Organizing Institutions:
Heinz von Foerster Gesellschaft / Wien
ASC – American Society for Cybernetics
WISDOM – Wiener Institut für
  sozialwissenschaftliche Dokumentation und Methodik
Institut für Zeitgeschichte | Universität Wien
AINS – Austrian Institute for Nonlinear Studies

Bei-Lok Hu

Emergence, Gravity and Thermodynamics

University of Maryland, College Park

I present some latest thoughts on three inter-related subjects:

1) Emergence: After describing three different senses of emergence, I point out that effective
field theory (EFT) or renormalization group (RG) is a necessary but not sufficient set of conceptual
means for describing emergence. EFT or RG [1] may suggest how different physics manifest at
different scales, but we also need to identify the mechanisms or processes whereby different
levels of structures and the laws governing them, including the symmetry principles, emerge.
That depends on deeper interplay of collectivity, complexity, stochasticity and self-organization.

2) Emergent Gravity: There are at least two intimately related veins in viewing gravity as emergent:
a) `General Relativity as Hydrodynamics’ [2] in the sense that gravity is an effective theory valid
only at the long wavelength, low energy limit of some underlying theory (quantum gravity) for the microscopic structures of spacetime and matter. b) Gravity as Thermodynamics [3], where such a view is often shaped by considering the effects of an event horizon on the quantum fluctuations of a field, first shown by Bekenstein and known today as the holography principle.

3) Gravity and Thermodynamics: Since both gravity and thermodynamics are classical theories of
macroscopic structures, if a deep connection exists, we should be able to see their direct relation
at this level, without relying on arguments invoking the microscopic structure of matter
(quantum fluctuations). This is posed as a challenge [4], one which physicists in the 19c
in principle may be able to resolve. If we can meet this challenge we may see the simpler and
deeper connection between gravity and thermodynamics without invoking quantum mechanics. If we fail we will perhaps see more clearly the essential role of quantum physics in explaining gravity and the necessary implication that a) either the macroscopic world is fundamentally quantum [5] or
b) quantum mechanics is also emergent from a deeper structure or as a form of organizational rules like statistical mechanics.

[1] For a recent meeting on this topic, see, e.g.,
http://www.perimeterinstitute.ca/Events/Emergence_and_Effective_Field_Theories/ Schedule/
[2] This view can be traced to Sakharov (1968). For references to writings of the major proponents see, e.g., B. L. Hu, “Emergent /Quantum Gravity: Macro/Micro Structures of Spacetime” Invited talk
DICE 2008. J. Phys. Conf. Ser. 174 (2009) 012015 [arXiv:0903.0878]
[3] For references and critiques, see, e.g., B. L. Hu, “Gravity and Nonequilibrium Thermodynamics of Classical Matter” Int. J. Mod. Phys. D 20:697-716 (2011) [arXiv:1010.5837]
[4] B. L. Hu, “Gravity and Thermodynamics: What exactly do we want?” Invited talk at ESF Exploratory Workshop: Gravity and Thermodynamics, SISSA Sept 8, 2011
[5] E.g, B. L. Hu, “Can Spacetime be a Condensate?” Int. J. Theor. Phys. 44 (2005) 1785-1806
[arXiv:gr-qc/0503067v2]