Physics is the study of the behavior of nature, exclusive of animate objects. It is the ultimate basis for other natural sciences, notably chemistry, which in turn forms the basis for biology. Physics is expressed in the language of mathematics, though physics is a discipline distinct from mathematics. In essence physicists seek to understand nature by formulating mathematical models that accurately predict future natural phenomena given knowledge of the present state.

Classical physics is that branch of physics based on the mechanics originally formulated by Isaac Newton, including Newtonian gravity and electromagnetism as formulated by James Clerk Maxwell and later codified by Oliver Heaviside. It includes classical mechanics in all formulations (Newtonian, Lagrangian and Hamiltonian),Maxwell’s wave formulation of electrodynamics and models based on these fundamental theories such as optics, fluid dynamics and the theory of elasticity.

http://en.wikipedia.org/wiki/Classical_mechanicshttp://math.ucr.edu/home/baez/classical/Relativity, both the special and general theories, developed by Albert Einstein, extends the classical mechanics of Newton to include behavior of bodies in relative motion at speeds approaching that of light and large gravitational phenomena. They are deterministic theories and are therefore sometimes classified as “classical”. Fundamentally relativity provides a revolutionary insight into the nature of time and space and departs from the Newtonian picture of everyday experience. One no longer has either “time” or “space” but rather an amalgamation of the two, spacetime, with a geometry that departs from the Euclidean model. The underlying mathematics is differential geometry, which itself is a subject of advanced mathematics. General relativity provides the best current model f gravitation. A large body of experimental evidence supports the accuracy of relativity.

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Quantum mechanicsarose in the early twentieth century as a result of contradictions that arose from attempts to apply classical mechanics to questions of the radiation of heat. The theory of the atom arose from this work, and also a profound change in our understanding of nature. Whereas classical mechanics and relativity produce definite predictions of natural phenomena, quantum mechanics predicts only probabilities. Subsequent theoretical and experimental work has confirmed that bat the most fundamental level, at atomic and sub-atomic scales nature is indeed stochastic. Quantum mechanics forms the basis for modern solid state physics which has provided us with the transistor and now the integrated circuit which makes modern computers possible.

http://www2.slac.stanford.edu/vvc/theory/quantum.htmlThe incorporation of special relativity into quantum theory resulted in quantum field theories. The first quantum field theory, developed independently by Feynman, Schwinger and Tomogawa was quantum electrodynamics (QED) which remains the basis of understanding of light and the electron. The later electroweak theory of Weinberg and Salaam unified QED with the theory of the weak interaction. Quantum chromodynamics (QCD), developed by Gell-Mann and Zweig explains the structure of protons and neutrons, and some other particles in terms of quarks and the forces that bind quarks. The electroweak theory and QCD comprise the Standard Model of particle physics. Notably excluded from the Standard Model is gravitation. No one has yet successfully formulated a theory that combines quantum field theories with general relativity.

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There are two further steps in the unification of fundamental theories. A unification of the electroweak theory and QCD would be called a grand unification theory (GUT). Several have been proposed, but none has been supported by experiment. A unification of general relativity with the electroweak theory and QCD would be called a “theory of everything” (TOE). String theory, cum M Theory, and loop quantum gravity are attempts at such a theory. Neither has yet been rigorously formulated from a mathematical perspective nor made definitive experimentally testable predictions.

http://en.wikipedia.org/wiki/String_theoryhttp://en.wikipedia.org/wiki/Loop_quantum_gravityhttp://en.wikipedia.org/wiki/Quantum_gravity