Relativity theory has had a profound influence on our picture of matter by forcing us to modify our concept of a particle in an essential way. (46) In classical physics, the mass of an object had always been associated with an indestructible material substance, with some "stuff" of which all things were thought to be made. Relativity theory showed that mass has nothing to do with any substance, but is a form energy. Energy, however, is a dynamic quantity associated with activity, or with processes. (47) The fact that the mass of a particle is equivalent to a certain of energy means that the particle can no longer be seen as a static _object, but has to be conceived as a dynamic pattern, a process involving the energy which manifest itself as the particle’s mass. (48) This new view of particles was initiated by Dirac when he formulated a relativistic equation describing the behavior of electrons. Dirac’s theory was not only extremely successful in accounting for the fine details of atomic structure, but also revealed a fundamental symmetry between matter and anti-matter. It predicted the existence of an anti-matter with the same mass as the electron but with an opposite charge. This positively charged particle, now called the positron, was indeed discovered two years after Dirac had predicted it. The symmetry between matter and anti-matter implies that for every particle there exists an antiparticles with equal mass and opposite charge. Pairs of particles and antiparticles can be created if enough energy is available and can be made to turn into pure energy in the reverse process of destruction. (49) These processes of particle creation and destruction had been predicted from Dirac’s theory before they were actually discovered in nature, and since then they have been observed millions of times. The creation of material particles from pure energy is certainly the most spectacular effect of relativity theory, and it can only be understood in terms of the view of particles outlined above. (50) Before relativistic particle physics, the constituents of matter had always been considered as being either elementary units which were indestructible and unchangeable, or as composite objects which could be broken up into their constituent parts. And the basic question was whether one could divide matter again and again, or whether one would finally arrive at some smallest indivisible units.