Ernst Mach

Ernst Mach (1838–1916) was a physicist and philosopher born in Moravia, then part of the Austrian Empire, and active during the transition from classical physics to relativity. He is remembered for the Mach number, which compares the speed of an object with the speed of sound, and for a philosophy of science that shaped logical positivism and American pragmatism.

He studied in Vienna and earned his doctorate in physics in 1860 under Andreas von Ettingshausen. He taught mathematics at Graz and then, from 1867 to 1895, experimental physics at the university in Prague. In 1895 he moved to Vienna, where he held a newly created chair in the history and philosophy of the inductive sciences. A stroke in 1898 left him partly paralysed, and in 1901 he left the university to take a seat in the Austrian House of Lords. He died in 1916, a day after his 78th birthday, near Munich.

His early work covered optics, acoustics and thermodynamics. With the physicist Peter Salcher he described the conical shock wave that forms in front of a projectile travelling faster than sound. Independently of Josef Breuer, he also worked out in 1873 how the inner ear senses the tilting of the head, through the movement of fluid in the semicircular canals.

His critique of Newtonian mechanics is perhaps his most lasting contribution. He questioned the idea of absolute space and time and argued that effects such as inertia should be understood as the result of how matter is distributed across the universe. Einstein later called this idea Mach's principle, and he credited Mach as a forerunner of general relativity, although he eventually moved away from Mach's empiricism.

In philosophy, Mach rejected metaphysics and held that our knowledge rests on sensations. Together with Richard Avenarius he developed empiriocriticism, which treated scientific laws as economical summaries of observed experience. For this reason he doubted the atomic hypothesis, since atoms cannot be observed directly. His ideas reached the Vienna Circle, Russian Marxists such as Bogdanov, and the Russian avant-garde. Lenin attacked them sharply in Materialism and Empirio-criticism, written in 1908.