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A list of all the posts and pages found on the site. For you robots out there is an XML version available for digesting as well.

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About me

** Published:**

This book aims to present a self-contained survey of important topics in classical mechanics. Starting from basic mathematical foundations, Newtonian mechanics is developed with an emphasis on problem solving methods and advanced topics. The later, increasingly sophisticated chapters go beyond the material usually covered in an introductory course. They delve into topics including gyroscopic motion, central forces and scattering, oscillations, wave analysis, and special relativity. A great deal of emphasis is placed on problem solving. Over 150 worked examples are distributed throughout the text and model a variety of useful techniques. Additionally, each chapter finishes with an extensive and difficult problem set. A special effort has been made to make these problem sets diverse and challenging; they should serve as rigorous tests of understanding, as well as avenues for further exploration. In addition to the main material, this book contains over 250 figures and detailed appendices on multivariable calculus, linear algebra, and differential equations. Sample material is available here.

Citation: Balasubramanian, S. & and Dempsey, R (2016). Mechanics: An Extended Introduction. CreateSpace Independent Publishing Platform. ISBN: 9781518659911.

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Published in *MNRAS*, 2018

In this paper, we aim to explain observations which indicate a strong relationship between the luminosity of a quasar source and the spatial extent of photoionization in the narrow-line region (NLR). Using a simple physical model of the clouds in the NLR, we are able to produce a theoretical size-luminosity relationship which matches several key features of the observed relationship. This model explains why a tight relationship is observed despite variations in the mass of the clouds, and the number of clouds present. Additionally, we use this population-based approach to study other features of AGN, such as the covering factor of the torus and the total mass of the NLR.

Citation: Dempsey, R. & Zakamska, N. (2018). "The size–luminosity relationship of quasar narrow-line regions" *MNRAS*. 477 (4), 4615-4626.

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Published in *MNRAS*, 2019

In this work, we revisit the size–luminosity relation of the extended narrow line regions (ENLRs) using a large sample of nearby active galactic nuclei (AGNs) from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. The ENLRs ionized by the AGN are identified through the spatially resolved BPT diagram, which results in a sample of 152 AGN. By combining our AGN with the literature high-luminosity quasars, we found a tight log-linear relation between the size of the ENLR and the AGN [OIII] λ5007 Å luminosity over four orders of magnitude of the [OIII] luminosity. The slope of this relation is 0.42 ± 0.02 which can be explained in terms of a distribution of clouds photoionized by the AGN. This relation also indicates that the AGNs have the potential to ionize and heat the gas clouds at a large distance from the nuclei without the aids of outflows and jets for the low-luminosity Seyferts.

Citation: Chen, J., Shi, Y., Dempsey, R., et al. (2019). "The spatial extension of extended narrow line regions in MaNGA AGN" *MNRAS*. 498 (1), 855 - 867.

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** Published:**

Semi-inclusive deep inelastic scattering, or SIDIS, is a nuclear scattering process which can be used to study the three-dimensional structure of hadrons. I presented the results of my summer 2018 reseach in the Jefferson Lab Theory Center, in which I developed a Monte Carlo analysis pipeline to study the SIDIS process.

** Published:**

I presented the results of my paper with Nadia Zakamska on a theoretical model of the size-luminosity relation for the narrow-line regions of quasars. The paper can be downloaded here.

** Published:**

The BCJ relations give a formal realization of the $(\text{gravity}) = (\text{gauge theory})^2$ correspondence, via a relationship between graviton and gluon amplitudes. In this talk, I reviewed the current understanding of the BCJ relations for tree-level amplitudes, and the conjectured extension to all loop orders. I also described ongoing research work into a classical consequence of gauge/gravity duality, the double copy relationship between Einstein gravity and Yang-Mills theory.

High school lectures, *Johns Hopkins University*, 2017

Splash is a program in which undergraduate students develop and teach short courses to local high school students. I taught a 7-part seminar on various topics in physics.

Undergraduate lectures, *Johns Hopkins University*, 2017

Based on the general interest of some undergraduate students in the JHU Department of Physics & Astronomy, I gave a series of lectures on special relativity, the least action principle, and classical field theory. The main point was to show how the Maxwell equations and the Einstein equations both arise naturally from an action principle.

High school lectures, *Johns Hopkins University*, 2018

Splash is a program in which undergraduate students develop and teach short courses to local high school students. I gave seven lectures on various topics in mathematics and physics.

Undergraduate lectures, *Johns Hopkins University*, 2018

Based on the general interest of some students at Johns Hopkins University, I gave lectures introducing quantum mechanics through a pedagogical route based on canonical quantization. Notes can be found here.

Undergraduate lectures, *Johns Hopkins University*, 2018

Continuing on the themes developed in two prior lecture series at Johns Hopkins, I gave lectures to the same group of students on the mathematics of classical gauge theories. The lectures covered physical aspects of gauge invariance in electrodynamics, as well as the mathematics of bundles and connections. Notes can be found here.