**Speaker:** Professor Bei-Lok Hu (University of Maryland)

**Date:** 11th March 2011

We begin by giving a simple description of our universe and trace back quickly in history to the very early universe where spacetime dynamics is described by Einstein’s general relativity and matter content is described by quantum field theory. This is the threshold to the unknown yet most challenging regime where a new ** theory for the microscopic structure of spacetime and matter – Quantum Gravity **is called for [1]. For more than half a century in the general relativity community it is believed that quantizing general relativity will produce such a theory. This view and this practice now face severe challenge because the basic assumption that quantizing a set of macroscopic variable shall lead to a microscopic theory is never justified for gravity. Now an increasing fraction of practitioners believe that

**valid only at the long wavelength, low energy limits, and the metric and connection forms are collective variables valid only in such limits. Quantizing them will thus lead to phonon dynamics of crystal lattices, in the analogy of the atomic theory for the structure of matter, not quantum electrodynamics of electrons and photons. In this new paradigm**

*general relativity is an effective theory***[2-3] and**

*general relativity is hydrodynamics***[4]. The primary task of**

*gravity is emergent***quantum gravity**is to find ways to unravel the underlying microscopic structures from the observed macroscopic structure, what is often called the “bottom-up” (from low energy up) approach, not unlike deducing the molecular constituents from hydrodynamics and kinetic theory [5-6] , or universalities of microscopic theories from critical phenomena. In contrast, the primary task of

**emergent gravity**is to explore the characteristic features of emergence, find the mechanisms and identify the processes whereby the physical phenomena in today’s macroscopic universe can be explained from candidate theories of the microscopic structure of spacetime (“top-down”). From this new perspective we inquire into the oldest puzzle humanity confronted — the Origin of our Universe [7].

[1] *For an introduction to different schools of thoughts on quantum gravity, read, e.g., **Approaches to Quantum Gravity**, ed. D. Oriti, (Cambridge University Press 2008) . Some key ideas presented in this talk are discussed in these reviews and essays:*

[2] B. L. Hu, “GENERAL RELATIVITY AS GEOMETRO-HYDRODYNAMICS” Invited talk at the Second Sakharov International Conference Lebedev Physical Institute, May, 1996. [gr-qc/9607070].

[3] B. L. Hu, “COSMOLOGY AS `CONDENSED MATTER’ PHYSICS” Invited talk given at the Third Asia-Pacific Physics Conference, Hong Kong, June 1988. Proceedings edited by K. Young [gr-qc/9511076]

[4] B. L. Hu, “EMERGENT / QUANTUM GRAVITY: Macro/Micro Structures of Spacetime”, DICE08 Proceedings in J. Phys. Conf. Ser. 174 (2009) 012015 [arXiv:0903.0878]

[5] “CAN SPACETIME BE A CONDENSATE?” Int. J. Theor. Phys. [gr-qc/0503067]

[6] B. L. Hu, “A KINETIC THEORY APPROACH TO QUANTUM GRAVITY” Int. J. Theor. Phys. 41 (2002) 2111 [gr-qc/0204069]

[7] B. L. Hu, “NEW VIEW ON QUANTUM GRAVITY AND THE ORIGIN OF THE UNIVERSE”, In *Where Do We Come From? — on the Origin of the Universe *(Book in Chinese) (Commercial Press, Hong Kong 2007) — A collection of essays based on public talks given by Stephen Hawking, Bei-Lok Hu, Robert Laughlin, Henry Tye, and others in Hong Kong, May-June 2006 [gr-qc/0611058]