[stouset]

Accelerating a rock to intercept a separate star system doesn’t need to generate any particularly noticeable emissions.

If it was intercepted someone could work backward from its trajectory and determine an origin, but the odds of noticing a cold, small, dark rock at relativistic speeds early enough to do anything about it seems slim.

[ordu]

It is a nice scheme, but what if your target is not bound to gravity wells anymore and the most of its economy is artificial structures orbiting its sun?

You will hit some ancient rocks orbiting the star, and even if people there dont need them anymore they are bound to become curious of the origin of your missiles.

[stouset]

Sending a relativistic chunk of tungsten is something we could accomplish at essentially current technology levels. Hell we could send a hundred of them. It’d be expensive and it would take awhile to get there but we could do it in a decade given sufficient motivation.

If caveman-level weaponry is sufficient to take out anyone not well on their way to becoming a Type II civilization, I’m betting on the cavemen.

[Intralexical]

I don't think we could do this at all, even theoretically.

If we took all the proven petroleum reserves in the world, and magically converted them into kinetic energy with 100% efficiency— With zero overhead for transportation, launch, agriculture, or obeying conservation of momentum­— That still wouldn't be enough to launch even a single planet killer. At most you could crater a small country, but not kill a civilization:

https://www.wolframalpha.com/input?i=2+trillion+barrels+of+o...

So let's say you do nuclear pulse propulsion like Project Orion. You've still got Tsiolkovsky's rocket equation to deal with. Assuming a speculative fusion bomb ISP of 75,000s, you would need a rocket with… over 50 orders of magnitude more mass than the entire observable universe, in order to accelerate a single proton to 0.9c:

https://www.wolframalpha.com/input?i=e%5E%280.9c%2F%2875000s...

Light sails will be huge, obvious/visible, and slow. Beamed power will run into issues with diffraction.

In fact, reaching 0.9c while you're still in the solar system plainly implies maintaining multiple hundreds of g's of acceleration over many dozen astronomical units of distance. That doesn't seem feasible at all. It's wildly beyond not only our best existing ion drives, but probably also any remotely feasible existing concept for space propulsion.

[perihelions]

This probably isn't true: we don't even have enough conventional weapons to destroy our own planet, and we've been optimizing that for centuries. A relativistic missile is a conventional weapon scaled up.

[ordu]

We couldn't accomplish this. Theoretically speaking, yes we could, but "theoretocally" means "ignoring the half of the reality". We can't even travel to Moon now practically.

To send a chunk of tungsten at a relativistic velocity would mean an effort for trillions of dollars. Don't forget that it is not enough to just get a chunk of tungsten moving, you want it to hit a moving target, and you'd better add some thrusters to it and a guidance system. Is there anyone willing to pay for that?

Any civilization will need to concentrate a lot of efforts to fire a chunk of tungsten, but why might it do it? There are better ways to utilize that effort. Maybe it is a rational thing to go aggressive nevertheless, but the benefits will be in a far future while politicians needs to justify spending now. People and societies are not rational. There is no real examples of rational agents, but people still insist on treating rational agents as something real. Theoretically speaking AGI might become a rational agent, but I doubt it from a practical standpoint: AGI will be limited by a computational power and by its abilities to gather data. So it will use heuristics, and it will be not rational. It can be closer to a platonic ideal of a rational agent then human, but even that is not free of doubt. People surpisingly well do with all their heuristics and when they appear irrational it is mostly due to inability of observers to understand the real motivation of people.

You need a much more advanced civilization to be as aggressive. A civilization that can do it by spending maybe 0.1% GDP for 10 years. At least looking at humanity, I'd say that any cost higher than that will not work definitely.

Such unprovoked and costly agression having no observable results easily could end a lot of political careers.

Theoretically speaking we can ignore all these difficulties and start with the assumption that it is possible to concentrate 100% GDP on a one task for years or even decades. Practically it is impossible.

Maybe another civilization will have another structure and will be able to concentrate efforts on a larger scale then humanity? Maybe. But could you imagine such a hypothetical civilization and estimate chances of it to get to a sufficiently advanced level? I cant neither. So while I keep in mind this theoretical dreams of rational civilization purging each other, I do not assign any credibility to them. I keep myself in an uncertain state, the best state to have an open mind, to be ready to absorb any evidence or reasoning.

[nmeagent]

They might notice the bits of interstellar dust that at the upper size range hit at 0.99c with kinetic energy on the order of magnitude of tactical nuclear weapons yield. This also makes targeting interesting without thrusters and fuel to correct their probably regularly perturbed trajectories. Those cold, small, dark rocks may not be quite so cold or dark if they're going to actually try to hit their targets.