Martin A. Fisher School of Physics

January 15, 2019

Brian Swingle, University of Maryland

Abstract: The speed of light sets a fundamental upper limit on the rate at which information can move through spacetime. However, in various systems found in nature, slower effective speed limits on information propagation can emerge. In the context of chaotic quantum many-body systems, I will discuss the relation between these emergent speed limits and the dynamics of entanglement. I will also describe how they are being probed experimentally and comment on some puzzles. In the context of a model of quantum gravity, we will see that these phenomena are connected to the emergence of spacetime itself. I will conclude by discussing some frontiers in our quest to understand locality, causality, and the structure of spacetime.

January 29, 2019

Robin Wordsworth, Harvard University

Abstract: The planet Mars has engaged the curiosity of scientists and amateurs alike for centuries, but many aspects of its evolution remain poorly understood. In particular, Mars contains abundant evidence for erosion by liquid water 3-4 Ga, but due to the faintness of the young Sun, basic climate theory suggests its surface should have been extremely cold. Here I describe our recent progress on this important planetary science problem. Based on 3D climate model results, I describe how adiabatic cooling under a thicker CO2 atmosphere would have led to stabilization of snow and ice deposits in the highland regions where most surface aqueous alteration is recorded. I then show that bursts of methane and hydrogen outgassed into this atmosphere, via mechanisms analogous to those that occur on Titan, could have caused intense intermittent warming on moderate (~100,000 y) timescales. This scenario fits many aspects of the geologic record and will be amenable to further in-situ testing by the Mars 2020 rover. Finally, I discuss the importance of Mars as a case study for planetary evolution in general, and argue that comparative solar system planetology can lead us to new insights into exoplanet habitability and biosignatures.

February 12, 2019

Gabriela González, Louisiana State University

Abstract: The first detection of gravitational waves in 2015 by the LIGO detectors, created by the merger of black holes more than a billion years ago, was followed by several other signals from black holes. In 2017, the merger of neutron stars was detected by LIGO and Virgo detectors and by gamma-ray telescopes, and was found by many electromagnetic observations too: a new era of gravitational wave astrophysics has started with very bright prospects for the future. We will describe the technology involved in the LIGO gravitational wave detectors, details of the latest discoveries and the exciting prospects for more detections in the next years.

February 26, 2019

Kerry Emanuel, MIT
Host: Bjoern Penning

Abstract: Hurricanes provide beautiful examples of many of the key physical processes important in geophysical systems. They are rare natural examples of nearly perfect Carnot heat engines with an interesting wrinkle: They recycle much of their waste heat into the front end of the engine, thereby achieving greater wind speeds than would otherwise be possible. They are driven by surface enthalpy fluxes made possible by the thermodynamic disequilibrium between the earth’s surface and atmosphere, a characteristic of radiative equilibrium in the presence of greenhouse gases. Their evolution, structure, and intensity all depend on turbulence near the ocean surface and in the outflow layer of the storm, high up in the atmosphere.

I will briefly describe these and other interesting aspects of hurricane physics, and also show how physics can be used to help assess hurricane risk.

March 12, 2019

Ethan Brown, Rensselear Polytechnic Institute
Host: Bjoern Penning

Abstract: Neutrinos are a unique type of matter that provides one of the underpinnings of the standard model of particle physics, but the observation of oscillations due to neutrino mass is specifically forbidden by the standard model. One resolution to this problem is a majorana neutrino that is both matter and antimatter. This predicts a rare neutrinoless double beta decay, which if observed, would provide information about the mass of neutrinos, the mass generation mechanism, and the matter antimatter asymmetry of the universe. The nEXO experiment will probe this hypothesis by measureing the half-life of this rare decay up to 1028 years, 18 orders of magnitude longer than the age of the universe, potentially discovering this exciting new physics. The physics of neutrinos, experimental methods, and some of the ongoing novel R&D to probe this rare decay will be presented.

March 19, 2019

William Wester, FermiLab
Host: Bjoern Penning

Abstract: The axion is a proposed particle whose existence might account for much of the dark matter of the universe. This same particle also arises as the solution to the strong-CP problem of particle physics. There have several decades of experiments that have attempted to detect new particles with many of the properties of the axion - but without being sensitive to the preferred region of parameter space. I will report on the Axion Dark Matter Experiment, ADMX, which is currently running and achieving the required sensitivity towards potential discovery.

