theory associated to area and volume have a discrete spectrum. Work on the semi-classical limit, the continuum limit, and dynamics was intense after this, but progress was slower. On the semi-classical limit front, the goal is to obtain and study analogues of the harmonic oscillator coherent states (candidates are known as weave states). Hamiltonian dynamics LQG was initially formulated as a quantization of the Hamiltonian ADM formalism, according to which the Einstein equations are a collection of constraints (Gauss, Diffeomorphism and Hamiltonian). The kinematics are encoded in the Gauss and Diffeomorphism constraints, whose solution is the space spanned by the spin network basis. The problem is to define the Hamiltonian constraint as a self-adjoint operator on the kinematical state space. The most promising work in this direction is Thomas Thiemann's Phoenix Project. Covariant dynamics Much of the recent work in LQG has been done in the covariant formulation of the theory, called "spin foam theory." The present version of the covariant dynamics is due to the convergent work of different groups, but it is commonly named after a paper by Jonathan Engle, Roberto Pereira and Carlo Rovelli in 2007–08. Heuristically, it would be expected that evolution between spin network states might be described by discrete combinatorial operations on the spin networks, which would then trace a two-dimensional skeleton of spacetime. This approach is related to state-sum models of statistical mechanics and topological quantum field theory such as the Turaeev–Viro model of 3D quantum gravity, and also to the Regge calculus approach to calculate the Feynman path integral of general relativity by discretizing spacetime. See also History of string theory References Further reading Topical reviews Carlo Rovelli, "Loop Quantum Gravity," Living Reviews in Relativity 1, (1998), 1, online article, 2001 version. Thomas Thiemann, "Lectures on Loop Quantum Gravity," e-print available as gr-qc/0210094 Abhay Ashtekar and Jerzy Lewandowski, "Background Independent Quantum Gravity: A Status Report," e-print available as gr-qc/0404018 Carlo Rovelli and Marcus Gaul, "Loop Quantum Gravity and the Meaning of Diffeomorphism Invariance," e-print available as gr-qc/9910079. Lee Smolin, "The Case for Background Independence," e-print available as hep-th/0507235. Popular books Julian Barbour, The End of Time: The Next Revolution in Our Understanding of the Universe (1999). Lee Smolin, Three Roads to Quantum Gravity (2001). Carlo Rovelli, Che cos'è il tempo? Che cos'è lo spazio?, Di Renzo Editore, Roma, 2004. French translation: Qu'est ce que le temps? Qu'est ce que l'espace?, Bernard Gilson