Jethalal S. Zaveri
Prof. Muni Mahendra Kumar
The picture of atom, which emerged from the study of its nuclei, showed that the matter was concentrated in microscopic drops, separated by huge distances. In the vast ocean of space between the fast moving nuclear drops, moved the electrons, which gave matter its solid aspect. Historically, during the course of our descent into the subatomic world a landmark was reached in 1930s when physicists thought that they had finally discovered the "basic building blocks" of matter consisted of atoms and that all atoms consisted of "elementary particles", viz. protons, neutrons and electrons. These were accepted as the ultimate indestructible units of matter. With further refinement of techniques and development of new devices for particle-detection, their number increased from 3 to 6, and then to 18 by 1955. By 1970s, over two hundred elementary particles were known to exist. How can such a large number be called elementary? In fact, a widespread belief among physicists that none of them deserves this title, prevailed.
Theoretical developments, which paralleled the discovery of new particles, reinforced the belief. It became increasingly clear that nuclear phenomena demanded the incorporation of relativity theory with the quantum theory, mainly because of the high speed of the subatomic particles, coming close to the speed of light. Relativity established unification of mass and energy, which was summed up in the famous equation E-mc2. This unification of mass and energy by the relativity theory forced the physicists to radically modify the concept of a particle. In classical physics, where matter and energy were separate entities, mass was associated with some "stuff" of which all material objects were made. Relativity showed that mass was a form of energy. Now energy being a dynamic entity, the particle can no longer be conceived as a static object but as a dynamic pattern. In 1930, Paul Dirac, an English physicist, who imposed the requirement of relativity on quantum physics, and formulated relativistic equations, describing the behaviour of electrons, put this new view of particles forth. His theory revealed a fundamental symmetry between matter and anti-matter. It predicted the existence of an 'anti-electron' with the same mass as the electron, hut with a positive charge. Two years later, in 1932, Carl Anderson actually discovered this new particle and called it 'positron'. Physicists later discovered that every particle has a counterpart, which is exactly like it but opposite in several major aspects. These counterparts were called antiparticles (equal mass but opposite charge). An anti-particle, despite, its name is a particle. Some particles have other particles as anti-particles. A few particles are their own anti- particles (like the photon).
Whenever a particle meets its anti-particle, they annihilate each other. When an electron meets a positron, for example, both of them disappear and in their place are created two photons which instantly depart from the scene at the speed of light If there is sufficient energy, a pair of particle and anti-particle can be created conversely, they can be made to turn into pure energy by mutual annihilation. These processes of particle-creation and annihilation, predicted from Dirac's theory have since been observed millions of times by experiments.
