Colloquium

One of the cool, frustrating things about quantum theory is how the once-innocuous concept of "measurement" gets really complicated. I'd like to understand how we find out about the universe around us, and how to reconcile (a) everyday experience, (b) experiments on quantum systems, and (c) our theory of quantum measurements. In this talk, I'll try to braid three [apparently] separate research projects into the beginnings of an answer. I'll begin from the premise that you make a measurement to find something out, then attack some specific questions: "How do we find out about quantum systems?", "What can we find out about quantum systems?", and finally "What do we actually know, afterward?" I'll give precise statements of these questions, then present [partial] answers.

Quantum field theory in curved spacetime (QFTCS) is the theory of quantum fields propagating in a classical curved spacetime, as described by general relativity. QFTCS has been applied to describe such important and interesting phenomena as particle creation by black holes and perturbations in the early universe associated with inflation. However, by the mid-1970\'s, it became clear from phenomena such as the Unruh effect that \'particles\' cannot be a fundamental notion in QFTCS. By the mid-1980\'s it was understood how to give a mathematically rigorous formulation of the theory of a free quantum field in curved spacetime. During the past decade, major progress has been made in providing a completely mathematically satisfactory formulation of renormalization in interacting QFTCS, thereby overcoming the difficulties caused by the absence of Poincare symmetry as well as the lack of a preferred vacuum state and a fundamental notion of \'particles\'. This talk will describe these developments and some of the insights that have thereby been attained.

I will discuss an alternative approach to simulating Hamiltonian flows with a quantum computer. A Hamiltonian system is a continuous time dynamical system represented as a flow of points in phase space. An alternative dynamical system, first introduced by Poincare, is defined in terms of an area preserving map. The dynamics is not continuous but discrete and successive dynamical states are labeled by integers rather than a continuous time variable. Discrete unitary maps are naturally adapted to the quantum computing paradigm. Grover's algorithm, for example, is an iterated unitary map. In this talk I will discuss examples of nonlinear dynamical maps which are well adapted to simple ion trap quantum computers, including a transverse field Ising map, a non linear rotor map and a Jahn-Teller map. I will show how a good understanding of the quantum phase transitions and entanglement exhibited in these models can be gained by first describing the classical bifurcation structure of fixed points.

Modern motivations for extra spacetime dimensions will be presented, in particular the surprising AdS/CFT connection to particle compositeness. It will be shown how highly curved, "warped", extra-dimensional geometries can naturally address several puzzles of fundamental physics, including the weakness of gravity, particle mass hierarchies, dark matter, and supersymmetry breaking. The possibility of direct discovery of warped dimensions at particle colliders such as the CERN Large Hadron Collider will be discussed. Some current questions in warped cosmology will also be introduced.

The progress in neutrino physics over the past ten years has been tremendous: we have learned that neutrinos have mass and change flavor. I will pick out one of the threads of the story-- the measurement of flavor oscillation in neutrinos produced by cosmic ray showers in the atmosphere, and its confirmation in long distance beam experiments. I will present the history, the current state of knowledge, and how the next generation of high intensity beam experiments will address some of the remaining puzzles.

Among the possible explanations for the observed acceleration of the universe, perhaps the boldest is the idea that new gravitational physics might be the culprit. In this colloquium I will discuss some of the challenges of constructing a sensible phenomenological extension of General Relativity, give examples of some candidate models of modified gravity and survey existing observational constraints on this approach. I will conclude by discussing how we might hope to distinguish between modifications of General Relativity and dark energy as competing hypotheses to explain cosmic acceleration.

The theory of strong interactions is an elegant quantum field theory known as Quantum Chromodynamics (QCD). QCD is deceptively simple to formulate, but notoriously difficult to solve. This simplicity belies the diverse set of physical phenomena that fall under its domain, from nuclear forces and bound hadrons, to high energy jets and gluon radiation. In this talk I show how systematic limits of QCD, known as effective field theories, provide a means of isolating the essential degrees of freedom for a particular problem while at the same time supplying a powerful tool for quantitative computations. The adventure will take us from the fine structure of hydrogen, to weak decays of B-mesons, to the behavior of energetic hadrons and jets in QCD.

Shear viscosity is a transport coefficient in the hydrodynamic description of liquids, gases and plasmas. The ratio of the shear viscosity and the volume density of the entropy has the dimension of the ratio of two fundamental constants - the Planck constant and the Boltzmann constant - and characterizes how close a given fluid is to a perfect fluid. Transport coefficients are notoriously difficult to compute from first principles. Recent progress in string theory, in particular the development of the gauge-gravity duality, has enabled one to approach this problem from a totally unexpected perspective. In my talk, I will describe the connection between the dynamics of black hole horizons (encoded in their quasinormal spectra) and the hydrodynamics of certain strongly coupled plasmas. I will comment on the relevance of this approach for the interpretation of data obtained in experiments on heavy ion collisions.

Theories of physics beyond the Standard Model predict the existence of relativistic strings, either as composite objects, or as fundamental constituents of matter. If they were created in the Big Bang, they would very likely still be present in the universe today. This talk reviews the thirty year history of cosmic strings, and describes the latest work which finds intriguing hints in the Cosmic Microwave Background data that the universe is filled with string.

Some of the speculations on new physics, beyond what is in the standard model are reviewed. Particular attention is paid to ideas that try to address the hierarchy puzzle, i.e., why is the weak scale so much smaller than the Planck scale. These new theories will be tested at the large hadron collider in the near future.