Why Does E=mc2_ - Brian Cox [14]
Michelson and Morley set themselves the challenge of measuring the speed of light at different times of the year. They and everyone else firmly expected that the speed would change over the course of a year, albeit by a tiny amount, because the earth (and along with it their experiment) should be constantly changing its speed relative to the ether. Using a technique called interferometry, the experiments were exquisitely sensitive, and Michelson and Morley gradually refined the technique over six years before publishing their results in 1887. The result was unequivocally negative. No difference in the speed of light in any direction and at any time of year was observed.
If the ether hypothesis is correct, this result is very hard to explain. Imagine, for example, that you decide to dive into a fast-flowing river and swim downstream. If you swim at 5 kilometers per hour through the water, and the river is flowing at 3 kilometers per hour, then relative to the bank you will be swimming along at 8 kilometers per hour. If you turn and swim back upstream, then relative to the bank you will be swimming at 2 kilometers per hour. Michelson and Morley’s experiment is entirely analogous: You, the swimmer, are the beam of light, the river is the ether through which the light is supposed to travel, and the riverbank is Michelson and Morley’s experimental apparatus, sat at rest on the earth’s surface. Now we can see why the Michelson-Morley result was such a surprise. It was as if you always travel at 5 kilometers per hour relative to the riverbank, irrespective of the river’s speed of flow and the direction in which you decide to swim.
So Michelson and Morley failed to detect the presence of an ether flowing through their apparatus. Here is the next challenge to our intuition: Given what we have seen so far, the bold thing to do might be to jettison the notion of the ether because its effects cannot be observed, just as we jettisoned the notion of absolute space in Chapter 1. As an aside, from a philosophical perspective the ether was always a rather ugly concept, since it would define a benchmark in the universe against which absolute motion could be defined in conflict with Galileo’s principle of relativity. Historically, it seems likely that this was Einstein’s personal view, because he appears to have been only vaguely aware of Michelson and Morley’s experimental results when he took the bold step of abandoning the ether in formulating his special theory of relativity in 1905. It is certainly the case, however, that philosophical niceties are not a reliable guide to the workings of nature and, in the final analysis, the most valid reason to reject the ether is that the experimental results do not require it.1
While the rejection of the ether may be aesthetically pleasing and supported by the experimental data, if we choose to take this plunge then we are certainly left with a serious problem: Maxwell’s equations make a very precise prediction for the speed of light but contain no information at all about relative to what that speed should be measured. Let us for a moment be bold, accept the equations at face value, and see where the intellectual journey leads. If we arrive at nonsense, then we can always backtrack and try another hypothesis, feeling satisfied that we have done some science. Maxwell’s equations predict that light always moves with a velocity of 299,792,458 meters per second, and there is no place to insert the velocity of the source of the light or the velocity of the receiver. The equations really do seem to assert that the speed of light will always be measured to be the same, no matter how fast the source and the receiver of the light are moving relative to each other. It seems that Maxwell’s equations are telling us that the speed of light is a constant of nature. This really is a bizarre assertion, so let us spend a little more time exploring its meaning.
Imagine that light is shining