Scientist of the Day - Ernst Mach

Portrait of Ernst Mach, age 67, photograph (Wikimedia commons)

Ernst Mach, an Austrian physicist, was born Feb. 18, 1838, in Chilitz, Moravia, now part of the Czech Republic. He was educated at home until he was 14, and then at the University of Vienna, where he studied physics, especially the newly discovered Doppler effect, which he experimentally confirmed. He obtained a position in Graz (where Johannes Kepler had started his own career 260 years earlier), and then was appointed to the professorship of experimental physics at the Charles-Ferdinand University in Prague, where he taught for nearly 30 years. He eventually returned to Vienna.

Shock wave produced by a supersonic bullet, photograph by Ernst Mach, ca 1888 (Wikimedia commons)

Mach's name is known to the general public primarily through the term "Mach number," the ratio of the speed of an object (usually a plane or a jet car) to the speed of sound, as in "Chuck Yeager reached Mach 2.4 in the X-1A in 1953." But the work Mach did on shock waves, especially those produced by projectiles moving faster than the speed of sound, and for which the term "Mach 1" was his reward, was the least interesting part of his career, although his photograph of a supersonic bullet, taken in 1886, 1887, or 1888 (authorities disagree), is a classic scientific photograph (second image). To historians of science, Mach is better known for denying the existence of atoms right up until his death in 1916, which was well beyond the time when Albert Einstein (1905) and Ernest Rutherford (1911) were demonstrating the reality of atoms, at least to their satisfaction.

Mach was a positivist, which means he rejected any scientific infrastructure that is not directly observable, and to him atoms were just hypothetical entities that scientists postulated for explanatory purposes but whose existence we are not entitled to assume. Many people were skeptical about atoms in the mid-19th century, but when Einstein showed that Brownian motion – the random motion of tiny particles in solution – is the expected result of the thermal motion of molecules, and when Rutherford bounced alpha particles off atomic nuclei, most scientists were convinced of the reality of atoms. But not Mach. If we cannot see them, we cannot assume their existence, he maintained to the end.

Bust of Ernst Mach, sculpted by Jakub Vlček, 2016, and inspired by Mach’s 1888 photograph of a supersonic bullet, Charles University, Prague (historyofscience.cz)

Mach was also known for formulating that deadly and shifty entity known as "Mach's principle." Isaac Newton, in his Principia (1687), had proposed a hypothetical experiment, involving a spinning bucket half-filled with water. Newton said we all know that water in a rotating bucket will move outwards and climb the walls of the bucket, forming a concave surface, because of centrifugal force. Newton then asked: what if we removed everything else in the universe, all those things with which we determine that the bucket is rotating? If the bucket then is not rotating with respect to any other object, would the water still climb the walls of the bucket? Is it even meaningful to say that the bucket is rotating, in the absence of any reference frame? And Newton said yes, because the bucket is still rotating with respect to absolute space. In fact, this is how Newton argues for the existence of space (and time) than is independent of matter.

Bust of Ernst Mach, Rathauspark, Vienna (Wikimedia commons)

Many of his contemporaries disagreed with Newton's conclusion, especially that arch-relativist, Gottfried Wilhelm Leibniz. But the most famous critic of the bucket experiment was Ernst Mach. Mach said essentially that local physics is determined by the large-scale structure of the universe; water climbs the rotating bucket because it is rotating with respect to the stars. In other words, the stars cause the centrifugal effect, and if there were no stars, then the bucket would remain flat. Mach's principle is often called vague and amorphous, but that is because Mach never formulated the principle. It was Einstein who first used the term "Mach's principle," referring to something that he had read in Mach, and it inspired much of his work on general relativity. If for no other reason, that makes Mach's principle special, even if we aren’t sure exactly what that principle is.

Mach suffered a stroke in 1898 and was paralyzed in the right half of his body, so he had to retire from teaching in Vienna, but he stuck around to make sure that the local atomists, such as Ludwig Boltzmann, did not have it too easy. Mach met Einstein around 1909 – Einstein was very influenced by Mach’s Science of Mechanics (1883) – and Einstein thought he had convinced Mach that atoms are real – but we know from Mach’s private correspondence that this was not the case.

Detail of the title page of Il pianeta marte, by Giovanni Schiaparelli, 1893, with presentation inscription to Ernst Mach (Linda Hall Library)

In addition to dozens of editions of Mach’s books on mechanics, we have one special piece of Machiana in our collections – an offprint from Mach’s library. It was written by Giovanni Schiaparelli and contains one of the first printed versions of his map of the canals of Mars (which you can see at our post on Schiaparelli). It is inscribed to Mach on the title page (fifth image). God only knows what Mach, arch-positivist, thought about the reality of the Martian canals.

William B. Ashworth, Jr., Consultant for the History of Science, Linda Hall Library and Associate Professor emeritus, Department of History, University of Missouri-Kansas City. Comments or corrections are welcome; please direct to ashworthw@umkc.edu.