Mach, Ernst

(born Feb. 18, 1838, Chirlitz-Turas, Moravia—died Feb. 19, 1916, Haar, Ger.) Austrian physicist and philosopher. After earning a doctorate in physics in 1860, he taught at the Universities of Vienna and Graz as well as Charles University in Prague.

Interested in the psychology and physiology of sensation, in the 1860s he discovered the physiological phenomenon known as Mach's bands, the tendency of the human eye to see bright or dark bands near the boundaries between areas of sharply differing illumination. He later studied movement and acceleration and developed optical and photographic techniques for measuring sound waves and wave propagation. In 1887 he established the principles of supersonics and the Mach number, the ratio of the velocity of an object to the velocity of sound. He also proposed the theory of inertia known as Mach's principle. In Contributions to the Analysis of the Sensations (1886), he asserted that all knowledge is derived from sensory experience or observation.