Theory, phenomenology, and ‘who ordered that?’

Abstract

The issue of Physics Today with Harry Lipkin’s provocative letter, “Who Ordered Theorists?” arrived by chance at the time I was reading Brian Greene’s The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory (Vintage Books, 2000) in which the author tells of the promise and excitement of string theory. Reading such dissimilar views about theory and experiment induces me to comment on an older theorist–experimentalist matter that Greene brings up in his survey of pre-string physics. He states: “… Maxwell’s theory showed, quite unexpectedly, that electromagnetic disturbances travel at [the speed of light]” (p 24).This statement about electromagnetic waves falling out of theory is exactly as it was presented to me nearly 50 years ago, and seems to me to be nearly universal, so there is no reason to criticize Greene for it. Yet the facts are just the opposite. Maxwell knew that his equations had to produce wave-like solutions because in 1856, W. Weber and F. Kohlrausch 1 1. See J. C. Maxwell, A Treatise on Electricity and Magnetism, (article 771). Oxford, England, Clarendon Press (1892). had measured the ratio of electrostatic to electromagnetic units, a quantity known from dimensional analysis to be a velocity, and had found it equal to the velocity of light. In the experiment, a Leyden jar of known charge capacity had had its potential determined by an electrometer, thereby establishing its charge in electrostatic units; it was then discharged through a ballistic galvanometer calibrated in magnetic units.Michael Faraday had shown a bit earlier that polarized light was affected by magnetism, furnishing a hint that light and magnetism were related, but this new result went far beyond a hint. Its significance was hardly lost on Maxwell, who wrote, “We can scarcely avoid the inference that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena.” 2 2. C. W. F. Everitt, James Clerk Maxwell: Physical and Natural Philosopher, New York, Scribner’s (1975) p. 99. His manipulation of the equations that described the laws of Gauss, Faraday, and Ampere had a definite goal, one that forced the bold assumption he made.Books on electricity for the last decades of the 19th century referred frequently to the Weber and Kohlrausch experiment, which was often reproduced as experimental techniques improved, but when electricity and magnetism began to be taught as derivative from Maxwell’s equations, the significance of the experiment was lost and the implication grew that it was all the consequence of a desire for symmetry. The replacement of gaussian by SI units removed c from its rightful place, and the trip to the memory hole was complete.REFERENCESSection:ChooseTop of pageREFERENCES <<1. See J. C. Maxwell, A Treatise on Electricity and Magnetism, (article 771). Oxford, England, Clarendon Press (1892). Google Scholar2. C. W. F. Everitt, James Clerk Maxwell: Physical and Natural Philosopher, New York, Scribner’s (1975) p. 99. Google Scholar© 2001 American Institute of Physics.