Seminar Abstracts - 2011

Tuesday, September 13, 2011
Cesar Hidalgo, Assistanct Professor, MIT Media Laboratory
"Economic Complexity: The Driver of Economic Growth"

In this talk I present an empirical method, quantitative model, and theoretical framework that can be used to quantify the complexity of a country's economy. I show that economic complexity can explain differences in the income distribution of countries, and their dynamics, since it is highly predictive of future economic growth. Additionally, I present predictions for the next decade and provide a preview of new online visualization tools that can be used to explore the productive structures of countries and their evolution.

Tuesday, September 20, 2011
Yevgeniy Vorobeychik, Sandia National Laboratories
"HOT or NOT? Decentralized Strategic Optimization in Complex Systems"

We introduce noncooperatively optimized tolerance (NOT), a generalization of highly optimized tolerance (HOT) that involves strategic (game theoretic) interactions between parties in a complex system. We illustrate our model in the forest fire framework. As the number of players increases, our model retains features of HOT, such as robustness, high yield combined with high density, and self-dissimilar landscapes, but also develops features of self-organized criticality (SOC) when the number of players is large enough. For example, the forest landscape becomes increasingly homogeneous and protection from adverse events (lightning strikes) becomes less closely correlated with the spatial distribution of these events. While HOT is a special case of our model, the resemblance to SOC is only partial; for example, the distribution of cascades, while becoming increasingly heavy-tailed as the number of players increases, also deviates more significantly from a power law in this regime. Surprisingly, the system retains considerable robustness even as it becomes fractured, due in part to emergent cooperation between neighboring players. At the same time, increasing homogeneity promotes resilience against changes in the lightning distribution, giving rise to intermediate regimes where the system is robust to a particular distribution of adverse events, yet not very fragile to changes.

Tuesday, September 27, 2011
Baruch Meerson, Professor of Physics, Hebrew University
"Extinction of established populations: A physicist's view"

Extinction of an isolated population after maintaining a long-lived established community is a dramatic phenomenon. It ultimately occurs, even in the absence of detrimental environmental variations, because of the intrinsic stochasticity of birth and death processes.

I will show how one can use a variant of WKB approximation (after Wentzel, Kramers and Brillouin) to evaluate the mean time to extinction of an established stochastic population. In this approximation the most likely path of the population to extinction is given by a separatrix (a heteroclinic orbit) of an underlying classical Hamiltonian.

Tuesday, October 4, 2011
Frank Schweitzer, ETH Zurich, Switzerland
"Open source software as a complex network"

Open source software (OSS) can be seen as a evolving complex network. On the structural level the nodes are given by software modules (such as classes), whereas the links between nodes may describe the type of connection (such as usage or inheritance). This network is highly dynamic because of the addition/deletion of nodes or links and the propagation of changes. Understanding the development of OSS puts a challenge on different sciences: physics, to reveal the structural features and the abstract dynamics of the network; computer science, to elucidate the software engineering principles underlying OSS; economics/management sciences, to understand the social interaction of developers and users. In this talk, using a highly data driven approach, we analyse the community dynamics in more than 100 projects, the dependency structure and the change records of 35 Java projects and the evolution of the dependency network. Our investigations show remarkable regularities in the structure and dynamics of OSS which can be reproduced by simple models, this way challenging established paradigms in software engineering.

Tuesday, October 11, 2011
Sinan Aral, Assistant Professor, New York University Stern School of Business
"Content and Causality in Influence Networks"

Many of us are interested in whether "networks matter." Whether in the spread of disease, the diffusion of information, the propagation of social contagions, the effectiveness of viral marketing, or the magnitude of peer effects in a variety of settings, two key questions must be answered before we can understand whether networks matter: 1) how the content that flows through networks affects the patterns of outcomes we see across nodes and 2) whether the statistical relationships we observe can be interpreted causally. Aral will review what we know and where research might go with respect to content and causality in networks. He will provide two examples from each area to structure the discussion: One from an analysis of email networks and the information content that flows through them at a mid-sized executive recruiting firm and the other from a randomized field experiment on a popular social networking website that tests the effectiveness of "viral product design" s trategies in creating peer influence and social contagion among the 1.4 million friends of 9,687 experimental users.

Tuesday, October 25, 2011
Taehun Lee, Assistant Professor, Mechanical Engineering, City College of New York
"High-order Method for Mesoscale Simulation of Liquid Drop Coalescence and Impact"

The lattice Boltzmann method (LBM) is a mesoscale approach, which can accommodate coarse-grained, molecular-level information into the macroscopic description of complex interfacial phenomena. This is achieved by introducing a phase field function into a single-phase lattice Boltzmann formulation to distinguish between phases (i.e. liquid/vapor, liquid/liquid), together with a phenomenological free energy functional of the solid-liquid-vapor system whose dissipative minimization constrains the temporal evolution of the phase field. LB equation is generally derived from the discrete Boltzmann equation by discretizing it on uniform rectangular mesh and usually comprises collision and streaming steps. While this greatly facilitates numerical procedure, it limits shapes of the computational domain that LBM can be applied to. This limitation could substantially increase computational effort for flows of boundary-layer type and in complex geometries with strong interactions between solid surface and contact line. To overcome geometric constraint of LBM and to improve its numerical stability at high Reynolds number, we have recently proposed high-order Galerkin/Discontinuous Galerkin LBM. In these computational frameworks, LB equation is regarded as a special space-time discretization of the discrete Boltzmann equation in the characteristic direction, and is solved by higher-order accurate schemes on unstructured mesh. In this presentation, a brief introduction to the temporal and spatial discretizations of the discrete Boltzmann equation will be given, with emphasis on the Galerkin/Discontinuous Galerkin approximations on unstructured mesh. Applications of the new LBM will be discussed in the simulations of single- and two-phase flows including flow past a cylinder, drop coalescence, and drop impact on thin liquid layer and flat/heterogeneous substrates.

