Physics Colloquia Spring 2014

(usually Fridays 2:00 PM in SE 319)

Titles link to the abstracts.

Date Speaker Title
Tuesday, Feb 18, 5:00PM
Rani Al-Senan
Feb 21
Ivan Agullo
Feb 28
Yasha Neiman
Tuesday, Mar 11, 10:00AM
Alessandra Corsi
Thursday, Mar 13, 10:00AM
Benjamin Owen
(Penn State)
Thursday, Mar 20, 10:00AM
Mark Scheel
Tuesday, Mar 25, 10:00AM
Eric Thrane
Mar 28
Stanimir Bonev
(Dalhousie University)
Mar 31
Ilya Mandel
(University of Birmingham)
Apr 11
Armin Fuchs
Thursday, Apr 17, 5:30PM
Polad Shikhaliev
Apr 25
Muxin Han, Centre de Physique Theorique

Colloquium Abstracts

Analyzing CT Dose Index (CTDI): Review and Experiment
Rani Al-Senan (UAB), Feb 18
Computed Tomography (CT) has become a crucial diagnostic tool in medical imaging. The continuing advances in CT scanner technology have shown significant improvement in image quality and scan time. Recent epidemiologic studies, however, have shown strong association between radiation doses from CT exams and cancer risk. While efforts continue to improve CT image quality and reduce (or maintain) the doses absorbed by the patient, the study of radiation dose measurements in CT is also growing. This talk will shed some light on the concept of CT dose index (CTDI); what it actually represents and how it is measured. The talk will also present the results of CTDI measurements on standard and custom-made acrylic phantoms which were performed to estimate both primary and scatter contributions to the CTDI.
Quantum Gravity and the Observable Universe
Ivan Agullo (LSU), Feb 21
An important difficulty in the search for a satisfactory theory of quantum gravity is the absence of experimental guidance. The astonishing improvement in cosmological observations attained in the last few years offers an exciting opportunity to change this situation. It is believed that the anisotropies observed in the CMB and in the distribution of galaxies were originated in the very early universe. Observing their details could therefore tell us about physics in such extreme conditions. In this talk I will review the physics of the genesis of cosmic non-uniformities, paying special attention to the interplay between quantum effects and gravitation. I will describe how cosmological observations can provide detailed information about processes where the relationship between gravity and quantum mechanics plays a crucial role.
Space is entangled - and the action knows.
Yasha Neiman (PSU), Feb 28
It is suspected that the well-known formula for black hole entropy in fact computes the *entanglement* entropy across surfaces in spacetime. After reviewing this conjecture, I relate it to some newly-discovered surprising properties of the gravitational action. Specifically, the action has a non-additive imaginary part, in contrast with the usual expectations from amplitude composition and unitarity. This imaginary part is given universally by the black hole entropy formula, regardless of matter content or higher-curvature couplings. I explain these classical results and show how they support the entanglement entropy conjecture. I also mention some implications for loop quantum gravity.
Gamma-ray bursts, Gravitational Waves, and Multi-messenger Exploration of the Transient Sky
Alessandra Corsi (The George Washington University), Mar 11
Progress in our understanding of gamma-ray bursts (GRBs) has been spectacular. We know that these events are the most relativistic explosions of stellar origin. However, fundamental questions such as the link between GRBs and supernovae, the geometry and composition of the GRB fireball, remain a mystery. In addition, the nature of GRB progenitors is yet to be probed directly. In this talk, I will present the work I am carrying out to help answer these intriguing questions. In view of the upcoming advanced ground-based gravitational wave detectors, I will describe how multi-messenger studies will soon provide us a completely new view of GRBs and of the transient sky.
Gravitational waves from young neutron stars
Benjamin Owen (Penn State University), Mar 13
Continuous gravitational waves can tell us a lot about their sources - rotating neutron stars - because they couple to more of the physics than any other type of signal. Whether the emission mechanism is hydrostatic perturbations (mountains) or dynamic ones (oscillating r-modes) it couples to the elasticity of the solid crust, superfluidity of the mantle, viscosity of the non-super fluid, and indeed connects to all ten volumes of the Landau-Lifschitz course on theoretical physics. I give a brief tour of the complicated physics associated with continuous gravitational-wave emission and describe recent efforts to hunt for it in the most likely places: young neutron stars and likely hiding places such as pulsar wind nebulae and supernova remnants.
Exploring the Collision of two Black Holes
Mark Scheel (Caltech), Mar 20
Gravitational-wave detectors such as LIGO are expected to be regularly detecting waves from astrophysical phenomena within a few years. A key source of these gravitational waves is a binary system consisting of two black holes in orbit around each other. Recently it has become possible to develop numerical codes that accurately solve Einstein's equations on a computer. In this way one can follow in detail the orbit of two black holes, the decay of this orbit via energy loss to gravitational radiation, the collision and merger of the two black holes into one larger black hole, and the relaxation of this remnant black hole to a quiescent state. We discuss one of these numerical codes, and how we use it to investigate the dynamical behavior of event horizons, the strong-field dynamics of black hole collisions, and the generation of gravitational waves. We discuss how numerical simulations can assist gravitational-wave observations to learn about gravitational-wave sources such as black hole binaries.
The dawn of gravitational-wave astronomy with Advanced LIGO
Eric Thrane (Caltech), Mar 25
The LIGO experiment seeks to detect gravitational waves--minute ripples in the fabric of spacetime caused by some of the most energetic events in the universe. Commissioning of the newly upgraded Advanced LIGO is well underway, and we expect our first science quality data in the coming 1-2 years. Advanced LIGO will see gravitational-wave sources ten times further than the previous initial LIGO experiment, facilitating the detection of potentially dozens of events every year. The coming era of gravitational-wave astronomy will yield important results for astrophysics, cosmology, and fundamental physics. In this talk, I will describe Advanced LIGO and the compelling science of observational gravitational-wave astronomy. I will present my work searching for different gravitational-wave signals in initial LIGO data, characterizing detector performance, and understanding problematic sources of instrumental noise.
