Sound of Silence

Sound of Silence

We hear the crack of a bullwhip because its tip is moving faster than the speed of sound, creating a shock wave, a small Sonic boom. A thunderclap has a similar origin. It was once thought that airplanes could not travel faster than sound. Today supersonic flight is commonplace. But the light barrier is different from the sound barrier. It is merely an engineering problem like the one the supersonic airplane solves. It is a fundamental law of Nature, as basic as gravity. And there are no phenomena in our experience – like the crack of the bullwhip or the clap of thunder for sound – to suggest the possibility of traveling in a vacuum faster than light. On the contrary, there is an extremely wide range of experience – with nuclear accelerations and atomic clocks, for example – in precise quantitative agreement with special relativity.

The problems of simultaneity do not apply to sound as they do to light because sound is propagated through some material medium, usually air. The sound wave that reaches you when a friend is talking is the motion of molecules in the air. Light, however, travels in a vacuum. There are restrictions on how molecules of air can move which do not apply to a vacuum. Light from the Sun reaches us across the intervening empty space, but no matter how carefully we listen, we do not hear the crackle of sunspots or the thunder of the solar flares.

Sound of silence. The loudest sound I’ve ever heard, was the sound of silence (quotations from Megan Jorgensen). Image : © Megan Jorgensen (Elena)