Eric Sembrat's Test Bonanza

Dark matter plays an important role in our current understanding of the universe.  Low surface brightness spiral galaxies are particularly excellent laboratories for probing the dark matter distribution and placing constraints on theoretical models.  I will discuss the observational techniques used to study these galaxies and their dark matter halos.  I will also discuss how well current dark matter models describe the observations and suggest how they might need to be updated.

The establishment and maintenance of boundaries and compartments between cell populations is essential for multicellular life. This process occurs reliably despite constant mechanical perturbations. For instance, during embryonic development and in wound healing, cells tug on each other as they rearrange and migrate. Similarly, mature tissue like the skin and muscle are exposed to continuous mechanical assault from their external environment. How cell populations maintain their integrity in the presence of mechanical stress is not understood at the molecular level. Cadherins are a family of cell-adhesion proteins that play a key role in mediating tissue integrity. Their principle function is to bind cells together and resist mechanical force. Here we use single molecule force measurements and computer simulations to identify how cadherins modulate their adhesion in response to mechanical stress. We show that in response to mechanical stimuli, cadherins alter their conformation and switch between three types of adhesive bonds: catch bonds which, counter-intuitively, become longer lived and lock in the presence of tensile force, slip bonds which become shorter lived when pulled and ideal bonds which are insensitive to tugging. Catch, slip and ideal bonds serve as a general mechanism that adhesive proteins use to withstand tensile force and tune the mechanical properties of intercellular junctions.

mso-fareast-font-family:"Times New Roman"">The annual Astro-GR meetings are to develop and strengthen the links between the astrophysics and relativity communities, by highlighting outstanding scientific issues which can only be resolved through collaboration of these groups. In that spirit, this workshop will dwell on questions which are intimately related to the formation, growth, and interaction of black holes with their environments, from cosmological scales all the way to the event horizon. - For more information please visit the official site at:

Astro-GR Meetings Site

I discuss "simple" dynamical systems on networks and examine how network structure affects dynamics of processes running on top of networks.  I consider results based on "locally tree-like" and/or mean-field and pair approximations and examine when they seem to work well, what can cause them to fail, and when they seem to produce accurate results even though their hypotheses are violated fantastically.  I'll also present a new model for multi-stage complex contagions--in which fanatics produce greater influence than mere followers--and examine dynamics on networks with hetergeneous correlations.  (This talk discusses joint work with Davide Cellai, James Gleeson, Sergey Melnik, Peter Mucha, J-P Onnela, Felix Reed-Tsochas, and Jonathan Ward.)

Recently¹ we have shown that a  "global phase space" (GPS) approach provides valuable understanding of the long-time coherence and Einstein-Podolsky-Rosen entanglement of a Bose-Einstein Condensate (BEC) trapped in a double-well optical lattice ("BEC dimer"). In particular, the GPS approach allows one to distinguish purely quantum effects from those which are captured by semi-classical methods.

The GPS approach in Ref. (1) was applied in the limit of zero dissipation. After reviewing the key results in this limit, we extend the approach to allow for dissipation and again compare the results with relevant experiments. Surprisingly, although consistent with some prior exploratory studies, we find that dissipation can actually enhance coherence in certain instances, particularly around self-trapped modes, corresponding to fixed points in the classical phase space. We explain a number of interesting features of this enhancement and argue that, in spatially extended systems (corresponding to multi-well optical lattices), these localized, self-trapped modes may also play a role in enhancing coherence.

^{*  With Ted Pudlik (BU), Holger Hennig (Harvard), and Dirk WItthaut (MPIDS-Göttingen)}

¹ Holger Hennig, Dirk Witthaut, and David K. Campbell, Phys. Rev. A 86,  051640 [R] 2012.

Forty years ago, Apollo astronauts placed the first of several retroreflector arrays on the moon.  Laser range measurements between the earth and the moon have provided some of our best tests to date of general relativity and gravitational phenomenology--including the equivalence principle, the time-rate-of-change of the gravitational constant, the inverse square law, and gravitomagnetism.  A new effort called APOLLO (the Apache Point Observatory Lunar Laser-ranging Operation) is now collecting measurements at the unprecedented precision of one millimeter, which will produce order-of-magnitude improvements in a variety of gravitational tests, as well as reveal more detail about the interior structure of the moon.  This talk will include an overview of the science, a description of the instrument and its performance, evidence for dust accumulation on the lunar surface, re-discovery of a lost Soviet reflector, and an outlook for advancing the state of gravity tests in the solar system.

