We've heard blasé dialog about Mach numbers from fighter pilots in movies, but do you know what Mach numbers actually mean? Among other things, it means that two planes going at Mach 1 can actually be going at very different speeds.

Ernst Mach was a brilliant nineteenth century scientist who came up with good work in pretty much every field he deigned to study. He worked in optics and in cosmology, but his most famous contribution had to do with the speed of sound, and what happens when objects exceed it. He wrote a paper describing the speed of sound and the shock wave produced at supersonic speed. He even got a picture of the waves produced when a bullet broke the sound barrier.

But breaking the sound barrier isn't always achieved at the same speed. The air around us has a certain temperature and pressure, and the speed of sound varies as those factors vary. A plane going slower than the speed of sound at ground level will go faster than the speed of sound high up in the air, even if both are going at the same speed. Mach numbers reflect the plane's speed relative to the speed of sound in the medium it's moving through. Mach 1 means it's going the speed of sound in that particular medium. Mach 2 means it's going twice the speed of sound.