Newtonianism

Rob Iliffe Isaac (1642/3-1727)

• Born in Woolsthorpe, Lincolnshire, England; attended Grantham Grammar School, 1655-61, and then Trinity College Cambridge. • In the mid-1660s ( the ‘marvelous’ – or plague – years) he discovered the integral and differential , the Binomial Theorem, • and the heterogeneity of white light – white light is composed of a mixture of more basic primary rays. • Shortly afterwards he showed that laws of force on Earth and in the heavens were governed by the same inverse-square (1/r2) law. • But – (a) his demonstration did not satisfy him and • (b) this was very far away from the later theory of universal gravitation. • He built first working reflecting telescope (1668), which gave rise to his research into alchemy and chemistry.

Newton’s Theory of Light and Colour

• In 1664-5 Newton read Robert Boyle’s book on colours and Robert Hooke’s Micrographia, and started his own research program on light, colour and vision. • Discovered in c. 1666 (during plague) that different coloured rays emerging from a prism had specific, unchanging refractive indexes. • He then spent years producing many experiments proving that white light was composed of a mixture of such (primary) rays. • This was his theory of the heterogeneity of white light. • Visible objects seemed to be one or other colour depending on their propensity to reflect rather than absorb specific colours. The Crucial Experiment

• Newton sent off a version of his private writings to Henry Oldenburg in February 1672 for publication in the Philosophical transactions, • he described how he had passed white light through a prism and was amazed to find that the image produced as a result was elongated, • and not round, as one would have expected given other theories. • To test whether light was produced/modified as a result of white light passing through a prism, or constitutive of the light before it reached the prism, • Newton described a ‘crucial experiment’ with two prisms, which was supposed to provide a demonstration of the truth of his theory. • However, to his frustration, it turned out to be hard to reproduce. Newton’s only drawing of the crucial experiment: 1. Light enters through a slit in a covered window on the right, and passes through a lens before encountering the first prism in the middle of the drawing. 2. After refraction it makes an elongated image on a board with 5 small circles; a hole that allows a primary ray to encounter a second prism to the left. 3. On passing through the second prism, the colour and the angle of refraction ‘nec variat lux fracta colorem’: ‘light does not change remain the same. [i.e. is not modified] when broken into colour’ Newton’s mathematical - June 1672

• Made novel methodological and disciplinary claims, arguing that his theory of colours was a mathematically certain ‘science’. • Significantly, he defined a ray of light phenomenologically or ‘abstractedly’ (i.e. without reference to the physical structure of light) and in terms of its measured index of refraction • The heterogeneity of white light was a theory but it was established by the crucial experiment and was not an ‘hypothesis’. • Hypotheses and unwarranted references to the internal physical structure of bodies were imaginary and inevitably caused disputes, • while mathematically certain statements were part of a well ordered scientific community. The comet of 1680-1

• The so-called ‘Great Comet’ appeared at end of 1680 and another appeared at start of 1681. • many theories, generally magnetical, were offered about their history, nature, and orbit. • And there were discussions about where they were the same comet. • In 1682, the English Astronomer Royal John Flamsteed argued that they had to be the same comet, which had turned in front of the Sun due to magnetic repulsion, • but Newton disagreed, adhering to older notion that comets generally travelled in straight(ish) lines.

• Newton’s model of the path of the comet, entering picture from lower right. • Comet travels BKC at which point some unknown force pulls it to D and then F. • The force cannot be magnetic, according to Newton. • It was therefore driven by an unknown species of force Newton and the Great Comet

• Newton argued that: • (a) according to the observations, a single comet would have had to slow down if it changed direction in front of the Sun; • (b) Hot magnets did not work, so the Sun could not be a magnet; • (c) There was no known mechanism, magnetic, vortical or otherwise, that explained how it could turn in its observed path either in front of, or behind the Sun; • A path behind the Sun was the best fit for the data, but there was no known mechanism to account for it, and so it made no sense. The route to Universal Gravitation • Discussions about inverse-square force law, the elliptical orbits of planets and the physical cause of their motion had taken place in London in early 1680s. • Edmond Halley visited Newton summer 1684 and asked him whether he could prove that the inverse-square law of motion gave rise to an elliptical orbit. • Over next 2 years Newton wrestled with problem and gradually developed 3 laws of motion and concepts of ‘force’, ‘mass’, and Universal Gravitation (‘attraction’). Naturalis Philosophiae Principia Mathematica (1687)

• Newton showed that measurable motions of the terrestrial and celestial spheres were subject to the same fundamental and mathematical general law. • He articulated the Law of Universal Gravitation in the equation F = G.mm’/r2 • The attractive force linking any two bodies in the world can be measured in terms of a constant (G) multiplied by the product of their masses divided by the distance between them. • Newton’s revolutionary book linked numerous types of measurable celestial and terrestrial phenomena, • and key data in favour of Universal Gravitation came from astronomers and others from around the globe.

