Emergent/quantum gravity: macro/micro structures of spacetime
Abstract
Emergent gravity views spacetime as an entity emergent from a more complete theory of interacting fundamental constituents valid at much finer resolution or higher energies, usually assumed to be above the Planck energy. In this view general relativity is an effective theory valid only at long wavelengths and low energies. We describe the tasks of emergent gravity from any ('topdown') candidate theory for the microscopic structure of spacetime (quantum gravity), namely, identifying the conditions and processes or mechanisms whereby the familiar macroscopic spacetime described by general relativity and matter content described by quantum field theory both emerge with high probability and reasonable robustness. We point out that this task may not be so easy as commonly conjured (as implied in the 'theory of everything') because there are emergent phenomena which cannot simply be deduced from a given microtheory. Going in the opposite direction ('bottomup') is the task of quantum gravity, i.e., finding a theory for the microscopic structure of spacetime, which, in this new view, cannot come from quantizing the metric or connection forms because they are the collective variables which are meaningful only for the macroscopic theory (valid below the Planck energy). This task looks very difficult or almost impossible because it entails reconstructing lost information. We point out that the situation may not be so hopeless if we ask the right questions and have the proper tools for what we want to look for. We suggest pathways to move 'up' (in energy) from the given macroscopic conditions of classical gravity and quantum field theory to the domain closer to the micromacro interface where spacetime emerged and places to look for clues or telltale signs at low energy where one could infer indirectly some salient features of the microstructure of spacetime.
 Publication:

Journal of Physics Conference Series
 Pub Date:
 June 2009
 DOI:
 10.1088/17426596/174/1/012015
 arXiv:
 arXiv:0903.0878
 Bibcode:
 2009JPhCS.174a2015H
 Keywords:

 General Relativity and Quantum Cosmology
 EPrint:
 15 pages. Essay for invited talks at the Symposium on "Foundations of Physics", University of Maryland, April 2427, 2008, Workshop on "Emergent Gravity", MIT, August 2529, 2008. In Proceedings of DICE 08 Conference, J. Phys. Conf. Ser. (2009)