I have tried to write a non-technical tour through the principles of physics. The theme running through this tour is that progress has often consisted in uncovering “hidden unities”. Let me explain what I mean by this phrase, taking the example (from Chapter 3) of electricity and magnetism. The unity here is hidden, because at first sight there seemed to be no connection between the two. The invention of the electric battery at the beginning of the nineteenth century ushered in a new period of research that showed that elec- tricity and magnetism are interconnected when they change with time. This did notmean that electricity andmagnetismare the same thing. They.
