Schwarzschild radius
The point of no return.
Named after Karl Schwarzschild, a German astrophysicist. Schwarzschild, a clever fellow, and with his compass and protractor he calculated the event horizon for a non-rotating black hole, a flash of inspiration that came to him in a bierkeller in Düsseldorf during a heated argument about the relative merits of male and female tennis players. In a dark place next morning he sketched out the broad parameters for the Schwarzschild radius of alcohol consumption. The lady with whom he had been arguing, a handsome Bavarian rechtsanwältin quickly derived the Schwarzschild radius of document comprehension — the two phenomena being highly correlated. It was only later that Schwarzschild appreciated its application in cosmology.
The signal from that night in Düsseldorf can still be faintly apprehended.