ABSTRACT
This chapter examines the notion of discrete symmetries and their implications for particle physics. Parity is the act of reflecting a system in a mirror. If the mirror-system has all the same physical properties as the original, then we say the system is invariant under parity. At a macroscopic level the asymmetry associated with the arrow of time is a consequence of initial conditions, which according to the laws of thermodynamics always become less ordered overall as time increases. At the microscopic level of collisions between the fundamental particles in our example of the explosion, the situation is quite different. If electromagnetic charge is reversed for all particles in a given system, then the sign of the electromagnetic field in the system must also be reversed. An ideal place to strengthen our understanding of discrete symmetries is with positronium.
