As in this book. More here, and here, including a disaster porn excerpt from the novel itself, and this:
So, there’s no direct defense against one of these things. But what about guidance? On an insterstellar scale, a planet is a teeny tiny moving object. And you certainly wouldn’t want to toss some of these at another civilization and miss.
So I assume that at the very least you’d have to do extensive testing. No weapons system works exactly right the first time you use it.
And if anyone were doing that nearby, we’d see some evidence of it, right?
I don’t see guidance as the biggest feasibility problem. The issue I see is that energy-in equals energy-out. So in order to accelerate a weapon to that speed, you have to put in 1.5 million megatons multiplied by whatever you actual efficiency is. If you’re a civilization that can wield that kind of energy, surely guidance is simple. (Guidance might require a local spotter/forward observer for course corrections. So it follows my general view of warfare: the more advanced you become, the more dangerous the guy with the laser pointer is. And thus, stopping these relativistic weapons is probably a matter of finding and killing the spotters.)
As for testing it… I’m not sure we would see testing and know what it was. In our own system, we might see a big impact on, say, Neptune, but we wouldn’t expect to have seen the object that caused the impact, so we’d conclude it was a comet or something similar, rather than a alien weapon. If testing was done outside our solar system, we would not expect to see it at all.
I don’t quite see how one follows from the other, it seems to me that guidance is a problem distinct from getting the energy to accelerate the thing, altogether.
I am not quite clear on “… its exact motion and position can never be determined …” Relativity is believed to be consistent, so if we observe an object traveling at .30c, its location and trajectory can be determined by adjusting for relative time dilation. Otherwise, GPS would not work right.
And, of course, something traveling really, really fast is incredibly vulnerable to anything that might get in its way. Throw a barrage of pebbles into its path, it might never make it here.
As I recall, much of the problem is accelerating such a projectile without vaporizing it; you need either a method with infinitesimal inefficiency, or an accelerator light years long. Or an interstellar drive on board the projectile itself, but then people can see it coming by the light of its own acceleration. And an on-board drive also limits the maximum velocity attainable too, due to fuel limitations.
It’s also easier to defend against than its proponents think for a space dwelling civilization; make sure that any space colonies randomly shift their location slightly on a regular basis, and the same light speed limitations that make it such a surprise render it incapable of changing course in time to hit. Solar or magnetic sails can be used for that jinking to render fuel moot.
There’s also the tiny problem that you’ve just drawn a big bright line right back to yourself that might as well have the label “genocidal lunatics here, please kill us” in letters a light year tall. Even if your victims don’t kill you with what you missed, you’re betting that there’s no civilization equal to yours or better within viewing range that’s inclined to destroy you as a threat.
I believe that’s in one of the links I put up, I was going to save that discussion for the IMHO version of this thread. For this one, I was really just looking for some reassurance that relativistic projectiles aren’t as scary a superweapon as the book makes them out to be. The idea kinda freaks me out a little…
One thing about an object moving at a relativistic speed is that it will be moving in a straight-ish line. It takes an enormous amount of force to change direction, so you can basically predict where it will be going. And, as I said, the defense is pretty simple, just throw some stuff in its path. A pebble hitting you at .30c would be very bad for you and for the pebble, you hitting a pebble at .30c is exactly the same event.
As in The Killing Star, these things work best against planets. You might need to make a few mid-course corrections, but the planet isn’t going anywhere fast. To launch them you can use something like Jordin Kare’s Sailbeam system in an adjacent system
if you are worried about your missiles being traced back to their point of origin, you could use a nearby red or brown dwarf system as a source of energy; obviously a long-term strategy.
Hopefully a long term offensive strategy like this would give you and your enemies plenty of time to work out a non-violent solution to your differences.
The pebble defence isn’t much good against a relativistic missile, unless it is deployed a long way from the target; all it does is spread the missile out into a cone, which hits the target with just as much momentum as the original.
Probably the best defence against a relativistic missile is a suitably large laser. You could put enough energy into the laser to spread the incoming missile into a relativistic cloud of vapour, far enough to make that cloud harmless. No anti-missile defence made of matter could reach as far away as a beam of e-m radiation (preferably with a very short wavelength).
Well, maybe I’m jumping ahead with some assumptions, but it seems to me that any feasible relativistic weapon is self-propelled and therefore able to correct its course.
I don’t really have time to go into the details of why I think unguided weapons are a problem, but basically, you are either
using a system like a rail gun that limits the total energy in the projectile to how quickly you can impart power to it. If you can deliver millions of megatons/second of power, then you don’t need relativistic weapons at all. Just skip the middle man.
using a system like a carrier ship that accelerates projectiles. Any configuration of this always means that the carrier+projectile can do more damage than the projectile alone and that the carrier+projectile is more maneuverable than the projectile alone. I have a hard time seeing any argument in favor of a projectile-only configuration unless we suppose some kind of engineering or financial limitations about propulsion systems.