Physics

or over three decades, the giant elliptical galaxy Messier 87 in the Virgo Cluster has hosted the most massive known black hole in the local universe. New observational data and improved stellar orbit models in the past several years have substantially expanded and revised dynamical measurements of black hole masses at the centers of nearby galaxies.

I will describe recent progress in discovering black holes up to twenty billion solar masses in ongoing surveys of massive elliptical galaxies.  I will discuss the implications of this new population of ultra-massive black holes, including its impact on our understanding of the symbiotic relationships between black holes and galaxies and on the gravitational waves signals from merging supermassive black hole binaries targeted by ongoing pulsar timing array experiments.

In  his 2000 Buhl Lecture, Barry Barish, then the director of LIGO, discussed gravitational waves, the ripples in the fabric of space and time whose existence was predicted by Einstein in 1916.  At the  time, LIGO had recently been constructed under Barish’s leadership and had begun to collect data.  On February 11, 2016, it was announced that LIGO’s upgraded detectors had made the first-ever observation of gravitational waves from a pair of merging black holes.  Barish returns to CMU for an encore Buhl Lecture in which  he will discuss the physics of gravitational waves, the techniques used to detect gravitational waves, and the implications of the new observations.

Barry Barish is the Linde Professor of Physics, Emeritus, at the California Institute of Technology.  He is a leading expert on gravitational waves, having led the Laser Interferometer  Gravitational-Wave Observatory(LIGO) project as the principal investigator and director from the beginning of construction in 1994 until 2005.  During that period, LIGO detectors reached design sensitivity and set many significant limits on astrophysical sources.  The more sensitive Advanced LIGO proposal was developed and approved while Barish was director, and he continues to play an active leading role in LIGO.  His other noteworthy experiments include an experiment at Fermilab using high-energy neutrino collisions to reveal the quark substructure of the nucleon.  These experiments were among the first to observe the weak neutral current, a linchpin of the electroweak unification theories of Glashow, Salam, and Weinberg.  Barry Barish is also the former director of the Global Design Effort for the international Linear Collider(ILC), the highest priority future project for particle physics worldwide.

Sponsored by the Carnegie Mellon University Department of Physics.

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Since the federal government is the primary source of support for research in physics and astronomy, every member of our community should know what is going on in Washington. This talk will cover the policy and political context for R&D at the federal level, relevant current events in Washington, and what our community can do to help shape future events.

Dr. Joel Parriott has been the Director of Public Policy at the American Astronomical Society for three years. He brought to the AAS a decade of experience at the White House Office of Management and Budget (OMB), where he was responsible for overseeing both the National Science Foundation and the Department of Energy Office of Science (including Ames Lab) on behalf of the President. Prior to his service at OMB, Dr. Parriott spent four years at the National Research Council on the staff of the Board on Physics and Astronomy, where he supported numerous high-level studies, including the 2001 Decadal Survey of Astronomy and Astrophysics and Connecting Quarks with the Cosmos. He earned his doctorate in astronomy and astrophysics at the University of Michigan.

Faculty Host: Diane Turnshek

In a decade that has already seen the first direct detection of gravitational waves and the discovery of the Higgs boson, many eyes are now focused deep underground on experiments seeking to make the first direct detection of dark matter.  The existence of dark matter is indisputable, supported by observations ranging from single galaxies to the entire visible universe, but the identity of dark matter remains a mystery, one which direct detection experiments hope to solve by observing dark matter particles from our own galaxy as they pass through detectors here on earth.  This hunt is defined by the fight against radioactive backgrounds, and has inspired detector technologies ranging from cryogenic semiconductors to superheated freons.  I will describe the physics behind the unique ways in which the leading experiments solve the background problem, give a few new ideas to get to the next level of dark matter sensitivity, and show the importance of a diverse field of experiments, not just for discovering dark matter but for understanding the dark matter signal after a discovery is made. Additional information.

About the Speaker.

Faculty Host: Jim Russ

A predictive theory of galaxy formation remains elusive, even after more than 50 years of dedicated effort by many renowned astrophysicists. The problem of galaxy formation is made difficult by the large range in scales involved and the many non-linear physical processes at work. This talk describes a new generation of numerical models that are designed to overcome these difficulties based on novel schemes for solving the fluid equations on a moving mesh. Initial results from this study provide insight into many aspects of galaxy assembly and the relationship between galaxies and cosmologically-distributed baryons.

Lars Hernquist is the Mallinckrodt Professor of Astrophysics at Harvard University. He received his Ph.D. in 1984 from Caltech, followed by postdoctoral fellowships at UC Berkeley and Princeton, before joining the faculty at UC Santa Cruz. He moved to Harvard in 1998, where he is past chair of the Astronomy Department. He is best known for his theoretical and computational work on galaxy mergers and the formation of cosmological structure. The Hernquist profile is widely used to describe the shape of the dark matter content of galaxies. He is a member of the American Academy of Arts and Sciences and the National Academy of Sciences.

Reception to Follow.

Highly symmetric nanoshells are found in many biological systems, such as clathrin cages and viral shells. Many studies have shown that symmetric shells appear in nature as a result of the free-energy minimization of a generic interaction between their constituent subunits. Here, we study the physical basis for the formation of symmetric shells, and by using a minimal model, demonstrate that these structures can readily grow from the irreversible addition of identical subunits. Our model of nanoshell assembly shows that the spontaneous curvature regulates the size of the shell while the mechanical properties of the subunit determine the symmetry of the assembled structure. Understanding the minimum requirements for the formation of closed nanoshells is a necessary step toward engineering of nanocontainers, which will have far-reaching impact in both material science and medicine.

