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Emergent / Quantum Gravity:  Macro / Micro Structures of Spacetime

 

Bei- Lok Hu  胡悲樂

 

Maryland Center of Fundamental Physics, University of Maryland, College Park, USA

email address:  blhu@umd.edu,  hubeilok@gmail.com

 

Abstract

 

It is generally agreed that the primary goal of quantum gravity is to find the  microscopic structure of spacetime.  However, for the last 50 years the cardinal principle upheld by most general relativists has been to find ways to quantize Einstein’s theory which has proven to be highly successful in describing the macroscopic structure of spacetime.  A tacit assumption in this existing paradigm is that doing so will yield the micro-structures of spacetime.  We challenge this supposition and present a different paradigm.     If general relativity is an effective theory valid only at the long wavelength and low energy limits, and the metric and connection forms are collective variables, then quantizing a classical theory such as general relativity valid in the macroscopic domain is unlikely to yield a theory of the microscopic structures of spacetime.  

To uncover the microscopic structures one needs to deal with issues of quantum to classical transition and find ways to unravel the underlying microscopic structures from observed macroscopic phenomena, not unlike deducing the molecular constituents from hydrodynamics or  universalities of microscopic models from critical phenomena. The macro to micro road poses a new and perhaps more serious challenge to the next generation of theorists, phenomenologists and experimentalists in quantum gravity.  We discuss issues at the quantum-classical and micro-macro interfaces such as quantum fluctuations and correlations, coarse-graining and backreaction, and use the well-formulated theory of stochastic gravity constructed from semiclassical gravity with contributions of fluctuations of the quantum field sources and the metric in a `bottom-up’ approach or a `grass-root’ road to quantum gravity.  The key ideas are expressed in these essays:

 

[1] B. L. Hu,  “Emergent / Quantum Gravity:  Micro / Macro Structures of Spacetime” J. Phys. Conf. Ser. 174 (2009) 012015 [arXiv:0903.0878]

 

[2] “CAN SPACETIME BE A CONDENSATE?”  Int. J. Theor. Phys. 44 (2005) 1785 [gr-qc/0503067]

 

[3] “STOCHASTIC GRAVITY: THEORY AND APPLICATIONS”,   B. L. Hu and E. Verdaguer, in  Living Reviews in Relativity 7 (2004) 3. lrr-2004-3 [update in arXiv:0802.0658]

 

[4] “A KINETIC THEORY APPROACH TO QUANTUM GRAVITY”  Invited talk given at the 6th   Peyresq Meeting,  France, June 2001.  Int. J. Theor. Phys. 41 (2002) 2111-2138 [gr-qc/0204069]

 

[5] “SEMICLASSICAL GRAVITY AND MESOSCOPIC PHYSICS" Invited talk at the International Symposium on Quantum Classical Correspondence,  Drexel University, Philadelphia, Sept. 1994. Proceedings eds D. H. Feng and B. L. Hu  (International Publishers, Boston, 1997)   [gr-qc/9511077]

 

[6] “GENERAL RELATIVITY AS GEOMETRO-HYDRODYNAMICS"  Invited talk at the Second Sakharov International Conference, Lebedev Physical Institute, Moscow, May, 1996. [gr-qc/9607070].

 

[7] “COSMOLOGY AS `CONDENSED MATTER' PHYSICS" 

Invited talk given at the Third Asia-Pacific Physics Conference, Hong Kong, June 1988.  Proceedings edited by K. Young  (World Scientific Publishing Co., Singapore, 1989).  [gr-qc/9511076]