Mach 1, and the
wall that wasn't there

Two broken ribs and a broom handle

The pilot was Captain Chuck Yeager of the US Air Force, 24 years old, a small-town boy from West Virginia who had become a fighter ace in the Second World War.

That weekend, just two days before the flight, he and his wife Glennis went riding in the desert near the airfield. Yeager's horse threw him. He broke two ribs on his right side.

If the doctors on the base found out, he would be grounded, and someone else might get the flight. So he kept quiet. According to NASA's history of the programme, he told no one at the base except his close friend and fellow engineer-pilot, Captain Jack Ridley.

There was one problem the two friends could not ignore. To seal himself into the rocket plane, Yeager had to reach across and slam a locking handle on the door - with his right arm. The pain made it impossible.

Ridley's fix was wonderfully low-tech. As Yeager told the story afterwards, Ridley cut off a short length of broom handle. Yeager could use it as a lever to push the handle home.

So on 14 October 1947, the first person to fly faster than sound climbed into the plane taped up, in pain, carrying a piece of a broom.

The needle jumped

At about 10:26 that morning, over the Mojave Desert, the bomber let go.

Yeager fired the rocket engine and climbed. Higher, faster. The plane shook as it neared the speed of sound, just as everyone feared.

Then, high above the desert at about 43,000 feet, the shaking stopped.

In his flight report, Yeager wrote that the needle on his Machmeter - the dial measuring speed compared with the speed of sound - hesitated, and then jumped past 1, to 1.06.

He was flying at around 700 miles per hour. He stayed supersonic for only about 18 seconds.

"There was no buffet, no jolt, no shock. Above all, no brick wall to smash into."

- Chuck Yeager, writing about the flight later

The wall had never been there.

A speed that changes with the weather

343 metres per second is not a fixed speed. It is the speed of sound in warm air near the ground. Sound travels faster through warmer air, and slower through cold air - because the molecules passing on the push are moving about faster when it is warm.

At 0 °C, sound travels at about 331 m/s. At −40 °C - the cold where Celsius and Fahrenheit agree (see /minus-40) - it slows to about 306 m/s.

High up where Yeager flew, the air was bitterly cold, far below freezing. Up there, Mach 1 was only about 295 m/s - roughly 660 mph.

So Yeager's 700 mph was faster than sound at his height, but it would not have been faster than sound down at ground level on a mild day, where Mach 1 is about 767 mph.

That is why pilots do not talk about miles per hour when they talk about sound. They use Mach numbers: your speed divided by the speed of sound wherever you are. Mach 1 is not a number on a speedometer. It is a ratio: the moment your speed and sound's speed are equal.

Walls that turn out to be doors

The story of Mach 1 is really the story of a word.

"Barrier" made people imagine something solid, a line that could not be crossed. The physics was real - the shaking, the shock waves, the planes that broke up. But the wall was a picture in people's heads, and it gave way to careful engineering, a bullet-shaped plane, and one very stubborn pilot.

Many limits in science look like this at first: frightening, absolute - until someone understands what is really happening, and they turn out to be difficult rather than impossible.

And the number itself turned out to be less fixed than it sounded. 343 is not the speed of sound. It is the speed of sound here, today, in this air.

Compare it with the speed of light - exactly 299,792,458 metres per second (see /299792458), which never changes with the weather. Light is about 874,000 times faster than sound in mild air, which is why you see the lightning before you hear the thunder.