Scientific Colloquium
November 19, 2010
BEI-LOK HU
UNIVERSITY OF MARYLAND
"New View on Quantum
Gravity and the Origin of the Universe "
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 active searches have been conducted in the past 60 years
for a new theory for the microscopic structure of spacetime – Quantum
Gravity [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 general relativity is an effective theory
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 hydrodynamics [2-3] and gravity is
emergent [4]. 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 [5-6] , 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”). Finally we explore from this new
perspective the oldest puzzle of human inquiries -- on 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] B. L. Hu, “CAN SPACETIME BE A CONDENSATE?” Int.
J. Theor. Phys. 44 (2005) 1785 [gr-qc/0503067]
[6] “A KINETIC THEORY APPROACH TO QUANTUM GRAVITY” Int. J. Theor. Phys.
41 (2002) 2111 [gr-qc/0204069]
[7] “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]
Bei- Lok Hu, Professor of
Physics
Maryland Center for Fundamental Physics and Joint Quantum Institute,
University of Maryland, College Park, Maryland 20742 USA