Computations in Science Seminars
Apr
23
Wed 12:15
Robert Rosner, University of Chicago
e-mail:
Host: Leo Kadanoff ()
“Clashing cultures: Science and Public Policy in the realm of Climate Change”

As part of its mission, the Panel on Public Affairs (POPA) of the American Physical Society semi-regularly develops statements for the APS on matters of public interest. One such matter is climate change, and over the past 6 months, POPA has been involved in a re-examination of the existing APS statement on climate change. I will discuss our approach, focusing on the dual issues of what we as physicists can say about this topic with some assurance, especially in the realm of prediction - and how do we explain this to a public that is highly polarized on this subject, not tolerant of nuance, and poorly educated on risk assessment and risk tolerance. All of these issues relate closely to how physicists deal with uncertainty quantification of models, and how these may translate - or not - to modeling in the social sciences.

Apr
30
Wed 12:15
Alex Robel, Harvard
e-mail:
Host: Wendy Zhang and Dorain Abbot
Dynamics of Antarctic Ice Stream Variability and Implications for Ice Sheet Stability

Ice streams are concentrated regions of fast flow within continent-scale ice sheets that can account for over 90% of an ice sheet’s internal mass transport. Ice stream flow exhibits variability at a range of temporal scales, with variability at hundred- to thousand-year time scales having a significant influence on net ice sheet mass balance, and as a result, global sea level. A dynamical systems approach is adopted to analyze a simplified box model of ice stream flow and hydrology. Within a range of parameters relevant to modern West Antarctica, there lies a subcritical Hopf bifurcation between stable ice streaming and oscillatory ice stream behavior. The associated hysteresis in ice stream behavior has implications for the response of ice streams to climate change. Combining these ice stream physics with a one-dimensional ice flow model produces shock-like “activation waves”. The associated oscillations in grounding line position complicate canonical grounding line stability theory and the idea of a vulnerable West Antarctic Ice Sheet.

May
7
Wed 12:15
Esperanza Linares, Caltech
e-mail:
Host: Wendy Zhang ()
Experimental study on liquid saturated granular flow.

Anyone who has played with sand has noticed the peculiar characteristic of this material; it can behave like a fluid but also behave like a solid. It is this duality that makes granular materials so interesting and yet so challenging to understand and predict. In addition to enabling both existence and failure of sand castles, granular materials and their suspension in liquids are prevalent in a wide range of natural and man-made processes. These include the industrial handling of seeds and slurries, clogging of drilling wells, and geological phenomena such as landslides and debris flows. However, most of our understanding of how these materials flow is based on empirical observations because of the complexity of having more than one phase (the solid and the fluid one), hampering, for example, the design of efficient transport of a suspension of solids in a fluid medium. The goal of my research is therefore to help develop constitutive models that predict how granular materials behave when sheared as a function of physical parameters, using carefully controlled experiments to validate and refine such models. My current research focuses on liquid-solid mixtures, and unlike the mechanics of dry granular material flows which are dominated by collisions and friction, the mechanics for these mixtures involve the interaction between the solid particles, the inertial effects from both liquid and solid phase, and viscous effects of the liquid. The experiments use a specially designed Couette cylindrical rheometer that allows probing the transition from transporting a pure liquid to transporting a dense suspension of particles. In particular, I will discuss the effects of particle concentration and the density ratio between the 2 phases under shear conditions where particle collisions might become important.

May
20
Tue 02:00 PM
Zorana Zeravcic, Harvard
e-mail:
Host: William Irvine ()
Associative memory through self-assembly
SPECIAL COMPUTATIONS IN SCIENCE SEMINAR ROOM W301-303 - 2:00 P.M.

Recently we have been developing a new connection between self-assembly and neural networks, where a multi-component particle system with specified interaction rules between its components is mapped onto a multi-state Hopfield neural network model. Within this framework, a fixed interaction pattern of neurons representing a “memory” maps to particle interactions encoding a certain structure. Properties of neural networks motivate new types of questions for self-assembly: Can the interaction energies of particles code for multiple structures at the same time? Can stored structures be self-assembled by throwing in a nucleation seed (i.e., a small assembly of particles) and have it complete into the desired stored structure? Can we define a capacity, i.e., a maximal number of structures that can be retrieved with limited error? In this talk I will address these questions and show that there exists a parameter regime where the answers to these questions are affirmative. This model has immediate application to synthetic systems with controllable binding specificity such as DNA tiles. Moreover, it suggests a new paradigm for programming cellular structures in biology: interactions between components are evolved in order to assemble different structures sharing some, if not all, of the components.

