concentrations in the projectile, imbalances in the magnetic field caused by minor variations in conductivity along the barrel - or targeting parameters - minute gravitational pull from other planets in your system, the impact of solar radiation - anything that could throw its trajectory off by so much as a nanometer as it leaves the barrel, you're doing no better than shooting blind.

(This is assuming your projectile's width is negligible, which it probably would be if it's a railgun, unless someone has plans for a hyper-accurate railgun with kilometer-wide slugs. A gas cloud, plasma, or beam of light would expand as it goes, making hits more likely... but spreading its force of impact across the whole area. It would still need to be extremely accurate to stand a chance of hitting, though.)

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    $\begingroup$ @jamesqf: It’s extremely hard to see small, cold, black objects in space. $\endgroup$
    – Michael
    Mar 19, 2021 at 7:48
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    $\begingroup$ +1, but a guidance computer and some tiny, stealthy thrusters on the projectile would probably mitigate this. The same reasoning applies: if it corrects its course while sufficiently far away then barely any thrust is needed, which would also make it easy to remain undetected. $\endgroup$
    – N. Virgo
    Mar 19, 2021 at 8:23
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    $\begingroup$ @Nathaniel I don't know how you plan to use a piece of sophisticated electronics with moving parts and interstellar sensors as part of rail gun ammo without all of those things getting destroyed. $\endgroup$ Mar 19, 2021 at 8:51
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    $\begingroup$ To be fair, a 10km spread (if you can keep it at that) isn't absolutely horrible if you're firing at the average interstellar fleet and you're shooting projectiles akin to a shotgun blast (i.e. many pellets) rather than a sniper rifle (i.e. one true shot). $\endgroup$
    – Flater
    Mar 19, 2021 at 12:19
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    $\begingroup$ @Michael They don't need to see anything to dodge. The gun is at least 3 light years away, so all the fleet needs to do is spend 0.01 m/s of delta-V in a random direction once every year, and it'll be up to 14,000 km from the location that would have been projected at the time the gun was fired. (Even if they use a laser instead of a railgun, they'll be 2800 km off-course). (Edit: actually more, since the observations they're basing the shot on are three years out of date.) $\endgroup$
    – Ray
    Mar 19, 2021 at 18:34

Railguns have to be useless if there is relativistic travel

Unfortunately, absolutely base table-stakes for being to travel at any significant fraction of C is some kind of detection, deflection, or shielding technology that can shrug off incoming projectiles, because by traveling at that speed, you have turned everything you run into into a projectile traveling at a similar fraction of C. As per the very-well-worth-reading paper, The interaction of relativistic spacecrafts with the interstellar medium, mono-atomic collisions can cause non-negligible wear and tear over time. A single 10-micron speck of dust can crater the surface of the ship. And while collision with something larger is unlikely, over large interstellar distances, with sufficient ships, it becomes a real problem that needs to be confronted. “How do we not die if we collide with a fist-sized meteorite” is a question that will have to have been long-since resolved for the setup of your universe.

Therefore, we have to presume that if ships are traveling at that speed, they have technology that makes those collisions pain-free. As a result, tossing projectiles at them will have no effect, even if you could hit an elephant at that range.

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    $\begingroup$ That doesn't quite follow. Shielding for relativistic gas and dust can be clever; you can use many very thin layers of defense in depth, lasers to sweep space ahead of you, magnetic fields to deflect ions, other fields to confine plasma released from impacts so as not to wear away the shields to soon and so on. Not one of those things will work well on a cloud of golf-ball sized projectiles, let alone something bigger, and if they do work well enough to save the ship (unlikely) a substantial amount of shielding will be lost, jeopardising the rest of the journey. $\endgroup$ Mar 19, 2021 at 10:54
  • $\begingroup$ @starfish prime: edited for clarity, thank you for the feedback. $\endgroup$
    – Daniel B
    Mar 19, 2021 at 11:51
  • $\begingroup$ @StarfishPrime Quite a lot of the intragalactic dust is neutral molecular gases. Magnetic fields could protect you from, say, charged particles from stellar winds, but a good ol' molecular cloud will flip you the bird and rip you apart. And if you've got energy enough to try to constantly laser every molecule that might be in your way in an attempt to deflect or ionize or whatever it, I don't think you're exactly worried about regenerating a shield. $\endgroup$ Mar 19, 2021 at 14:59
  • $\begingroup$ @zibadawatimmy people have thought about this before. You can find their justifications and workings if you look, I'm sure. $\endgroup$ Mar 19, 2021 at 18:30

Fast moving space ships will be watching for incoming impactors.

