Eric Sembrat's Test Bonanza

Abstract

The imaging of black-hole shadows with the Event Horizon Telescope has opened a new window into the strong-field spacetimes of these extreme astrophysical objects. I will first discuss the technological and theoretical advances that led to the first images of the black holes in the centers of the M87 galaxy and of the Milky Way. I will describe how this observation allows us to perform new tests of General Relativity. I will explore the connection of the new results to tests of gravity with other astrophysical and cosmological probes. I will conclude with a prognosis on what ground-based observations of shadows can tell us about black-hole metrics and the underlying theory of gravity.

Abstract

Asymptomatic transmission represents a double-edged sword. Asymptomatic infections can improve individual-level outcomes by reducing severe cases but lead to more cases overall and even more fatalities at population scales. The first part of the talk will explain the dynamical mechanisms underlying the impact of asymptomatic transmission on epidemic dynamics, including challenges in characterizing the strength, severity, and long-term outcomes of an emerging epidemic. The second part of the talk will highlight ways that epidemic models served as a guide for real-time mitigation and action-taking to reduce asymptomatic transmission of Covid-19 at both local and national scales.

Abstract

Inflating a rubber balloon leads to a dramatic shape change: a property that is exploited in the design of soft robots and deployable structures. On the one hand, fluid-driven actuators capable of complex motion can power highly adaptive and inherently safe soft robots. On the other hand, inflation can be used to transform seemingly flat shapes into shelters, field hospitals, and space modules. In both cases, just like the simple balloon, only one input is required to achieve the desired deformation. This simplicity, however, brings strict limitations: soft actuators are often restricted to unimodal and slow deformation and deployable structures need a continuous supply of pressure to remain upright. Here, we embrace multistability as a paradigm to improve the functionality of inflatable systems. In the first part of this seminar, I exploit snapping instabilities in spherical shells to decouple the input signal from the output deformation in soft actuators–a functionality that can be utilized to design a soft machine capable of jumping. In the second part of the seminar, I draw inspiration from origami to design multistable and inflatable structures at the meter scale. Because these deployable systems are multistable, pressure can be disconnected when they are fully expanded, making them ideal candidates for applications such as emergency sheltering and deep space exploration. Together, these two projects highlight the potential of multistability in enabling the design and fabrication across various scales of multi-form, multi-functional, and multi-purpose materials and structures.

Bio

Katia Bertoldi is the William and Ami Kuan Danoff Professor of Applied Mechanics at the Harvard John A.Paulson School of Engineering and Applied Sciences. She earned master degrees from Trento University (Italy) in 2002 and from Chalmers University of Technology (Sweden) in 2003, majoring in Structural Engineering Mechanics. Upon earning a Ph.D. degree in Mechanics of Materials and Structures from Trento University, in 2006, Katia joined as a PostDoc the group of Mary Boyce at MIT.  In 2008 she moved to the University of Twente (the Netherlands) where she was an Assistant Professor in the faculty of Engineering Technology. In January 2010 Katia joined the School of Engineering and Applied Sciences at Harvard University and established a group studying the mechanics of materials and structures. She is the recipient of the NSF Career Award 2011 and of the ASME's 2014 Hughes Young Investigator Award. She serves as Editor for the journals Extreme Mechanics Letters and New Journal of Physics. She published over 150 peer-reviewed papers and several patents. For a complete list of publication and research information: https://bertoldi.seas.harvard.edu/

Dr Bertoldi’s research contributes to the design of materials with a carefully designed meso-structure that leads to novel effective behavior at the macroscale. She investigates both mechanical and acoustic properties of such structured materials, with a particular focus on harnessing instabilities and strong geometric non-linearities to generate new modes of functionality. Since the properties of the designed architected materials are primarily governed by the geometry of the structure (as opposed to constitutive ingredients at the material level), the principles she discovers are universal and can be applied to systems over a wide range of length scales.

Abstract: Some geometrically frustrated magnets are claimed to host a quantum spin liquid (QSL) state. The QSL is a theoretically-proposed many-body state that is fully quantum coherent and, if achievable, may offer a new route to quantum computation. We show that the response of such materials to small amounts of disorder cannot be explained in terms of the canonical source-field construct and suggests a temperature-dependent permeability with a  hidden energy scale where short-range order among spins is established. This particular type of short range order mediates long range interactions and is called the eminuscent phase, best described within the Coulomb representation. The instability of the eminuscent phase to spin glass formation in 3D and random singlets in 2D, in addition to the short range order itself, pose serious challenges to the viability of the QSL state.

Bio: Arthur P. Ramirez joined Bell Labs in 1984 after completing his Ph.D. at Yale on spin solitons in a 1D ferromagnet. At Bell he studied a variety of topics including heavy fermions, cuprate and C60 superconductivity, geometrical frustration, colossal magnetoresistance, and organic electronics. After a stint at Los Alamos, he returned to Bell in 2003. He entered academia in 2009 at UC Santa Cruz as dean of engineering and is now professor of physics.

