Colloquium

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.

We show that if a condensed matter system (a quantum qbit system) is in a string-net condensed state, then the low energy excitation in such a system can be gauge bosons (such as photons) and fermions (such as electrons). Such a system is actually the ether that we have been looking for 150 years. We will also discuss a quantum qbit system that may even give rise to emergent gravitons.

We review how renormalization, born in quantum field theory has evolved into a rather universal tool to analyze the change of physical laws under scaling. Recent developments in non commutative geometry with hopefully potential applications to the quantization of gravity will be discussed.

The boundary object is an ethnographic term that describes objects, processes, or words that cross between cultures or disciplines. Boundary objects are often the currency and the result of cross disciplinary practices. All manner of things, from software, to maps, to theories can provide a rich terrain for misunderstanding, tentative agreements or new insights. Case studies of cross-disciplinary art and science collaborations or design and engineering projects will provide examples.

The “clock ambiguity” is a general feature of standard formulations of quantum gravity, as well as a much wider class of theoretical frameworks. The clock ambiguity completely undermines any attempt at uniquely specifying laws of physics at the fundamental level. In this talk I explain in simple terms how the clock ambiguity arises. I then present a number of concrete results which suggest that a statistical approach to physical laws could allow sharp predictions to emerge despite the clock ambiguity. Along the way, I get to ask some interesting questions about what we expect of fundamental laws of physics, and give some surprising answers.

Theoretical neuroscience, like theoretical physics, attempts to discover and quantify the basic principles governing the systems it studies. Currently, however, there are very few attempts at unification across the levels of organization found in the brain. In this talk, I will describe the biological mechanisms of interest to neuroscientists, and describe a quantitative method for constructing sophisticated models of these mechanisms. Through a series of examples, I will show how the three principles that make up this method are general, allowing us to better understand a broad range of complex behaviour in a unified manner.