The Day the Universe Changed - James Burke [140]
Having worked the problem out in old-fashioned Newtonian terms, Maxwell dumped the entire apparatus. In the early 1860s he had read of experiments by Wilhelm Weber and Rudolph Kohlsrausch, carried out in 1856, which had shown that the speed of a current moving along a wire was close to the measured speed of light. Armand-Hippolyte-Louis Fizeau had established light speed in 1849 by sending a beam through the spinning teeth of a cog-wheel, reflecting it, and measuring at what speed the wheel had to turn in order to block the returning beam with the next cog.
Maxwell’s model. Hexagons rotate around lines of force: the axes of rotation indicate the direction of the force, the velocity indicates its magnitude. Balls representing electric particles prevent the hexagons from interacting with each other. The balls’motion represents electric current.
Fizeau’s apparatus for determining the speed of light (below). When the rate at which the cog-wheel spins is increased, the light passing between the teeth (above left) is first partially eclipsed (centre) and then totally disappears (right).
Maxwell became convinced that the similarity between the speed of light and that of current was too close to be ignored. He too was a Naturphilosoph and looked for continuity through all phenomena. This led him to seek the simplest explanation. In 1865 he published A Dynamical Theory of the Electromagnetic Field, in which he said that light, like electricity and magnetism, consisted of transverse waves of the ether. In the space between electrified bodies lay some kind of matter which went into motion when the phenomena occurred.
Maxwell examined all forms of conducting material to see how much ‘strain’they took to start and continue the movement of a current. His feeling was that the energy being transferred was in the field itself, not only in the bodies. In all this Maxwell had made one supreme advance. He had removed the force from the area of mechanics and placed it in the field of optics, uniting all three phenomena: light, electricity and magnetism. But the question still remained regarding the medium through which this united force was moving. It was made of matter, Maxwell was sure. The ether, as he saw it, was the intangible, infinitely rigid yet infinitely flexible material with which space had to be filled if the electromagnetic effect were to take place at all.
Maxwell tried examining the ether with light beams. He passed the light from a star through a prism, set first in the direction of the movement of the earth through space and then perpendicular to that path. There was no apparent difference in what happened to the light. It was as if the ether did not exist. Maxwell, however, was convinced that it existed and was ‘certainly the largest, and probably the most uniform body of which we have any knowledge’.
Moreover, if it did exist a force moving within it would take time to propagate. This would destroy Newton’s concept of simultaneous action. It may have been the realisation of this possibility that led Maxwell to make a statement which, although he did not then know it, was of great portent. With the possible disappearance of Newtonian simultaneity from the universe, the absolutes were in danger of disappearing too. He said:
Our primitive notion may have been that to know absolutely where we are, and in what direction we are going, are essential elements of our knowledge as conscious beings. But this notion… has been gradually dispelled from the minds of the students of physics. There are no landmarks in space… we may compute our rate of motion with respect to the neighbouring bodies, but we do not know how these bodies may be moving in space.
In fact, something was known. Ever since the earth had been displaced from the centre of Aristotle’s universe by Copernicus, there had been the possibility that the new centre, the sun, might itself be moving through space. In 1805 a musician-turned-astronomer from Hanover called