Plans for the plane that would become the Concorde-the first commercial “supersonic transport,” or S.S.T.-began in the nineteen-fifties. The boom sweeps over everything below it-a kind of sonic broom that is about a mile wide for every thousand feet of plane altitude. Contrary to what you might imagine, a plane causes a sonic boom not just once, when it breaks the sound barrier, but continuously for the entire time that it’s supersonic. Bullets travel fast enough to cause sonic booms, as do the tails of whips.
(Often, sonic booms go boom-boom.) It’s no coincidence that sonic booms sound like thunder thunder is a sonic boom, caused by shock waves expanding around lightning bolts. A zone of low pressure follows-the trough of the wave-and then normal air pressure returns, creating its own sound. They begin to build up, and this single, merged wave reaches the ground all at once, creating a boom. But when the plane itself exceeds that speed-at around seven hundred and seventy miles per hour at sea level, or around six hundred and sixty at cruising altitude-it catches up to the waves expanding in front of it. The principle behind the boom is simple: sound travels through the air in the form of compression waves, so called because they occur as air gets denser and sparser as a plane flies, the waves expand in all directions at the speed of sound. Like all supersonic flyers, Yeager trailed a sonic boom behind him. He did it in a tiny, orange-colored plane called the Bell X-1-essentially, a cockpit and two wings connected to a rocket engine.
In 1947, Chuck Yeager, the Air Force test pilot, became the first person to break the sound barrier.
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