Emergence: Gravity as Thermo- and Hydro-dynamics
Abstract
Renewed interest in viewing gravity from thermodynamics considerations is stirred up by a recent proposal that ‘gravity is an entropic force’. Even though the speaker finds the arguments justifying such a claim rather ad hoc and simplistic compared to the original one, he supports the call to explore deeper the relation between gravity and thermodynamics. It is in the same spirit as an old proposal of the speaker to view general relativity as hydrodynamics, i.e., the low-energy long-wavelength limit of some underlying theories for the microscopic structure of spacetime, be it string theory or loop quantum gravity. These proposals, together with that of G. E. Volovik and Xiao-Gang Wen, brought out the emergent nature of gravity. In this talk the speaker will define the tasks of quantum versus emergent gravity, explore different senses of emergence, suggest how nonequilibrium ideas and methodology are suited for exploring emergent phenomena and the consequences of viewing spacetime as a condensate. A way of exploring the theme of gravity as thermodynamics is also presented using nonequilibrium thermodynamics, first of classical matter without invoking any quantum considerations such as Bekenstein-Hawking entropy, holography or Unruh effect. The nonequilibrium nature of classical gravity shared with other systems with long range interaction, such as its negative heat capacity, is at the root of many salient features of say, black hole physics. The speaker will end with a challenge he recently posed to all proponents of this theme: “Derive gravity from thermodynamics without invoking quantum physics”. Because both are classical theories about macroscopic phenomena, it would be intellectually more satisfying to see a direct link. Though not easy, this should in principle be possible for even a 19c physicist. More interestingly, what would a failure to meet this challenge say about the inter-relation of gravity, thermodynamics and quantum physics?
About the speaker
Bei-Lok Hu got his PhD in theoretical physics from Princeton University in 1972 under the late Prof John A. Wheeler. After postdoctoral work at Stanford University, University of California, Berkeley and Santa Barbara in mathematics, physics and astrophysics, he was appointed an honorary research fellow at Harvard University in 1979 before he assumed his current position at the University of Maryland in 1980.
Prof. Hu’s research in the 70’s was on quantum field theory in curved spacetime with applications to quantum processes in the early universe, for that work he was elected Fellow of the American Physical Society. Prof. Hu began pioneering work on nonequilibrium quantum field theory in the 80’s which resulted in a book with Dr. Calzetta by this title published in 2008 in the Cambridge Monograph in Mathematical Physics series. In 1990 Prof. Hu began his seminal work on quantum decoherence and non-Markovian processes of open quantum systems. Since 2000 he has been studying quantum entanglement dynamics in atomic-optical systems with applications to quantum information processing. He is a founding fellow of the Joint Quantum Institute dedicated to the advancement of quantum science and its applications. He is also the chief architect in the inauguration of the International Society for Relativistic Quantum Information in 2010. His current research interest is on foundational issues of quantum and statistical mechanics behind macroscopic quantum phenomena and quantum thermodynamics.
Prof. Hu is a world-renowned leader in quantum gravity research. His long-held critically independent viewpoint that general relativity is a hydrodynamic theory first presented at the Second Sakharov Conference in 1996 has, alongside with his Maryland colleague Jacobson’s 1995 paper on viewing Einstein’s equation as an equation of state, as well as work from the condensed matter community by Volovik and Wen, helped ushered in a vibrant field known today as emergent gravity. This is the theme of his lecture.