Materials under extreme conditions
Stanimir Bonev (Dalhousie University), Mar 28
Knowledge of materials under extreme pressure (1-104 gigapasals) and temperature (102-104 K) conditions is of fundamental scientific importance and crucial for understanding the physics and chemistry of planets - their structure, formation and climate. Extreme conditions can also offer unique ways for the synthesis of materials with novel mechanic, optical, and bonding properties. The state of a compressed many-body system often depends on a delicate balance between various energy terms. Minor variations in their relative significance can lead to dramatic phase transitions. In some cases, striking phenomena can be explained with subtle changes in microscopic structure and electronic properties. This puts stringent requirements for the accuracy of the theory used to predict high-pressure phenomena. Interpretation of experiments can be challenging as well for the same reasons, especially at high or low temperatures, where collecting in situ data under well-controlled environment is hard. In this talk, I will describe some of our theoretical studies of materials under extreme conditions. Our goals include the discovery of novel energetic solids, prediction of phase stability at high pressure and finite temperature, and understanding the role of semi-core electron interactions and lattice dynamics at extreme compressions. I will illustrate these topics with examples from recent work - the prediction of polymeric metallic nitrogen, establishing the stability of carbon dioxide at planetary conditions, and studies of light metals, where the interplay between core electron interactions and lattice dynamics gives rise to rich structural diversity.
Gravitational wave astrophysics of compact binaries
Ilya Mandel (University of Birmingham), Mar 31
The first direct detections of gravitational waves from mergers of black holes and neutron stars by ground-based interferometric detectors LIGO and Virgo are anticipated in the next 5 years. They will provide us with a new probe on the Universe and enable us to explore compact-binary astrophysics and strong-field general relativity. In this talk, I review the current knowledge of the coalescence rates and parameter distributions of merging neutron-star and black-hole binaries and the data-analysis challenges associated with finding their signatures in noisy data. I report on ongoing efforts to develop a framework for converting gravitational-wave observations into improved constraints on astrophysical parameters and tests of general relativity, and discuss future developments necessary to the success of gravitational-wave astronomy.
Diffusion Tensor Imaging and Its Application to Mild Traumatic Brain Injuries
Armin Fuchs (FAU), Apr 11
Mild traumatic brain injuries (MTBI), in most cases, cannot be detected using imaging modalities like CT or MRI. However, diffusion tensor imaging (DTI) reveals subtle changes in white matter integrity as a result of head trauma and plays an important role in refining diagnosis and management of MTBI. We use DTI to detect the microstructural changes in collegial football players induced by axonal injuries and to monitor their evolution during the recovery process. Three players suffered a MTBI during play or practice and underwent scanning within 24h with follow-ups after one and two weeks. Scalar diffusion indices were derived from diffusion tensors and analyzed using tract-based spatial statistics (TBSS) and voxel-wise t-tests to detect brain regions showing significant group differences between the injured subjects and controls. Both analyses revealed overlapping regions in the corticospinal tract with significant increase in fractional anisotropy and decreases in transverse and mean diffusivity within 24h. In voxel-wise t-tests strong indications for recovery were found spatially and temporally. For mean and transverse diffusivity, regions showing significant differences shrunk between the first and the follow-up scans. Although the sample size is small, these findings are remarkably consistent across all subjects and scans.
Low-dose, material selective CT with photon counting technology
Polad Shikhaliev, Apr 17
Computed Tomography (CT) was introduced in 1973 by Hounsfield, who received Nobel Prize for this work. Since this time, CT became a gold standard for medical imaging. Despite that, current CT systems exhibit some major limitations. They use so-called energy integrating x-ray detectors that are suboptimal for CT imaging applications. These detectors absorb x-ray photons, sum (integrate) the energies of all photons, and provide a single analog signal. Thus, the information about energy of each x-ray photon is lost. Lack of the energy information prevents material selective CT imaging and results in higher image noise which requires increasing radiation dose to the patient. Because x-ray photons are naturally discrete, each photon could be detected (counted) separately, and also, its energy could be measured. Such a photon counting energy selective data acquisition could allow for material selective CT imaging at a single CT scan and fixed x-ray tube potential. In addition, electronics noise could be rejected, and image noise could be further decreased by photon energy weighting. As a result, patient dose could be decreased substantially. Although these possibilities were well known previously, it was technologically challenging to count large numbers of x-ray photons by detector pixels at very high count rates, to measure the energy of each photon, and to save large amount of digital data, all during several seconds allocated for clinical CT scans. Remarkable advancements have occurred in the last decade or so in chip-level micro-electronics developments, computer speeds and memories, and detector technologies. These advancements brought the photon counting CT technology to the level where it can meet many of the demands of clinical applications. Thus, photon counting CT research became a hot topic.
In this talk we will review history of the photon counting CT, its current status, existing challenges, and our contributions to this field.
Loop Quantum Gravity and Quantum Geometry
Muxin Han, Apr 25
Loop Quantum Gravity is a promising theory of non-perturbative quantum gravity. It is also a theory of quantum space and time. In this talk, the motivations, ideas, and concepts in Loop Quantum Gravity will be introduced in a non-technical language. The properties and consequences of quantum spacetime geometry will be introduced. Some of the main results and open issues will be reviewed in the end.