This fleet is moving fast and can't dodge. Space junk and rocks are a threat to them. They will be watching for such and they will have countermeasures as have frequently been bandied about on this stack. Space is not hazy and you can see a long way using various EMR frequencies. 0.2c is fast but radar and lasers are faster. As regards the fastest of those two they argue but races always wind up a tie.

In any case these ships will have countermeasures to divert or disable incoming trash which includes your railgun rounds.

  • $\begingroup$ They're limited in both ammunition and energy, which cannot be said for the planetary system the attacker is in. $\endgroup$ Mar 19, 2021 at 10:55
  • $\begingroup$ I would expect they budget for a certain expected amount of incoming hazards. But a planetary system hurling megatons of projectiles at them is probably well outside their operational expectations unless they routinely encounter militarised star-systems with more gun than sense. $\endgroup$
    – Ruadhan
    Mar 19, 2021 at 11:40
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    $\begingroup$ They can easily dodge. Any travel in a future, where there is the threat of attack, will likely include some random course adjustments. Even minimal thruster nudges that won't have a significant effect on travel time, will result in not being able to predict exactly where your target will be. $\endgroup$ Mar 19, 2021 at 16:45
  • $\begingroup$ They'd be shielded against and watching for normal-albedo objects coming from their direction of travel, not for radar-absorbing, nano-black fletchettes sneaking up from behind their engine bell. And any ship which must carry a useful payload of crew, fuel, and ordnance should necessarily be less manoeuvrable than a railgun projectile which can devote almost its entire mass to maneuvering thrusters. $\endgroup$
    – Will Chen
    Mar 21, 2021 at 13:25
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    $\begingroup$ @MSalters The density of the interstellar medium isn't even enough to sustain a Bussard ramjet. I'm skeptical that there will be enough dust to pose an insurmountable problem— Although I could potentially see a fleet deliberately projecting a dust cloud around them precisely to be able to detect this kind of attack. $\endgroup$
    – Will Chen
    Mar 21, 2021 at 23:50

You could never hit, at that range, with a rail gun - but not for the reasons listed. You're firing projectiles with mass on fight paths that last years. Every single object in area will interact with them, in terms of gravity.

They'll be attracted in very slightly different directions, depending on the flight path. Unless you have perfect knowledge of all mass concentrations in a volume measuring cubic light years and the capability to predict the motions and interactions of all these objects... you're projectiles are going to drift this way and that... Pulled by that rogue planet, slowly moving off course...

  • $\begingroup$ unless the projectiles have in built course correction. Its possible that they could even have sub projectiles. Imagine a container with thrusters on it which aims itself towards the enemy fleet, and prior to impact just opens up, spilling out thousands of slugs of depleted uranium into a cloud. $\endgroup$
    – Innovine
    Mar 21, 2021 at 11:52
  • $\begingroup$ Big problem it would have (assuming it survives being fired out a rail gun) would be determining it's position to +/- 50m after traveling light-years... with no object inside of light minutes. $\endgroup$ Mar 21, 2021 at 16:24
  • $\begingroup$ @user2702772 Exactly so. Innovine: clearly, but how does that relate to the Question as Asked, or the exposition beneath it? $\endgroup$ Mar 25, 2021 at 21:02

Railguns are unlikely to produce velocities higher than a few km/s. The physics involved in electromagnetically propelling the projectile by an electrical current passed through the rails and projectile mean they will be slow in comparison to a simple chemical rocket. I will assume some translation error and take "rail gun" as referring to some other mass driver/beam launch technology capable of reaching the desired velocity.

Apart from that, any mechanism for accelerating a projectile to 0.2c is likely to be somewhat noisy. Even a tiny fraction of the total power being emitted as electromagnetic radiation will be noticeable from a great distance. If they have the slightest reason to be on guard, they are likely to notice you shooting decades before your projectiles arrive. However limited their propulsive abilities might be, they're probably sufficient to dodge given this much warning.

Your projectiles likely need some tracking ability just to enable them to hit their targets at such extreme range, but this can be countered by firing projectiles to intercept them, damaging or destroying their tracking and propulsion systems. The advantage is on the side of the defenders: they can launch smaller projectiles at slower speeds to cripple your projectiles. It comes down to an arms race between weapons, countermeasures, and counter-countermeasures.

  • $\begingroup$ The advantage isn't quite with the defenders; they only have the reserves of matter and power they brought with them, whereas the OP has an entire planetary system and a star to power their weapons with. Attrition is very much not on the side of the isolated fleet that has no hope of hiding, escaping or resupply. $\endgroup$ Mar 18, 2021 at 19:21
  • $\begingroup$ The advantage I was speaking of is on a per-encounter basis. The attacker needs to accelerate large projectiles to a sizeable fraction of the speed of light to hit a target lightyears away. The defender just needs to put something in the path of that projectile at a sufficient distance that they can evade the debris with reasonable propellant consumption. The attacker does have vastly more resources available, but will have to devote those resources to the attack before their target passes out of reasonable range. Whether that gives them an advantage overall depends on other factors. $\endgroup$ Mar 19, 2021 at 16:30

While it has been mentioned in passing in a couple of the answers, I don't feel like the gravity problem hasn't been given enough airtime here.