Tuesday, November 8, 2011
Marcel Filoche, Director of Research, CNRS
"The complex geometry of the lung airway system: an optimal but fragile system?"

The lung airway system is a complex transportation system in charge of delivering the oxygen from the air to the blood while clearing at the same time the carbon dioxide from the blood. Its geometrical structure is that of a space-filling dichotomous branching tree. From the point of view of ventilation, this tree can be modeled as an arrangement of pipes which has to simultaneously satisfy several constraints:

-Its aerodynamic resistance must be low in order to minimize the energy dissipation of the system, which implies large airway diameters.

-Also, its volume must as small as possible since it does not participate to gas exchange.

-It must transport the oxygen to the terminal units in a limited time defined by the duration of the inspiratory phase; The compliant structure must be able to exhale all the air contained in the lung.

-Last but not least, since this structure is the result of a biological growth process, all its performances must be robust against physiological and morphological variability.

We will show that these multiple constraints drastically narrow down the domain of admissible geometries. As a consequence, what could be defined as the "optimal" tree is also fragile against any type of physiological variability. This suggests that regulation mechanisms must be at work in the lung in order to keep up the ventilatory performance.

Tuesday, November 15, 2011
Forrest Stonedahl, Professor, Centre College
"Query-Based Model Exploration: Parameters and Paradigms"

In the past few decades, agent-based modeling (ABM) has emerged as a powerful computer simulation technique in which many agents interacting according to simple rules can give rise to complex aggregate-level behavior. However, as ABM is increasingly employed in both the natural and social sciences, the methods and tools for understanding, exploring, and analyzing the behavior of agent-based models have not kept pace. I see this as a large outstanding problem (and challenge) for the ABM research community.

In this seminar, I will provide an overview of my dissertation research, which addresses one aspect of this challenge: the exploration of model behavior as model parameters are varied. Specifically, I will report on a comprehensive investigation of the use of genetic algorithms (and other metaheuristic search algorithms) for exploring the range of behaviors produced by agent-based models. This investigation is comprised of a series of in-depth case studies (including models of collective animal motion, viral marketing in social networks, ancient Puebloan civilizations, and online news story consumption), as well as the development of a broader benchmark suite of exploration tasks. More generally, I will argue for the benefits of a paradigm shift in exploration methodology. However, the success of this paradigm shift depends on new tools. Thus, I will also discuss the design of BehaviorSearch, a software tool that I have developed to support this methodology. This work has important implications for the calibration, verification, and validation of agent-based models, and I hope that this and similar projects will nudge (or entice) ABM practitioners toward more robust model analysis, and ultimately result in better science.

Monday, November 21, 2011
Atzumi Ohara, Professor, Fukui University, Japan
"Information geometry of nonextensive statistical mechanics with Tsallis entropy (q-exponential family)"

A q-exponential family is a set of probability distributions, which is a natural generalization of the standard exponential family, and is related to many physical phenomena in complex systems that obey power-laws. The talk presents new geometric aspects of the behaviors of solutions to the porous medium equation (PME) and its associated equation. First we discuss thermostatistical structure with information geometry on a manifold of generalized exponential densities. A dualistic relation between the two existing formalisms is elucidated. Next by equipping the manifold of q-Gaussian densities with such a structure, we derive several physically and geometrically interesting properties of the solutions. The manifold is proved invariant and attracting for the evolving solutions, which play crucial roles in our analysis. We demonstrate that the moment-conserving projection of a solution coincides with a geodesic curve on the manifold. Further, the evolutional velocities of the second moments and the convergence rate to the manifold are evaluated in terms of the Bregman divergence. Finally we show that the self-similar solution is geometrically special in the sense that it is simultaneously geodesic with respect to the mutually dual two affine connections.

Reference: Ohara and Wada, J. Phys. A: Math. Theor., 43035002 (2010)

Tuesday, November 29, 2011
Tom Witten, Dept. of Physics, University of Chicago
"Orienting colloidal objects by programmed sedimentation"

Colloidal particles in a liquid sink or rise under gravity; that's why we have to "shake well before using." Unless these particles are especially symmetric, they also rotate as they drift. The rotation at any moment is proportional to the force via an Onsager "twist matrix" that depends on its orientation. The equations of motion resemble the Euler equations for a freely rotating rigid body, though dissipative and chiral effects alter the motion markedly. Under constant forcing a particle generally attains a state of uniform rotation. In this talk we explore what happens when gravity changes direction in a programmed way. We describe two protocols that lead to complete alignment of an ensemble of identical particles: all have the same orientation and all rotate with the same phase. Such phase coherence resembles that of spins in a pulsed nuclear magnetic resonance setup. Increasingly, as we seek to manipulate microscale biological and man- made objects en masse, it will be useful to manipulate their orientation by a uniform dynamical protocol.

Tuesday, December 6, 2011
Damon Centola, Assistant Professor, MIT Sloan School of Business