Events Archive

Mar
06
2012
Low-order models for control of fluids
The ability to effectively control a fluid would enable many exciting technological advances, such as the design of quieter, more efficient aircraft.  Model-based feedback control is a particularly attractive approach, but the equations governing the fluid, although known, are typically too complex to apply standard tools for dynamical systems analysis or control synthesis.  This talk addresses model reduction techniques, which are used to simplify existing models, to obtain low-order models tractable enough to be used for analysis and control, while retaining the essential physics.
Mar
05
2012
Nobel Colloquium: Surveying the Accelerating Universe with Supernovae
In 1998 two rival teams of astronomers studying exploding white dwarf stars, called type Ia supernovae, came to the surprising conclusion that the expansion of the Universe is speeding up. This discovery of "the accelerating Universe" ushered in a revolution in our cosmological understanding. I will describe the steps leading to this discovery, and how observations of supernovae from telescopes on the ground and in space can be used to trace the history of cosmic expansion. The continued study of these stellar explosions will shed light on the mysterious "dark energy" that dominates and drives our accelerating Universe.
Feb
24
2012
Diffusion and weak ergodicity breaking within living cells
Diffusion of single molecules and organelles in living cells has attracted considerable interest. The motion so essential for intra- and intercellular transport, regulation, and signaling, and hence for the life within cells exhibits surprising deviations from normal Brownian motion. Using optical tweezers combined with single particle tracking inside living cellular organisms we study intracellular diffusion of nano-sized organelles inside living cells. The temperature increase caused by absorption by the laser light as well as the potential physiological damage are important also to consider and will be addressed [1,2]. Lipid granules inside living...
Feb
24
2012
"Shocks in fragile matter"
In ordinary solids, acoustic shocks are extreme mechanical phenomena: they occur when rigid materials are subjected to violent impacts. But in soft materials things are different. Granular media, foams and polymer networks can all be prepared in a state of vanishing rigidity in which even the tiniest perturbation elicits an extreme mechanical response. When that happens these materials are not just soft, they have become fragile. In this talk, we present simulations in which two-dimensional jammed granular packings are dynamically compressed, and demonstrate that the elementary excitations are strongly nonlinear shocks, rather than ordinary...
Feb
23
2012
Angular momentum, stability and fragmentation in 3D attractive Bose-Einstein condensates
Trapped attractive atomic Bose-Einstein condensates (BECs) in three spatial dimensions are known to exist for some finite time only. This is because the gas is prone to self-collapse, due to the attractive nature of the interaction. The 'mainstream' way to describe the state of the condensate is a mean-field (MF) theory, that assumes total condensation of the system.  In this talk I will introduce the notion of fragmentation, in contrast to coherence, and show that the states of definite angular momentum of the 3D many-body system cannot be condensed MF states. With this at hand, I examine the impact of the angular momentum to the stability of the attractive gas and show that there...
Feb
22
2012
Stochasticity in gene expression and its regulation by non-coding RNAs
One of the fundamental problems in biology is understanding how phenotypic variations arise in individuals. Phenotypic variation is generally attributed to genetic or environmental factors. However, in several important cases, phenotypic variations are observed even among genetically identical cells in homogeneous environments. Recent research indicates that such `non-genetic individuality' can arise due to intrinsic stochasticity in the process of gene expression. Correspondingly there is a need to develop a framework for quantitative modeling of stochastic gene expression and its regulation. Of particular interest is modeling of regulation by non-coding...
Feb
21
2012
Fish robotics: understanding the diversity of fish locomotion using mechanical devices
There are over 28,000 species of fishes, and a key feature of this remarkable evolutionary diversity is a great variety of propulsive systems used by fishes for maneuvering in the aquatic environment.  Fishes have numerous control surfaces (fins) which act to transfer momentum to the surrounding fluid.  In this presentation I will discuss the results of recent experimental kinematic and hydrodynamic studies of fish fin function, and their implications for the construction of robotic models of fishes.  Recent high-resolution video...
Feb
20
2012
Strong-field atomic physics in the x-ray regime
The advent of x-rays sources with unprecedented intensity will enable the study of nonlinear physics in the high frequency regime. In 2009, a physicist dream became reality with the commissioning of the world’s first x-ray free-electron laser, the LCLS, at SLAC. In contrast to low frequency strong-field physics where valence electrons react to the optical field, at high frequency the atom will be ionized from the inside out. The question remains as to whether the atomic response to x-rays will be adequately described by low-order perturbation theory or necessitate a non-perturbative description which is more commonly used at low-frequency. In this talk, these issues will...
Feb
16
2012
Low metallicity star formation: a nursery for compact binary mergers?
Most predictions for binary compact object formation are normalized to the present-day Milky Way population. In this talk, I suggest the merger rate of black hole binaries could be exceptionally sensitive to the ill-constrained fraction of low-metallicity star formation that ever occurred on our past light cone. I discuss whether and how observations might distinguish binary evolution uncertainties from this strong trend, both in the near future with well-identified electromagnetic counterparts and in the more distant future via third-generation gravitational wave detectors.
Feb
15
2012
Gene Regulation by Riboswitches: From Folding Landscape to System Level Description

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