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Ivancevic_Applied-Diff-Geom

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Chapter 6<strong>Geom</strong>etrical Path Integrals and TheirApplicationsThe machinery of applied differential geometry, as presented so far, is: (i)rigorous, (ii) elegant, and (iii) powerful – as a tool for understanding, predictionand control of complex nonlinear systems. However, due to itssmooth nature, it is limited to modelling of deterministic and continuous–time dynamical systems only. Naturally, the question arises: is it possibleto extend this smooth machinery, so to be able to effectively deal alsowith probabilistic and discrete–time dynamical systems, like e.g., Markovchains? And the answer is: Yes. Namely, in the very core of the XX Centurygeometrodynamics, there is a powerful conceptual and computationaltool that is ‘by default’ used as a starting point for virtually every new physicaltheory – the celebrated Feynman path integral. In the path–integralformalism, we first formulate the specific classical action of a new theory,and subsequently perform its quantization by means of the associatedamplitude. This action–amplitude picture is the core structure in any newphysical theory. Unlike mathematical manifolds, bundles and jets, the pathintegral is an invention of the physical mind of Richard (Dick) Feynman.Its virtual paths are in general neither deterministic not smooth, althoughthey include bundles and jets of deterministic and smooth paths, as well asMarkov chains. Yet, it is essentially an applied differential geometry, withits Riemannian and symplectic versions, among many others. At the beginning,it worked only for conservative physical systems. Today it includesalso dissipative structures, as well as various sources and sinks. Its smoothpart reveals all celebrated equations of the 20th Century, both classicaland quantum. It is the core of modern quantum gravity and string theory.It is arguably the most important construct of mathematical physics. Atthe edge of a new millennium, if you asked a typical theoretical physicist:what will be your main research tool in the new millennium, he/she would983

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