Ernst Mach

The Mach number, named in honor of Ernst Mach, expresses the ratio of an object's speed to the local speed of sound. The term was coined by Swiss engineer Jakob Ackeret in 1929 and is now universally used in aerodynamics and aviation to describe supersonic and hypersonic flight speeds.

Mach was a radical empiricist who argued that all scientific knowledge must ultimately be reducible to observable sensations and experiences. His 'economy of thought' principle held that science's purpose was to provide the simplest and most efficient description of sensory phenomena, rejecting any unobservable entities as unnecessary.

Mach was famously skeptical of the reality of atoms because they could not be directly observed, maintaining that they were merely useful fictions for calculating experimental results. This put him in direct conflict with Ludwig Boltzmann and other proponents of atomic theory during the late 19th century.

Mach's critique of Newton's concepts of absolute space and time, articulated most thoroughly in his 1883 book 'The Science of Mechanics,' directly influenced Einstein's development of the theory of relativity. Einstein initially credited Mach's philosophical perspective as a key inspiration, though he later distanced himself from Mach's stricter empiricist views.