Though light was finally accepted to exist in the form of waves in the 19th century, other problems awaited the immediate attention of physicists.

The 19th century was a vitally important period in the history of light. It began with Thomas Young’s final proof that light existed as a wave, which had become scientific dogma about halfway through the century.

In 1864, British Physicist James Clerk Maxwell presented his findings to the Royal Society in England that light and electromagnetism were one in the same thing, and could be defined using a series of equations (which were, of course, the famous “Maxwell Equations”). In essence, Maxwell had finally determined mathematically just how light waves work, with a sort of “leap-frog” motion, wherein electricity and magnetism work together in a “mutual embrace,” as Maxwell himself put it, to propel themselves through space.

How Does Light Travel?

And yet, there was another issue of concern involved even in this new, much more scientific, explanation of light. That was the question of a medium. By definition, a wave needs a medium through which to propagate (move). Sound waves need air or other matter through (which is why there’s no sound in outer space), water waves need… well… water. How can light move through space if it has nothing to travel in?

It was because of this problem that there arose the theory of luminiferous ether. According to scientists, space must consist of an invisible, almost non-existent substance which is, by definition, entirely omnipresent. In fact, the theory of ether had been around since Aristotle (who considered it the “Fifth Element” or “Quintessence,” the first four being earth, fire, air and water), but after the equations of Maxwell, it found itself being put on a new scientific foundation.

Michelson, Morley and the Speed of Light

In 1887, two American experimenters named Albert Mickelson and Edward Morley came up with a very clever experiment to finally prove once and for all the existence of the ether. They did this by splitting a beam of light and sending it in two directions, one in the direction the Earth was moving relative to the ether, and one perpendicular, the theory being that the light moving against the ether would slow down slightly. The beams were then recombined in order to see if they were still aligned or if they had developed interference (which would mean that a change of speed had, indeed, occurred).

The result of the experiment turned up a couple of very interesting things. The first was that there was no ether (making it one of the most important “failed” experiments in history). Second, it was realized that, while it may sound absolutely crazy on the surface, the speed of light appeared to remain absolutely constant, no matter what reference frame it was being viewed from.

It is this last point which is crucial to understanding where Einstein was coming from when he developed his theory of Special Relativity in 1905 (and which will be explained in greater detail in the next part of this examination of Einstein’s theory, so if you don’t understand it, don’t worry).

Einstein would base his entire revolutionary idea on the fact that, as Michelson and Morley proved, the speed of light (in a vacuum) is constant. ~300,000 kilometers per second. Always.

For more information on physics, see Also:

Galilean Relativity

A History of Light

Relativity and Electromagnetism

Simultaneity and Relativity

Time and Space Dilation

The Addition of Velocities

References:

Robinson, A. (2006). The Last Man Who Knew Everything. New York: Pi Press.

The New York Public Library. (1995). Science Desk Reference. New York, NY: Macmillan.

Gribbin, J. (2002). The Scientists: A History of the Science Told Through the Lives of its Greatest Inventors. New York: Random House.

Einstein, A. (1920, May 5). Ether and the Theory of Relativity. Address at the University of Leyden . Leyden.