Even if you solve all of the aiming problems (pointing it in the right direction, getting exactly the right energy output, etc.) and even if you have total knowledge of the distribution of gasses, radiation, and so on in the path, you still need to solve the N-body problem to accurately predict the path of the projectile. Not only do you have to solve a problem we can't even consider an answer to (except in certain very limited example cases), you have to solve it for every gravitational source in the vicinity, for the entire flight time of the projectile.

Which is another way of saying: nope, not going to happen.

For the sake of argument, let's assume that you have solved the N-body problem. You have a Matrioska Brain capable of the calculation, and some way of actually measuring all of the mass in the system with sufficient accuracy to get the input data the calculation needs. You calculate the trajectory and make the shot.

Once the shot is made any disturbance that wasn't already part of the calculation introduces errors. A ship changes course unexpectedly, an asteroid collision breaks up a couple of rocks in an unexpected way, a slight change in solar emission alters the net force by a tiny amount... and the projectile misses it's appointment.

Railguns are fantastic because of their kinetic potential, not their accuracy. Getting hit with a mass travelling at an appreciable fraction of light speed is going to ruin your day. But over long distances? Either it's going to miss or you'll have enough warning to get out of the way. Even if you don't have detectors sensitive enough to see them coming in all you need to do is make tiny adjustments to your engine output at random and you're effectively impossible to hit at relativistic ranges, even with SOL weapons like lasers.

If you really want to hit them that far out, send some AI-controlled torpedoes with exciting sub-munitions like nuclear pumped lasers, or drones with railguns that will do the damage but from close in instead of impossibly far away.


Relativistic shotgun (only works on approaching ships).

You don't need to get anything up to speed, the incoming ships are already moving plenty fast. What you need is to overwhelm their automatic impact defense systems and also keep them from avoiding your weapon. Bring in the shotgun approach...

Figure out how good their defense systems are, how accurate your guess is as to where they'll be, and how much maneuvering they can do to avoid you. This gives you the mass you need and the volume you need to put it in.

Then find at least enough mass to equal that. Something that's not very solid works well: one of the asteroids that's basically a pile of rubble would be a good bet (note that you'll need some way to contain it during transportation).

Drag it out to the area you anticipate them passing through.

Then shake it. Pieces everywhere, big huge cloud of crud filling up enough volume that they won't be able to avoid it with enough density that it will overwhelm their defenses.

Sit back and await results.


If there have been Questions more loose, someone might be able to point them out.

In building weapons targeting any great distance, the viability of railguns seems like a reasonable consideration… until you spoil it with all that other stuff.

To stay on topic, a weapon targeting interstellar distances has three basic requirements:

Launch power; accuracy; warhead power… who sees another, please chip in.

May we ignore the warhead? It gets there, or it doesn’t…

As posed, your Question seems to ignore accuracy, leaving the launch power of rail-guns.

Are you asking whether a rail-gun could do the job at all or how efficient it might be, as compared to other technologies? If this Worldbuilding technology, why not simply state that your railguns have that capability?

If this is about real-world technology, can you explain how SE Worldbuilding permits that?

“Ordinary” guns have put shells 111 miles high and whether at that height they came close to escape velocity, you might like to research. Apparently, explosive-powered guns can’t readily give their projectiles a muzzle velocity of more than ≈2 km/s, while railguns might exceed 3 km/s.

If you’re in a timeline tens of thousands of years advanced from ours, why are you asking about such trivia?

How could it matter that humans have colonized a significant portion of the stellar neighborhood, umpty light-years across?

Why is it “sadly” that the laws of physics seem to mostly have held up? More usefully, how have those laws failed? Can you explain, or would you rather drop that?

How could even you, let alone any ordinary person, “extrapolate” technology tens of thousands of years advanced?

What is the timeline communicator you’d like readers to ignore?

What shows that your Great God of Plot is more than a dream you had?

How in thousands of years is the greatest invention fusion energy and torch-ships? Is that limitation your own, or what?

When your ships are capable of supporting a constant 1g or more “upto Plot” what does that mean? 3G? 15G? 315G? If you’re dealing with a mere 1G, how could any period of time matter?

What kind of interstellar object could not also be a weapon of mass destruction due to its mass and velocity or as you yourself said, kinetic energy?

Did anyone else retain the will to live even up to this point?


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