Ernst Mach

Physics

Most of Mach's initial studies in experimental physics concentrated on the interference, diffraction, polarization and refraction of light in different media under external influences. From there followed explorations in 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_p/v_s is called the Mach number after him. It is a critical parameter in the description of high-speed fluid movement in aerodynamics and hydrodynamics. Mach also contributed to cosmology the hypothesis known as Mach's principle.

Philosophy of science

Empirio-criticism

From 1895 to 1901, Mach held a newly created chair for "the history and philosophy of the inductive sciences" at the University of Vienna. In his historico-philosophical studies, Mach developed a phenomenalistic philosophy of science that became influential in the 19th and 20th centuries, empirio-criticism, a rigorously positivist and radically empiricist philosophy established by the German philosopher Richard Avenarius and further developed by Mach, Joseph Petzoldt, and others, according to which all we can know is our sensations.

Mach originally saw scientific laws as summaries of experimental events, constructed for the purpose of making complex data comprehensible, but later emphasized mathematical functions as a more useful way to describe sensory appearances. Thus, scientific laws, while somewhat idealized, have more to do with describing sensations than with reality as it exists beyond sensations:

The goal which it (physical science) has set itself is the simplest and most economical abstract expression of facts.

When the human mind, with its limited powers, attempts to mirror in itself the rich life of the world, of which it itself is only a small part, and which it can never hope to exhaust, it has every reason for proceeding economically.

In reality, the law always contains less than the fact itself, because it does not reproduce the fact as a whole but only in that aspect of it which is important for us, the rest being intentionally or from necessity omitted.

In mentally separating a body from the changeable environment in which it moves, what we really do is to extricate a group of sensations on which our thoughts are fastened and which is of relatively greater stability than the others, from the stream of all our sensations.

Suppose we were to attribute to nature the property of producing like effects in like circumstances; just these like circumstances we should not know how to find. Nature exists once only. Our schematic mental imitation alone produces like events.

Mach's positivism influenced many Russian Marxists, such as Alexander Bogdanov. In 1908, Lenin wrote a philosophical work, Materialism and Empirio-criticism, in which he criticized Machism and the views of "Russian Machists". His main criticisms were that Mach's philosophy led to solipsism and to the absurd conclusion that nature did not exist before humans:

If bodies are "complexes of sensations," as Mach says, or "combinations of sensations," as Berkeley said, it inevitably follows that the whole world is but my idea. Starting from such a premise it is impossible to arrive at the existence of other people besides oneself: it is the purest solipsism. ...if [Mach] does not admit that the "sensible content" is an objective reality, existing independently of us, there remains only a "naked abstract" I, an I infallibly written with a capital letter and italicised, equal to "the insane piano, which imagined that it was the sole existing thing in this world." If the "sensible content" of our sensations is not the external world, then nothing exists save this naked I engaged in empty "philosophical" acrobatics.

— Chapter 1.1, "Sensations and Complexes of Sensations"

Lenin also accused Mach of plagiarizing George Berkeley's views on this basis:

It is a sheer plagiarism on Berkeley. Not a single idea, not a glimmer of thought, except that "we sense only our sensations." From which there is only one possible inference, namely, that the "world consists only of my sensations." .... For if the "assumption" of the existence of the external world is "idle," if the assumption that the needle exists independently of me and that an interaction takes place between my body and the point of the needle is really "idle and superfluous," then primarily the "assumption" of the existence of other people is idle and superfluous. Only I exist, and all other people, as well as the external world, come under the category of idle "nuclei." Holding this point of view one cannot speak of "our" sensations; and when Mach does speak of them, it is only a betrayal of his own amazing half-heartedness. It only proves that his philosophy is a jumble of idle and empty words in which their author himself does not believe.

— Chapter 1.1, "Sensations and Complexes of Sensations"

In accordance with empirio-critical philosophy, Mach opposed Ludwig Boltzmann and others who proposed an atomic theory of physics. Since one cannot observe things as small as atoms directly, and since no atomic model at the time was consistent, the atomic hypothesis seemed unwarranted to Mach, and perhaps not sufficiently "economical". Mach had a direct influence on the Vienna Circle philosophers and logical positivism in general.

Several principles are attributed to Mach that distill his ideal of physical theorization, called "Machian physics":

1. It should be based entirely on directly observable phenomena (in line with his positivistic leanings)
2. It should completely eschew absolute space and time in favour of relative motion
3. Any phenomena that seem attributable to absolute space and time (e.g., inertia and centrifugal force) should instead be seen as emerging from the distribution of matter in the universe.

The last is singled out, particularly by Einstein, as "the" Mach's principle. Einstein cited it as one of the three principles underlying general relativity. In 1930, he wrote, "it is justified to consider Mach as the precursor of the general theory of relativity" and "the whole direction of thought of this theory conforms with Mach's". Einstein further reported that he had read David Hume and Mach's work "with eagerness and admiration shortly before finding relativity theory" and that "very possibly, I wouldn't have come to the solution without those philosophical studies". Before his death, Mach apparently rejected Einstein's theory. Einstein knew his theories did not fulfill all Mach's principles, and no subsequent theory has either.

Epistemological constructivism

According to Alexander Riegler, Mach's work was a precursor to the influential perspective known as constructivism. Constructivism holds that all knowledge is constructed rather than received by the learner. He took an exceptionally non-dualist, phenomenological position. The founder of radical constructivism, Ernst von Glasersfeld, gave a nod to Mach as an ally.

On the other hand, there is also a reasonable case for viewing Mach simply as an empiricist and a precursor of the logical empiricists and the Vienna Circle. On this view, the purpose of science is to detail functional relationships between observations: "The goal which it (physical science) has set itself is the simplest and most economical abstract expression of facts."

Influence

Friedrich Hayek wrote that, when he attended the University of Vienna from 1918 to 1921, "as far as philosophical discussion went it essentially revolved around Mach's ideas". Mach's work has also been cited as an influence on the Vienna Circle, being described as a "major precursor of logical positivism". Members of the Circle organized the "Ernst Mach Society" as a vehicle for discussion of their ideas.

Mach's work was a "forerunner" of Gestalt psychology.

Physiology

In 1873, independently of each other, Mach and the physiologist and physician Josef Breuer discovered how the sense of balance (i.e., the perception of the head's imbalance) functions, tracing its management by information the brain receives from the movement of a fluid in the semicircular canals of the inner ear. That the sense of balance depends on the three semicircular canals was discovered in 1870 by the physiologist Friedrich Goltz, but Goltz did not discover how the balance-sensing apparatus functions. Mach devised a swivel chair to test his theories, and Floyd Ratliff has suggested that this experiment may have paved the way to Mach's critique of a physical conception of absolute space and motion.

Psychology

In the area of sensory perception, psychologists remember Mach for the optical illusion called Mach bands. The effect exaggerates the contrast between edges of the slightly differing shades of gray as soon as they make contact, by triggering edge-detection in the human visual system.

More clearly than anyone before or since, Mach made the distinction between what he called physiological (specifically visual) and geometrical spaces.

Mach's views on mediating structures inspired B. F. Skinner's strongly inductive position, which paralleled Mach's in the field of psychology.

Eponyms

In homage his name was given to:

• 3949 Mach, an asteroid
• Mach, a lunar crater
• Mach bands, an optical illusion
• Mach diamonds, seen in supersonic exhausts
• Mach Five, the car used by Speed Racer
• Mach number, the unit for speed relative to the speed of sound