Portrait of the Austrian physicist, Ernst Mach, 1838-1916. Mach studied at Vienna and was strongly interested in psycho-physics. As a philosopher of science he argued that information comes through sensations, and that the world consists of data; that which may not be sensed is meaningless. Mach influenced authors of quantum mechanics and his book: Mechanics, influenced Einstein. Mach's prin- ciple deals with space, mass, and the relationship of one body to another. Experiments in airflow, for which Mach is best known, led to the Mach angle and Mach number (ratio of a projectile's speed to the speed of sound) and thus sonic and supersonic speed measurements.