March 26, 2019

Thomas Fai, Brandeis University

Abstract: The single cell biflagellate Chlamydomonas reinhardtii has proven to be a very useful model organism for studies of size control. The lengths of its two flagella are tightly regulated. We study a model of flagellar length control whose key assumption is that proteins responsible for the intraflagellar transport (IFT) of tubulin are present in limiting amounts. In the case of two simultaneously assembling flagella, regardless of the details of how the flagella are coupled, we find that the widely-used assumption of a constant disassembly rate is inconsistent with experimental results. We therefore propose a model in which diffusion gives rise to a length-dependent concentration of depolymerizer at the flagellar tip. This model is found to be consistent with experimental results and generalizes to other situations such as arbitrary flagellar number.

April 2, 2019

Shuo Zhang, MIT
Host: Elisabeth Mills

Abstract: Supermassive black holes (SMBHs) may spend the majority of their lives accreting at low rates through radiatively inefficient, advection-dominated accretion flow. Therefore, low-luminosity SMBHs could greatly outnumber their more active cousins, active galactic nuclei, thus are essential for our understanding of SMBH activity cycles and their relationship to galaxy evolution. In particular, SMBHs harbored in local galaxies are found to be remarkably under-luminous. The best studied under-luminous SMBH is the closest such object to Earth, Sagittarius A*, located in the nucleus of our Milky Way galaxy. In this talk, I will discuss how I have been probing the outburst history of Sagittarius A* and how I plan to apply these techniques to SMBHs in nearby galaxies such as Andromeda. The Galactic nucleus of the Milky Way galaxy also serves as an ideal lab to probe exotic physics. I would like to introduce a new research direction of mine, to probe Galactic cosmic-rays at MeV through PeV energy scales. This research has far-reaching implications for a range of fields of study: the origin of Galactic cosmic-rays, particle acceleration mechanisms and dark matter searches, aiming to eventually answer the question whether Galactic cosmic-rays are from ordinary astrophysical sources or of more exotic origin.

April 9, 2019

Oskar Hallatschek, UC Berkeley
Host: Jane Kondev

Abstract: Luria and Delbrück discovered that mutations that occur early during a growth process lead to exceptionally large mutant clones. These mutational “jackpot” events are thought to dominate the genetic diversity of growing cellular populations, including biofilms, solid tumors and developing embryos. In my talk I show that jackpot events can be generated not only when mutations arise early but also when they occur at favourable locations, which exacerbates their role in adaptation and disease. I will also consider the impact of recurrent jackpot events, which lead to a bias favoring alleles that happen to be present in the majority of the population. I argue that this peculiar rich-get-richer phenomenon is a general feature of evolution driven by rare events.

April 16, 2019

Jasna Brujic, NYU
Host: Ben Rogers

Abstract: From a broad perspective, the challenge is to understand how different interaction potentials and driving forces can be combined to produce a useful designed structure. Here, we will use biological molecules, particularly lipids and DNA strands, to functionalize emulsions in order to self-assemble arbitrarily designed structures [1,2]. This versatile system allows us to control the fluidity, specificity, valency, and logical programming of interactions between droplets, which in turn facilitates the assembly of complex structures, including emulsion polymers, loops and clusters with particular geometries [3]. In particular, we will focus on the sequential self-assembly of linear droplet chains, in which we will pre-program the strength and specificity of the interactions along the chain [4]. Secondary structure formation will be revealed by confocal microscopy over time, in response to temperature cycling, this `beads-on-a-string’ experimental model is analogous to protein folding on the molecular scale, but provides access to the configurational fluctuations on the droplet scale. Learning the principles of emulsion folding will allow us to design and construct folds that result in complex 3D objects from one-dimensional chains. The droplets can readily be solidified, therefore they offer a route to hands-off manufacturing of objects with inbuilt hierarchies. The long-term goal is to design sequences in order to build unlikely crystalline symmetries, aperiodic crystals, and disordered structures with desirable optical and mechanical properties.

April 30, 2019

Mark Van Raamsdonk, University of British Columbia
Host: Matthew Headrick

Abstract: The AdS/CFT correspondence provides a quantum theory of gravity in which spacetime and gravitational physics emerge from an ordinary non-gravitational quantum system with many degrees of freedom. In this talk, I will explain how quantum entanglement between these degrees of freedom is crucial for the emergence of a classical spacetime, and describe progress in understanding how spacetime dynamics (gravitation) arises from the physics of quantum entanglement.

