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This Week at Physics

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Events During the Week of November 11th through November 17th, 2018

Monday, November 12th, 2018

Plasma Physics (Physics/ECE/NE 922) Seminar
Advances and discoveries en route to magnetically confined electron-positron plasmas
Time: 12:05 pm
Place: 2241 chamberlin hall
Speaker: Dr. Eve Stenson, IPP Garching
Abstract: The large mass imbalance between ions and electrons produces a
separation of the two species' length and time scales that is a
cornerstone of traditional plasma physics. To consider the behavior of a
"pair plasma", comprising particles with opposite charge but equal mass,
is to revisit much of plasma physics from the ground up. Since the idea
was first introduced four decades ago, on the order of 1000 papers have
explored this topic via a variety of analytical and computational
treatments, but the experimental side of the investigation is still in
its nascence. Laboratory studies of matter-antimatter plasmas will
enable new tests of simulation and theory predictions, with implications
for our understanding of fundamental plasma science, astrophysical
phenomena in which pair plasmas play a role, and also traditional
electron-ion plasmas.

Toward these ends, the goal of the APEX (A Positron Electron eXperiment)
collaboration is to create and study electron-positron plasmas confined
in the magnetic field of a levitated dipole. A key milestone that was
recently achieved is the demonstration of lossless injection of
low-energy (e.g., 5-eV) positrons into a prototype dipole trap, in which
the positrons can then be confined for several seconds (corresponding to
hundreds of thousands of toroidal transits). Another recent result of
note is the discovery that phosphor screens (long used to diagnose both
matter and antimatter) produce significantly more luminescence from
incident positrons than incident electrons. This has potential to be a
new means of investigating luminescent materials, in addition being of
significant utility for nonperturbatively diagnosing low-energy positron
beams and plasmas.

SUPPORTED IN PART BY A GENEROUS GRANT FROM THE WOMEN'S SCIENCE &
ENGINEERING LEADERSHIP INSTITUTE (WISELI)
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Tuesday, November 13th, 2018

Chaos & Complex Systems Seminar
Artificial general intelligence
Time: 12:05 pm
Place: 4274 Chamberlin (Refreshments will be served)
Speaker: Bill Hibbard, UW Space Science and Engineering Center
Abstract: In the early 1960s Ray Solomonoff combined Turing's theory of computation with Shannon's information theory to create algorithmic information theory. The Kolmogorov complexity of a binary string is defined as the length of the shortest program that computes the string. Solomonoff used a related measure as the basis for an (uncomputable but approximable) universal induction algorithm for predicting arbitrary binary strings. In the early 2000's Marcus Hutter combined this induction algorithm with sequential decision theory to define his universal AI that maximizes expected rewards from arbitrary environments, and to define a formal measure of intelligence. This work led to conferences and journals dedicated to the mathematical study of properties of artificial general intelligence (AGI) systems, including ways that they may fail to conform to the intentions of their designers and ways to design systems that do conform to their design intentions. These problems are not resolved and research is very active. While some mainstream AI developers criticize AGI theory, the creators of some of the most successful AI systems (e.g., Google DeepMind) are also deeply involved in this AGI research. Practical versions of Hutter's universal AI are called Bayesian program learning and in some ways they outperform the deep learning algorithms that are revolutionizing AI.
Host: Clint Sprott
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Council Meeting
council meetings
Time: 3:30 pm
Place: 2314 Chamberlin Hall
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Wednesday, November 14th, 2018

Department Meeting
Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
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Thursday, November 15th, 2018

