Title: Dimensional Reduction in Quantum Gravity & Fractal Spacetime from the Quantum Browian Motion Perspective.

Speaker: Bei-Lok Hu (University of Maryland).

Abstract: Many approaches to quantum gravity find that spacetime near the Planck scale (10^33cm) becomes effectively two-dimensional. Recently there are proposals that the structure of spacetime becomes fractal at such a scale. Rather than relying on a postulated theory for the microscopic structure of spacetime as with strings, loops or sets, here we examinine this issue starting from low energy known theories, such as semiclassical gravity, where the vacuum expectation values of the stress energy tensor of a quantum matter field act as a source in driving the classical spacetime dynamics. Quantum Brownian motion has been a useful paradigm for the inclusion of matter field fluctuations in the establishment of stochastic gravity based on the Einstein-Langevin equation. When only the two point functions of the stress tensor are included, solutions for the metric fluctuations (“spacetime foam”) have been found for Minkowski and de Sitter spacetimes Higher moments of the stress-energy tensor have been shown to be just as important which leads to novel phenomena even at today’s low energy. Here we ask the question, would including non-Gaussian noise sources, or higher moments of the stress energy tensor in a self-consistent backreaction study of the spacetime dynamics reveal the underlying fractal structure of spacetime? This is motivated by the anomalous diffusion processes where multi-scale wave operators need be introduced to account for the effects deviating from the usual Brownian motion. This approach offers a useful key to unlocking the mesoscopic behavior of spacetime, which is a more meaningful and accessible regime, being the common denominator of all theories of quantum gravity and a connector to more verifiable low energy phenomena.

Time: Friday 7th October 2016, 2pm.

Place: Lecture Room, 1st Floor, School of Theoretical Physics, DIAS, 10 Burlington Road, Dublin 4.