Eric Sembrat's Test Bonanza

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Abstract

The fundamental constants of nature must fall within a range of values in order for the universe to develop structure and ultimately support life. This talk considers the current constraints on these quantities and assesses the degree of fine-tuning required for the universe to be viable. The first step is to determine what parameters are allowed to vary. In the realm of particle physics, we must specify the strengths of the fundamental forces and the particle masses. The relevant cosmological parameters include the density of the universe, the cosmological constant, the abundance of ordinary matter, the dark matter contribution, and the amplitude of primordial density fluctuations. These quantities are constrained by the requirements that the universe lives for a sufficiently long time, emerges from its early epochs with an acceptable chemical composition, and can successfully produce galaxies. On smaller scales, stars and planets must be able to form and function. The stars must have sufficiently long lifetimes and hot surface temperatures.  The planets must be large enough to maintain atmospheres, small enough to remain non-degenerate, and contain enough particles to support a biosphere.

We also consider specific fine-tuning issues in stars, including the triple alpha reaction that produces carbon, the case of unstable deuterium, and the possibility of stable diprotons. For all of these issues, the goal of this enterprise is to delineate the range of parameter space for which universes can remain habitable.

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There is currently a strong movement worldwide toward portable, battery operated wirelessly connected devices such as GPS receivers, cellular telephones and laptop computers and tablets. This new generation of electronics enables vast new capability, but also comes with new challenges such as bandwidth limitations, sensitivity to jamming and reduced access to calibration. A new generation of miniature, low-power, low-cost precision instruments is being developed at NIST for use in such portable technologies. These include clocks, magnetometers, gyros and wavelength references, all based on precision atomic spectroscopy and using emerging new fabrication capabilities such as microelectromechanical systems and photonics.

This talk will describe the design, fabrication and performance of these instruments, as well as touch on several applications to which they are well-suited. Finally, we will speculate on future opportunities for these types of devices such as compact instruments based on laser-cooled atoms and a broader view on chip-scale metrology.

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The surface of eukaryotic cells is covered with a dense layer of glycans that affect numerous physiological processes and aberrant cell-surface glycosylation has been implicated in cancer, inflammation and other diseases. Using a combination of metabolically labeled glycans with bioorthogonal click reactions, we implement single molecule tracking and super-resolution imaging of cell-surface glycans on live cells. We have developed a form of confocal interference microscopy, based upon laser feedback interferometry, to simultaneously measure the topography of the membrane.

We seek to experimentally verify the predictions of our model of energetics that govern the nucleation, growth and disassembly of integrin adhesions in the presence of long glycan molecules. Our experimental approach has enabled the visualization of dynamic tunneling nanotubes connections between cells and the tracking of glycosylated receptors along the surface of these membranes.

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Thermoacoustic oscillations occur when a flame is confined within an acoustic cavity, such as a combustion chamber. Their amplitude grows if the oscillating flame releases more heat at times of higher pressure and less heat at times of lower pressure. The phase between the pressure and the additional heat release is critical. It can vary from cycle to cycle, resulting in quasiperiodic, multi-periodic, or chaotic oscillations, as observed in experiments and numerical simulations.

Simulations also reveal a multitude of periodic and quasiperiodic unstable attractors, which attract the system in many directions in phase space and repel in one direction. The system's state can pass within the vicinity of several unstable attractors before arriving at a stable attractor, which has similar features to bypass transition to turbulence in hydrodynamics. Sometimes small differences in initial states lead to diverging paths in phase space and different final states.

In some linearly stable thermoacoustic systems, thermoacoustic oscillations can be triggered by a small pulse. A simple thermoacoustic system containing a stable fixed point, an unstable periodic solution and a stable periodic solution is examined. The `minimal seed' is found with nonlinear adjoint looping. Growth to the stable periodic solution is shown to exploit non-normal transient growth around the unstable periodic solution, rather than non-normal transient growth away from the fixed point.
 

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Bharath Hebbe Madhusudhana, M.S. in Mathematics, Ph.D. in Physics

Wednesday, December 12, 2018

While in high school, Bharath Hebbe Madhusudhana wanted to be a mathematician or a physicist. Now, he takes home degrees in the two fields he esteems the most: an M.S. in Mathematics and a Ph.D. in Physics.

The mathematics degree was almost an afterthought. When Bharath began his Ph.D. program in physics, he also started taking one graduate-level class in mathematics per semester. Before long, he needed only a few more, as well as a thesis, to complete the requirements of the master’s degree.

