Originally published 7 July 1987
Ask the man in the street what is the greatest scientific discovery of the twentieth century and he will as likely as not reply, “the theory of relativity.” Many scientists will agree. Relativity underlies our present understanding of atoms and stars. It is the basis for our ideas about the origin and evolution of the universe. It is the very warp of the fabric of physics, the threads on which all else is woven.
Most histories of relativity begin with an experiment that was performed exactly 100 years ago [in 1887] this summer, in the basement of the Case School of Applied Science in Cleveland, by the physicist Albert A. Michelson and the chemist Edward Morley. The Michelson-Morley experiment stands as one of the great milestones of physics.
Michelson was arguably the finest experimentalist of his time, and the remarkable instrument he devised with Morley — called an interferometer — was a model of precision and ingenuity. With that instrument, the two scientists hoped to demonstrate the motion of the Earth through space by measuring the effect of that motion on the observed speed of light.
To understand the experiment, consider this analogy. You are sitting in a motionless boat on a still pond. You drop a pebble into the water. Some time later you observe the circular ripple, or wave, that has moved out from the place where the pebble fell. Since you have not moved, you are at the center of the circular wave.
Like a boat in a pond
Now imagine that the boat is moving. Drop a pebble into the pond and sometime later observe the disturbance. As before, the wave is a circle, centered on the place where the pebble fell into the water. But now you are not at the center of the circle because, even as the wave moved, you moved away from the place where you dropped the stone.
Finally, imagine you are in a boat on a broad still lake out of sight of shore. Is the boat drifting? Here is one way to tell. Drop a pebble in the pond. If, later, you see yourself at the center of the circular wave, then you are at rest. If you are off-center, then you have drifted.
It was something like this that Michelson and Morley attempted to do. Instead of a wave in water, they used a light wave, which was presumed to move through a medium called the “luminiferous ether” which filled all of space. The “boat” was the Earth itself. Because the Earth moves in an orbit around the sun, it must be moving through the ether. If we emit a light beam in the direction the Earth is moving, we should follow along behind the wave and it won’t seem to get away so quickly. If we emit the light beam in the opposite direction, it should move away from us more quickly.
But to everyone’s surprise, including Michelson’s and Morley’s, the light beam moved away at the same speed in all directions, just as if the Earth were at rest in the “pond” of space. So were Copernicus and Galileo wrong? Is the Earth at rest in the universe? Unthinkable! Then what?
During the next eighteen years many people tried to explain away the strange result of the Michelson-Morley experiment. There was lots of tinkering and fudging with theories, to no one’s complete satisfaction. The negative result of the experiment was a standing embarrassment to physicists who thought that they had pretty much figured out all of the secrets of nature.
Jettisoning old notions
Then in 1905 young Albert Einstein published a paper called “On the Electrodynamics of Moving Bodies.” What Einstein did in this paper was so bold, so daring, that it bowled over his contemporaries. In effect, he said by fiat that all observers, regardless of their motion relative to each other, will measure the same speed for light. In my analogy, it is as if the observer in a moving boat and the observer in a non-moving boat both find themselves at the center of the same circular wave, even though they are not in the same place.
To make this apparently impossible thing possible, Einstein jettisoned the venerable notions of absolute space and absolute time. In his new physics, space and time become inextricably bound up with each other and with motion. A meter-long stick that is moving appears shorter. Moving clocks slow down. Even the notion of “simultaneous events” was tossed out the window; two events which are simultaneous for one observer will not be simultaneous for another observer.
These Alice-in-Wonderland marvels were just the beginning. Following his audacious intuition wherever it led him, Einstein unfolded a new theory of gravity and demonstrated the equivalence of matter and energy. And experiment soon proved him right. Physics was revolutionized from the ground up. Almost nothing of 19th century physics survived unchanged.
Historians continue to debate the extent to which Einstein was influenced by the work of Michelson and Morley. Apparently the influence was slight. But that does not lessened the significance of the famous experiment whose centenary we celebrate this year. What Michelson and Morley did in that Cleveland basement 100 years ago was the first experimental “proof” of a theory that was not yet born. When Einstein published his paper on relativity in 1905, the Michelson-Morley experiment made it that much easier for his contemporaries to say, “But of course!”
