Physics

One of the great triumphs of 19th- century science was the emergence of thermodynamics. This is a subject of great power and generality, setting down the rules for what is possible and, even more crucially, what is not possible: there can be no perpetual motion machines, heat flows from hot bodies to cold bodies, and any effort to convert energy from one form to another always involves a bit of waste. A central, if slightly mysterious concept in thermodynamics is the entropy, which is introduced first as a bookkeeping device but then becomes fundamental. In the formulation of statistical mechanics, the bridge connecting our microscopic description of atoms and molecules to the macroscopic phenomena of our everyday experience, entropy reappears as a measure of the number of states that are accessible to all the atoms and molecules.

In the mid-twentieth century, entropy makes yet another appearance, first as a quantitative measure of information, and then as a limit on the amount of space that we need to record that information. It is astonishing that the same concept reaches from steam engines to the internet, and from molecules to language. In this lecture I will try to give a sense for these four different notions of entropy, and their connections with one another, hoping to give a sense for the unifying power of mathematics.

As endpoints of cosmic structure formation that emerge in the era of dark energy domination, the population of clusters of galaxies offers insights into cosmology and the gravitational growth of large-scale structure. The composition of clusters — dark matter and baryons in multiple phases co-evolving within a hierarchical cosmic web of massive halos — is being scrutinized observationally across the electromagnetic spectrum and with increasingly sophisticated numerical simulations.  In this presentation, I will outline the phenomenological framework of cluster cosmology, emphasizing multi-wavelength population statistics and support from astrophysical simulations, then discuss some of the challenges associated with early 21st century reality.

About the Speaker.

Nearly 400 years ago, Galileo gave us the image of the great Book of Nature, lying open before us. We could read it, he said, only if we understood its language - the language of mathematics. The search for a mathematical description of nature, an activity we now call theoretical physics, has been extraordinarily successful. In a real sense, what we see around us are the consequences of equations that can be written on one sheet of paper. This tremendous success encourages physicists to keep searching for simplicity, even in apparently complex systems. Why do we believe that the world should be described by simple models? Is this just an extrapolation from past successes, liable to fail at any moment? Faced with the evident complexity of the world, is the search for simple mathematical descriptions just a matter of guessing, or are there principles to guide our search? I’ll address these questions with lessons from history of the subject, then turn to one of the modern frontiers: the search for a physicist’s understanding of the brain and mind.

Discussions of the infamous measurement problem of quantum foundations tend to focus on how the output of a measurement, the pointer position, can be thought of in consistent quantum mechanical terms, while ignoring the equally important issue of what this outcome says about the earlier microscopic situation the apparatus was designed to measure. An experimental physicist is typically much more interested in the path followed by a particle before it triggered his detector than in what happened later, and if quantum mechanics cannot provide a clear explanation, how can one claim that this theory has been confirmed by experiment? The talk will use Wheeler's delayed choice paradox to identify the fundamental conceptual issues underlying this second measurement problem, and then sketch the resolution provided by the consistent histories interpretation, using a modification of Birkhoff and von Neumann's quantum logic.

For 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.

Additional Info

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

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