Ernst Mach of experimental physics concentrated on the interference, diffraction, polarization and refraction of light in different media under external influences. From there followed important explorations in the field of supersonic fluid mechanics. Mach and physicist-photographer Peter Salcher presented their paper on this subject in 1887; it correctly describes the sound effects observed during the supersonic motion of a projectile. They deduced and experimentally confirmed the existence of a shock wave of conical shape, with the projectile at the apex. The ratio of the speed of a fluid to the local speed of sound "v"/"v" is now called the Mach number. It
Ernst Mach Mach are attributed a number of principles that distill his ideal of physical theorisation—what is now called "Machian physics": The last is singled out, particularly by Albert Einstein, as "the" Mach's principle. Einstein cited it as one of the three principles underlying general relativity. In 1930, he stated that "it is justified to consider Mach as the precursor of the general theory of relativity", though Mach, before his death, would apparently reject Einstein's theory. Einstein was aware that his theories did not fulfill all Mach's principles, and no subsequent theory has either, despite considerable effort. According to Alexander Riegler, Ernst