What Einstein Did in 1905
12:00 AM, Oct 5, 1998 • By JONATHAN V. LAST
Papers three and four -- "On the Electrodynamics of Moving Bodies" and "Does the Inertia of a Body Depend on Its Energy Content?" -- dealt with Einstein's first truly revolutionary observations: that the speed of light is absolute and that physics centers around the precept of special relativity. His final paper, "On a Heuristic Point of View Concerning the Production and Transformation of Light," cemented yet another revolutionary idea about light: that it travels in particles, or quanta.
Like all studies of elemental phenomena, the study of light began with observations. Light is so basic that early thinkers had no inkling of what it was, so they took note of how it behaved. Euclid developed a Law of Reflection; Hero of Alexandria, observing Euclid's work and evidence of the rectilinear propagation of light, proposed that light always travels the shortest distance between two points. Aristotle gave brief thought to how light might work and proposed a primitive method of conduction via the medium of a mysterious "aether," and his thought was refined by Rene Descartes in his "Law of Refraction." Descartes and others proposed that "the energy of a light ray emitted from a point source continuously spreads out over an ever increasing volume" and exhibits "a rapid vibratory motion of the medium propagating at a very great speed" -- which is to say that light is a wave.
Waves are physical disturbances, and in order to travel, they must have some substance to disturb. This presents no problem for light on Earth, where the air is made up of a complex mix of gasses. But it does create difficulties in the vacuum of space. And to get around these difficulties, scientists postulated the existence of an invisible, luminiferous aether permeating the universe and providing a medium through which waves of light may travel. Newton was suspicious of the notion of the aether and proposed briefly in his treatise on optics that light might be not a wave but a string of particles that needed no medium. But while his corpuscular theory held some sway for a while, the hypothesis of the aether carried the day. Indeed, it was an enormously helpful scientific hypothesis. It could support the fantastically high frequencies of light, yet allow planets to move through it unimpeded. It even established a universal clock by creating a single inertial frame of reference.
The descent into the aether represented one of physics' most notorious wrong turns since Ptolemy posited that the sun revolves around the earth. From the beginning there was evidence to the contrary, but for three centuries scientists put themselves through all manner of contortions to explain the aether.
In 1725, the astronomer James Bradley tried to triangulate the distance to a star by observing its position at two different times during the year. Bradley was shocked to find that the star appeared to move in the direction of Earth's orbit. But instead of drawing the obvious conclusion that the star's light was behaving like particles, Bradley decided that his work was proof that the aether not only was ubiquitous and invisible, but also had the amazing ability to remain wholly undisturbed as the planets moved through it.
More than 150 years after Bradley's work, Albert Michelson and Edward Morley attempted to measure the earth's motion through this aether. They reasoned that if the earth moves in relation to the aether (which it must since the aether is perpetually at rest), then the earth's motion should influence the speed of the light perceived on the earth. They found it did not.
The Michelson-Morley experiment should have rocked the world, but it was instead for the most part willfully ignored. A few scientists did begin to question the existence of the aether, but most concluded that the experiment itself must have been flawed in some way.
And so it fell to Einstein to fix the broken concepts of not only how light behaved, but what light was. Using the work of Hendrik Lorentz and James Maxwell, Einstein took as established that the speed of light was an absolute. He then began to think about the nature of simultaneous events.
Whether or not two things happen at "the same time" depends on the proximity of the events to the observer. If the events and the observer are all in the same frame of reference, then they may happen simultaneously. But if they are not all in the same frame of reference, the possibility of two events' happening "at the same time" becomes very difficult to hold.