Physics and Astronomy Calendar

Week of Monday, March 25th 2019

Monday, March 25th 2019
12:15 pm:
Speaker: TBD

Tuesday, March 26th 2019
1:25 pm:
Space Physics Seminar in Tate 301-20
Speaker: Trevor Knuth
Subject: Fast time variations in solar flare X-ray flux - A probe for particle acceleration
Speaker: Vipin Kumar
Subject: Physics Guided Machine Learning: A New Paradigm for Modeling Dynamical Systems

Physics-based models of dynamical systems are often used to study engineering and environmental systems. Despite their extensive use, these models have several well-known limitations due to incomplete or inaccurate representations of the physical processes being modeled. Given rapid data growth due to advances in sensor technologies, there is a tremendous opportunity to systematically advance modeling in these domains by using machine learning (ML) methods. However, capturing this opportunity is contingent on a paradigm shift in data-intensive scientific discovery since the “black box” use of ML often leads to serious false discoveries in scientific applications. Because the hypothesis space of scientific applications is often complex and exponentially large, an uninformed data-driven search can easily select a highly complex model that is neither generalizable nor physically interpretable, resulting in the discovery of spurious relationships, predictors, and patterns. This problem becomes worse when there is a scarcity of labeled samples, which is quite common in science and engineering domains.

This talk makes a case that in a real-world systems that are governed by physical processes, there is an opportunity to take advantage of fundamental physical principles to inform the search of a physically meaningful and accurate ML model. Even though this will be illustrated in the context of modeling water temperature, the paradigm has the potential to greatly advance the pace of discovery in a number of scientific and engineering disciplines where physics-based models are used, e.g., power engineering, climate science, weather forecasting, materials science, and biomedicine.

Faculty Host: Vuk Mandic

Wednesday, March 27th 2019
1:25 pm:
Speaker: Itamar Kimchi, University of Colorado - Boulder
Subject: Dirty Entangled Quantum Magnets
Faculty Co-Host: Alex Kamenev

Studying quantum entanglement over the past 1--2 decades has allowed us to make remarkable theoretical progress in understanding correlated many-body quantum systems. However electrons in real materials experience random heterogeneities ("dirt") whose theoretical treatment, including strong correlations, has been a challenge. I will describe how synthesizing ideas from quantum information theory, statistical mechanics, and quantum field theory gives us new insights into the role of randomness in 2D correlated quantum spin systems. First I will outline our results on weak bond-randomness in two theoretically controlled cases (valence-bond-solids and classical dimer models) and apply them to random quantum magnets to show that topological defects with free spins necessarily nucleate and control the low energy physics. Second I will describe how the results lead us to conjectures in 2D, and a proved theorem in 1D, of Lieb-Schultz-Mattis-type constraints on all possible low-energy fates of quantum magnets, that hold even with randomness. Third I will describe how the theory predicts a scaling collapse of the temperature and magnetic-field dependence of thermodynamic quantities that is consistent with experimental observations from multiple materials, suggesting that these materials exhibit randomness-driven long range entanglement.

Faculty Host: Fiona Burnell

Thursday, March 28th 2019
10:10 am:
Biophysics Seminar in 120 PAN
Speaker: Jerome C. Mertz, College of Engineering, Boston University
Subject: TBD
Faculty Host: Jochen Mueller
12:10 pm:
Speaker: Jin-Ah Kim and Liliya Williams
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Jason Hogan, Stanford
Subject: TBD
Faculty Host: Roger Rusack

Friday, March 29th 2019
11:00 am:
Nuclear Physics Seminar in Physics Tate 301-20
Speaker: James Austin Harris, Oak Ridge National Lab
Subject: The multidimensional character of nucleosynthesis in core-collapse supernovae

The intrinsically multi-dimensional neutrino-driven explosion mechanism of core-collapse supernovae (CCSNe) is notoriously difficult to model self-consistently.

As a matter of either computational expediency or necessity, nuclear burning, when included at all, is traditionally constrained to a small reaction network consisting only of the (α,γ) reactions necessary in linking 4He to 56Ni.

Feedback between the evolving hydrodynamics and changing composition, and resulting energy generation, precludes the deficiencies of this simplification from being entirely resolved with post-processing calculations.

Using a much more realistic, in situ reaction network capable of accurately tracking nuclear energy generation and neutronization, we examine the nucleosynthesis in multidimensional, self-consistent, neutrino-driven supernova models.

We find differences between the in situ and post-processing approaches, indicating that such rigor in evolving the nuclear composition is needed to accurately calculate the nucleosynthesis of matter that has been ejected from the inner regions of the explosion mechanism.

This has implications for some of the most interesting nucleosynthetic processes in CCSNe, specifically α-rich and α-poor freeze-out, which produces several isotopes particularly relevant to observations (e.g.44Ti, 48Ca, and 92Mo).

12:20 pm:
Speaker: Zhen Jiang
Subject: TBA
12:30 pm:
Speaker: Vitaly Vanchurin (U. Minnesota, Duluth)
Subject: TBA
2:30 pm:
Speaker: Vuk Mandic
Subject: TBD
Faculty Host: Evan Skillman
4:40 pm:
Speaker: Roger Rusack, High Energy

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