The crossword solution for the term PHYSICIST MACH is ERNST. Ernst Mach was an Austrian physicist and philosopher who made important contributions to the fields of physics, philosophy, and psychology. He is best known for his work on supersonic motion, which led to the development of the Mach number, a measure of the speed of an object relative to the speed of sound. He also developed the concept of Mach's principle, which states that the inertia of a body is determined by the gravitational interaction of all other bodies in the universe.