Public Talks and Colloquia

Here is a selection of colloquia and public lectures / outreach talks that I’ve given over the past few years. Those which were given at Perimeter Institute may be viewed online at PIRSA.


Colloquia are typically given in a physics department, and are aimed at upper level undergraduates and graduate students. The goal is to communicate developments in a specialization to a wider audience of scientists.

  • Modifying Gravity: The View from Below   We live at a time of contradictory messages about how successfully we understand gravity. General Relativity seems to work very well in the Earth’s immediate neighbourhood, but arguments abound that it needs modification at very small and/or very large distances. This talk tries to put this discussion into the broader context of similar situations in other areas of physics, and summarizes some of the lessons which our good understanding of gravity in the solar system has for proponents for its modification over very long and very short distances. The main message is that effective theories (in the technical sense of effective) provide the natural (and arguably only known) precise language for framing proposals. Its framework is also useful, inasmuch as it makes some modifications seem more plausible than others, though there are also some surprises. Amongst the surprises is evidence that in some ways gravity behaves more like condensed matter physics or optics than particle physics, raising the possibility that tools from these areas may be useful for understanding cosmology and black holes.
  • Finding the Gravity in any Situation: the potential impact of the AdS/CFT correspondence Evidence has been building since the mid-1990s that the set of field theories is much smaller than had been thought, with the strongly coupled limit of one theory turning out to be identical to the weakly coupled limit of another. In particular, specific extra-dimensional string theories appear to be precisely equivalent to more garden-variety four-dimensional theories not involving gravity or strings at all. Since field theories provide the mathematics used to describe nature in many areas of physics, this observation may ultimately have enormous consequences for physics, if for no other reason than providing unusual new tools for extracting the predictions of strongly coupled systems. This colloquium aims to describe, for nonspecialists, the nature of this equivalence, why extra dimensional anti de Sitter space (AdS) appears to be related to conformally invariant field theories (CFTs) in lower dimensions, and some of the present evidence on which belief in their correspondence is based.
  • What is the Universe Made Of? The Case for Dark Matter and Dark Energy For the first time in human thought it is now possible to observationally determine how much matter is in the Universe as a whole. These observations strongly support the “Concordance Model” of Hot Big Bang Cosmology, and reinforce earlier indications that ordinary matter (atoms, nuclei and electrons) make up at present at most 4% of the total of the Universal energy density. The big surprise was that the rest consists of *two* kinds of unknown forms of matter: the so-called Dark Matter and Dark Energy. This talk summarizes for non-specialists the various lines of evidence for their existence, and some of the theoretical ideas which have been proposed to account for their properties.
  • The Dog that Didn’t Bark: The Cosmological Constant Problem and the Puzzle of Dark Energy The (‘old’) Cosmological Constant Problem, which asks why the vacuum zero-point energy appears not to gravitate, was a theoretical thorn in our collective sides even before the Dark Energy was discovered. This talk reviews why it is regarded to be a problem, and argues that its solution is likely to be the biggest clue we have about the nature of Dark Energy. A set of minimal criteria are formulated against which proposed solutions to the problem can be judged, and most proposals found to fail. Finally I argue why I believe that a solution to the Cosmological Constant Problem may yet exist and is likely telling us why the LHC should not see superpartners for ordinary particles, even though it also implies that supersymmetry must exist and be important at low energies.


Public lectures and outreach talks are similar in content, but with slightly different audiences in mind. As the name implies, public lectures are aimed at the general public, though this is often a self-selected audience that chooses to spend an hour of their evening attending a talk about science.

The audience for an outreach talk, on the other hand, is usually a group of teachers or high-school students, who have a more reliable interest in and knowledge of basic physics.

  • Gravitational Waves: The Sounds of Spacetime Gravitational waves were recently seen coming from the coalescence of two black holes a great distance away. This talk intends to convey three things: 1. why modern theories of forces always require the existence of waves; 2. why this requirement is partly driven by relativity (largely because of its requirement that nothing goes faster than light, which in turn is founded on the impossibility of being able to sends signals into one’s own past); and 3. why seeing gravitational waves from black holes is likely to teach us much about the universe as a whole in addition to learning about the waves and black holes themselves.
  • Engineering the Vacuum: The Higgs Boson discovery and what it says about what the Large Hadron Collider might find The Large Hadron Collider (LHC) has now been running for several years (after a rocky start), but will shut down for major maintenance early next year. Although proposed as a no-lose proposition, so far the finding of a new boson has been the sole major discovery. This talk reviews why the start was rocky and how this constrains the physics program over the next few years. It then briefly surveys three things: what is the evidence that this new boson is the Higgs boson; why theoretical expectations are high for there being more to discover; and why the theoretical proposals fall broadly fall into three categories (and how each is faring in view of the data so far). If time permits I will close by telling you why I personally think the cosmological constant problem implies the LHC will strike paydirt and make quantum gravity an experimental science.
  • Physicists at CERN: Angels or Demons? The movie Angels and Demons, based on one of Dan Brown’s (Da Vinci Code) books, is partially set at the Large Hadron Collider (LHC) in CERN in Geneva. This talk provides a scorecard of how well the movie does at getting the physics right; and of how well Hollywood does at portraying science as a whole. It closes with a very brief summary of what the LHC is, how well it is performing so far, and what it is trying to achieve.
  • String Theory: Quantum Mechanics and Gravity – the Start of a Beautiful Relationship? String theory is our best candidate for a theory of the physics at very short distances, and is the only known candidate theory which handles quantum corrections to gravity in a reasonable way at the shortest distances. In this talk I (loosely) use the movie “Casablanca” to summarize some of the ideas behind string theory, and some problems and opportunities to which it leads.
  • Are We Alone? How Likely is Extraterrestrial Intelligence? More than 40 years ago astronomer Frank Drake estimated how likely it would be that there might be other intelligent species in the galaxy. He did so in order to see whether or not it would be worth the effort to search the skies with radio telescopes for evidence of their existence. He concluded that it was, and the resulting search is known as SETI (Search for ExtraTerrestrial Intelligence). In this talk I will review this estimate, with an eye towards whether or not we can provide a better estimate today for the likelihood of extraterrestrial life, and what intelligent guesses we can reasonably make about their properties. Come along and find out if we are likely to be alone.
  • Before the Big Bang? Cosmic Inflation: The Ultimate Free Lunch The Hot Big Bang model of cosmology is just now being redundantly tested for the first time, and so far provides our best understanding of the shape and evolution of the Universe after it was a few seconds old. But it all works under the assumption that the initial conditions at this time were very carefully chosen to be very flat and homogeneous. Surprisingly, recent observations provide some evidence as to what kinds of physics during earlier epochs might produce such a special initial condition, and in so doing may bring us to another “Copernican” revolution concerning our picture of how special our place in the Universe really is.