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.

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.

PDF PPT

*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.

PDF PPT

*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.

**Seminars**

*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.

PDF PPT

*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.

PDF PPT

*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.