The crack of the whipSound goes 343 m/s through the air out front. Beat it and you leave your own racket behind — and that is the whole of the sound barrier.
Lightning flashes. You count — one Mississippi, two — and thunder comes at five seconds to the mile. That count is the speed of sound and you have been measuring it since you were a kid: about 343 m/s in dry air at room temperature[8], slower when it is cold, because sound in air is nothin' but the air bumpin' into itself, and cold air bumps lazier.
What the barrier actually is
Anything movin' through air makes pressure waves and they run out ahead at the speed of sound, tellin' the air up yonder to get out of the way. That is why air flows around a slow airplane instead of pilin' up on it — it got word.
Go faster than the news and the news stops arriving. The air out front learns about you when you hit it. The waves you already made cannot get away, so they stack into one shock, and that stack is what folks called a barrier. Nothing solid about it. It is paperwork that did not get delivered.
The cone drags along behind. When its edge sweeps across your yard you hear the boom — not once at the moment of breakin' through, but continuously, anywhere the cone touches ground. A supersonic airplane lays a strip of boom about a mile wide for every thousand feet of altitude, the whole way it flies[34]. That strip is why they are not allowed over land in most places.
The first thing anybody made go supersonic was a whip
Chuck Yeager got the credit in 1947 with a rocket plane[28]. A drover with a bullwhip beat him by a few thousand years and did not know it.
The physics is a taper. A whip is thick at the handle and thin at the end, and the energy you put in the thick part has to keep goin' when the rope runs out of mass to carry it. Same energy, less and less whip, so the little bit that is left goes faster and faster. Goriely and McMillen ran the numbers in 2002 and found the tip passes twice the speed of sound[10]. The crack is a sonic boom you can hold in your hand.
Three ways to hear this for yourself
Count the thunder. Seconds between flash and rumble, divided by five, gives miles. Divided by three gives kilometres. You are timing 343 metres a second.
Listen to a rifle downrange. A .30-06 leaves the muzzle around 853 m/s — Mach 2.49[11]. Stand off to the side and safe and you hear two noises: the bullet's own boom arriving first from close by, then the powder's bang from the gun.
Watch a jet land. On approach it is subsonic and quiet-ish, because the air ahead still gets the word.
Why the barrier was hard anyway
Nothing was solid about it, but something was true about it: right around Mach 1 the drag on a wing spikes, and where the shock sits on the wing moves, so the controls change their mind about what they do. Early airplanes went into the transonic band and came out with the stick doing the opposite of what it had done a minute before. Men died of that before the shape of a wing for those speeds was worked out. The barrier was never a wall in the air. It was a wall in the engineering.
Fifty years on, the standing record for an air-breathing aeroplane with a person in it is still 980.4 m/s — the SR-71, 1976[12] — and the fastest car is still 341.1 m/s, ThrustSSC on the salt in 1997[9]. Going fast in air gets expensive in a hurry, and the next page is the reason.