The success of the Principia

• Newton used the theory of universal gravitation to account for tides, cometary motion and the oblate spheroidal shape of the earth, • Making predictions about future, minute motions in the solar system • Crucial elements of Principia are quantification and measurement; • Like his earlier work on light and colours, the Principia was an example of a mathematically-based science, and a demonstration that natural philosophy (as Galileo had said) was fundamentally mathematical. • Newton understood this in theological terms; God was a consummate geometer, who had embedded mathematical order into the structure of the infinite universe. • It required a mathematician like Newton to read the Book of Nature. The Methodology of the Principia

• Newton’s methodology (i.e. the way in which he set out his demonstrative argument) was very sophisticated: • 1. Due to an infinite number of Newtonian forces always in operation, less general laws (Galilean/ Keplerian) could never be exact. • 2. Discrepancy between theory (e.g. idealized, perfect Keplerian orbits) and robust data would always point to potential causal factors – it was evidence of the operation of such forces and thus a prompt for further research. • 3. It could make detailed predictions (e.g. return of Halley’s Comet in 1758) • 4. was Universal Gravitation falsifiable? • 5. or comprehensible? Privately Newton thought the direct cause was God. Criticism of Principia

• Critics such as Christiaan Huygens and Gottfried Leibniz were committed to physical/ mechanical explanations of natural phenomena as a key part of what natural philosophy was; • and Leibniz in particular argued that the notion of ‘attraction’ was an incomprehensible ‘occult quality’ (and thus a medieval voodoo force). • Others saw Principia as merely an exquisite piece of geometry, a position that was complicated by Newton’s explicitly mathematicist approach in the first two Books of the Principia. • Or, like Louis-Bertrand Castel, they saw Principia as a monster that was neither mathematics nor physics.

Newtonianism: The Divided Legacy

• Tin the Principia, the 3 laws of Motion and the definitions of ‘force’ and ‘mass’, implied homogeneity of all matter. • Newton was an atomist and a vacuist; evidence from both celestial and terrestrial phenomena suggested that there was no matter in empty space that retarded planetary or cometary motions; • He attacked various physical explanations and their related systems as mere ‘hypotheses’ (especially the Cartesian system) • And his brief claim ‘’ in his attack on vortices and hypotheses in the ‘’ of 1713 became the great anti-fictivist credo of Enlightenment.

Newton’s (1704, 1706, 1717/18)

• First major publication of Newton’s theories of reflection, refraction – originally in English, with Latin edition in 1706 and second English edition in 1717. • ‘Queries’ added to successive editions were extensions of questions and concepts from earlier alchemical and scientific programmes. • became most influential statements in Eighteenth century about existence of an aether, or two aethers, to explain light, heat, electricity and even gravity. Physics at the Royal Society under Newton’s presidency (1703-27); Hauksbee’s electric machine

French Newtonians everywhere

• French philosophers were generally supportive of the Cartesian- Leibnizian view, and hostile to Newton’s doctrines. • This especially true of shape of the Earth – so 2 scientific expeditions were devised to test the issue: • In Peru (Ecuador) (1735-44): • Charles Marie de La Condamine, Pierre Bouguer, Louis Godin • In Lapland (1736-7): • Pierre Maupertuis, Aléxis-Claude Clairaut, Pierre Le Monnier (and Anders Celsius) Pierre Louis Moreau de Maupertuis (1698-1759) first Head of Prussian Academy of under Frederick the Great:

Inventor of Principle of Least Action Francesco Algarotti, Newtonianism Per le Dame (1737, 1739)

Voltaire (with help of Madame du Châtelet), Eléments de la Philosophie de Neuton, (1738)

Newton and Enlightenment Sense Experience and the Real world

• Like many of his predecessors, Newton denied in the Principia that ordinary sense experience gave a true account of the real world. • The experience of untrained people was fallible, and gave a ‘relative’ or ‘common’ account of the world (and it was the language of the Bible); • The real world therefore lay behind the veil of ordinary perception. • Expert researchers (mathematically-astute astronomers) could produce and assess reliable, quantitative information. • By making inferences from measured motions to the underlying forces, robust data helped experts derive the True and Absolute system of the world, which was ultimately structured by Universal Gravitation.