Basic Law of Physics

 It is simple and easy to state—there is no experiment that you can perform that allows you to detect your own uniform motion. Yet it is general—there is no experiment that can be done at any time, at any place, using anything, that can detect your uniform motion. And most important, it is completely subject to experimental test on an all-or-nothing basis. Just one verifiable experiment detecting one’s own uniform motion, and the principle of relativity is no longer a basic law. It may become a useful approximation, but not a basic law.

Once a fundamental law like the principle of relativity is discovered or accepted, it has a profound effect on the way we think about things. In this case, if there is no way that we can detect our own uniform motion, then we might as well ignore our motion and always assume that we are at rest. Nature is usually easier to explain if we take the point of view that we are at rest and that other people and things are moving by. It is the principle of relativity that allows us to take this self-centered point of view.
It is a shock, a lot of excitement is generated, when what was accepted as a basic law of physics is discovered not to be exactly true. The discovery usually occurs in some obscure corner of science where no one thought to look before. And it will probably have little effect on most practical applications. But the failure of a basic law changes the way we think.
Suppose, for example, that it was discovered that the principle of relativity did not apply to the decay of an esoteric elementary particle created only in the gigantic particle accelerating machines physicists have recently built. This violation of the principle of relativity would have no practical effect on our daily lives, but it would have a profound psychological effect. We would then know that our uniform motion could be detected, and therefore on a fundamental basis we could no longer take the point of view that we are at rest and others are moving. There would be legitimate debates as to who was moving and who was at rest. We would search for a formulation of the laws of physics that made it intuitively clear who was moving and who was at rest.
This is almost what happened in 1860. In that year, James Clerk Maxwell summarized the laws of electricity and magnetism in four short equations. He then solved these equations to predict the existence of a wave of electric and magnetic force that should travel at a speed of approximately 3 × 10⁸ meters per second. The predicted speed, which we will call c, could be determined from simple measurements of the behavior of an electric circuit.
Before Maxwell, no one had considered the possibility that electric and magnetic forces could combine in a wavelike structure that could travel through space. The first question Maxwell had to answer was what this wave was. Did it really exist? Or was it some spurious solution of his equations?

The clue was that the speed c of this wave was so fast that only light had a comparable speed. And more remarkably the known speed of light, and the speed c of his wave were very close—to within experimental error they were equal. As a consequence Maxwell proposed that he had discovered the theory of light, and that this wave of electric and magnetic force was light itself.
Maxwell’s theory explained properties of light such as polarization, and made predictions like the existence of radio waves. Many predictions were soon verified, and within a few years there was little doubt that Maxwell had discovered the theory of light.
One problem with Maxwell’s theory is that by measurements
of the speed of light, it appears that one should be able to detect one’s own uniform motion. In the next section we shall see why. This had two immediate consequences. One was a change in the view of nature to make it easy to see who was moving and who was not. The second was a series of experiments to see if the earth were moving or not.
In the resulting view of nature, all of space was filled with an invisible substance called ether. Light was pictured as a wave in the ether medium just as ocean waves are waves in the medium of water. The experiments, initiated by Michaelson and Morley, were designed to detect the motion of the earth by measuring how fast the earth was moving through the ether medium.
The problem with the ether theory was that all experiments designed to detect ether, or to detect motion through it, seemed to fail. The more clever the experiment, the more subtle the apparent reason for the failure. We will not engage in any further discussion of the ether theory, because ether still has never been detected. But we will take a serious look in the next section at how the measurement of the speed of a pulse of light should allow us to detect our own uniform motion. And then in the rest of the chapter we will discuss how a young physicist, working in a patent office in 1905, handled the problem.