Title： What is Quantum Gravity? – Spacetime micro-structure or Quantum Macro- Phenomena?

Speaker: Bei - Lok Hu (胡悲樂)
    Maryland Center for Fundamental Physics and Joint Quantum Institute, University of Maryland, College Park, USA

Abstract:

  The short answer to the title question which most practitioners, in a score of approaches to quantum gravity (QG), would agree upon is: QG are theories about the microscopic structures of spacectime at the Planck scale 10^{-35} m. However, despite a good three quarters of a century’s effort, no honest researcher can claim that this goal is met. One can posit these micro-structures to be strings, loops, sets, simplices, groups, cells, networks, matrices. But until one can show from the interaction of their micro-constituents that a smooth manifold appears with some notion of metricity, or that the Minkowski spacetime is a viable vacuum state, in a robust way – call this the`Top-Down’ approach (`top’ referring to ultra high energy 10^19Gev) -- one cannot claim success. This effort will likely go on for many more decades because experimental evidences are not easily accessible. In this talk I shall raise three groups of issues related to QG: 1) I’ll begin with my critiques on some popular claims: Many theories attributed to QG such as AdS-CFT duality, albeit having important foundational theoretical values, are not about QG. Much less truth value is there in the claims of recent proposals of laboratory experiments that measuring two entangled masses would be a proof of QG. 2) I’ll share with you my own view, namely, General relativity is in the nature of a hydrodynamic theory [arXiv:gr-qc/9511077], valid only in the long wavelength, low energy domain. GR is an effective theory emergent from quantum gravity [arXiv:0903.0878]. This view opens up a new dimension of inquiry hitherto largely ignored, namely, starting with the observable macro phenomena, try to induce the concealed micro structures. I’ll suggest avenues for this `Bottom-Up’ approach. 3) A more fruitful measure is not to coerce Quantum (Q) into Gravity (G), but to expose and understand their contradictions. Instead of G x Q, pay more attention to G+Q. There is more hope to check your theory against experimental (laboratory by design) or observational (astro-cosmological) data. And, there are equally important foundational questions, such as the role of quantum in macroscopic phenomena, making use of concepts and perspectives from quantum information theories. This can add new insights to our understanding of the interplay of Q and G.

2024年5月23日3：30PM，物理学院422室