The Physical World
The whole physical world is made of fundamental particles. Most are transient, and are seen only in violent particle-collisions. Some may exist as 'exchange particles' holding others together. This chapter proposes to ignore dark matter, and regard the whole universe as being made up of atoms. They consist of three kinds of stable and enduring particle: the proton, the neutron and the electron. There are a few hundred stable non-radioactive atomic nuclei. Astronomy depends on detecting radiation from the sky very sensitively. It has done wonders with visible light, and radio astronomy is advancing rapidly. Both matter and radiation exist in physical space. Space is strange stuff. It exists to hold material— that is why the notion was invented. James Maxwell showed that electric and magnetic forces should generate 'electromagnetic radiation', and calculated that it travelled at the speed of light. Both space and time seem to be infinite, in the sense that one cannot easily imagine an 'edge' to either.