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Ivancevic_Applied-Diff-Geom

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714 <strong>Applied</strong> <strong>Diff</strong>erential <strong>Geom</strong>etry: A Modern Introductioncoefficient. An invariant tensor is also called an intertwining tensor.All invariant tensors are given by the standard Clebsch–Gordon theory.More precisely, for fixed s 1 . . . s n , the invariant tensors form a finite dimensionallinear space. Pick a basis v j is this space, and associate oneof these basis elements to the node. Notice that invariant tensors existonly if the tensor product of the representations s 1 . . . s n contains thetrivial representation. This yields a condition on the coloring of thelinks. For n = 3, this is given by the well known Clebsh–Gordan condition:each color is not larger than the sum of the other two, and thesum of the three colors is even.We indicate a colored graph by {Γ, ⃗s, ⃗v}, or simply S = {Γ, ⃗s, ⃗v}, anddenote it a ‘spin network’. (It was R. Penrose who first had the intuitionthat this mathematics could be relevant for describing the quantum propertiesof the geometry, and who gave the first version of spin network theory[Penrose (1971a); Penrose (1971b)].)Given a spin network S, we can construct a state Ψ S (A) as follows. Wetake the propagator of the connection along each link of the graph, in therepresentation associated to that link, and then, at each node, we contractthe matrices of the representation with the invariant tensor. We get a stateΨ S (A), which we also write asOne can then show the following.ψ S (A) = 〈A|S〉. (4.200)• The spin network states are normalizable. The normalization factor iscomputed in [DePietri and Rovelli (1996)].• They are SU(2) gauge invariant.• Each spin network state can be decomposed into a finite linear combinationof products of loop states.• The (normalized) spin network states form an orthonormal basis forthe gauge SU(2) invariant states in H (choosing the basis of invarianttensors appropriately).• The scalar product between two spin network states can be easily computedgraphically and algebraically.The spin network states provide a very convenient basis for the quantumtheory.The spin network states defined above are SU(2) gauge invariant. Thereexists also an extension of the spin network basis to the full Hilbert space.

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