R. G. Herb Condensed Matter Seminar
Optically-pumped dynamic nuclear polarization under ambient conditions via nitrogen-vacancy centers in diamond
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Prof. Carlos Meriles, City College of New York
Abstract: A broad effort is underway to improve the sensitivity of nuclear magnetic resonance through the use of dynamic nuclear polarization. Nitrogen-vacancy (NV) centers in diamond offer an appealing platform because these paramagnetic defects can be optically polarized efficiently at room temperature. This presentation surveys alternative NV-based 13C spin polarization protocols, with emphasis on recent schemes designed for powder geometries. Through experimental, analytical, and numerical work, we show that 13C spins polarize efficiently for virtually all orientations of the NV axis relative to the applied magnetic field and over a broad range of hyperfine couplings. We will also discuss the mechanics of the polarization of 13C spins through the interaction of the NV with other ‘proxy’ paramagnetic defects, with attention to the interplay that emerges between spin cross-relaxation and mechanical rotation of the crystal as a whole.
Host: McDermott
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Cosmology Journal Club
Time: 12:00 pm
Place: 5242 Chamberlin Hall
Abstract: Please visit the following link for more details:
http://cmb.physics.wisc.edu/journal/index.html
Feel free to bring your lunch!
If you have questions or comments about this journal club, would like to propose a topic or volunteer to introduce a paper, please email Ross Cawthon (cawthon@wisc.edu) and Santanu Das (sdas33@wisc.edu).
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Astronomy Colloquium
"The Journey or the Destination: Isolating the Origin of the Physics Driving Gas Conditions in Galaxy Nuclei"
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and Cookies 3:30 PM, Talk begins at 3:45 PM
Speaker: Elisabeth Mills, Brandeis University
Abstract: Centers of galaxies are some of the most extreme objects in our universe: hosting starbursts and active supermassive black holes that can launch jets and winds far outside the compact galaxy nucleus. While there are relics of an active past in the center of our own Milky Way, at present it does not exhibit any of this activity. However, the central 300 parsecs of our Galaxy does contain a sizable reservoir of molecular gas that is the fuel for future star formation and black hole accretion. Constraining the physical conditions of this gas is critical for understanding how this reservoir will evolve to influence future activity in the Milky Way’s nucleus. Determining the origin of these conditions is also key to determining whether the same physics that govern gas conditions in this region can help us interpret more distant and active galaxy nuclei. I will present the results of my recent work following the changes in physical properties of this gas as it approaches the black hole; increasing in temperature, density, and turbulence, while largely resisting the onset of star formation. This work provides evidence that the extreme gas conditions in this region are driven largely by infall processes: the journey it takes to reach the central parsecs, rather than the energetic phenomena (supernovae, cosmic rays, massive star winds, UV radiation, and occasional X-ray flaring) encountered at its destination. However, as our Galactic center is relatively inactive, the next challenge is determining the extent to which the understanding gained from a detailed study of this region can be applied to more active systems. I will discuss early results from my ALMA program to make parsec-scale observations of the ionized and molecular gas in the center of NGC 253, a nearby galaxy with an order of magnitude more star formation and molecular gas that hosts a massive molecular outflow. Comparison of these two galaxy centers will isolate the gas conditions that both govern and are influenced by a nuclear starburst, and allow the definition of local templates for understanding the physics of this feedback process.
Host: Professor Snezana Stanimirovic
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Special Joint Condensed Matter - Atomic Molecular Optical Physics Seminar
Time-varying shear strain as an ultrafast symmetry switch in a Weyl semimetal
Time: 4:30 pm
Place: 5310 Chamberlin Hall
Speaker: Aaron Lindenberg, Standford University
Host: Jim Lawler
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Friday, November 16th, 2018

Theory Seminar (High Energy/Cosmology)
Dynamical field range and mass hierarchies
Time: 2:00 pm
Place: 5280 Chamberlin Hall
Speaker: Aitor Landete, University of Wisconsin-Madison
Abstract: Several swampland conjectures suggest that there is a critical field range beyond which the effective field theory description breaks down in quantum gravity. In this talk we will review applications of these conjectures to axion monodromy models. We argue that the field range of interest is the field space distance traced by the physical trajectory that solves the equations of motion. A sufficiently large mass hierarchy can delay the breakdown of the effective field theory and allow simple techniques of moduli stabilization. In absence of such hierarchy multi-field techniques should be used. We illustrate these subtleties in Type II string compactifications.
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Physics Department Colloquium
Terahertz frequency topological switches
Time: 3:30 pm
Place: 2241 Chamberlin Hall
Speaker: Aaron Lindenberg, Standford University
Abstract: Novel characterization techniques developed over the past two decades have revolutionized our ability to visualize the microscopic, atomic-scale processes that determine the functional properties of materials. The overarching challenge here is that the relevant time-scales and length-scales for these processes are typically 10^-13 seconds (100 femtoseconds) and 10^-10 m (1 Angstrom) such that our view of how a material or device functions is often blurred out in time or in space. In this talk I will describe femtosecond-resolution crystallographic measurements probing dynamical switching responses in topological Weyl semimetals. First I will provide a brief introduction to the unique aspects of these materials. I will then show that terahertz frequency light pulses can be used to induce large amplitude interlayer shear oscillations with ~1% strain amplitudes, leading to a topologically distinct metastable phase. Separate nonlinear optical measurements show that this transition is associated with a symmetry change from a non-centrosymmetric to centrosymmetric structure and therefore corresponds to a transition to a topologically trivial phase. We further show that such shear strain serves as an ultrafast, energy-efficient means to induce more robust, well-separated Weyl points or to annihilate all Weyl points of opposite chirality. This work defines new possibilities for ultrafast manipulation of the topological properties of solids and for a topological switch operating at THz frequencies. Reference: "Time-varying shear strain as an ultrafast symmetry switch in a Weyl semimetal,” E. Sie et al., Nature (2018) (in press)
Host: Jim Lawler
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