O.M. Stewart Colloquium

Every Monday, at 4 PM the department of Physics and Astronomy hosts the O. M. Stewart Colloquium, in rm 120, Physics Bldg.

Refreshments are served starting at 3:30 PM in the Physics Library (rm 223, second floor).

Spring 2021
Date Speaker/Title/Abstract
4/19/21 John F. Marko, Northwestern University
Loop extrusion, chromatin crosslinking, and the geometry, topology and mechanics of chromosomes and nuclei

The chromosomes of eukaryotic cells are based on tremendously long DNA molecules that must be replicated and then physically separated to allow successful cell division.  I will discuss what we have learned about chromosome structure from our group's biophysical experiments and mathematical modeling of chromosome structure. A key emerging feature of chromosome organization is the role of active chromatin loop formation, or "loop extrusion" as a mechanism leading to chromosome compaction, individualization, and segregation.  I will discuss a number of aspects of the SMC complexes thought to be the loop-extruding elements. I will also discuss our group's studies of the role of chromosomal epigenetic marks in control of the structure and integrity of the cell nucleus.

4/12/21 Smitha Vishveshwara, UIUC
Exploring anyons and black holes-like dynamics in flatland

The world and the Universe we live in are composed of fermions and bosons. The quantum statistics of these particles overwhelmingly governs what we see around us. But  one could wonder, can other kinds of quantum particles exist? I will begin this colloquium with an introduction of quantum statistics and the fascinating possible existence of anyons, particles which obey 'fractional' statistics.  

The quantum Hall system forms a marvelous two-dimensional realm for hosting many rich phenomena, including fractional statistics. I will describe how anyons can emerge in this setting, how they could be detected borrowing from beam-splitter and other principles used to detect bosons and fermions, and how landmark experiments of last year did perform such detection. I will also illustrate how the same setting can probe dynamics akin to that found in the astrophysical realm of black holes. Specifically, point-contact geometries can exhibit phenomena parallel to Hawking-Unruh radiation and black hole quasinormal modes associated with ringdowns in gravitational wave detection.  

Join Zoom Meeting: https://umsystem.zoom.us/j/99580334685?pwd=ZFFudlkwOGxKMUhSamFiRUpnVXpQUT09

4/5/21 Prof. Dam Thanh Son, University of Chicago
Strongly-interacting systems: from fractional quantum Hall effect to field-theoretic dualities

Quantum systems with strong interaction exist in many branches of physics and present a challenge for theory. We will discuss some recent methods to solve the problem, focusing on one  particular example: the fractional quantum Hall fluid. Many phenomena occurring in this fluid can be explained by postulating a new type of quasiparticle - the composite fermion. I will describe theories of this composite fermion, in particular the recently proposed "Dirac composite fermion theory", and how research in condensed matter physics has stimulated the discovery of a large number of new dualities between quantum field theories, previously unknown to high-energy physics. 

2/22/21 Dr. Alan Robock, Rutgers University
Climatic and Humanitarian Impacts of Nuclear War

A nuclear war between any two nations, such as India and Pakistan, with each country using 50 Hiroshima-sized atom bombs as airbursts on urban areas, could inject 5 Tg of soot from the resulting fires into the stratosphere, so much smoke that the resulting climate change would be unprecedented in recorded human history.  Our climate model simulations find that the smoke would absorb sunlight, making it dark, cold, and dry at Earth’s surface and produce global-scale ozone depletion, with enhanced ultraviolet radiation reaching the surface.  The changes in temperature, precipitation, and sunlight from the climate model simulations, applied to crop models show that these perturbations would reduce global agricultural production of the major food crops for a decade. Since India and Pakistan now have more nuclear weapons with larger yields, and their cities are larger, even a war between them could produce emissions of 27 or even 47 Tg of soot.

My current research project, being conducted jointly with scientists from the University of Colorado, Columbia University, and the National Center for Atmospheric Research, is examining in detail, with city firestorm and global climate models, various possible scenarios of nuclear war and their impacts on agriculture and the world food supply.  Using six crop models we have simulated the global impacts on the major cereals for the 5 Tg case.  The impact of the nuclear war simulated here, using much less than 1% of the global nuclear arsenal, could sentence a billion people now living marginal existences to starvation.  By year 5, maize and wheat availability would decrease by 13% globally and by more than 20% in 71 countries with a cumulative population of 1.3 billion people.  In view of increasing instability in South Asia, this study shows that a regional conflict using <1% of the worldwide nuclear arsenal could have adverse consequences for global food security unmatched in modern history. The greatest nuclear threat still comes from the United States and Russia.  Even the reduced arsenals that remain in 2020 due to the New START Treaty threaten the world with nuclear winter.  The world as we know it could end any day as a result of an accidental nuclear war between the United States and Russia.  With temperatures plunging below freezing, crops would die and massive starvation could kill most of humanity.

As a result of international negotiations pushed by civil society led by the International Campaign to Abolish Nuclear Weapons (ICAN), and referencing our work, the United Nations passed a Treaty to Ban Nuclear Weapons on July 7, 2017.  On December 10, 2017, ICAN accepted the Nobel Peace Prize “for its work to draw attention to the catastrophic humanitarian consequences of any use of nuclear weapons and for its ground-breaking efforts to achieve a treaty-based prohibition of such weapons.”  Will humanity now pressure the United States and the other eight nuclear nations to sign this treaty?  The Physicists Coalition for Nuclear Threat Reduction is working to make that happen.

Bio: Dr. Alan Robock is a distinguished professor of climate science in the Department of Environmental Sciences at Rutgers University, associate editor of the journal Reviews of Geophysics, Lead Author of the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, and a former Peace Corps Volunteer in the Philippines, and has been researching the climatic and agricultural impacts of nuclear war for the past 35 years.

Join Zoom Meeting: https://umsystem.zoom.us/j/94685134794?pwd=c0x4bzlmNjlrck10Wmp6SWp2YzkvUT09

Speaker(s):  Dr. Alan Robock, Rutgers University Host:  Prof. Deepak Singh