May
21
Wed 12:15
Katia Bertoldi, Harvard
e-mail:
Host: William Irvine ()
Soft structures: turning instabilities into an opportunity

Materials capable of undergoing large deformations like elastomers and gels are ubiquitous in daily life and nature. An exciting field of engineering is emerging that uses these compliant materials to design active structures and devices, such as actuators, adaptive optical systems and self-regulating fluidics. Compliant structures may significantly change their architecture in response to diverse stimuli. When excessive deformation is applied, they may eventually become unstable. Traditionally, mechanical instabilities have been viewed as an inconvenience, with research focusing on how to avoid them. Here, I will demonstrate that these instabilities can be exploited to design materials with novel, switchable functionalities. The abrupt changes introduced into the architecture of soft structure by instabilities will be used to change their shape in a sudden, but controlled manner. Possible and exciting applications include materials with unusual properties such negative Poisson’s ratio, phononic crystals with tunable low-frequency acoustic band gaps and reversible encapsulation systems.

May
28
Wed 12:15
Jim Sethna, Cornell
e-mail:
Host: Leo Kadanoff ()
Sloppy models, differential geometry and how science works

“With four parameters I can fit an elephant; with five I can make it wag its tail.” Systems biology models of the cell have an enormous number of reactions between proteins, RNA, and DNA whose rates (parameters) are hard to measure. Models of climate change, ecosystems, and macroeconomics also have parameters that are hard or impossible to measure directly. If we fit these unknown parameters, fiddling with them until they agree with past experiments, how much can we trust their predictions? Multiparameter fits are sloppy; the parameters can vary over enormous ranges and still agree with past experiments. Nonetheless, they can often make useful predictions about future experiments, even allowing for these huge parameter uncertainties: a few stiff combinations of parameters govern the behavior. These sloppy models all appear strikingly similar to one another – for example, the stiffnesses in every case we’ve studied are spread roughly uniformly over a range of over a million. We will use ideas and methods from differential geometry to explain what sloppiness is and why it happens so often. Finally, we shall show that models in physics are also sloppy – that sloppiness is a kind of parameter compression which makes science possible, both in physics and in other fields.

Jun
4
Wed 12:15
Daniel Fabrycky, University of Chicago
Host: Daniel Holz ()
Dynamics of Planetary Systems
Jun
11
Wed 12:15
Cheng Chin, University of Chicago
e-mail:
Host: Leo Kadanoff ()
Jun
18
Wed 12:15
OPEN
Jun
25
Wed 12:15
OPEN
Jul
2
Wed 12:15
Amy Kolan, St. Olaf College
e-mail:
Host: Leo Kadanoff ()
Jul
9
Wed 12:15
Sayantan Majumdar, University of Chicago
e-mail:
Host: Leo Kadanoff ()
Jul
16
Wed 12:15
Ivo Peters, University of Chicago
e-mail:
Host: Leo Kadanoff ()
Jul
23
Wed 12:15
Jun Zhang, New York University
e-mail:
Host: Leo Kadanoff ()
Jul
30
Wed 12:15
Brian Skinner, Argonne
e-mail:
Host: Leo Kadanoff ()
Aug
6
Wed 12:15
Andrew Gronewold, Great Lakes Environmental Research Laboratory
e-mail:
Host: Leo Kadanoff ()
Aug
13
Wed 12:15 PM
William A. Dembski, Discovery Institue
e-mail:
Conservation of Information in Evolutionary Search

Conservation of Information (CoI) asserts that the amount of information a search outputs can equal but never exceed the amount of information it inputs. Mathematically, CoI sets limits on the information cost incurred when the probability of success of a targeted search gets raised from p to q (p < q), that cost being calculated in terms of the probability p/q. CoI builds on the No Free Lunch (NFL) theorems, which showed that average performance of any search is no better than blind search. CoI shows that when, for a given problem, a search outperforms blind search, it does so by incorporating an amount of information determined by the increase in probability with which the search outperforms blind search. CoI applies to evolutionary search, showing that natural selection cannot create the information that enables evolution to be successful, but at best redistributes already existing information. CoI has implications for teleology in nature, consistent with natural teleological laws mooted in Thomas Nagel's Mind & Cosmos.

Aug
20
Wed 12:15
Glen Weyl, University of Chicago
e-mail:
Host: Leo Kadanoff ()
Aug
27
Wed 12:15
OPEN
Oct
8
Wed 12:15
Bob Batterman, University of Pittsburgh
e-mail:
Host: Leo Kadanoff ()
Oct
15
Wed 12:15
Jane Wang, Cornell
e-mail:
Host: Leo Kadanoff ()
Oct
22
Wed 12:15
Andy Ruina, Cornell
e-mail:
Host: Leo Kadanoff ()
Oct
29
Wed 12:15
OPEN
Nov
5
Wed 12:15
Guenter Ahlers, UC Santa Barbara
e-mail:
Host: Leo Kadanoff ()
Nov
12
Wed 12:15
Luis Bettencourt, Santa Fe Institute
e-mail:
Host: Leo Kadanoff ()
Nov
19
Wed 12:15
OPEN
Dec
3
Wed 12:15
OPEN
Dec
10
Wed 12:15
OPEN
Jan 2015
21
Wed 12:15
Heinrich Jaeger, University of Chicago
e-mail:
Host: Leo Kadanoff ()