Interstellar "rods from god": https://en.wikipedia.org/wiki/Kinetic_bombardment

A civilization powerful enough to make mega-structures is almost certainly able of landing on one of its endemic large meteors, attach thrusters and over the course of decades accelerate it towards us in a collision path with the earth at a decent fraction of the speed of light.

By the time we'd notice it would essentially be impossible to stop.

I'd recommend reading the book https://en.m.wikipedia.org/wiki/The_Engines_of_God

I'd also recommend The Three Body Problem trilogy, specifically the second book, The Dark Forest.

I'm about halfway through, and can second the recommendation. I think I see a potential twist, but I want to see how it actually plays out in the end.

I wonder how you steer that thing. As in, earth is a pretty small target from a few light years away. In the case where you want to visit some planet peacefully, you spend the second half of the journey decelerating so you can do the needed corrections easily due to your lower speed, but if you are close to your target at a significant fraction of light speed, it's hard to change your course.

I've watched Armageddon. All you need is a nuke and Bruce Willis.

Mr Willis is 67 years old. I suspect that any incoming attack asteroids are more than 13 years out (it just seems statistically improbable that one will show up sooner than that - I don't have any data on incoming bogies), and asking a >80 yo person to go take out asteroids seems like a plan destined to fail.

Point being, we should hurry up and create a bunch of Bruce Willis clones before it's too late. The future of humanity depends on it!

Please keep comments like this on reddit. Thank you

No, you need Harry Stanton. Bruce is just an actor.

Doh! Harry Stamper

Missile as in asteroid. Why bother with anything but the engines, big rocks are more than enough for interstellar warfare. Perhaps that's what got the dinosaurs and there's another one on its way right now :)

They could probably also cover such an asteroid in some sort of stealth coating relatively cheaply, so we wouldn't even detect it until a few hours before impact.

They wouldn't even have to, really. We see almost nothing out there, coming towards us or not, and even if we saw it a year early, what could we do about it?

> even if we saw it a year early, what could we do about it?

You could try sticking various forms of interceptors on it's path, at those speeds it's probably not maneuvering so relatively easy to intercept if you have warning.

I'm not sure to what degree we could deflect it or mitigate the damage, but I wouldn't be surprised if it was a sizeable one. Any collision is going to convert a ton of energy from kinetic energy to thermal energy, which should in large part radiate away before impacting earth. We could also do various clever things like putting hydrogen targets in the way which would undergo fusion upon collision with an object moving at relativistic speeds.

My thinking is that if you have a rock headed your way at say 10% of lightspeed or so, pretty much anything you can do to it past a certain point is mostly just going to change what form of matter smacks into your planet.

Maybe my mental model for how that works is wrong, but I'm just picturing a dust/rubble cloud at 10% of c irradiating the planet with xrays or something due to whatever extreme friction interaction happens in the atmosphere.

A sample size of 1 suggests this wouldn’t happen.

Humans are the most aggressive species we know of capable of mass destruction. We would (and do) certainly destroy other species to get some resources in order to enrich a small but highly privileged portion of our population. However—despite our aggression—we would not destroy an alien world at the mere sight of it without any prospects for profit.

Our most destructive era was probably the era of nuclear missile testing. It certainly did an enormous needless destruction of non-human (and human) habitat. This era only lasted a couple of decades and ended with a comprehensive test ban in the 1990s. Despite our capabilities we never tested these weapons of horror in space or on alien planets, and we probably never will.

> Despite our capabilities we never tested these weapons of horror in space

I’m afraid that is not true.

https://en.m.wikipedia.org/wiki/Starfish_Prime

“The explosion took place at an altitude of 250 miles (400 km), above a point 19 miles (31 km) southwest of Johnston Atoll.”

That’s roughly the same altitude the international space station flies at.

And then there's "Project K" where the Soviets EMP'd their own population for science.

Oh! Very interesting. I didn’t know about that one. Thank you.

Yes, you are right. I forgot about that one. Of course the Partial Nuclear Test Ban Treaty banned those in 1963.

> It certainly did an enormous needless destruction of non-human (and human) habitat. This era only lasted a couple of decades and ended with a comprehensive test ban in the 1990s.

PTBT happened in the early 60s, so almost all surface tests were conducted within the first twenty years of developing nukes. I don't think making "craters" in desert mines really qualifies as enormous destruction of habitats.

Yes the second and third decades of nuclear weapons testing were definitely more destructive then the following. The Partial Nuclear Test Ban Treaty of 1963 definitely helped slowing down the destruction by a lot. However e.g. France—a non signatory—continued detonating their nuclear bombs above ground on and around their pacific island territories well into the 70s. The environmental damage (and damage to the nearby human communities) is still very much present today.

But the fact that the PTBT was signed by most nuclear states which slowed down this pointless destruction—and later the CTBT, which almost stopped it in 1997—shows that even humans with our demonstrable aggressiveness do work to limit our destruction when said destruction doesn’t contribute to the enrichment of a subset of other members of the species.

Why would you go through all that effort for a lower grade civ that had zero chance of attacking you and you couldn’t get (and probably don’t need) the resources.

Next question: Are all distant civilisations friendly because there’s no reason not to be?

Likely not.

Convergent evolution. As soon as you get animals you get resource competition, violence, and a food chain with a predator at the top. In more sophisticated species you also get dominance/submission hierarchies.

It's possible at some point a species might transcend that and become wholly benevolent. It's also possible that some species are cooperative colony organisms which somehow evolve intelligence, self-awareness, and technology.

But at a guess it's more likely that most species remain aggressive and competitive for a very long time, and they'll only ignore humans if we aren't complex enough to be a threat - now or in the future - and have nothing worth stealing or harvesting.

The precautionary principle. Imagine that you are an alien and you don’t destroy every fledgling spacefaring civilization that you detect.

Best case: you save some marginal resources.

Worst case: that civilization “grows up” and develops the means to destroy you.

But what would be their motivation to do so?

It would still be a huge effort - and for what? Why destroy a distant civilization?

For humans in a hypothetical advanced society the most likely reason would be preemptive self defence.

I think it's a mistake to assume that aliens will be overly similar in their thought process.

We don't worry too much about killing an ant colony. Which always feels like it should be different when "sentient" beings are involved, but humans are so limited that beyond 150 people we struggle to conceptualize of other sentient minds as "real".

So really it could go either way: alien intelligence so advanced and vast that we don't seem meaningfully alive to it, or alien intelligence so advanced and vast that man's inhumanity to man makes us seem impossibly savage and dangerous.