Some Recent Talks

Here are the slides for a few talks I have given recently, in PDF and PPT format. You can use Adobe Acrobat Viewer to view them, which can be obtained for free from  here .

Public Lectures

The Theory Formerly Known as String Theory: At Last a Quantum Theory of Gravity?

String theory is our best candidate for a theory of the physics of
very short distances, and is the only known candidate theory which
handles quantum corrections to gravity in a reasonable way at the
highest energies. In this talk I summarize the advances which have
been made in understanding string theory, and how this has led to
its extension to a broader theory known as M Theory. I also summarize
the problems which remain to be solved.



Strings and Branes: Quantum Gravity Meets Experimental Physics?
(CAP Lecture Tour, 2004)

String theory is our best candidate for a theory of the physics of
very short distances, and is the only known candidate theory which
handles quantum corrections to gravity in a reasonable way at the
highest energies. In this talk I summarize the advances which have
been made in understanding string theory, and how this has led to
the discovery of extended objects in the theory called D-Branes.
Besides describing how this has changed our understanding of
string theory, I summarize how the existence of branes changes
how we think about the experimental implications of any theory
of very short distances, including a discussion of some of the
outstanding problems and opportunities which remain.


Neutrinos: What We Think We Know, and Why It May Yet Be Wrong
(CAP Lecture Tour 2004)

Over the past few years the evidence has become compelling (most
convincingly from Canada's own Sudbury Neutrino Observatory) that neutrinos
may not be behaving as they are supposed to according to the otherwise
brilliantly--successful Standard Model of particle physics. This talk is meant
as a non-technical introduction to the neutrino's properties; both as they are
understood in the Standard Model, and how this understanding
is now changing in view of the current experiments.


String Theory and Inflation: The Start of a Beautiful Relationship?

String theory is our best candidate for a theory of the physics
at very short distances, but is very much a theory in search of
an observable application. Inflation is a very successful
phenomenological theory of cosmological initial conditions, but
has proven difficult to embed into a real theory of short distances.
Is each one the answer to the other's problem? I will describe the
recent progress which has been made in bringing these two theories together.


Fighting the Split Brain: Why Renormalization is a Good Thing

The incompatibility of General Relativity and Quantum
Mechanics --- both of which have strong experimental support
in their respective regimes --- is often lamented as one of the great
failures of modern physics. I argue in this talk that this
incompatibility is not so drastic, and that General Relativity and
the Standard Model of particle physics together provide an excellent
quantitative *quantum* description of all physics --- including
gravity --- over  the complete range of distance scales we understand.
This perspective follows from the modern picture of the physics
which underlies renormalization. It provides a powerful, unified
point of view whose application has borne fruit  in many
branches of physics.

Can Extra Dimensions Be as Large as 100 Microns Across?

Considerable attention has been focussed recently on the possibility
that extra dimensions can be as large as 100 microns across, and
the fundamental length scale of gravity, such as described by string theory,
is much larger than the Planck scale -- 10^(-32) cm-- and possibly might
even be as large as 10^(-17) cm, making it close to the resolution of
upcoming high-energy experiments.  This talk is a reasonably opinionated
description (for nonspecialists) of the arguments underlying this possibility,
emphasizing those parts of the proposal you might want to take seriously, and why.


What Can Solar Neutrinos Tell Us About the Solar Radiative Zone?

The original motivation for searching for solar neutrinos was to
acquire a direct probe of solar properties deep within the solar
interior. Now that the wild ride in which we learned more about
neutrinos than the sun is coming to an end, we can again ask what
we might learn about solar physics. This talk is meant as a first
small step in this direction, where I'll argue that the existence of
resonant neutrino oscillations excludes certain kinds of density
fluctuations in the solar interior. I'll argue that the kinds of
fluctuations which are excluded may arise from magnetic fields in
the solar radiative zone (without requiring a neutrino magnetic moment)
and so their absence provides an new constraint on the kinds of
magnetic fields which can be present there.


(Plenary talk given at AHEP 2003, Valencia, October 2003: PDF  PPT)

String Inflation

The discovery of D-Branes in string theory and the realization that all
known particles (except perhaps the graviton) may be localized on
surfaces within a larger-dimensional spacetime has dramatically
changed how we think about the low-energy implications of
fundamental theories of very small distances. This `Brane World'
picture has many implications for cosmology, of which I summarize
two. I first summarize the recent progress towards finding brane inflation
within string theory, and second (if there is sufficient time) I describe a
recent brane-world proposal understanding the size of the recently
discovered dark energy.


Dark Energy from Supersymmetric Large Extra Dimensions

All dynamical explanations of dark energy in terms of the evolution
of a scalar field (`quintessence models') should explain how a degree
of freedom can remain light enough to be cosmologically active at
present, and why such a light degree of freedom is not seen in
precision tests of gravity, but very few attempt to do so. This
talk summarises these constraints, and explores the cosmological
implications of a model which uses the novel features of the
brane-world scenario to address these issues.


Are Inflationary Predictions Sensitive to Very High Energy Physics?

It has been proposed that the successful inflationary
description of density perturbations on cosmological scales is
sensitive to the details of physics at extremely high
(trans-Planckian) energies. I will critically analyse this idea by examining how
inflationary predictions depend on higher-energy scales within a
simple model where the higher-energy physics is well understood.
The result is the best of all possible worlds: inflationary predictions
are robust against the vast majority of high-energy effects, but
*can* be sensitive to some effects in certain circumstances,
in a way which does not violate ordinary notions of decoupling.
This implies both that the comparison of inflationary predictions
with CMB data is meaningful, and that it is also worth searching
for small deviations from the standard results in the hopes of
learning about very high energies.


Brane-Antibrane Inflation

Recent observations of the cosmic microwave background seem to support
the picture that the very early universe underwent a dramatic expansion
during an inflationary phase, yet convincing detailed models of inflation
remain elusive. Studies of brane inflation try to see whether the difficulties
encountered can be overcome by appealing to string theory -- our best guess as to the
physics relevant to these early times. Long-standing difficulties in finding
inflation within string theory are now being overcome with the realization
that all known particles and interactions may appear within string theory
trapped on a surface (or "brane") within the higher-dimensions which string
theory predicts. This talk is meant to summarize this recent progress.

Supersymmetry Breaking and Moduli Stabilization from 6D Supergravity

Most of the interesting questions concerning whether string theory
can describe the low-energy physics of the lab or the very early universe
require some understanding of how supersymmetry breaks and how the many
string degrees of freedom get masses. This talk will summarize the
obstacles which make this difficult, and will resurrect an old
six-dimensional supergravity model which has many of the features which we
would like to find in a real string vacuum. As such it provides a toy
model in which to explore the issues, and may be a first step towards
constructing realistic string compactifications along the lines now being
explored using nonzero fluxes.

Two-dimensional Electron Systems: Duality in the Laboratory?

Many features of Quantum Hall systems seem to be well described
by the assumption that the important charge carriers are weakly
interacting particles and vortices, which enjoy a large duality
symmetry group (a level two subgroup of SL(2,Z)) which is based
on particle-vortex interchange. This talk is meant as an introductiona
to Quantum Hall systems with an emphasis on the evidence for the
existence of this duality symmetry. A generalization of the
symmetry to other two-dimensional systems is then proposed.