Mar
11
Teaser:

I will describe several models for running insects, from an energy-conserving biped, through a muscle-actuated hexapod driven by a neural central pattern generator, to a reduced phase-oscillator model that captures the dynamics of unperturbed gaits and of impulsive perturbations. I will argue that both simple models and large simulations are necessary to understand biological systems.  The models show that piecewise-holonomic constraints due to intermittent foot contacts confer asymptotic stability on the...

Mar
07
Teaser:

I will review ideas that may be useful in identifying electromagnetic (EM) emission from supermassive black hole (SMBH) binaries. In particular, any detectable EM emission is likely to be time-variable, which should aid in its identification. I will discuss four possibilities for such variable emission: (i) roughly periodic signals due to the orbital motion prior to coalescence, (ii) a transient pre-cursor caused by the gas trapped inside the binary's orbit, and transients "after-glows" produced by (iii) post-merger gas accretion and (iv) by merger-induced shocks in a circumbinary disk. I will argue that these...

Mar
06
Teaser:

Understanding the locomotion of animals and robots can be a challenging problem, involving nonlinear dynamics and the coordination of many degrees of freedom. Geometric mechanics offers a vocabulary for discussing these dynamics in terms of lengths, areas, and curvatures. In particular, a tool called the *Lie bracket* combines these geometric concepts to describe the effects of cyclic changes in the locomotor's shape, such as the gaits used by walking or crawling systems.

In this talk, I will introduce some basic principles of geometric mechanics, and show how they...

Mar
05
Teaser:

Matter placed in a strong magnetic field provides a fascinating laboratory in which to study exotic quantum phenomena in a highly controllable manner.  This talk will summarize our recent findings of novel magnetic properties of carbon nanotubes, graphene, and graphite, probed via high-field magneto-optical spectroscopy.  A magnetic field applied parallel to a nanotube introduces an Aharonov-Bohm phase to the electronic wave function, which leads to band gap oscillations, magnetic brightening of dark excitons, and extremely large magnetic...

Mar
04
Teaser:

The ATLAS Experiment at the Large Hadron Collider with its sister experiment CMS reported a discovery last summer of a new boson which is consistent with the Standard Model Higgs boson.  The Higgs particle has been searched for decades. It is the final jewel in the Standard Model of particle physics, a crowning achievement of 20th century science that gives a powerful understanding of fundamental particles and their interactions. In the Standard Model, the Higgs is the quantum of a field that accounts for the masses of those particles.  We will describe the apparatus, the data and other searches....

Mar
04
Teaser:

The ATLAS Experiment at the Large Hadron Collider with its sister experiment CMS reported a discovery last summer of a new boson which is consistent with the Standard Model Higgs boson.  The Higgs particle has been searched for decades. It is the final jewel in the Standard Model of particle physics, a crowning achievement of 20th century science that gives a powerful understanding of fundamental particles and their interactions. In the Standard Model, the Higgs is the quantum of a field that accounts for the masses of those particles.  We will describe the apparatus, the...

Feb
28
Teaser:

The control of quantum systems is limited by unwanted interactions with the environment and uncertainties in the applied control fields.  For ions these uncertainties include unwanted fluctuations in the intensity and the frequency of the electromagnetic field used to control the qubit. For unknown but static errors on the time scale of the experiment, compensating composite pulses sequences can be used to minimize the effect of these errors. In this talk, I will describe the general method of compensating composite pulse sequences for single qubit and multi-qubit systems. I will...

Feb
26
Teaser:

The physics of granular flow is of widespread practical and fundamental interest, and is also important in geology and astrophysics. One challenge to understanding and controlling behavior is that the mechanical response is nonlinear, with a forcing threshold below which the medium is static and above which it flows freely. Furthermore, just above threshold the response may be intermittent even though the forcing is steady. Two familiar examples are avalanches on a heap and clogging in a silo. Another example is dynamical heterogeneities for systems...

Feb
25
Teaser:

What could possibly be new in the Ising model, arguably the most-studied model of statistical physics?  Plenty!  Consider the Ising model initially at
infinite temperature that is suddenly cooled to zero temperature and evolves by single spin-flip dynamics.  What happens?  In one dimension, the ground state is always reached and the evolution can be solved exactly.  In two dimensions, the ground state is reached only about 2/3 of the time, and the long-time evolution is characterized by two distinct time scales, the longer of which arises from topological defects...

Feb
20
Teaser:

PLEASE NOTE: This is a WEBINAR

We investigate the effect of anharmonicity and interactions on the dynamics of an initially Gaussian wavepacket in a weakly anharmonic potential. We find that repeated perturbations can create revivals, echoes, and revival-echoes, with properties that can be controlled via the strength and symmetry of the perturbations.  We also find that depending on the strength and sign of interactions and anharmonicity, the quantum state can be either...

Feb
18
Teaser:

I present inelastic neutron scattering data from one- two- and three-dimensional insulating magnetic materials at low temperatures that do not display a coherent resonant mode of excitation. Instead, momentum resolved spectra take the form of bounded continua. I interpret the spectra as evidence for fractionalization of a spin flip into distinct quasi-particles.

Feb
14
Teaser:

The U.S. Naval Observatory provides the master clock for the DoD.  To support this mission, we have built and fielded 4 rubidium atomic fountain clocks at our Washington D.C. site.  This ensemble of clocks has been running continuously for slightly less than two years and is contributing to our larger ensemble of atomic clocks.

Feb
12
Teaser:

Many organisms fly in order to survive and reproduce. I am fascinated by the mechanics of flying birds, insects, and autorotating seeds. Their development as an individual and their evolution as a species are shaped by the physical interaction between organism and surrounding air. It is critical that the organism’s architecture is tuned for propelling itself and controlling its motion. Flying macroscopic animals and plants maximize performance by generating and manipulating vortices. These vortices are created close to the body as it is driven by the action of muscles or gravity, then are...

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