Ernst Mach

Ernst Mach was trained as a physicist in Vienna but ended up something closer to a philosopher of science. He spent his career at the University of Prague and later Vienna, working across acoustics, optics, fluid dynamics, and the theory of perception.

In 1887 he and his student Peter Salcher captured the first photographs of a supersonic bullet, using a schlieren apparatus to make the compressed air around the projectile visible. The V-shaped shock wave trailing the bullet matched his geometric prediction exactly. The half-angle of that cone depends only on the ratio of projectile speed to sound speed — the quantity now called the Mach number.

His writings on inertia and reference frames — Mach's principle — profoundly influenced Einstein, though Mach himself never accepted general relativity. He is the rare physicist whose name survives as a unit, a cone, and a philosophical position.