The Man Who Stood Still by Moving Very Fast

Here is a trick worth thinking about. A truck barrels down a road at 80 kilometres per hour. Mounted in its bed is a spring-loaded launcher aimed straight backward. At the right moment, the launcher fires a man out the back at exactly 80 kilometres per hour — the same speed, in the opposite direction.

The two velocities cancel. To anyone watching from the ground, the man does not fly backward at all. He hangs in the air, motionless, and then drops straight down like a dropped sack of flour, while the truck carries on without him.

This is Galilean relativity performing a party trick. Galileo worked it out in the seventeenth century using ships rather than trucks: if you drop a ball below deck on a smoothly sailing vessel, it lands at your feet exactly as it would on dry land. The ball is already moving with the ship, so relative to you it may as well be standing in harbour. Nobody inside the ship can tell, from the physics of a dropped ball, whether they are moving or still. Velocities add and subtract cleanly. Match one to another and you get zero.

The launched man is the same idea run in reverse. The truck gives him forward motion; the launcher subtracts an equal amount. The ground gets the difference, which is nothing.

The catch is that nothing here is smooth or gentle. The whole illusion depends on the launcher accelerating him to precisely the truck's speed in the short length of a truck bed — a violent shove packed into a few metres. Miss the target speed and he does not float; he tumbles off the tailgate at whatever the difference happens to be. A helmet and body armour are not decorative. Physics is generous about the concept and merciless about the execution.

Galileo, incidentally, never had a spring launcher or a diesel engine to test his idea with, only ships and a talent for imagining what he could not yet build. Four centuries later, someone bolted the principle to a pickup and threw a human being out of it to watch him hang, briefly, in a pocket of stillness carved out of two speeds pointed at each other.