September 4, 2018

Marcelle Soares-Santos, Brandeis University

Abstract: Motivated by the exciting prospect of a new wealth of information arising from the first observations of gravitational and electromagnetic radiation from the same astrophysical phenomena, the Dark Energy Survey (DES) has established a search and discovery program for the optical transients associated with LIGO/Virgo events using the Dark Energy Camera (DECam). This talk presents the discovery of the optical transient associated with the neutron star merger GW170817 using DECam and discusses its implications for the emerging field of multi-messenger cosmology with gravitational waves and optical data.

September 18, 2018

David Roberts, Brandeis University

Abstract: The collision and coalescence of supermassive black holes (SMBHs), such as those that lie at the heart of every galaxy, are expected to produce an enormous flux of gravitational waves (GWs) at very low frequencies (on the order of nanohertz, wavelengths of parsecs). The required pairs of SMBHs are presumably formed after the merger of two large galaxies to create a single larger galaxy initially containing two SMBHs. These each are expected to travel to the dynamical center of the resulting galaxy and eventually merge. The ensemble of such collisions should produce a GW background that is detectable using pulsar timing arrays (PTAs). However, current observational limits from the Australian PTA on the low frequency GW background lie below the levels expected based on theoretical considerations of the rate of such SMBH collisions. We have undertaken a project to study the fraction of large galaxies that harbor a pair of SMBHs using the structure of radio galaxies (RGs) as signposts. Out of a sample of over 1600 bright RGs Cheung selected 100 candidates that showed structures that suggested possible disruption of the central BHs. We have used the Jansky Very Large Array to make multi-frequency multi-array images of these "X-shaped radio galaxies" (XRGs) and created a well-studied sample of 87. Analysis of their structures suggests that most are created by backflow from the extended lobes. However, a small fraction (>=4%) show clear evidence for precession of the BH axis, presumably driven by interactions with a nearby second SMBH. We conclude that study of the structure of XRGs is useful in the search for binary SMBHs, and that such searches can help set limits on the expected low-frequency GW background.

October 2, 2018

Stefan Söldner-Rembold, University of Manchester

Abstract: The Deep Underground Neutrino Experiment (DUNE) will be an international observatory for neutrino science, designed, constructed and operated by a global collaboration of scientists. Primary science drivers are the discovery of CP violation in the neutrino sector, the detection of neutrinos from supernovae, and the search for baryon number violation. DUNE will consist of two neutrino detectors placed in the world’s most intense neutrino beam. A near detector will record particle interactions near the source of the beam at Fermilab close to Chicago. A second, much larger, far detector operating with more than 40 kt of liquid-argon will be installed a mile underground in South Dakota. Several mid-size liquid-argon detectors at Fermilab and CERN are being constructed to demonstrate the potential of the cutting-edge liquid-argon technology employed for DUNE. I will introduce the technology and give an overview of the current status and future discovery potential of the DUNE programme.

October 9, 2018

Kerstin Perez, Massachusetts Institute of Technology

Abstract: Cosmic-ray antiprotons have been a valuable tool for dark matter searches since the 1970s. Recent years have seen increased theoretical and experimental effort towards the first-ever detection of cosmic-ray antideuterons, in particular as an indirect signature of dark matter annihilation or decay in the Galactic halo.  In contrast to other indirect detection signatures, which have been hampered by the large and uncertain background rates from conventional astrophysical processes, low-energy antideuterons provide an essentially background-free signature of dark matter, and low-energy antiprotons are a vital partner for this analysis.  I will discuss the upcoming balloon-borne GAPS experiment, which exploits a novel detection technique utilizing exotic atom capture and decay to provide sensitivity to antiproton, antideuteron, and antihelium cosmic-ray signatures.  In particular, I will detail the fabrication of the lithium-drifted Silicon detectors that are essential to its success.

October 23, 2018

Greg Landsberg, Brown University

Abstract: With any evidence for new physics yet to be found at the LHC, there has been a significant shift in both the direction of searches for new physics and the methods used to purse them. With no clear peaking excesses and only mild hints for any deviations from the standard model in the LHC data, we are pursuing new avenues of searches by looking at the places missed both by the early LHC running and by previous machines. Many of these new searches required a significant development of new experimental and theoretical tools. Using mainly CMS as an example, I'll discuss these new directions in our quest for new physics and show a few recent results illustrating their power and novelty.

