Eric Sembrat's Test Bonanza

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

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.

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.

Research on precise control of quantum systems occurs in many laboratories throughout the world, for fundamental research, new measurement techniques, and more recently for quantum information processing. I will briefly describe experiments on quantum state manipulation of atomic ions at the National Institute of Standards and Technology (NIST), which serve as examples of similar work being performed with many other atomic, molecular, optical (AMO) and condensed matter systems across the world. This talk is the “story” of my involvement in these subjects that I presented at the 2012 Nobel Prize ceremonies.

I will discuss three fluid-mechanics problems: fluid motions related to drinking, clapping, and bouncing, which you might have experienced or observed once during daily activities.

Drinking: Drinking is defined as the animal action of taking water into the mouth, but to fluid mechanists, is simply one kind of fluid transport phenomena. Classical fluid mechanics show that fluid transport can be achieved by either pressure-driven or inertia-driven processes. In a similar fashion, animals drink water using pressure-driven or inertia-driven mechanisms. For example, domestic cats and dogs lap water by moving the tongue fast, thereby developing the inertia-driven mechanism. We will investigate how cats and dogs drink water differently and discuss the underlying fluid mechanics.

Clapping: Droplets splash around when a fluid volume is quickly compressed. This phenomenon has been observed during common activities such as kids clapping with wet hands. The underlying mechanism involves a fluid volume being compressed vertically between two objects. This compression causes the fluid volume to be ejected radially and thereby generate fluid threads and droplets at a high speed. In this study, we designed and performed laboratory experiments to observe the process of thread and drop formation after a fluid is squeezed.

Bouncing: When two fluid jets collide, they can bounce off each other, due to a thin film of air which keeps them separated. We describe the stable non-coalescence phenomenon between two jets of the same fluid, colliding obliquely with each other. Using a simple experimental setup, we carry out a parametric study of the bouncing jets by varying the jet diameter, velocity, collision angle, and fluid viscosity, which suggests a scaling relation that captures the transition of colliding jets from bouncing to coalescence. This parameter draws parallels between jet coalescence and droplet splashing (crown-splash), indicating that the transition is governed by a surface instability.

Combining knowledge of physics with a healthy willingness to estimate and approximate, we can say some rather profound things about future paths available to our society in terms of energy and resources.  Topics such as growth, global warming, fossil fuels and their potential replacements, and energy storage are ripe targets for back-of-the-envelope quantification, and will be explored in this talk.  A subtext is that we should not take 

for granted that superior substitutes will replace fossil fuels.

Since the discovery of superconductivity - the ability of certain materials to conduct electricity without dissipation - at the laboratory of Kamerlingh Onnes in 1911, the phenomenon has become ubiquitous in Nature. Over the span of a century, innumerous superconductor s – also called charged superfluids - have been discovered starting from the element Mercury to more complex materials such as Copper Oxides.  In addition, several neutral superfluids were discovered ranging from liquid Helium to ultra-cold atoms.   A key characteristic of neutral or charged superfluids is that they allow the flow of energy through the material without dissipation.  Such exotic property has been found in certain metals, neutron stars, nuclei and ultra-cold atoms, but they are still of limited use.  In order to engineer superfluid systems and take advantage of their properties, it is necessary to understand them and learn how to control them – a task that requires time, substantial investments, extensive research, and often good luck. Over the last several decades, scientists made important fundamental and technological advances that made possible the control of some desirable properties of charged and neutral superfluids, which can now be used in medical imaging, levitating trains, submarine propulsion, generators, gyroscopes and space applications.  Calibri;mso-fareast-theme-font:minor-latin;mso-hansi-theme-font:minor-latin;
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Galaxy collisions and mergers are a common consequence of the structure formation in the universe. We know that they happen because we see a number of beautiful examples on the sky through the "eyes" of many astronomical observatories. It is also thought that almost every galaxy (including our own, the Milky Way) harbors a supermassive black hole at its center. I will discuss the "knowns" and "unknowns" in the evolution of supermassive black hole pairs that form in collisions of their host galaxies and end their cosmic journey when the two black
holes merge due to the emission of gravitational waves.

Dr. Robert Liu trained as a condensed matter experimentalist working on quantum transport and noise in mesoscopic semiconductors, but always believed that the true value of a physics doctorate was in learning how to think through and solve problems. As a graduate student at Stanford, he became fascinated with questions about the brain and memory, and decided to apply his quantitative training from physics to the study of neuroscience. He moved to the University of California at San Francisco’s Sloan-Swartz Center for Theoretical Neurobiology to begin postdoctoral work on studying the neural code used in sensory systems. What is the brain signaling about the outside world? How are stimuli that are behaviorally relevant to us represented in neural activity? How do we evaluate this code quantitatively? Is it efficient? These are some of the questions that initially drew his curiosity, and he pursued a principled approach to addressing them grounded in studying how natural stimuli are processed in the brain. At UCSF, he collaborated with others to develop a mouse electrophysiology preparation to study these in the context of auditory processing of natural communication sounds. He expanded this work from an anesthetized mouse preparation to recordings in awake mice.

He will discuss both his own scientific trajectory and some selected results at the intersection of interests from both neuroscience and physics perspectives.

I present a new numerical code constructed to obtain accurate simulations of encounters between a star and a massive black hole. The relativistic tidal interaction is calculated in \emph{Fermi normal coordinates} (FNC). This formalism allows the addition of an arbitrary number of terms in the tidal expansion. Although Newtonian hydrodynamics and self-gravity is assumed for the star, there are several significant terms in the expansion that should be retained. I give the relevant orbital post-Newtonian terms. The three-dimensional parallel (MPI) code includes a PPMLR hydrodynamics module to treat the gas dynamics and a Fourier transform-based method to calculate the self-gravity. Results are given for a white dwarf ($n=1.5$ polytrope) with comparisons between simulations and predictions from the linear theory of tidal encounters. The encounters are at the threshold of disruption ($\eta=1-6$) for white dwarf to black hole mass ratios $\mu \sim 10^{-5}-10^{-3}$. It is shown that the inclusion of the octupole ($l=3$) tidal term will cause the center of mass of the star to deviate from the origin of the FNC. Also shown is a relativistic suppression in the amount of energy deposited onto the star. Finally, I estimate the new orbital parameters for the star after it passes by the black hole.