WARNING: MAY BE CONSIDERED EDUCATIONAL IN SOME JURISDICTIONS
Also, WARNING: INSPIRED BY POINTLESS INTERNET DISCUSSION
Space is big. This is a well-known fact.
How big…now that’s where things get interesting.
Let’s say that you are the captain of the warship Belligerence, assigned to orbit Mars. Your long-range sensors bleep as a hostile ship is detected thirty light minutes away, near Jupiter. You grab the weapons systems and open fire, right?
Thirty light minutes means that unless you have some kind of magic way of detecting people without needing to worry about speed of light delay, the location of the hostile ship is half an hour out of date. It could be moving at the speed of a Matchbox car and still throw off your firing solution at that distance.
Then there’s the weaponry you’re using. This is where things start to get silly.
Speed-of-light weapons. It’s rare to find a sci-fi series without some kind of laser, phaser, maser, or bananafanabobaser. These will take another half hour to reach the target. Meaning that you need to aim at where the ship will be in an hour, rather than where it is now. There’s also the question of effective range. A ten megawatt X-ray laser will screw with their electronics…if you can hit them…but its actual killing range is about one light minute, even though the beam will keep going until it hits something.
RKVs. The RKV, or Relativistic Kill Vehicle, is essentially a weaponised form of general relativity. At 99.9% of the speed of light, a 1kg mass is equivalent to about 132 megatons of TNT when it hits you (for context, the Tsar Bomba, the largest nuke ever detonated, was about 100 megatons). And if you’re familiar with Mass Effect, you know that high-velocity projectiles blasting through vacuum don’t experience much friction, unlike stuff in an atmosphere. The net result is that you still have to worry about speed of light delay, and you probably won’t be able to fit it with a guidance system because steering at 99.9% of lightspeed is just a tiny bit difficult. You could theoretically rig them with a time delay, but it’s important to remember that time dilates at high speeds, and an RKV that isn’t going at those speeds isn’t much of an RKV. Also, it takes a hell of a lot of energy to get an RKV going, because you can’t just get energy out of nowhere.
Non-RKV projectiles. These are just straightforwardly hopeless. If they’re going at half the speed of light, they will take an hour to get near the target, meaning that you have essentially an hour and a half of delay to deal with. The slower they move, the dumber it gets.
So, if you’re in orbit around Mars and you detect a hostile ship near Jupiter, either ignore it or use your handwave-powered FTL to go and fight it at a much shorter range than thirty light minutes.
– OSM out.