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The ESA Gaia mission has delivered amazingly accurate spatial and kinematic measurements for millions of stars, and rapidly improving information on a billion more. Ground-based spectroscopic surveys deliver additional important information about the birth conditions of these stars. Orbital times in the Galaxy are measured in tens of millions of years, so how do we use this snapshot of kinematic and birth information to learn the gravitational force law in the Milky Way, and, in turn, the distribution of dark matter? There are classical stellar-dynamics approaches that (more-or-less) find consistency relationships among second moments (velocity dispersions). But new approaches have appeared in the last few years that deliver images of stellar orbits. I discuss these new orbit-imaging methods and their promise and limitations. They might end up being the most precise tools we will ever have for constraining the dark matter and its dynamics.
Amid rapidly escalating tension between the United States and China, professors, scientists, and students of Chinese ethnic origin as well as those engaging in academic collaborations with China are under heightened scrutiny by the federal government. In 2015, Dr. Xi became a casualty of this campaign despite being innocent. This experience gave him insights into the challenges Chinese scientists face and the immediate threat to the open environment in fundamental research.
Millisecond pulsars are rapidly rotating neutron stars with phenomenal rotational stability. The NANOGrav collaboration monitors an array of about 80 of these cosmic clocks in order to detect perturbations due to gravitational waves at nanohertz frequencies. These gravitational waves will most likely result from an ensemble of supermassive black hole binaries. Their detection and subsequent study will offer unique insights into galaxy growth and evolution over cosmic time. I will present our most recent dataset and the results of our gravitational wave analysis, which suggests the presence of a common spectral signature in the data that could be the first hints of a gravitational wave background. I will then describe the gains in sensitivity that are expected from additional data, discoveries of millisecond pulsars, more sensitive instrumentation, and international collaboration and discuss prospects for detection in the next several years.
Ultracold atoms offer a unique platform to study spin physics. When atoms are arranged in an optical lattice in form of a Mott insulator, the atomic motion is frozen out and the study and control of the spin degree of freedom emerges as a new frontier. Heisenberg spin models, where only neighboring spins interact, are the paradigmatic model for many interesting phenomena. Until very recently, all experimental studies with cold atoms addressed the special case of an isotropic Heisenberg model. Using lithium-7 atoms and Feshbach resonances to tune the interactions, we have created spin ½ Heisenberg models with adjustable anisotropy, including the special XX-model which can be exactly solved by mapping it to non-interacting fermions. Spin transport changes from ballistic to diffusive depending on the anisotropy. For transverse spin patterns, we have found several new dephasing mechanisms related to a superexchange induced effective magnetic field. Using rubidium atoms and two atoms per site, we have realized spin 1 models. The onsite interactions give rise to a so-called-single-ion anisotropy term proportional to (S_z)^2, which plays an important role in stabilizing magnetism for low-dimensional magnetic materials. Our studies of spin dynamics illustrate the role of ultracold atoms as a quantum simulator for materials and for elucidating fundamental aspects of many-body physics.
For 130 years, a cylinder made of a platinum-iridium alloy stored near Paris was the official definition of a kilogram, the basic unit of mass. This all changed on May 20, 2019: a kilogram is now defined by a fundamental constant of nature known as the Planck constant h, which relates the energy of a photon to its frequency: h= 6.62607015 10-34 kilograms times square meters per second.
Sounds complicated? In this talk, I will provide the reasons for changing the definition of the kilogram, give simple explanations what the new kilogram is conceptually, and explain how objects with exactly known masses can be realized using advanced technology.
We are excited to announce that our distinguished speaker is Prof. Wolfgang Ketterle, the John D. MacArthur Professor of Physics at the Massachusetts Institute of Technology. Prof. Ketterle works on experimental research in atomic physics and laser spectroscopy and focuses currently on Bose-Einstein condensation in dilute atomic gases. He was among the first scientists to observe this phenomenon in 1995, and realized the first atom laser in 1997. His earlier research was in molecular spectroscopy and combustion diagnostics.
Prof. Ketterle's awards include the Rabi Prize of the American Physical Society (1997), the Gustav-Hertz Prize of the German Physical Society (1997), the Discover Magazine Award for Technological Innovation (1998), the Fritz London Prize in Low Temperature Physics (1999), the Dannie-Heineman Prize of the Academy of Sciences, Göttingen, Germany (1999), the Benjamin Franklin Medal in Physics (2000), and the Nobel Prize in Physics (2001, together with E.A. Cornell and C.E. Wieman).
Prof. Ketterle will also give the Applied Physics/Physics colloquium at 4:30 PM on Tuesday, April 13, 2021. Detailed information about both lectures and our speaker can be found on Physics site (link below). As in previous years, both lectures are free and open to the public via Zoom webinar.
In the 1970s, Hawking showed that black holes, like any finite-temperature system, radiate energy and so eventually evaporate away entirely. However, his calculations showed something very weird: unlike any other physical system, the radiation seemed to contain no information about the initial state of the black hole. Instead the information that fell into the black hole was simply lost forever. This contradiction between Hawking's calculations and the ordinary rules of quantum mechanics has been a driving force behind much of the research in quantum gravity over the ensuing decades. Finally, in the last couple of years, we have begun to understand where Hawking's calculation went wrong, and to derive precise predictions, consistent with unitary quantum mechanics, for the information content of Hawking radiation. However, the new calculations, which involve weird spacetime topologies called 'spacetime wormholes', lead to as many new questions as answers.