The Apparent Need for Ether

Thomas Young, an English physicist and physician, performed his famous experiments on the interference of light in 1802. A decade later, the French physicist and engineer Augustin Fresnel published his calculations showing the detailed understanding of interference, diffraction, and polarization. Because all known waves (other than light) require a medium in which to propagate (water waves have water, sound waves have, for example, air, and so on), it was naturally assumed that light also required a medium, even though light was apparently able to travel in vacuum through outer space. This medium was called the lu mi - nif er ous ether or just ether for short, and it must have some amazing properties. The ether had to have such a low density that planets could pass through it, seemingly for eternity, with no apparent loss of orbit position. Its elasticity must be strong enough to pass waves of incredibly high speeds! The electromagnetic theory of light (1860s) of the Scottish mathematical physicist James Clerk Maxwell shows that the speed of light in different media depends only on the electric and magnetic properties of matter. In vacuum, the speed of light is given by v c 1/1m0P0, where m0 and P0 are the permeability and permittivity of free space, respectively. The properties of the ether, as proposed by Maxwell in 1873, must be consistent with electromagnetic theory, and the feeling was that to be able to discern the ether’s various properties required only a sensitive enough experiment. The concept of ether was well accepted by 1880. When Maxwell presented his electromagnetic theory, scientists were so confi dent in the laws of classical physics that they immediately pursued the aspects of Maxwell’s theory that were in contradiction with those laws. As it turned out, this investigation led to a new, deeper understanding of nature. Maxwell’s equations predict the velocity of light in a vacuum to be c. If we have a flashbulb go off in the moving system K, an observer in system K measures the speed of the light pulse to be c. However, if we make use of Equation (2.1) to find the relation between speeds, we find the speed measured in system K to be c v, where v is the relative speed of the two systems. However, Maxwell’s equations don’t differentiate between these two systems. Physicists of the late nineteenth century proposed that there must be one preferred inertial reference frame in which the ether was stationary and that in this system the speed of light was c. In the other systems, the speed of light would indeed be affected by the relative speed of the reference system. Because the speed of light was known to be so enormous, 3 108 m/s, no experiment had as yet been able to discern an effect due to the relative speed v. The ether frame would in fact be an absolute standard, from which other measurements could be made. Scientists set out to find the effects of the ether.