Colloquium

Physicists are often so awestruck by the lofty achievements of the past, we end up thinking all the big stuff is done, which blinds us to the revolutions ahead. We are still firmly in the throes of the quantum revolution that began a hundred years ago. Quantum gravity, quantum computers, qu-bits and quantum phase transitions, are manifestations of this ongoing revolution. Nowhere is this more so, than in the evolution of our understanding of the collective properties of quantum matter. Fifty years ago, physicists were profoundly shaken by the discovery of universal power-law correlations at classical second-order phase transitions. Today, interest has shifted to Quantum Phase Transitions: phase transitions at absolute zero driven by the violent jigglings of quantum zero-point motion. Quantum, or Qu-transitions have been observed in ferromagnets, helium-3, ferro-electrics, heavy electron and high temperature superconductors. Unlike its classical counterpart, a quantum critical point is a kind of 'black hole' in the materials phase diagram: a singularity at absolute zero that profoundly influences wide swaths of the material phase diagram at finite temperature. I'll talk about some of the novel ideas in this field including 'avoided criticality' - the idea that high temperature superconductivity nucleates about quantum critical points - and the growing indications that electron quasiparticles break up at a quantum critical point.

Entanglement is one of the most fundamental and yet most elusive properties of quantum mechanics. Not only does entanglement play a central role in quantum information science, it also provides an increasingly prominent bridging notion across different subfields of Physics --- including quantum foundations, quantum gravity, quantum statistical mechanics, and beyond. Arguably, the property of a state being entangled or not is by no means unambiguously defined. Rather, it depends strongly on how we decide to regard the whole as composed of its part or, more generally, on the restricted ways in which we are able to observe and control the system at hand. Acknowledging the implications of such an operationally constrained point of view naturally has led to a notion of 'generalized entanglement,' which is directly based on quantum observables and offers added flexibility in a variety of contexts. In this talk, I will survey some of the main accomplishments of the generalized entanglement program to date, with an eye toward recent developments and open problems.

The fundamental laws of physics are very simple. The world about us is very complex. Living things are very complex indeed. This complexity has led some thinkers to suggest that living things are not the outcome of physical law but instead the creation of a designer. Here I examine how complexity is produced naturally in fluids.

The vacuum landscape of string theory can solve the cosmological constant problem, explaining why the energy of empty space is observed to be at least 60 orders of magnitude smaller than several known contributions to it. It leads to a 'multiverse' in which every type of vacuum is produced infinitely many times, and of which we have observed but a tiny fraction. This conceptual revolution has raised tremendous challenges in particle physics and cosmology. To understand the low-energy physics we observe, and to test the theory, we will need novel statistical tools and effective theories. We must also solve a long-standing fundamental problem in cosmology: how to define probabilities in an infinite universe where every possible outcome, no matter how unlikely, will be realized infinitely many times. This 'measure problem' is inextricably tied to the quantitative prediction of the cosmological constant.

Quantum graphity is a background independent condensed matter model for emergent locality, spatial geometry and matter in quantum gravity. The states of the system are given by bosonic degrees of freedom on a dynamical graph on N vertices. At high energy, the graph is the complete graph on N vertices and the physics is invariant under the full symmetric group acting on the vertices and highly non-local. The ground state dynamically breaks the permutation symmetry to translations and rotations. In this phase the system is ordered, low-dimensional and local. The model gives rise to an emergent U(1) gauge theory in the ground state by the string-net condensation mechanism of Levin and Wen. In addition, in such a model, observable effects of emergent locality such as its imprint on the CMB can be studied. Finding the right dynamics for the desired ground state is ongoing work and I will review some of the basic results with an emphasis on the use of methods from quantum information theory such as topological order and the use of the Lieb-Robinson bounds to find the speed of light in the system.

Astronomers have discovered many candidate black holes in the universe and have studied their properties in ever-increasing detail. Over the last decade, a few groups have developed observational tests for the presence of event horizons in candidate black holes. The talk will discuss one of these tests, which indicates that the supermassive black hole at the center of our Galaxy must have a horizon.

In the 60’s, the analytic S-matrix program was developed in an attempt to describe the strong interactions – at the time, this was a theory of massive particles like pions. The S-matrix is an object that encodes the information of the probability of producing a certain set of final particles from a given set of initial particles. Eventually, the S-matrix program was replaced by Quantum Field Theory and in particular by Quantum Chromo Dynamics as the description of the strong interactions. In recent years there has been a resurrection of the S-matrix paradigm. The current view is that S-matrix techniques are most natural and powerful in theories of massless particles! Moreover, from this new perspective, the simplest quantum field theory to consider is now believed to be the maximally supersymmetric gravity theory. If the expectation is correct then N=8 supergravity will turn out to be a finite theory of gravity in perturbation theory.

The PAMELA satellite-borne experiment was launched from the Baikonur cosmodrome on the 15th of June 2006. It has been collecting data since July 2006. The instrument is composed of a silicon-microstrip magnetic spectrometer, a time-of-flight system, a silicon-tungsten electromagnetic calorimeter, an anticoincidence system, a shower tail counter scintillator and a neutron detector. The primary scientific goal is the measurement of the antiproton and positron energy spectrum in order to search for exotic sources, such as dark matter particle annihilations. PAMELA is also searching for primordial antinuclei (anti-helium), and testing cosmic-ray propagation models through precise measurements of the energy spectra of light nuclei and their isotopes. Moreover, PAMELA is investigating phenomena connected with solar and earth physics. The first results obtained in the explored research fields and in particular for antiproton-proton and positron-electron ratios will be presented.

We are currently in the throes of a potentially huge paradigm shift in physics. Motivated by recent developments in string theory and the discovery of the so-called \'string landscape\', physicists are beginning to question the uniqueness of fundamental theories of physics and the methods by which such theories might be understsood and investigated. In this colloquium, I will give a non-technical introduction to the nature of this paradigm shift and how it developed. I will also discuss some of the questions to which it has led, and the nature of the controversies it has spawned.

According to general relativity, space-time ends at singularities and classical physics just stops. In particular, the big bang is regarded as The Beginning. However, general relativity is incomplete because it ignores quantum effects. Through simple models, I will illustrate how the quantum nature of space-time geometry resolves the big bang singularity. Quantum physics does not stop there. Indeed, quantum space-times can be vastly larger than what general relativity had us believe, with unforeseen physical effects in the deep Planck regime.

The discovery and understanding of superconductivity has provided important paradigms for physics, including spontaneous gauge symmetry breaking and the Anderson-Higgs mechanism. More recent discoveries in superconductivity have given us examples of doped Mott insulators, still an unsolved theoretical problem, as well as superconductors which spontaneously break time reversal symmetry and which support Majorana fermions and non-Abelian statistics. The latter are of potential interest to quantum computing. This talk will review our current understanding of, and the open issues related to, these new supercondutors, including the high T_c cuprates, iron pnictides, and strontium ruthenate.