Seeing Shock Waves

In this still image from a video of a 2008 demonstration of a U.S. Navy railgun, the shock waves in front of the projectile are momentarily visible. When travelling faster than the speed of sound in air, information (in the form of pressure waves) is unable to travel ahead of the projectile, meaning that the air cannot deform around the object as it does at low speeds.  Instead, a front known as a shock wave forms on or in front of the object, depending on its speed and shape. Across this shock wave, thermodynamic properties of the gas are discontinuous; the pressure, temperature, and density of the air rise drastically, but the air is also deformed so that it passes around the object. (See also: bullet from a gun.)

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