Do you have a dumb question that you're kind of embarrassed to ask in the main thread? Is there something you're just not sure about?
This is your opportunity to ask questions. No question too simple or too silly.
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Notes -
Point of order: a 10g bullet at 99.99999% c would impact with the power of 480 megatons. I wouldn't want to be at ground zero, but the Earth and even humanity would trundle along just fine.
There are more-ridiculously-overpowered weapons humanity has thought of, though, even discounting "add more nines". The Nicoll-Dyson beam is basically something any civilisation with a Dyson Swarm can just do - specifically, use half the satellites' power beams as a phased-array laser, for a galactic-range beam that burns the atmosphere and oceans off a planet within an hour. There's always the black hole gun; drop a black hole into a star and it goes nova (wiping out everything in the system; space habitats won't save you here). And, of course, there's always the option of actually sending forces (directly or via something analogous to a Commander from Total Annihilation).
Short answer is that actual alien attack in the reasonably-near future is basically a "you die, no save".
Of course, this is not to say that space colonisation is a bad deal from an X-risk perspective! There are lots of risks that becoming multiplanetary removes. Grey goo, ice-nine, terrorist geoengineering, most bioweapons (the exception being intelligent ones), and so on. And, of course, in the limit if we don't leave Earth at some point in the next billion years we all die when the oceans boil. Alien attack is just not one of them.
The one thing I would say is that I'd prefer we colonised, um, almost literally anywhere in the Solar System that isn't Mars. Mars is one of the few places we can check lithopanspermia as a theory, and it's also kind of a shitty choice in general (it's got an atmosphere, but not enough of an atmosphere to block radiation well enough or survive without pressure suits/domes, and the relatively-high gravity prevents easy deep excavation to block radiation that way or easy return to Earth due to the gravity well). Deep cities on asteroids (or even Luna!) and cloud cities on Venus are better ideas; Mars just doesn't have much to recommend it and the Mars-like places in the system to check for lithopanspermia are very few so destroying one just for the meme value seems crazy.
Motion denied. I said this:
This has the convenient ability to be whatever I want, including ∞ megatons. Your planet is destroyed. Please try again with a different civilization.
Other than that, I mostly agree. Any single-system civilization would be easy to kill, but two planets is better than one. And truly advanced hostile beings could probably wipe the entire universe. (Or at least you can't prove otherwise).
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