Most post-apocalyptic stories are, at their core, somewhat optimistic. The nukes have fallen, the zombies are rampaging and popular fashion tastes are trending toward skull necklaces and assless chaps, but somehow, life still exists – and may even be flourishing. A nuclear war doesn’t prevent humanity from returning to the stars in A Canticle for Liebowitz, nor does it stop nation-states from reforming in Fallout. Even the Mad Max films feature towns and the trappings of cultural development. If a little bit of life survives, they contend, it’ll only be a matter of time before we continue our grand march of progress.
The following scenarios do not allow for such a rosy outlook:
RELATIVISTIC KILL VEHICLES
“The more efficient a reaction drive, the more effective a weapon it makes.”
– “The Warriors,” by Larry Niven
Most every story involving an alien invasion or war in space has, in large part, lied to you. Something as complex as an Independence Day-level invasion or a Death Star is not necessary if your goal is to simply erase a particular planet from the star map. Civilizations capable of engaging in interstellar travel would need to be able to harness enormous amounts of energy, and with that ability comes the possibility of using said energy as a weapon. In short, if you’re able to move a ship at the speed of light, you’re able to move a missile at the speed of light, and anything moving at the speed of light is going to apply rather a lot of force to whatever it strikes.
Even the word “missile” here is misleading – the ideal relativistic kill vehicle, as such a nightmare weapon would be called, could be something as simple as a small asteroid.
Say a particularly nasty alien race decided that they’d rather not see what happens when a species as vicious as humanity takes to the stars. All they’d really need to do is strap a big enough engine to a big enough rock, point that rock at Earth (or where Earth will be when it arrives), turn it on, and then basically forget about it. Moving at the speed of light, something the size of, say, a tub of ice cream would be more than enough to wipe out a major city and cause devastating environmental damage. Scale it up to a car to wipe out a country, a city to make the planet totally unusable, and a state to crack it in half like a walnut.
Even worse, since our theoretical killer rock is moving at the speed of light, you literally cannot see it coming. As soon as your eyes are able to perceive it, it’s already there. Barring technology far beyond what we’ve been able to reasonably conceive, there would be no way to stop it.