As a plot device, the Bat Signal is kind of cool. We live in an age saturated with countless ways to communicate: everything from Skype to Twitter to Internet sites letting us chat with anyone in the world. But none of that would work with Bruce Wayne. He'd need to let Verizon or somebody in on his little secret, and we can't have that. (In the old '60s TV show, they ran a phone line from Commissioner Gordon's office to the Batcave, which we always thought was pretty silly.)

So in a world where every means of communication leaves a fingerprint that enterprising bad guys can track, how do you get in touch with Batman when you need his help? You mount a spotlight on the roof and turn it on. It's so low-tech it might as well be a caveman's fire, but it does the job, so nobody questions how absurdly simple the whole thing is.

Symbolically, it holds a very profound meaning: representing the trust between Jim Gordon and his pointy-eared buddy. Harvey Dent uses the signal at one point, and it feels like he's breaking the rules, like he's violated some kind of personal connection between them. It's wrong: only Jim gets to use the signal because only Jim has earned the right to ask for Batman's help.

And of course, the Signal gets shattered when Jim and Bruce both have to pretend that Batman's gone bad. Gordon actually invites the media up to the rooftop to watch him smash the signal with an axe. That trust needs to appear to have vanished—even if the dark secret they share holds them together—and there's no more potent way of demonstrating that than destroying their one reliable means of connecting with each other. By destroying it, Gordon really is severing his ties with Batman, leaving the hero of Gotham alone in the night with no one else to turn to.