Approaches to Quantum Gravity

The causal set approach to Quantum Gravity 411to reconcile the two, 7 it has been argued here that, at present, the causal set isthe only proposal that does so. The causal set program is an active and growingone. Many projects in progress have not been mentioned above. Attempts toidentify the “atoms” that carry the black hole entropy have been made [77; 78],and this work is currently being extended to higher dimensions by Fay Dowkerand Sara Marr. Further work on the question of “observables” has also beencarried out [79]. Pros and cons of an amplitude-based dynamics are also beinginvestigated. As well as this work, the basic causal set idea continues to inspireother approaches [80; 81]. Every statement of a result given here raises manymore questions, only some of which are being pursued. This multiplicity of unansweredquestions, the relatively small set of prerequisites needed to contribute tothem, and the comparative, “strategic” perspective on Quantum Gravity that theapproach offers, make causal sets an attractive field for both new and experiencedresearchers.References[1] L. Bombelli, J.-H. Lee, D. Meyer and R. Sorkin, Phys. Rev. Lett. 59 (1987) 521.[2] R. D. Sorkin, First steps with causal sets, in Proceedings of the Ninth ItalianConference on General Relativity and Gravitational Physics, Capri, Italy, September1990, edited by R. Cianci et al. (Singapore, World Scientific, 1991), pp. 68–90.[3] R. D. Sorkin, Space-time and causal sets, in Relativity and Gravitation: Classicaland Quantum, Proceedings of the SILARG VII Conference, Cocoyoc, Mexico,December 1990, edited by J. C. D’Olivo et al. (Singapore, World Scientific, 1991),pp. 150–173.[4] R. D. Sorkin, The causal set approach to quantum gravity, Talk given at the Santa FeWorkshop, New Directions in Simplicial Quantum Gravity, 28th July–8th August1997, http://t8web.lanl.gov/people/emil/Slides/sf97talks.html.[5] R. D. Sorkin (2003), gr-qc/0309009.[6] R. D. Sorkin (1989), gr-qc/9511063.[7] F. Dowker (2005), gr-qc/0508109.[8] http://physics.syr.edu/sorkin/.[9] S. W. Hawking, The path integral approach to quantum gravity, in GeneralRelativity: An Einstein Centenary Survey (Cambridge, Cambridge University Press,1979), pp. 746–789.[10] R. D. Sorkin, Int. J. Theor. Phys. 36 (1997) 2759, gr-qc/9706002. 9706002;[11] M. Y. Kalmykov, Class. Quant. Grav. 12 (1995) 1401, hep-th/9502152. 9502152;[12] C. Correia da Silva and R. M. Williams, Class. Quant. Grav. 16 (1999) 2197,gr-qc/9903003.[13] L. Bombelli, R. K. Koul, J.-H. Lee and R. D. Sorkin, Phys. Rev. D34 (1986) 373.[14] R. D. Sorkin (1997), gr-qc/9705006.[15] J. D. Bekenstein (1994), gr-qc/9409015.[16] T. Jacobson and R. Parentani, Found. Phys. 33 (2003) 323, gr-qc/0302099. GR-QC0302099;7 See for example [76].