Why would it need to be a good reason? Especially why would it need to be a good reason, from our perspective?
Hardly an original thought but it's entirely possible that an alien civilization pro-forma sends out an unmanned planet-killer-scale weapon at every alien civilization they detect, simply to avoid the possibility that we might grow up to be hostile to them, or even compete with them for resources.
An unmanned bomb like that wouldn't take many resources and it's possible they would view us as primitively as we view stomping on an ant hill in africa.
Now that's a depressing thought.
Chimpanzees 'go to war', and so do some non-great ape species like ants and termites. Interestingly, some ant species conquer other colonies and essentially 'enslave' the defeated worker ants using pheromones. It stands to reason that belligerence would arise elsewhere in the universe because it has on Earth several times.
It is unreasonable to assume aliens are anything like us.
Colonizing another solar system, that's massively expensive. Sending a single interstellar missile at the planet though... that could wipe us out... and definitely seems possible without a huge expenditure of resources (for a civilization with mega-structures).
Missiles being much simpler because they don't have to support life, they don't have to slow down - and therefore don't need on board propulsion past what is needed for manoeuvring (assuming some sort of "push it with lasers" style of propulsion), and apart from systems needed for manoeuvring they can really just be a hunk of metal that you accelerate really fast.
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.
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!
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.
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.
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.
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.
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.
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.
Next question: Are all distant civilisations friendly because there’s no reason not to be?
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.
Best case: you save some marginal resources.
Worst case: that civilization “grows up” and develops the means to destroy you.
It would still be a huge effort - and for what? Why destroy a distant civilization?
I think it's a mistake to assume that aliens will be overly similar in their thought process.
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.
Not many resources really. They can simply use their abundance of energy to hurl, propel, or divert some rock at us at some non-negligible fraction of light speed, and we would be powerless to stop it. Likely we wouldn't even see it coming. Make it two or a dozen to be sure.
It doesn't even take a very advanced civilization to pull off such a thing. Give it maybe a hundred years and humanity might be able to do it too.
Since it is a rather easy to end a civilization in such a way and it is near impossible to defend against, you arrive at a very... interesting game-theoretic problem: Did they already send kinetic weapons our way? Should we destroy them before they decide to destroy us? If we're about to die anyways, does it even matter if we fire back?
It's like the cold war except nobody is able to talk to another and you don't see the nukes coming.
Wouldn't you need to solve the n-body problem to hit a planet (earth) from many light years away with a rock? And to do this without exact knowledge of planetary bodies in our star system. And even if you could, a stray hydrogen atom hitting your rock early on would make it miss its target by some light hours. So you better account for every speck of dust on the way and everything with gravitational pull on your rock. An all the quantum fluctuations..
I would even wonder whether the space itself is fine grained enought to precisely target stuff light years away.
If humanity was doing this, we'd likely attach propulsion and accelerate the entire way anyways. Unless there's some new drive technology discovered by then or we hijack something natural/environs to do it for us.
You could colonize the galaxy in few hundred million years at most without FTL. Replicating machines which can build and launch many more of themselves after reaching each new resource rich system is all that is needed.
Humanity may not reach other systems, but either descendants of humanity or human derived machines improved over many generations during transit by AI(doesn’t have to be AGI) is likely to visit.
Yes, but it is quite likely human technology will some day.
Imagine you didn't care about the pristine land- let's ignore aesthetic reasons to hold back for a moment.
As technology improves, it becomes easier to exploit resources. You can dig deeper with automated robots etc. Now imagine a technologically mature civilization: one whose tech is pushing the limits of physics.
If our current ideas of physics are about right, you have to build things out of matter, and there's a finite amount of matter and negentropy out there. Conversely, right now, replication is cheap. Interstellar travel times are longer than civilisational lifetimes: ergo, most entities that arrive at other stars will have relatively mature technology.
If I'm a super-civilisation/large-scale AI digging in for the long haul (till the end of the universe), what do I do? I put out all the stars, if I can, because they're wasteful. Negentropy is finite: we want to eke out every tiny bit of use there, not let hydrogen atoms fuse randomly out of our control.
Large corporations viciously purse 0.1% efficiency gains because that amounts to millions of dollars at their scale. At the scale of the galaxy, 0.1% efficiency gains in your conversion of stars, planets, rocks and dust into things you care about is quadrillions of lives.
The high travel time means the dynamics are different. You don't have to move from star to star: you can just build (number of solar systems)*(safe number of sentient self-replicating factory probes per system) probes in your home system, and send them out all at once (or as they roll off the line). Every solar system is a massive prize, every star left burning a travesty. If I'm turning the galaxy into stockpiled resources, I'm not going to leave any out that are profitable- and with mature tech, it's all profitable. There's no labor cost, after all.
Anybody not in a hurry would have little more difficulty than we did launching Voyager or New Horizons. They would, of course, need to build it such that it would operate for long enough to get here, which would be harder.
Or, anyway, start operating again once it got near here. It ought not to be very difficult to preserve equipment cooled to 2.7 degrees above absolute zero, which is, notably, below the temperature where helium condenses. Maybe the boiling helium could be used to wake it up.
The dynamics of "generation ships" change drastically with changes in human lifetime, including both the dynamics on board such a ship, but also the motivation to pursue it.
People seriously underestimate how difficult it would be to travel between solar systems. The scales involved are so far beyond human experience that we can't properly visualize them. Short of discovering a practical FTL (which seems likely to be impossible thanks to the Fermi Paradox) it is unlikely that humans will ever visit a distant solar system.