12/24 The First Images of a Black Hole and General Relativistic Magnetohydrodynamical (GRMHD) Accretion/Jet : achievements and challenges

Time: 1:20pm, December 24 (Thursday)
Title: The First Images of a Black Hole and General Relativistic Magnetohydrodynamical (GRMHD) Accretion/Jet : achievements and challenges
Speaker: Prof. Hung-Yi Pu
(Department of Physics, National Taiwan Normal University)

Place: Science Building III 1F SC110

12/17 The emergence of collective modes, ecological collapse and directed percolation at the laminar-turbulent transition

Time: 1:20pm, December 17 (Thursday)
Title: The emergence of collective modes, ecological collapse and directed percolation at the laminar-turbulent transition
Speaker: Dr. Hong-Yan Shih
(Institute of Physics, Academia Sinica)

Place: Science Building III 1F SC110

Abstract: 

How a laminar flow becomes turbulence has been an unsolved problem for more than a century and is important in various industrial applications. Recently precise measurements in pipe flow experiments showed non-trivial spatiotemporal complexity at the onset of turbulence where lifetime and splitting time of metastable turbulence do not diverge asymptotically as would have been expected in a sharp transition.
Based on numerical evidence from the hydrodynamics equations, we discovered the surprising fact that the fluid behavior at the transition is governed by the emergent predator-prey dynamics of the important long-wavelength mode, leading to the mathematical prediction that the laminar-turbulent transition is analogous to an ecosystem on the edge of extinction. This prediction demonstrates that the laminar-turbulent transition is a non-equilibrium phase transition in the directed percolation universality class, and provides a unified picture of transition to turbulence emerging in systems ranging from turbulent convection to magnetohydrodynamics.

Reference:
[1] Hong-Yan Shih, Tsung-Lin Hsieh and Nigel Goldenfeld, Nature Physics 12, 245 (2016)
[2] https://www.nature.com/collections/rxsztdqblr

12/10 Observation of an Unusual Upward-going Cosmic-ray-like Event in the Third Flight of ANITA

Time: 1:20pm, December 10 (Thursday)

Title: Observation of an Unusual Upward-going Cosmic-ray-like Event in the Third Flight of ANITA

Speaker: Prof. TsungChe Liu
(Electrophysics Department, National Chiao Tung University)

Place: Science Building III 1F SC110

Abstract:

We report on an upward traveling, radio-detected cosmic-ray-like impulsive event with characteristics closely matching an extensive air shower. This event,
observed in the third flight of the Antarctic Impulsive Transient Antenna (ANITA), a NASA-sponsored long-duration balloon payload, is consistent with a similar event reported in a previous flight.
These events may be produced by the atmospheric decay of an upward-propagating τ-lepton produced by a ντ interaction, although their relatively steep arrival angles create tension with the standard model (SM) neutrino cross section.
Each of the two events have a posteriori background estimates of <∼ 10^{−2} events.
If these are generated by τ-lepton decay, then either the charged-current ντ cross section is suppressed at EeV energies, or the events arise at moments
when the peak flux of a transient neutrino source was much larger than the typical expected cosmogenic background neutrinos.

https://arxiv.org/pdf/1803.05088.pdf

未解微中子之謎：質量來源、超乎模型預測的高能量