The sound barrier got its name and its reputation by breaking apart airplanes and throwing them on the ground. It seemed you could fly right up to the speed of sound, but if you sped past it (which tended to happen when diving earthward), you wished you hadn't. The popular perception of hitting the sound barrier bore a similarity to a bird hitting a picture window.
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.
So what is this infamous sonic boom, if not the shattering of the picture-window-in-the-sky?
It's a stampede of traumatized air molecules, actually.
When a plane flies slower than the speed of sound (which is 750 mph/1,207 kph at sea level and 660 mph/1,062 kph at 6 miles/9.7 kilometers 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.
So when an airplane goes faster than the speed of air/sound, it stops slicing the air like a knife and begins pushing it like a plow, shoving a fat plug of chaotic molecules before it. As with an explosion, this insult to the molecules generates a shock wave that ripples outward in a sphere. When that shock wave, traveling at the speed of sound, reaches your ear, it sounds like "BOOM."
But despite what many believe, that "boom" isn't a one-time occurrence.