Mach, Ernst (1838–1916)
Ernst Mach was an Austrian physicist and philosopher whose name is commemorated in Mach numbers. His greatest influence was in philosophy where he rejected from science all concepts that couldn't be validated by experience. This freed Einstein from the absoluteness of Newtonian space and time, paving the way toward his theory of relativity, and helped inform the logical positivism of the Vienna Circle.
Mach's principle is a suggestion put forward by Mach in 1893 according to which the inertial effects of mass are not an innate property of a body, but instead are the result of the effect of all the other matter in the universe. In other words, it postulates that the local behavior of matter is influenced by the global properties of the universe. More specifically, Mach's principle claims that it is not absolute acceleration, but the acceleration relative to the center of mass of the universe, which determine the inertial properties of matter. If a body is accelerated with respect to the total mass of the universe, it experiences "forces" such as centrifugal and centripetal force. Mach's principle, however, does not offer an explanation of how the matter in the rest of the universe influences the inertial properties of of the body.
Mach's Principle was brought to widespread attention by Einstein, and undoubtedly played some role as Einstein formulated his general theory of relativity. However, the principle is now known to be incompatible with the general theory.