Why Does E=mc2_ - Brian Cox [13]
The attentive reader might notice a puzzle here, or at least some sloppy writing on our part. Given what we said in Chapter 1, it clearly makes no sense to quote a speed without specifying relative to what that speed is defined, and Maxwell’s equations make no mention of this problem. The speed of the waves—that is, the speed of light—appears as a constant of nature, the relationship between the relative strengths of the electric and magnetic fields. Nowhere in this elegant mathematical structure is there a place for the speed of the source of the waves, or indeed the receiver. Maxwell and his contemporaries knew this, of course, but it didn’t worry them unduly. This is because most, if not all, of the scientists of the time believed that all waves, including light, must travel in some kind of medium; there must be some “real stuff ” that is doing the waving. They were practical folk in Faraday’s mold, and to them things don’t just wave on their own with no support. Water waves can exist only in the presence of water, and sound waves travel only in the presence of air or some other substance, but certainly not in a vacuum: “In space, no one can hear you scream.”
So the prevailing view at the end of the nineteenth century was that light must travel through a medium, and this medium was known as the ether. The speed that appeared in Maxwell’s equations then had a very natural interpretation as the speed of light relative to the ether. This is exactly analogous to the propagation of sound waves through air. If the air is at a fixed temperature and pressure, then sound will always travel at a constant speed, which depends only on the details of the interactions between the air molecules, and has nothing to do with the motion of the source of the waves.
The ether must be a strange kind of stuff, though. It must permeate all of space, since light travels across the voids between the sun and earth and the distant stars and galaxies. When you walk down the street, you must be moving through the ether, and the earth must be passing through the ether on its yearly journey around the sun. Everything that moves in the universe must make its way through the ether, which must offer little or no resistance to the motion of solid objects, including things as large as planets. For if the ether did offer resistance to the motion of solid objects, the earth would have been slowed down during each of its 5 billion solar orbits, just as a ball bearing slows down when dropped into a jar of molasses, and our Earth years would gradually change in length. The only reasonable assumption must be that the earth and all objects move through the ether unimpeded. You may think that this would make its discovery impossible, but the Victorian experimentalists were nothing if not ingenious, and in a series of wonderfully high-precision experiments beginning in 1881, Albert Michelson and Edward Morley set out to detect the apparently undetectable. The experiments were beautifully simple in conception. In Bertrand Russell’s excellent book on relativity written in 1925, he likens the earth’s motion through the ether to going for a circular walk on a windy day; at some point you will be walking against the wind, and at some point with it. In a similar fashion, since the earth is moving through the ether as it orbits the sun, and the earth and sun together are flying through the ether in their journey around the Milky Way, then at some point in the year the earth must be moving against the ether wind, and at other times with it. And even in the unlikely event that the solar system as a whole is at rest relative to the ether, the earth’s motion will still generate an ether wind as it travels around the sun, just as you