system
Given the following post, respond as if you're commenting on Reddit:
human
Could someone explain how FTL violates causality?

I've done the wiki reading but it still doesn't make intuitive sense to me. Obviously reverse time travel does because of things like the Grandfather paradox, but I can't seem to grasp why FTL / instantaneous transmission breaks causality.
gpt
So Alice and Bob get fed up with each other and decide they're going to have a duel with tachyon pistols. The rules are thus: Each duelist will board his or her superadvanced spaceship and, on the count of three, accelerate away from each other for ten seconds. They will then turn (without stopping, that's an important technicality), and fire their tachyon pistols at each other.

Alice, filled to the brim with loathing for Bob, boards her spaceship and waits for the count. One … two … three and she's off at some substantial fraction of speed of light. She counts down ten seconds, turns and fires at Bob.

But since Bob and Alice have been receding from each other at high speed, Bob is time dilated in Alice's frame of reference. So when her clock says ten seconds have elapsed, only five seconds have elapsed for Bob. When she fires her magic instantaneous tachyon pistol, it hits Bob's spaceship when his clock reads five seconds.

Enraged that Alice fired early, Bob turns and shoots right back at her. But since they've been receding from each other at high speed, Alice is time-dilated in Bob's frame. So when he fires at the instant his clock reads five seconds, only two and a half seconds have elapsed for Alice. Bob's aim is better than Alice's, so his shot hits her spaceship and kills her … seven-and-a-half seconds before she fired the shot that caused Bob to shoot her back.

Faster-than-light anything and causality cannot coexist.