But people are stubborn, and they wanted to know what was on the other side.

"People knew it could be done -- every bullet goes supersonic," says Steve Robinson, who made a living studying turbulence before joining the astronaut corps, where he now makes turbulence for a living. "That's why the first supersonic airplane was shaped like a 50-caliber bullet."

Needless to say, airplanes were soon breaking the sound barrier, and the accompanying sonic boom was breaking windows.

When a plane flies slower than the speed of sound (which is 750 mph at sea level and 660 mph at six miles above sea level, where the air is colder), the air molecules have time to part and flow smoothly around it. But, like cheese molecules or water molecules, air molecules can only move so fast. They have a built-in speed limit. And since sound is a disturbance passing from molecule to molecule, like falling dominos, the speed of air molecules limits the speed of sound.

That's right: If people stood shoulder to shoulder from California to New York, and the Supersonic Transport Concorde flew over their heads, every single person would hear a sonic boom. It's really more of a "sonic bellow." The auditory fact of the matter is, however, that each person hears just one boom as the shock wave reaches his ears. Well, maybe he hears two booms.

Accidents do happen, Shepherd says. Because the speed of sound (and hence the sound barrier) does vary with air temperature, and because pilots never hear their own "sonic bellow," pilots can be ignorant of their crime until they get the bill for the windows.

Breaking the sound barrier isn't cheap, which is why airliners abstain. As you may know from daily experience, it takes far more energy to push your fist through cheese than to slice it with a knife. Seats on the Concorde are pricey because it takes a lot of jet fuel to plow cheese all the way to Paris.

So why bother going supersonic? There's a brief silence from Robinson's end of the phone, as though he's admiring the naiveté of the question.