Ernst Mach

Ernst Waldfried Josef Wenzel Mach was born on 18 February 1838 in Chirlitz, a village in Moravia near Brno that then belonged to the Austrian Empire and is now Chrlice, a district of the Czech city of Brno ^[1]. His father, Johann Mach, had studied at Prague and worked as a tutor; the family soon moved to Untersiebenbrunn, east of Vienna, where Johann farmed and educated his children at home ^[2].

That home schooling dominated Mach's childhood. Until the age of fourteen he learned almost entirely from his father, absorbing classics, grammar, and natural history alongside practical work on the farm ^[2]. A brief spell at a Benedictine gymnasium ended badly when the monks judged the boy untalented, an episode Mach later recalled with some irony. He also trained for a time as a cabinetmaker, a craft apprenticeship that left him with lasting respect for manual skill and, by his own account, a readiness to emigrate to America if academic life failed him ^[3].

In 1855 he entered the University of Vienna to study mathematics, physics, and philosophy. He completed a doctorate in physics in 1860 with a thesis on electrical discharge and induction, then stayed on as a Privatdozent, lecturing for meager fees while performing experiments on the Doppler effect, which some senior physicists at the time still doubted ^[1][3]. Short of money for elaborate apparatus, he turned to problems in the physiology of the senses, work that could be done cheaply and that would occupy him for the rest of his life.

Mach's first professorship came in 1864, when the University of Graz appointed him to a chair of mathematics; two years later he moved within Graz to a chair of physics ^[3]. In 1867 he married Ludovica Marussig in Graz, and the same year he accepted the chair of experimental physics at the Charles University in Prague, a post he would hold for twenty eight years ^[1][3].

Prague made his reputation. There Mach carried out sustained research on the perception of bodily rotation, describing how the inner ear senses acceleration, work that ran parallel to studies by Josef Breuer and Alexander Crum Brown on the vestibular system ^[4]. He also investigated an optical effect in which the eye exaggerates the contrast at boundaries between light and dark areas. The illusory bright and dark strips he described are now known as Mach bands, and the neurophysiologist Floyd Ratliff later devoted an entire book to Mach's role in explaining them ^[4].

Alongside laboratory work, Mach wrote for a broad audience. His lectures and popular essays argued that scientific concepts are tools for organizing sensations rather than descriptions of hidden metaphysical machinery. In 1883 he published Die Mechanik in ihrer Entwickelung, translated into English as The Science of Mechanics, a critical history that examined how the concepts of mass, force, and inertia had actually arisen ^[5]. The book went through many editions and became one of the most influential works on the foundations of physics written in the nineteenth century.

Among Ernst Mach achievements, the best known outside philosophy concerns bodies moving faster than sound. In 1887 Mach and Peter Salcher, an Austrian naval academy instructor in Fiume, published photographs of a projectile in supersonic flight, using a spark shadowgraph technique to capture the conical shock wave trailing from its tip ^[6]. The images settled a practical dispute about why certain projectiles wounded so severely, and they demonstrated that the ratio of a body's speed to the speed of sound governs the flow pattern around it. In 1929, long after Mach's death, the Swiss aerodynamicist Jakob Ackeret proposed calling that ratio the Mach number, which is why pilots today speak of flying at Mach 2 ^[6][7].

His second great contribution was conceptual. In The Science of Mechanics Mach attacked Newton's notions of absolute space and absolute time as concepts that no experiment could ever reach ^[5]. Analyzing Newton's rotating bucket experiment, he suggested that inertia might arise from the interaction of a body with the distant masses of the universe rather than from motion relative to space itself. Einstein found this line of thought so fertile that he coined the phrase Mach's principle for it and acknowledged Mach as a forerunner of general relativity, writing a warm obituary of him in 1916 ^[7][8].

