What is considered 'scientific knowledge' evolves. How much of todays scientific knowledge will still be 'fact' in a hundred years' time? Here we look back at various scientific theories that were orthodox in their time but are now defunct, though some still affect our ways of thinking about the world. This is a brief history of scientific errors or false theories. Just how wrong is it possible to be? How easily can we really tell what is rational from what is irrational? See also the essay on Pseudoscience. An entry from that, on Erroneous Theories, is incorporated here. Phlogiston is a favourite example of getting things the wrong way round. Theories often go out of fashion and come back in different forms. Caloric was the supposed fluid responsible for heat, but we still analyse heat-flow. Aether (or Ether) was the supposed medium in which light and other electromagnetic waves vibrated, but now we have vacuum fluctuations. Text books and popularised accounts are often responsible for oversimplification of theories. Is scientific fact stranger than fiction? Is there ever an end to a wrong theory?
Phlogiston. This was the leading theory of combustion from 1670 when it was proposed by J. J. Becher and developed by G. E. Stahl until 1800 when it was still defended by Joseph Priestley. It was hypothesised that in all flammable materials there is present a colorless, odourless, weightless (and therefore rather difficult to detect) substance called 'phlogiston' which is given off when the material is burnt. The resulting ash, having been 'dephlogisticated', is the pure form of the substance. The idea of 'purification by fire' had been long established. In fact combustion is the combination of elements, particularly with oxygen, giving off heat, and may produce several different compounds in the form of solid ash, gases or liquids.
Homeopathy. C. F. S. Hahnemann introduced 'homeopathy' in about 1790 in the belief that an illness can be cured by the use of minute quantities of some substance that produces symptoms similar to that of the illness. This is an application of a similar principle to 'sympathetic magic'. There are still advocates of homeopathy (though possibly not in its original form). Chemists now consider the doses to be too diluted to have any possible effect, other than as a placebo.
Powder of Sympathy. Less understandable today is the idea of the 'powder of sympathy' of Sir Kenelm Digby, an early Fellow of the Royal Society. Robert Lomas in The Invisible College (about the founding of the Royal Society) writes: "He proposed that all manner of wounds could be cured by the application of 'copperas' or 'green vitriol'. At first sight the idea has merit as the ferrous sulphate he described has both astringent and antiseptic properties, but Digby intended to apply the antiseptic, not to the wound but to the weapon that caused it! He called this 'cure' the powder of sympathy. He persisted in believing in this magical powder despite the fact it was rarely successful." (Or the weapon rarely available, I should have thought!)
The Four Humours. The Greek physician Galen (Claudius Galenus c.130 - c.200), who practiced his art on gladiators and served the Roman Emperor Marcus Aurelius, codified existing medical knowledge. His system of medicine, Galenism, based on his 84 surviving technical treatises, remained authoritative until the Renaissance. This included the theory of the four bodily humours, blood, phlegm, yellow bile (choler) and black bile (melancholy). The relative balance of these four fluids was thought to determine emotional and physical disposition. Hence 'out of humour' meaning in a bad mood.
Euclidean Geometry. Geometry was first systematised in the work called The Elements written around year −300 by the Greek mathematician Euclid, who studied in the great library at Alexandria. His text remained the authoritative source until the early 19th century, when 'non-Euclidean' geometries were discovered, in which the axiom about parallel lines was shown to be replaceable by others. Although Euclidean geometry is still very useful for many everyday purposes, and it was believed by its originators to be based on assumptions that were 'self-evident' truths about the geometry of the real world, it is now regarded as a purely formal mathematical system and is known to bear very little relation to the geometry of the real world on large or small scales. On the large scale Einstein replaced the three-dimensional space of Euclid by the four-dimensional space-time of Minkowski, and showed that 'straight lines', in the form of light rays, are curved when they pass close to massive objects like stars. In the small scale, Heisenberg's uncertainty principle maintains that the more accurately we try to fix the position of a particle (i.e. the coordinates specifying its position in Euclidean space) the less accurate are its other physical properties (i.e. its mass and velocity, which multiplied together make its momentum). Another way of putting this is that at such small scales the Euclidean idea of 'distance' no longer has any meaning. Mathematicians are still working on forms of 'Quantum Geometry' appropriate to these small scales. The 'points' in Euclidean geometry are pretty weird when you think about it. They have no size and between any two there can be found another one - in fact between any two can be found an infinity of other points - in fact a 'continuum' of other points (which is an even higher infinity).
