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Ivancevic_Applied-Diff-Geom

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298 <strong>Applied</strong> <strong>Diff</strong>erential <strong>Geom</strong>etry: A Modern Introductionphysical excitations of a field on space–time. They are excitations of space–time itself.Loop quantum gravity was briefly described by [Rovelli (1997)] as follows:Definition of theory. The mathematical structure of the theory has beenput on a very solid basis. Early difficulties have been overcome. Inparticular, there were three major problems in the theory: the lack ofa well defined scalar product, the overcompleteness of the loop basis,and the difficulty of treating the reality conditions.• The problem of the lack of a scalar product on the Hilbert spacehas been solved with the definition of a diffeomorphism invariantmeasure on a space of connections [Ashtekar and Lewandowski(1995)]. Later, it has also became clear that the same scalar productcan be defined in a purely algebraic manner [DePietri andRovelli (1996)]. The state space of the theory is therefore a genuineHilbert space H.• The overcompleteness of the loop basis has been solved by the introductionof the spin network states [Rovelli and Smolin (1995)].A spin network is a graph carrying labels (related to SU(2) representationsand called ‘colors’) on its links and its nodes.Each spin network defines a spin network state, and the spin networkstates form a (genuine, non-overcomplete) orthonormal basisin H.• The difficulties with the reality conditions have been circumventedby the use of the real formulation [Barbero (1994);Barbero (1995a); Barbero (1995b); Thiemann (1996)].The kinematics of loop quantum gravity is now defined with alevel of rigor characteristic of mathematical physics [Ashtekar andIsham (1992); Ashtekar et. al. (1995)] and the theory can bedefined using various alternative techniques [DePietri and Rovelli(1996); DePietri (1997)].Hamiltonian constraint. A rigorous definition version of the Hamiltonianconstraint equation has been constructed. This is anomaly free,in the sense that the constraints algebra closes (but see later on). TheHamiltonian has the crucial properties of acting on nodes only, whichimplies that its action is naturally discrete and combinatorial [Rovelliand Smolin (1988); Rovelli and Smolin (1994)]. This fact is at the roots

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