Prior to Tech, Bharath completed his undergraduate degree in physics in the Indian Institute of Technology (IIT) Kanpur, in Uttar Pradesh, India. He knew he would do a Ph.D. “I joined Georgia Tech in the pursuit of a place where cutting-edge research was being done,” he says.

At Tech, Bharath not only studied his major fields but also pushed himself to communicate his science well. In 2016, he participated in Georgia Tech’s Three Minute Thesis Competition. Competitors explained their research to a diverse audience in just three minutes.

At the time, Bharath was a fourth-year Ph.D. student. He had discovered something fundamental about rubidium atoms: When cooled to about 170 nanoKelvins – almost absolute zero – and exposed to a magnet that traces a circle around them, the very-low-energy rubidium atoms can remember something abstract. They can tell the area of an abstract surface – called the Boy’s surface – corresponding to the real traced circle.

For his spirited explanation of how atoms, when cooled to almost immobility, remember abstract geometric phenomena, the judges named Bharath the third-place winner and the audience voted him as one of two winners of the People’s Choice award.

What is the most important thing you learned at Georgia Tech?
Apart from the technical knowledge necessary to conduct scientific research in my area, I learned the art of academic communication and collaboration in research. The papers I wrote and the conferences I attended helped me learn the basics of communicating my research work. While working with multiple faculty members at Georgia Tech, I gained experience in scientific collaboration.  

What is your proudest achievement at Georgia Tech?
One of my research papers was rejected three times in a row by the same journal. However, with a carefully crafted rebuttal, I got it published after the fourth resubmission. The process was challenging, but I was supported extensively by the faculty members at Georgia Tech. 

Which professor(s) or class(es) made a big impact on you?
I gained a lot from the technical guidance of my advisor, Professor Michael Chapman. I owe my experimental skills and my intuitive understanding of atomic physics to him. He also provided valuable advice on crucial career-related decisions that I had to make in the later part of my Ph.D. work. His guidance has been pivotal in my professional development.

Professors Brian Kennedy and Carlos Sa de Melo also made a significant impact on my understanding of physics.

Professor Kennedy was always welcoming and available to talk about the theoretical aspects of our experiment. The discussions he had with me helped steer my research work into a productive direction. He also helped me extensively in writing a theoretical research paper and getting it published. During this process, with Professor Kennedy’s support, I learned how to respond to critical reviews of a research paper.

Being an experimental atomic physicist, I owe almost all my understanding of condensed-matter theory to Professor Sa de Melo. He is very friendly and always enthusiastic to talk about physics. I remember several late-night discussions with him in the laboratory, which resulted in a research paper that he and I wrote.  

I am grateful to two professors from the School of Mathematics, Greg Blekherman and John Etnyre.

As my master’s thesis advisor, Professor Blekherman is responsible for my technical knowledge in the area of convex optimization.  He was kind and accommodating as a thesis supervisor.

Professor Etnyre helped me understand the mathematical basis of my thesis project, which involved the fascinating subject of topology. He was always made himself available for discussions, from which I benefited greatly.   

What is your most vivid memory of Georgia Tech?
I have several.

Professor Sa de Melo would sometimes come to our lab at 9 PM. Along with a freshly brewed pot of tea, we talked about physics. Sometimes, we would lose track of time, only to realize that it is past 1 AM and we should call it a day. These discussions alone have resulted in a couple of research papers.

In the evenings, I would go on long walks, circling the campus area, occasionally stopping at the Campus Recreation Center for a swim or rock climbing or a game of ping-pong.

In what ways did your time at Georgia Tech transform your life?
Professionally, I now have a clear view of what I am going to do. At Georgia Tech, along with the acquiring the necessary technical skills, I developed an understanding of the goals of the specific research field. This understanding helped me decide what I want to do next.  

What unique learning activities did you undertake?
In 2016, Professor Chapman encouraged me to participate in the Three Minute Thesis (3MT) Competition at Tech. The challenge was to communicate my thesis work in three minutes to a nonexpert audience.

While preparing for 3MT, I learned the art of oral communication, and it changed the way I presented my work at conferences thereafter. I was fortunate to win prize money, which I used to attend a conference. Professor Chapman had the foresight to know that participating in 3MT would be a good step in my professional development.

What advice would you give to incoming graduate students at Georgia Tech? Georgia Tech has vast intellectual wealth, held by the numerous knowledgeable faculties in various disciplines. I would advise incoming graduate students to make use of this resource, as well as the facilities available on campus, to maximize their intellectual development during their time here.