Mach also helped found what became the philosophy of science. His book The Analysis of Sensations, first published in 1886, argued that the world scientists study is built from elements of sensory experience, and that the self is not an indivisible unit ^[9]. He rejected the atomic theory as it stood in his day, on the ground that atoms were then unobservable, a position that put him in famous opposition to Ludwig Boltzmann ^[1]. Though experiment later vindicated atoms, Mach's demand that physical concepts earn their keep through observation influenced Einstein's operational treatment of simultaneity and, later, the logical positivists of the Vienna Circle, whose organization was named the Verein Ernst Mach in his honor ^[9].

Mach and Ludovica, known as Louise, raised five children, four sons and a daughter ^[3]. Family life carried real sorrows: his son Heinrich, a promising chemist, died in 1894, a loss that contemporaries said darkened Mach's later years ^[3]. Another son, Ludwig, trained in medicine, became his father's closest assistant, helping with optical experiments in Prague and Vienna and managing his affairs after illness struck ^[2].

Colleagues described Mach as plain in manner and stubborn in argument, a man who preferred simple apparatus he could build or adapt himself. He read widely in psychology, history, and Eastern thought, and he held political views that leaned toward socialism and pacifism, unusual commitments for a senior Austrian professor of his era ^[2]. In Prague, where nationalist tension between Czech and German speakers ran high, he twice served as rector of the university and dealt with the disputes surrounding its division into separate Czech and German institutions in 1882 ^[3].

He was candid about the limits of his own philosophy's popularity. When critics accused him of dissolving the physical world into mere sensations, he replied that he was simply refusing to add anything to experience that experience itself did not contain. The debates he provoked, with Boltzmann over atoms and with Max Planck over the aims of physics, were sharp but conducted in print rather than through academic intrigue ^[9].

In 1895 the University of Vienna created a chair unusual for its time, a professorship in the philosophy of the inductive sciences, and called Mach home from Prague to fill it ^[1]. The appointment recognized that his real subject had become the nature of scientific knowledge itself. His Vienna lectures drew large audiences, and his ideas spread among physicists, psychologists, and writers, including the novelist Robert Musil, who wrote a dissertation on Mach's epistemology ^[9].

The Vienna period was cut short. In 1898 Mach suffered a stroke that paralyzed the right side of his body ^[1]. He continued working with his left hand and with Ludwig's help, but in 1901 he retired from teaching. In the same year the emperor appointed him to the upper house of the Austrian parliament, where the ailing scientist occasionally appeared to support progressive causes ^[3].

Retirement did not end his output. He revised his earlier books, published Knowledge and Error in 1905, a summing up of his theory of inquiry, and continued a wide correspondence, including exchanges with the young Einstein ^[8][9]. In 1913 he moved to the home of Ludwig in Vaterstetten, near Munich. He died there on 19 February 1916, one day after his seventy eighth birthday ^[1]. A final book on optics appeared posthumously, prepared from his manuscripts by his son.

Few scientists have a legacy split so evenly between hard engineering and abstract philosophy. Every supersonic aircraft, wind tunnel report, and spacecraft reentry calculation uses the Mach number, and the shock wave photography he pioneered with Salcher grew into the modern science of high speed flow visualization ^[6][7]. A large lunar crater on the Moon's far side carries his name, as does the Ernst Mach Institute for high speed dynamics in Freiburg, Germany.

In philosophy his influence ran through the twentieth century. The Vienna Circle treated him as a founding ancestor of logical empiricism, and his insistence on the economy of thought, the idea that theories are compact summaries of experience, shaped debates about scientific realism that continue today ^[9]. Einstein credited Mach's historical criticism of mechanics with loosening the grip of absolute space on his imagination, even though the two men's views diverged and Mach expressed reservations about relativity late in life ^[8].

Students compiling Ernst Mach facts often note the paradox at the center of any Ernst Mach biography: the man whose name is attached to supersonic speed doubted atoms, and the philosopher who distrusted theory helped inspire the most theoretical physics of the new century. His own explanation would probably have been simple. He asked of every concept what observations stood behind it, and that single habit, applied without fear of authority, turned out to be useful everywhere from ballistics ranges to the foundations of spacetime ^[5][9].