Electric Current. The earliest investigators of transmission of electricity through metals thought of it as a fluid 'current' flowing through a pipe. Unfortunately, for some reason they got the direction of flow wrong. We now know that a 'direct electric current' flowing from a positive to a negative electrode is in fact a flow of (negative) electrons in the opposite direction (or perhaps more accurately a wave-flow among these electrons).
Caloric. When heat was considered to be a fluid it was known as 'caloric'. Lavoisier included caloric among his list of elements. The transfer of heat from place to place is still described by 'heat flow' equations, but it is treated as a flow of the energy contained in the relative motions of the many molecules of the material.
The Luminiferous Ether. Newton thought light was made up of particles (his term was 'corpuscules'), but the 19th century experiments of Thomas Young at the Royal Institution (1809) showed it to have wave properties. Since ocean waves require water and sound waves require air (and of course are also transmitted through water and other materials) it seemed obvious that light waves must also be undulations in some medium, namely the light-bearing (luminiferous) 'ether' which was imagined to fill the whole of space. Unfortunately the 1887 experiments of A. A. Michelson and E. W. Morley to detect the motion of the earth through the ether showed instead that no such motion was detectable.
Action at a Distance. The supposed interaction of two separated bodies without any intervening medium was for a long time considered to be nonsensical. Descartes for example based his theory of vortices on direct action of bodies on one another. The success of Newton's theory of gravitation overcame this prejudice, but still presented philosophical difficulties. In modern theories all interactions are assumed to require some form of intervening 'force field' and gravity in Einstein's theory of general relativity is ascribed to distortion of space-time; but is this any more understandable?
Epicycles. The astronomy expounded by Claudius Ptolemy (c.150, based on earlier work of Hipparchus and others) in his 'Great Compendium of Astronomy' known as the Almagest was the generally accepted view of the universe until well beyond the time of Copernicus (1543). It is based upon the Greek reverence for the circle, which was considered the perfect geometrical form, and therefore the only form of motion appropriate in the celestial regions. Since the planets do not in fact move exclusively in circles it was necessary, in order to meet this requirement, to explain their actual motions by making them move on smaller circles (epicycles) that themselves moved on the larger circles, in a sort of mad clockwork. Nicholaus Copernicus although he made the sun the centre of the solar system, still used a system of cycles and epicycles to explain the actual orbits.
Harmony of the Spheres. Before he discovered any of his three laws of planetary motion Johannes Kepler, in his first book Mysterium Cosmographicum 1596, expounded the idea that the intervals between the orbits of the then known six planets must be related, for reasons of Pythagorean 'harmony of the spheres', to the ratios determined by the five regular solids (tetrahedron, cube, octahedron, dodecahedron and icosahedron). This fantastic scheme nearly worked, and a lesser man might have settled for it, but Kepler continued to struggle with the problems over the next twenty-four years. (His story is told very well in Arthur Koestler's The Sleepwalkers (1959) where Kepler features as 'The Watershed' between the medieval Copernicus and the modern Galileo). In his last book Harmonici Mundi (1618) he describes his attempts to relate the distances and speeds of the planets in their orbits to the Pythagorean musical scale. Eventually he comes up with his third law, in modern terms: The squares of the periods of revolution of the planets are proportional to the cubes of their mean distances from the sun. [Koestler wrote: "Not the least achievement of Newton was to spot the Three Laws in Kepler's writings, hidden away as they were like forget-me-nots in a tropical flowerbed." (p.401)]
The Titius-Bode Law.
Catastrophism and Gradualism have always been rival theories, though the real question is what the balance between them should be. 'Neptunian' or 'Wernerian' geology associated with Abraham Gottlob Werner (1749 - 1817), one of the first geologists to frame a classification of rocks, gave his name to a theory of deposition, which he advocated in controversy with James Hutton (1726 - 1797), whose 'Huttonian' theory emphasised the igneous origin of many rocks and deprecated the assumption of causes other than those we still see at work, as expounded in A Theory of the Earth (1785).
Lamarckism. The theory of organic evolution outlined by Jean Baptiste Pierre Antoine de Monet Chevalier de Lamarck (1744 - 1829) in Philosophie zoologique (1809), based on the principle that characteristics of an organism modified during its lifetime are inheritable. Also known as the 'theory of acquired characteristics' resulting from increased use or disuse of an organ, or the effects of the environment. Neo-Lamarckism is a modern theory of evolution based on Lamarckism and emphasising the influence of environmental factors on genetic changes.
This article derives in part from notes prepared for a discussion held at a meeting of the Outlanders (Leicester Science Fiction Group) on 6 June 2003, and is still evidently very incomplete.