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 electricity and magnetism are interconnected when they change with time. This did not mean that electricity and magnetism are the same thing. They are certainly different, but they are two aspects of a unified whole, “electromagnetism”. In general, it makes no sense to talk about one without the other.

This pattern of unification is fairly typical. Every time such a unification is achieved, the number of “laws of nature” is reduced, so that nature looks not only more unified but also, in some sense, simpler. More and more apparently diverse phenomena are explained by fewer and fewer underlying principles. This is the message I have tried to get across.

This book has a second theme. Quite often, different branches of physics have seemed to contradict each other when taken together. The contradiction is then resolved in a new, consistent, wider theory, which includes the two branches. For example, Newton's theory of motion and of gravitation conflicted with electromagnetism, as it was understood in the nineteenth century. The resolution lay in Einstein's theories of relativity. There are several other instances of progress by resolution of contradictions in this book.