October 30, 2018

Ian Crossfield, Massachusetts Institute of Technology

Abstract: Extrasolar planets and cool stars emit most of their light beyond the range of standard optical observations. These objects are often best studied using infrared spectroscopy. I will present recent results from my group on two topics: space-based IR spectroscopy of exoplanet atmospheres, and ground-based, high-resolution spectroscopy of both planets and stars. I will also conclude with a brief discussing of how future IR-optimized observatories will also enable exciting new science in these areas.

November 6, 2018

Prineha Narang, Harvard University

Abstract: Exciting discoveries during the past few decades in quantum science and technology have brought us to this next step in the quantum revolution: the ability to fabricate, image and measure materials and their properties at the level of single atoms is almost within our grasp. Yet, at the most fundamental level a tractable quantum mechanical description and understanding of these materials does not exist. The physics of quantum materials is rich with spectacular excited-state and non-equilibrium effects, but many of these phenomena remain poorly understood and consequently technologically unexplored. Therefore, my research focuses on understanding how quantum-engineered materials behave, particularly away from equilibrium, and how we can harness these effects for technologies of the future. I will present my approach, from a theoretical and computational standpoint, in this seminar. Electron-photon, electron-electron as well as electron-phonon dynamics and far-from-equilibrium transport are critical to describe ultrafast and excited-state interactions in materials. Ab initio descriptions of phonons are essential to capture both excitation and loss (decoherence) mechanisms, and are challenging to incorporate directly in calculations due to a large mismatch in energy scales between electrons and phonons. I will show results using a new theoretical method we have developed to calculate arbitrary electron-phonon and electron-optical interactions in a Feynman diagram many-body framework integrated with a nonequilibrium carrier transport method. Further, I will discuss a new formalism at the intersection of cavity quantum-electrodynamics and electronic structure methods, quantum-electrodynamical density functional theory (QEDFT), to treat electrons and photons on the same quantized footing. I will demonstrate how these ab initio techniques can guide the search for relevant quantum properties in 2D and 3D materials, including new quantum emitters. In the second part of the seminar, I will show recent results using newly developed theoretical methods to evaluate the linear optical properties of low dimensional and heterostructured quantum materials. Further I will discuss how we extend these methods as a computational probe of hydrodynamic materials, for which electronic transport behaves according to the laws of hydrodynamics over conventional scattering descriptions. Finally, I will discuss the linear and nonlinear optical properties of these hydrodynamic and other similar Dirac and Weyl compounds to better understand the effect of linear dispersion on overall transport and optical properties.

Nov. 13–Nov. 15, 2018

Spencer Bloch, Yau Mathematical Sciences Center, Tsinghua University

November 27, 2018

John Wilmes, Brandeis University

Abstract: In joint work with Santosh Vempala, we study the complexity of training neural network models with one hidden nonlinear activation layer and an output weighted sum layer. We obtain essentially matching upper and lower bounds. We begin with the observation that the concepts computed by simple neural networks are well-approximated by low-degree polynomials when inputs are drawn uniformly from the sphere, so regression problems for data labelled by an unknown neural network can be captured by agnostic polynomial learning. We give an agnostic learning guarantee for training neural networks via gradient descent, establishing convergence within error ϵ​​ of the optimum when the number of gates, samples, and iterations are all bounded by nO(k) log(1/ε)​​ for concepts approximated by degree-k​​ polynomials. On the other hand, any statistical query algorithm (including gradient descent in its many variations) requires nΩ(k)​​ queries for the same problem.

December 11, 2018

Gabriel Redner, Google
Host: Michael Hagan

Biography: Since completing his PhD in Physics at Brandeis in 2015, Gabriel has worked as a Software Engineer at Google, focusing on networking infrastructure and security.

Abstract: The internet is many contradictory things: complex yet fundamentally simple, indispensable yet dangerous, easy to use but hard to understand. Or is it? In this talk, I'll work to demystify what happens under the hood of the global internet, and show how just as in physics, complex behaviors rest on simple foundations. I'll also discuss how my career has taken me from physics to software engineering and back again, and then back again.