So there’s this alien invasion flick called Battle: Los Angeles. It’s getting mixed reviews. Ebert hates it – “an insult to the words “science” and “fiction,” and the hyphen in between them.” With the caveats that his judgment is usually questionable, I haven’t seen the movie, and I don’t plan to see the movie, I’m thinking I’m relatively safe in trusting his opinion in this particular case.
Now it’s also true that judging this kind of movie by its physics is an exercise in foolishness, akin to complaining about the lack of 8 consecutive 24 episodes of Jack Bauer enjoying a good night’s sleep. Nonetheless, every physics blogger from time to time has to deploy the Canonical Planetary Warfare Complaint. Pour encourager les autres, or something like that.
It goes like this: consider a rock. A big one, the size of a car. What’s that, maybe 5 cubic meters? I have no idea, but we’re talking a nice big rock. The density of granite is something like 2.7 g/cm^3, so our rock has a mass of around 13,500 kilograms.
This rock is sitting in the cargo bay of the evil alien ship in low earth orbit. In such an orbit, the ship and thus the rock are traveling at some 8 km/s. The kinetic energy of that rock is
Which works out to about 4.3 billion joules, which is conveniently just about the same amount of energy as a whopping
2000 pounds 100 tons of TNT (correction via Eric in comments – I misread the decimal point in the joules-per-ton conversion).
So any alien who wants to beat earthbound humanity into submission has a rather easy plan available: drop big rocks on power plants, railway hubs, highway chokepoints, and ports. Release a few dump trucks worth of gravel into low earth orbit to take out all the satellites just for kicks. Then pour a cup of tea and wait for a starving and suddenly stone-age humanity to wave the white flag. You’d need just a few thousand big rocks. Maybe a few hundred, judiciously applied.
The box office returns for my science fiction epic “Rocks Fall, Everyone Dies” are likely to be dismal*. But if you’re planning a planetary assault, you’d be hard-pressed to do better.
*Science fiction novelists are a hardier bunch, and use this trope all time time. Heinlein’s version is probably the most famous.