Forces of Nature

by Brian Cox

As we’ve already mentioned, there isn’t a ‘special’ inertial reference frame; all inertial reference frames are as good as each other. If you’re in an inertial reference frame, you are allowed to say that you are standing still, and there is absolutely no measurement you can make that will tell you otherwise. It is because we are approximately sitting in an inertial reference frame on the surface of the Earth that we don’t feel as if we are moving from moment to moment. Einstein elevated the requirement that all inertial frames are equivalent to a fundamental principle. This means that identical experiments carried out in different inertial frames will always lead to the same results. To put it another way, the laws of Nature do not change as we switch our point of view between inertial reference frames; if they did, we could tell the difference between the reference frames! I don’t want to give the game away early in the chapter, but this ultimate democracy between inertial frames turns out to be such a severe constraint on the laws of Nature that Newton’s laws and the laws of electricity and magnetism cannot both be right. This may not sound too serious, but we will see in Chapter Four that the laws of electricity and magnetism are one of the great pillars of physics alongside Newton’s laws. They describe so many things we take for granted in our everyday lives; the action of electrical generators and motors, the formation of a rainbow, the action of lenses, the optical ...