Ether
A long time ago, ether (or aether, if you're a Brittish vowel junky) was thought to be the medium through which light propagated. No one knew what it was, but the physics of the day required that it be there. But it wasn't. This dude named Michaelson proved that it wasn't there using a laser interferometer, and his results were recently duplicated in a physics laboratory in Fairbanks, Alaska. (Bailey, et al., 2004)
In other words, if you believe the ether exists, you should put the crack pipe down and go to this website for help.