The School of Physics Colloquium Committee

Cordially invites you to

The State of The School Address

Date: August 29, 2022

Time: 3:00 pm to 4:00 pm

Location: Marcus Nanotechnology Building

Conference Room: 1116-1118

***********

Immediately followed by a Reception

 Please join us for to welcome our new Chair Dr. Feryal Özel

Appetizers and Refreshments will be served

Location: Howey Building Courtyard (between main building and large Lecture Halls)

Abstract: 'Years have passed since the first detection of pulsed very-high-energy (VHE; E >100 GeV) gamma-rays from the Crab pulsar with VERITAS, yet much is still unresolved in relation to the nature of pulsar emission mechanisms and how they interact with the surrounding medium. No completely satisfactory model has been produced that can accurately describe all aspects of the pulsed gamma-ray emission observed from the Crab and other pulsars. Understanding the properties of VHE emission detected in observations made by many different experiments still poses a significant challenge. The crux of the issue remains; is the Crab pulsar unique, or do other pulsars also exhibit the same behavior in the VHE regime, and, in either case, what are the underlying mechanisms? To try and answer this question, while also learning more about the pulsar population and the physics of VHE gamma-ray production, this work will present the results of a search for pulsed emission in the VHE band from six Millisecond Pulsars (MSPs) in the archival VERITAS data-set, the first such survey of MSPs, and the most sensitive VHE measurements ever made for the targets. I test to see if significant pulsed emission is detected, report the observed VHE pulsed flux and gamma-ray conversion efficiency of these MSPs, to determine if there is an appearance of a VHE flux element at these energies, for the sources studied here. As the analyses result in non-detections, in every case, upper limits are placed on the aforementioned quantities. The upper limits are compared with a modern, comprehensive pulsar model energy spectrum and are found to be compatible with the proposed theoretical scenario, although we are limited by a lack of target-specific predictions. Pulsars are also sources of non-pulsed gamma-rays. However, at the time of writing, there has been no decisive detection of the TeV emission expected by current models from any pulsar tail that is also seen in the X-ray or radio bands. An observational campaign has been carried out by VERITAS to hunt for VHE gamma-ray emission from the candidate tail regions associated with three nearby pulsars (PSR B0355+54, PSR J0357+3205 and PSR J1740+1000) that move supersonically and exhibit significant X-ray tails. The results of this analysis provides quantification of the TeV flux and luminosity, from the tail regions of the targets, for comparison with other pulsar wind nebulae observations and the predictions of modern pulsar tail models. The results of this search also provide guidance for the selection of additional candidates, and quantifying the properties of pulsar tails, for new pulsars tails that may be observed in the VHE regime.'

Abstract: Atomic beams are a key technology for realizing navigation-grade timekeeping and inertial sensing instruments. Miniaturization of atom beam technology can enable new quantum sensor architectures benefitting from foundry production and microfabrication approaches. This thesis paves the way towards future atomic sensing devices using chip-scale thermal atomic beams enabled by MEMS (micro-electromechanical systems) technology in three steps – chip-scale atomic beam collimation, brightness enhancement, and vacuum packaging. We first demonstrated using a microfabricated thin capillary array to create highly collimated, continuous rubidium atom beams traveling parallel to a silicon wafer surface. Precise, lithographic definition of the guiding channels allowed us to shape and tailor atoms’ velocity distributions in ways not possible using conventional machining.

We then performed beam brightening via blue-detuned optical molasses following pre-collimation given by these microchannel arrays. Stimulated forces reduced the transverse velocity spread to below 1 m/s within a total travel distance of 4.5 mm upon a silicon substrate, consuming a cooling power of only 8 mW, 9 times lower power than earlier free-space experiments on cesium. Finally, we achieved a fully chip-scale atomic beam system containing an atom vapor reservoir and atomic beam drift region bridged by those thin silicon microchannels for differential pumping, in conjunction with graphite and non-evaporable getters embedded in the anodically bonded silicon-glass cell for sustaining the vacuum. In addition, we also performed free-space Ramsey interferometry with a two-zone separation as short as 8 mm, which mimicked the conditions and constraints for its future implementation on this chip-scale platform to unleash its potential in inertial sensing and timekeeping.

Reference:

Li, C., Chai, X., Wei, B., Yang, J., Daruwalla, A., Ayazi, F., & Raman, C. (2019). Cascaded collimator for atomic beams traveling in planar silicon devices. Nature communications, 10(1), 1-8.

Li, C., Wei, B., Chai, X., Yang, J., Daruwalla, A., Ayazi, F., & Raman, C. (2020). Robust characterization of microfabricated atomic beams on a six-month time scale. Physical Review Research, 2(2), 023239.

Wei, B., Crawford, A., Andeweg, Y., Zhuo, L., Li, C., & Raman, C. (2022). Collimated versatile atomic beam source with alkali dispensers. Appl. Phys. Lett. 120, 144001.

Li, C., Martinez, G., McGehee, W., Kitching, J., & Raman, C. (2022). A Microfabricated Chip-scale Atomic Beam System with Self-sustained Vacuum. Bulletin of the American Physical Society.