MACH

Mach is a name most often associated with Ernst Mach, an Austrian physicist and philosopher who lived from 1838 to 1916. His work spanned physics, physiology, and the philosophy of science, and he studied topics including shock waves, motion perception, and the methodology of scientific investigation. Several scientific terms and technologies are named in his honor.

The Mach number is a dimensionless quantity used in fluid dynamics to compare an object's speed with the local speed of sound. It is defined as M = v / a, where v is the object's velocity and a is the speed of sound in the surrounding medium. Subsonic flow has M less than 1, transonic around 1, and supersonic greater than 1; very high-speed flows are often described as hypersonic. The concept is central to aerodynamics and the design of aircraft, propulsion systems, and missiles, where compressibility effects become significant.

Mach bands refer to a visual illusion described by Ernst Mach in the 19th century. They are exaggerated lightness or darkness at boundaries between areas of slightly differing brightness, illustrating the brain's edge-detection processing.

The Mach kernel refers to an early microkernel developed at Carnegie Mellon University. It served as a basis for several research operating systems and, through its influence on later designs, contributed to the kernel architecture of modern systems such as macOS and iOS, where a Mach-inspired core forms part of the XNU kernel.