More than a century after Newton’s lifetime, new evidence for substantivalism—the view that space itself is a physical substance—came onto the scene. As you may recall from chapter 2, Maxwell’s equations allowed him to calculate the speed at which electromagnetic waves (light waves) should travel: approximately three hundred million meters per second. But speed is a relative quantity, right? As discussed earlier in this chapter, the speed of an object varies depending on the reference frame we use to describe its motion. In which reference frame, then, does light travel at the speed predicted by Maxwell’s equations? Light travels at approximately 300,000,000 meters per second, relative to … what?
Typically, the speed of a wave is measured relative to the medium (substance) through which the wave travels. The speed of sound is approximately 340 m/s relative to the air, for instance. Similarly, if you throw a rock into a pond, waves will ripple out from the splash in all directions, travelling at a certain speed relative to the water. Maxwell and other physicists of his day naturally assumed that there must be some medium through which light waves travel. The speed predicted by Maxwell’s equations, they thought, was the speed of light relative to that medium.
But what is that medium, exactly? It can’t be air, because light can travel through empty space, where there is no air. Perhaps “empty” space isn’t empty at all; perhaps it is filled with some unknown substance that we cannot see or feel. Or maybe—just maybe—space itself is the medium through which light travels. Maybe space itself is a physical substance, just as Newton thought! Either way, it seemed obvious to everyone that light must travel through some substance, just as all other waves do. Physicists called the unknown substance aether, borrowing this name from ancient Greek mythology. (In Greek mythology, Aether was a god who embodied heavenly air that other gods breathe.)
According to the aether theory, the speed derived from Maxwell’s equations was the speed at which light travels through aether, whatever that turns out to be—space itself, perhaps, or some unknown substance that permeates space. Light travels at approximately three hundred million meters per second, relative to aether. In other words, according to this theory, the reference frame of the aether is the only reference frame in which Maxwell’s equations hold precisely. If this theory is correct, then there is a uniquely privileged frame of reference after all!
Might there be a way to detect this mysterious aether, and thereby establish the truth of Newton’s substantivalism once and for all? American scientists Albert Michelson and Edward Morley intended to find out. Their famous experiment, performed in 1887, will be described on the next page.