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.

 

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