We are pleased to announce that the 3rd Annual SESICB Southeastern Regional conference will be held on Friday, September 27, 2013 at Georgia Institute of Technology, located in the midtown district of Atlanta, Georgia. The conference will be held in Technology Square Research Building, Auditorium, located at 85 Fifth Street NW, Atlanta, Georgia 30332. 

The meeting will begin at 9:00 A.M. with a presentation from the keynote speaker. Additionally, we anticipate a full day of 10-minute talks covering topics pertaining to morphology and biomechanics of vertebrates, invertebrates, and plants. We especially encourage student talks, as the regional meetings provide a friendly environment in which to share research progress and new ideas. Talks will be interspersed with coffee breaks, giving plenty of opportunity for informal discussion. The meetings will finish at approximately 4 P.M. on Friday, which will give people some time to decompress and enjoy the city.

"Registration is free but mandatory"

Please register via the conference homepage

In 1953, Enrico Fermi, John Pasta, and Stan Ulam initiated a series of computer studies aimed at exploring how simple, multi-degree of freedom nonlinear mechanical systems obeying reversible deterministic dynamics evolve in time to an equilibrium state describable by statistical mechanics. Their expectation was that this would occur by mixing behavior among the many linear modes. Their intention was then to study more complex nonlinear systems, with the hope of modeling turbulence computationally.

The results of this first study of the so-called Fermi-Pasta-Ulam (FPU) problem, which were published in 1955 and characterized by Fermi as a “little discovery, ” showed instead of the expected mixing of linear modes a striking series of (near) recurrences of the initial state and no evidence of equipartition. This work heralded the beginning of both computational physics and (modern) nonlinear science. In particular, the work marked the first systematic study of a nonlinear system by digital computers (“experimental mathematics”) and led directly to the discovery of “solitons,” as well as to deep insights into deterministic chaos and statistical mechanics.

In this talk, I will review the original FPU problem and trace several distinct lines of research that arose from it. Specifically, I will show how a continuum approximation to the original discrete system led to the discovery of “solitions” and how recent treatments of the FPU and related spatially extended discrete systems reveal the presence of “Intrinsic Localized Modes” (ILMs)” and of “q-breathers.”

I will then describe briefly the basic mechanism that allows the existence of ILMs, discuss some of their essential features, and illustrate a few of the wide range of physical systems in which they have recently been observed. I will show how “q-breathers” can give a plausible quantitative explanation for the recurrence phenomenon observed by behavior by FPU and how these results can be reconciled with mixing, equipartition, and statistical mechanics.

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mso-ansi-language:EN-US">For centuries, platinum metal has been highly valued for its luster and rarity. With the technological revolution, Pt has found many applications owing to its high chemical inertness and catalytic properties. Recently, electronic properties of Pt have attracted a significant interest for possible spintronic applications. By passing an electrical current through a device comprised of a bilayer of Pt with a ferromagnet, one can modify the dynamical magnetic characteristics of the ferromagnet by the pure spin current generated in Pt by the spin Hall effect. I will describe our recent measurements demonstrating that one can utilize the spin Hall effect in Pt to significantly suppress or enhance thermal magnetization fluctuations in an adjacent ferromagnet, modify the dynamical damping rates, induce magnetization auto-oscillations, and reverse the magnetization. Finally, I will describe measurements indicating that not only Pt can affect the properties of the adjacent ferromagnet, but also the ferromagnet can induce magnetism in Pt, suggesting the possibility of intricate interplay between magnetism and spin transport.

mso-fareast-font-family:"Times New Roman"">All muscles generate force by converting chemical energy into mechanical work.  They do so via protein enzymes (myosin molecules)  that undergo conformational changes upon release of stored energy.  How they change shape and how they interact with each other via elastic or viscous coupling mechanisms remains an open problem in biophysical analyses of force generation by systems of molecular motors.  And, intriguingly, we have neglected the potential for force generation in multiple dimensions.  To address these issues, we combine computational models of interacting molecular motors with force measurement at the whole muscle level.  Additional we use high speed, time-resolved, small angle x-ray diffraction to reveal real-time dynamics of the molecular players associated with force generation and energy storage.