Our journey to the very small has now brought us beyond the atoms, which are bulky and fragile objects compared with what we shall be occupied with next: the atomic nucleus and whatever is inside. The electrons, now seen ‘at a great distance’ circling around the nucleus, are themselves also small and extremely robust. I now invite you to have a look inside the nucleus, through the eyes of the scientists before 1970. I consider the years around 1970 as a crucial period, but I am now also choosing the year 1970 bcause this was the time I became acquainted with elementary particle physics myself, as a young graduate student at the State University of Utrecht in the Netherlands.

All the physics that I mentioned earlier (and of course a great deal more) was basic stuff for students of theoretical physics. Also, a lot was known about the structure of the atomic nucleus. The nucleus is built from two species of building blocks: protons and neutrons. The proton (Greek np{πρῶτoς = first) owes its name to the fact that the simplest atomic nucleus, that of hydrogen, consists of just one proton. It carries one positive unit of charge. The neutron resembles the proton as if it were its twin brother: its mass is practically the same, its spin is the same, but the electric charge is absent in a neutron; it is neutral.