Huygens light=waves

Christiaan Huygens was right over light, not Isaac Newton, so why did he not prevail in the 17th century?

Huygens’s principle that light is a wave phenomenon is fundamental physics – physical ‘things’ are of two types, particles and waves, the former spatially limited and the latter extended in space and travelling through it.

In this Huygens conflicted with Newton, who described light as particles. Huygens’ deeper difference was in formulating science as principles and relationships rather than Newton’s science of “laws”. Newton’s particle theory dominated for a century, until ~1830, while his science of laws or axioms has persisted in pedagogy, even though principles are more important in higher physics.

Andriesse’s story** touches on these rival fundamentals, while centering on Huygens’ colourful life and work in maths and physics, making many discoveries that are commonly ascribed to Newton. This first ‘complete’ English-language biography does go a good way to showing Huygens’ key role in the 17th century scientific revolution.

In the dynamics of collisions, he showed by experiment that momentum is conserved, then came up with the energy of motion as a second conserved quantity, arguing from Galilean relativity. This gave a pre-Newton alternative to the “laws of motion”. Huygens went on to analyse bodies as a combination of small particles, whence he solved the compound pendulum and invented the cycloidal pendulum that kept time whatever the swing amplitude.

The general infinitesimal analysis he developed was a precursor of differential calculus; indeed Huygens was teacher to Leibnitz and exchanged many letters with him, though did not grasp the import of Leibnitz’s new method.

Huygens’ magnificent study on light started from a belief that ‘light spreads in circles and not in an instant’. This predated Rømer’s 1676 explanation of the timing of Jupiter’s moons varying with the Earth-Jupiter distance, as due to the finite speed of light.  Huygens first pictured light waves as water waves, adding together to give wave fronts, but in a 3-dimensional ether of infinitesimal and invisible particles. He derived a description of mirror reflection and of refraction at air-glass or air-water interfaces. This depended on light travelling more slowly in glass or water – as in the earlier refraction hypothesis with Fermat’s principle of least time. Newton’s light-particles, on the contrary, speeded up when entering the glass. Measuring the speed of light directly was not possible at that time.

Huygens’ waves also explained diffraction at an edge (opposite to Newton’s repulsion from an edge) and the double refraction observed in crystalline quartz (then called Iceland spa). A second “extraordinary” image is seen, depending on the crystal orientation.  Huygens’ ingenious explanation was in terms of elliptical rather than circular expanding waves, and he progressed to dispensing with the idea of the ether.

There was plenty to show the superiority of the wave concept over Newton’s particles (see Marshall’s Wave Particle Duality in the Seventeenth Century). Thus Andriesse’s biography prepares the ground for a key issue for the history of science – why did Newton’s faulty physics prevail over Huygens?

Max Wallis, Cardiff University.

——- ** C. D. Andriesse – Huygens: The Man Behind the Principle, CUP 2005.  The above review was originally published Feb. 2007 by the British Society for History of Science  ——-

Wave particle duality in the seventeenth century  Commentary on Trevor Marshall’s paper on Philica

We now know that Newton did not destroy Hooke’s records, when he succeeded Hooke as Secretary of the Royal Society. The handwritten minutes of the Royal Society as recorded by Robert Hooke were discovered in an attic and purchased early in 2006 by the Society. The ~520 yellowing and stained pages are currently being restored and recorded (including the numerous comments in the margins) to be released for general study.

The article is useful, indeed important, in pointing out how far Newton was wrong in his optical theory – correcting the idea from school physics of Newton’s Rings (showing interference) and the colours of light (via a prism) that Newton was pre-eminent on this topic. That Newton rejected Fermat’s idea on the speed of light differing in air, glass and water – replacing it to explain refraction as acceleration of light particles at the air-water/glass interface – should be taught at school, in order to show how wrong great scientists can be.

Why did Newton’s view prevail over Fermat, Huygens and Gremaldi is an important question. Marshall suggests that it was a group around Newton who were proselytising for the Enlightenment. But why choose to follow a religious (if non-conformist) Newton who espoused a science of “laws”, rather than of principles and relationships? Compare Newton’s ‘laws’ with Fermat’s principle of least time and Huygens’s energy conservation. Surely the explanation lies more in the difficulty in discarding religious (god-the-creator) concepts or in challenging the authority of the Church.

The article makes abundant use of diagrams, which is very appropriate. Mathematical techniques were being developed at the time, particularly the maths of infinitesimals, the precursor of calculus. Huygens’s alternative (correct) wave theory of light was developed as geometrical constructions, so the diagrams are necessary to appreciate his ingenuity. Much of maths is indeed algebra, but geometry has its place and is actually more important in mathematical creativeness than generally acknowledged.

Max Wallis, Cardiff University  (December 2006)


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One Response to Huygens light=waves

  1. Pingback: Extremist Physics: equations rule the World | Crisis-in-Physics

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