2018 Fall Graduate Colloquium Series: Kai Yang/William Julius/Cooper Watson

DateNovember 9, 2018Time3:35 - 5:00 pm
LocationBaylor Sciences Building, Room E.125
2018 Fall Graduate Colloquium Series

Kai Yang/

Kai Yang
Effective Two-Dimensional Model for Granular Matter with Phase Separation

Granular systems confined in vertically vibrated shallow horizontal boxes (quasi two-dimensional geometry) present a liquid to solid phase transition when the frequency of the periodic forcing is increased. An effective model, where grains move and collide in two-dimensions is presented, which reproduces the phase transition. The key element is that besides the two-dimensional degrees of freedom, each grain has an additional variable ε that accounts for the kinetic energy stored in the vertical motion in the real quasi two-dimensional motion. This energy grows monotonically during free flight, mimicking the energy gain by collisions with the vibrating walls and, at collisions, this energy is instantaneously transferred to the horizontal degrees of freedom. As a result, the average values of ε and the kinetic temperature are decreasing functions of the local density, giving rise to an effective pressure that can present van der Waals loops. (Reference: D. Risso et al., Physical Review E 98, 022901, (2018))

William Julius
Soft Hair on Black Holes

In the mid 70s, Steven Hawking and Jacob Bekensteins study of radiating black holes suggested that the process of black hole evaporation via Hawking radiation destroyed information about the physical states which formed the black hole. This is the infamous black hole information paradox. Recently it has been shown that Bondi-van der Burg-Metzner-Sachs supertranslation symmetries imply an infinite number of conservation laws governing so called supertranslation charge. These conservation laws imply that black holes carry large amounts of soft (zero energy) supertranslational hair. An explicit description of this soft hair, in terms of soft gravitons and photons on the black hole horizon, is given. It is also demonstrated that complete information about the quantum state is stored on a holographic plate at the future boundary of the horizon. These conservation laws give rise to an infinite number of exact relations between evaporation products of black holes which differ only by their soft hair. While this work does not solve the information paradox, it does begin to highlight key issues with several of the previously unquestionable assumption that underlie it without invoking any new physics. (Reference: S. W. Hawking et al., Phys. Rev. Lett. 116, 231301 (2016))

Cooper Watson
How to Tell the Shape of a Wormhole by its Quasinormal Modes

Recent observations of gravitational waves from compact objects do not single out the geometry of their sources such as wormholes which can mimic the ringdown phase of Einsteinian black holes. One approach constructs wormhole metrics based on a concentration of exotic matter in a dense compact region near the throat of a wormhole. The concentration of exotic matter is described by effective potentials which have the form of a potential barrier. The tideless Morris-Thorne metric used to describe the wormhole can be expanded into Taylor series near the throat. This paper proposes the first unique method used to reconstruct the metric of an arbitrary traversable Lorentzian wormhole near its throat using the high frequency quasinormal modes (damped oscillations) of the system. The solution to constructing the metric from the maximum effective potential provides more information about a wormhole than a black hole because many of the processes such as Hawking radiation, accretion of mater, quasinormal ringing etc., occur at a distance away from the event horizon. The proposed solution is possible with the WKB approach for tideless wormholes or tidal forces known from independent methods such as analyzing the redshift effect. However, this approach does not lead to a unique solution for the metric. The unique approach can be applied to rotating wormholes given they are symmetric enough to guarantee the separation of variables with the known spherical harmonics. (Reference: R. A. Konoplya. Phys. Lett. B 784 (2018), 43-49)

For more information contact: Dr. Howard Lee, 254-710-2277
PublisherDepartment of Physics
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