The man behind the Mach number refused to believe atoms existed
Ernst Mach photographed shock waves from supersonic bullets, and anyone who quotes a speed as Mach 2 is using his name. He also shaped Einstein's thinking about space and inertia. Yet after hearing Ludwig Boltzmann lecture in 1897, he declared flatly that he did not believe atoms exist.
Mach was born on 18 February 1838 in Chrlice, Moravia, now part of Brno in the Czech Republic, and was taught at home by his parents until he was 14. He took a physics doctorate at the University of Vienna in 1860, with early work on the Doppler effect in light and sound. He declined a chair in surgery at Salzburg, became a professor at Graz, and in 1867 moved to Charles-Ferdinand University in Prague, where he stayed 28 years.
His signature physics was visual. He showed that a projectile moving faster than sound compresses the air ahead of it, and with his son Ludwig used schlieren photography to capture the shadows of these invisible shock waves; Ludwig improved an interferometer in the early 1890s to sharpen the images. The ratio of an object's speed to the speed of sound now carries his name. He also contributed to psychology and physiology, discovering the visual illusion called Mach bands and a non-hearing function of the inner ear that helps govern balance.
As a philosopher he held that only sensations are real, a phenomenalism that sat awkwardly with atoms as mind-independent objects. He saw scientific laws as compact summaries of experience. His critique of Newton's absolute space and time anticipated relativity, and his ideas influenced logical positivism and American pragmatism. The Mach principle, about the origin of inertia, was never written down by him, but Philipp Frank credited him with a vivid version: when the subway jerks, it is the fixed stars that throw you down.
Mach was liberal in politics and later socialist. He criticised European colonial conquests, joined the upper house of the Austrian Parliament in 1901 yet refused ennoblement as unfitting for a scientist, and left money to a Social Democrat newspaper. He was godfather to Wolfgang Pauli. After a stroke in 1898 he kept writing, and he died on 19 February 1916.
Source: Ernst Mach