Where are you headed after graduation?
I am starting a postdoctoral position at the Max Planck Institute of Quantum Optics, in Garching, Germany.

Professors Chapman, Kennedy, and Sa de Melo helped me develop the skills and confidence to continue in academia.They prepared me for an academic career, particularly for this postdoctoral position.

Media Contact: 

A. Maureen Rouhi, Ph.D.
Director of Communications
College of Sciences

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The ‘Wet-Dog Shake’ And Other Physics Mysteries

Thursday, November 8, 2018

Ever wondered why your dog’s back-and-forth shaking is so effective at getting you soaked? Or how bugs, birds, and lizards can run across water—but we can’t? Or how about why cockroaches are so darn good at navigating in the dark? 

Those are just a few of the day-to-day mysteries answered in the new book How to Walk on Water and Climb Up Walls: Animal Movement and the Robots of the Future, by Georgia Tech's David Hu. Hu is an associate professor in the Schools of Mechanical Engineering and of Biological Sciences and an adjunct associate professor in the School of Physics.  

Listen to the Science Friday episode that aired on Nov. 2, 2018.

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H-Index High Scorers

Tuesday, November 6, 2018

The h-index has become a widely cited measure of a researcher’s influence and citation impact. A researcher with an index of h has published h papers which have been cited in other papers at least h times. If someone has an h-index of 100, it means that 100 papers by this researcher have been cited  in at least 100 other research publications.

Scoring an h-index of more than 100 is a singular feat that is achieved by few researchers. According to a recent survey of public Google Scholar profiles, several professors in the College of Sciences have achieved this milestone:

  • Jean-Luc Bredas, School of Chemistry and Biochemistry
  • Mostafa El Sayed, School of Chemistry and Biochemistry
  • Uzi Landman, School of Physics
  • Seth Marder, School of Chemistry and Biochemistry
  • Zhong Lin Wang, School of Chemistry and Biochemistry, adjunct
  • Younan Xia, School of Chemistry and Biochemistry

Image Credit: en:user:Ael 2, vectorized by pl:user:Vulpecula - vectorized version of File:H-index_plot.PNG, Public Domain, https://commons.wikimedia.org/w/index.php?curid=8716387

Media Contact: 

A. Maureen Rouhi, Ph.D.
Director of Communications
College of Sciences
 

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Abstract

Will discuss chaos in quantum many-body systems, specifically how it is relates to thermalization and how it fails in many-body localized states. I will conjecture a new universal form for the spreading of chaos in local systems, and discuss evidence for the conjecture from a variety of sources including new large-scale simulations of quantum dynamics of spin chains.

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The human brain accounts for just 2% of the body's mass but metabolizes 25% of its calories, producing significant metabolic waste. However, waste buildup links to neurodegenerative diseases like Alzheimer's and Parkinson's. The brain removes waste via the recently-discovered glymphatic system, a combination of spaces and channels through which cerebrospinal fluid flows to sweep away toxins like amyloid-beta. With an interdisciplinary group of neuroscientists and physical scientists, I study the physical processes of the glymphatic system: Where does fluid flow, and how fast? What drives flow? Does flow shear cause waste accumulation? How can we improve waste removal? Can we use glymphatic flow to deliver drugs? The team combines physics tools like particle tracking and newly-invented front tracking with biological tools like two-photon imaging through cranial windows in order to address these questions with in vivo flow measurements. I will talk about recent results showing that glymphatic flow pulses with the heart, is driven by artery walls, and can be manipulated by changing the wall motion. 

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How Elephants Eat with Their Trunks

Tuesday, October 23, 2018

How do Elephants Eat With Their Trunks?

By Roni Dengler | October 23, 2018 6:01 pm

As the largest land mammal on the planet, elephants eat a lot of food. On average, the giants consume more than 440 pounds of vegetation per day, or the equivalent of about two corncobs per minute. And now, scientists have figured out how the beasts are able to eat so much so fast....To figure out how elephants pick up so much stuff at a time, David Hu and his colleagues filmed a 34-year-old female African elephant from the Atlanta zoo as she consumed loads of carrots and rutabagas cut into differently sized cubes, as well as heaps of bran.

Read the full story by Roni Dengler here.

David Hu is an associate professor in the Schools of Mechanical Engineering and Biological Sciences and an adjunct associate professor in the School of Physics. 

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