Date: 2011-5-10,3:00pm

Venue:Conference Hall 322，Science Building

Title: Emergent Gravity,Spacetime Condensate and Macroscopic Quantum Phenomena

Speaker: Bei- Lok Hu, Professor of Physics

Maryland Center for Fundamental Physics and Joint Quantum Institute, University of Maryland

Abstract:In the first lecture we described the new view towards 1) quantum gravity [1] which is defined as the theories for the microscopic structures of spacetime and matter, and 2) classical gravity described by general relativity, a theory valid only at the long wavelength, low energy limits of quantum gravity. For 1) we make the distinction between quantum gravity and theories obtained from quantizing general relativity. Combing with 2), that general relativity is not a fundamental, but an effective theory, quantization of the metric and connection forms which are viewed as collective variables will lead to phonon dynamics, in analogy to the atomic theory for the structure of matter, but not quantum electrodynamics of electrons and photons. Thus general relativity is hydrodynamics [2-3] and gravity is emergent [4]. In this new paradigm the primary task of 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], 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”). In this second lecture we shall a) discuss the key issues of emergence, b) present the view of spacetime as a condensate [6], what it says about dark energy and the `atoms of spacetime’; and from it c) explore the theoretical underpinnings of macroscopic quantum phenomena.

[1] 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] "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] “EMERGENT / QUANTUM GRAVITY: Macro/Micro Structures of Spacetime”,

DICE08 Proceedings in J. Phys. Conf. Ser. 174 (2009) 012015 [arXiv:0903.0878]

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

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

[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]

[8] C. H. Chou, B. L. Hu and Y. Subasi, “Macroscopic quantum phenomena from the large N perspective” 2011.