About the Speaker.

Faculty Host: Ira Rothstein

How do our brains make sense of a complex and unpredictable world? In this talk, I will discuss a physicist's approach to the neural topography of information processing in the brain. First I will review the brain's architecture, and how neural circuits map out the sensory and cognitive worlds. Then I will describe how highly complex sensory and cognitive tasks are carried out by the cooperative action of many specialized neurons and circuits, each of which has a simple function. I will illustrate my remarks with one sensory example and one cognitive example. For the sensory example, I will consider the sense of smell ("olfaction"), whereby humans and other animals distinguish vast arrays of odor mixtures using very limited neural resources. For the cognitive example, I will consider the "sense of place", that is, how animals mentally represent their physical location. Both examples demonstrate that brains have evolved neural circuits that exploit sophisticated principles of mathematics - principles that scientists have only recently discovered.

Vijay Balasubramanian received B.Sc. degrees in Physics and Computer Science from MIT and M.Sc. in Computer Science from MIT. Following a Ph.D. in Theoretical Physics at Princeton University, he was a Junior Fellow at the Harvard Society of Fellows. He then joined the physics faculty at the University of Pennsylvania where he has conducted research in theoretical physics and in neuroscience. He seeks to understand the principles that organize information processing by living systems, and especially by neural circuits. In theoretical physics he has written about the statistical inference of models, on the apparent destruction of information by black holes, and on the transformation of information between microscopic and coarse-grained descriptions of the world. Broadly, he seeks to understand how physical systems create, store and transform information.

The physics of black holes is amazingly rich, with deep connections to basic physical questions that nominally have nothing to do with gravity. For example, the response of a black hole to accreting mass is intimately connected to the response of a quantum liquid to being stirred.  In this colloquium I will describe this "holographic" connection and use it to draw lessons about turbulent fluids, disordered quantum systems and black hole dynamics. For instance, a detailed study of gravitational dynamics reveals the surprising fact that superfluid turbulence in two dimensions can decay and dissipate energy like normal fluid turbulence in three dimensions.  Conversely, familiar facts about turbulent flows imply that the horizons of certain accreting black holes behave like fractals with dimension 10/3.

Allan Adams has been an Assistant Professor at MIT since 2008. He earned his A.B. in physics from Harvard University in 1998, his M.A. from the University of California, Berkeley, in 2000, and his Ph.D. from Stanford University in 2003. After earning his Ph.D., Adams spent three years at Harvard as a Junior Fellow. In 2006, he came to MIT as a Principal Research Scientist before joining the faculty in 2008.

Faculty Host: Ira Rothstein

While there are four commonly observed states of matter (solid crystal, liquid, gas, and plasma), we have known for some time now that there exist many other forms of matter. For example, both quasicrystals and liquid crystals are states of matter that possess properties that are intermediate between those of crystals and conventional liquids. The focus of my talk will be disordered hyperuniform many-body systems [1], which can be regarded to be new distinguishable states of disordered matter in that they behave more like crystals or quasicrystals in the manner in which they suppress large-scale density fluctuations, and yet are also like liquids and glasses because they are statistically isotropic structures with no Bragg peaks. Thus, disordered hyperuniform systems can be regarded to possess a "hidden order" that is not apparent on short length scales while being structurally rotationally invariant.

I will describe a variety of different examples of such disordered states of matter, both equilibrium and nonequilibrium varieties. I will demonstrate that there exist classical ground states that are hyperuniform and disordered in a high-density regime down to some critical density, below which the system undergoes a phase transition to ordered states [2]. Disordered hyperuniform systems appear to be endowed with novel physical properties, including complete photonic band gaps comparable in size to those in photonic crystals [3] and improved electronic band-gap properties. Moreover, we have recently shown that photoreceptor cell patterns (responsible for detecting light) in avian retina have evolved to be disordered and hyperuniform [4].

1. S. Torquato and F. H. Stillinger, "Local Density Fluctuations, Hyperuniform Systems, and Order Metrics," Phys. Rev. E, 68, 041113 (2003).
2. R. D. Batten, F. H. Stillinger and S. Torquato, "Classical Disordered Ground States: Super-Ideal Gases, and Stealth and Equi-Luminous Materials," J. Appl. Phys., 104, 033504 (2008).
3. M. Florescu, S. Torquato and P. J. Steinhardt, "Designer Disordered Materials with Large, Complete Photonic Band Gaps," Proc. Nat. Acad. Sci., 106, 20658 (2009).
4. Y. Jiao, T. Lau, H. Haztzikirou, M. Meyer-Hermann, J. C. Corbo, and S. Torquato, "Avian Photoreceptor Patterns Represent a Disordered Hyperuniform Solution to a Multiscale Packing Problem," Phys. Rev. E, 89, 022721 (2014).

HAWC is a second generation wide-field TeV gamma-ray observatory. HAWC is just being completed in the high mountains of Mexico at 4100m a.s.l. The modularity of the detector enabled us to begin taking science data with a detector that was significantly more sensitive than previous instruments even while the detector was under construction. This talk will describe the science goals of the project, the current status of the detector and present science results from operations with the partially completed detector.

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