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What is the particle theory?

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11/14/2009

Particle theory states that all matter is made of particles. It is an ancient concept, going as far back as Greek philosophers in 500BC who believed in atomism - the notion that all matter comprises tiny, indestructible particles. Skipping ahead to the 1800's, John Dalton postulated that each element of nature was made of unique, distinct, and basic particles, which he called atoms after the Greek term. The atom was considered to be an indestructible, elementary particle that made up all matter in the universe. The 1900's witnessed some revolutionary developments in our thinking and understanding of the particle theory. In 1913, Neils Bohr constructed the first scientific theory of the atomic structure, and nearly 100 years later, his Nobel winning theory still underpins all atomic model classroom instruction across the world. His theory states that all atoms comprise a positively charged nucleus with negatively charged electrons in a surrounding orbit. Ernest Rutherford confirmed Bohr's theory in 1919 by proving the existence of a proton through experimental results, and James Chadwick completed the Bohr model in 1931 when he discovered the neutron. By the early 1930's, the scientific community declared the proton, electron, and neutron as the most basic, elementary, and indivisible particles in the universe. Paul Dirac, a well known physicist of his time, was even quoted as triumphantly declaring that physics, as we know it, would soon be over. Dirac was right, but for all the wrong reasons. Before the decade closed, physicists were exploring the decay of protons and neutrons into other particles, thus slamming the door shut on Dirac's conceited view of physicists' mastery. In 1964 Murray Gell-Mann and George Zweig postulated the concept of quarks, more as a mathematical convenience than as a physical concept. Five years later James Bjorken and Richard Feynman proved the existence of quarks through experimental results, and in doing so they solidified the basis for the current atomic theory, known as the Standard Model. The Standard Model divides all elementary particles into two general categories: Fermions and Bosons. Fermions make up all particles of the universe, such as protons and neutrons, and bosons are exchanged between particles to transfer energy, such as the electromagnetic force. The simple Standard Model particles include: * Fermions * ** Quarks (6 in all: up, down, charm, strange, top, bottom) ** Leptons (6 in all: electron, muon, tauon, and 3 neutrinos) * Bosons * ** Gauge Bosons (4 in all) ** *** Gluon (Nuclear force) *** Photon (Electromagnetic force) *** W and Z Bosons ( Weak force) The Standard Model adequately explains three of the four known forces in the universe: weak force, strong force (Nuclear force), and electromagnetic force. Gravity has eluded the Standard Model since the Model's inception. Physicists are seeking the Grand Unification Theory - the Holy Grail of particle theories - that would unify the theory of gravity with the Standard Model into a single, master particle theory.