Invariant classification of vacuum PP-waves

We solve the equivalence problem for vacuum PP-wave spacetimes by employing the Karlhede algorithm. Our main result is a suite of Cartan invariants that allows for the complete invariant classification of the vacuum pp-waves. In particular, we derive the invariant characterization of the G2 and G3 sub-classes in terms of these invariants. It is known [Collins91] that the invariant classification of vacuum pp-waves requires at most the fourth order covariant derivative of the curvature tensor, but no specific examples requiring the fourth order were known. Using our comprehensive classification, we prove that the q<=4 bound is sharp and explicitly describe all such maximal order solutions

What is General Relativity?

General relativity is a set of physical and geometric principles, which lead to a set of (Einstein) field equations that determine the gravitational field, and to the geodesic equations that describe light propagation and the motion of particles on the background. But open questions remain, including: What is the scale on which matter and geometry are dynamically coupled in the Einstein equations? Are the field equations valid on small and large scales? What is the largest scale on which matter can be coarse grained while following a geodesic of a solution to Einstein's equations? We address these questions. If the field equations are causal evolution equations, whose average on cosmological scales is not an exact solution of the Einstein equations, then some simplifying physical principle is required to explain the statistical homogeneity of the late epoch Universe. Such a principle may have its origin in the dynamical coupling between matter and geometry at the quantum level in the early Universe. This possibility is hinted at by diverse approaches to quantum gravity which find a dynamical reduction to two effective dimensions at high energies on one hand, and by cosmological observations which are beginning to strongly restrict the class of viable inflationary phenomenologies on the other. We suggest that the foundational principles of general relativity will play a central role in reformulating the theory of spacetime structure to meet the challenges of cosmology in the 21st century.Comment: 18 pages. Invited article for Physica Scripta Focus issue on 21st Century Frontiers. v2: Appendix amended, references added. v3: Small corrections, references added, matches published versio

Death and the Pearl Maiden

Shows how English responses to the Black Death were hidden in plain sight—as seen in the Pearl, Cleanness, Patience, and Sir Gawain and the Green Knight poems

Projected performances: the phenomenology of hybrid theater

Throughout the 20th century, mediatized forms gained prominence and eclipsed the theater as a site of cultural power and popularity. Because of this tension, performance theorists like Peggy Phelan framed the definition of theater through its inherent differences from film and television. Other theorists like Philip Auslander problematized this distinction, particularly due to television’s similarities to live performance. The cinema, however, has remained an opponent to performance, ignored in favor of technologies that more readily promote a sense of “liveness.” In Projected Performances, I argue that film projection is more closely related to performance than previously thought, particularly when viewed in light of their phenomenological similarities. Projection is a live act that generates a kind of presence that approximates what is felt with a live performer. The theatrical setting of most film viewings foregrounds this phenomenological frame, despite the prerecorded nature of the content. Despite the seemingly static nature of film, the exhibition of it is most often decidedly theatrical. Hybrid theater, in which productions incorporate film projection alongside live performers, highlights these similarities in a much more explicit way, creating a unique sensory experience. This blending of effects is evident in theatrical broadcasts like the Metropolitan Opera’s “Live in HD” series, which capitalizes on the liveness of theater to draw people to the cinema. I also investigate hybrid productions that use projected scenery, such as The Woman in White and The Elephant Vanishes, as well as productions that feature projected bodies, like the work of Lemieux.Pilon 4d Art. Finally, I interrogate the use of projections in the monumental spectacles of the opening ceremonies at the 2008 and 2010 Olympics in Beijing and Vancouver, respectively.Throughout, I examine the ways in which these hybrid productions trouble the assumed distinction between performance and media, demonstrating that projection is a kind of performance that can share the stage with live performers without damaging the unique essential qualities of theater

Observational Challenges for the Standard FLRW Model

We summarise some of the main observational challenges for the standard Friedmann-Lemaitre-Robertson-Walker cosmological model and describe how results recently presented in the parallel session `Large--scale Structure and Statistics' (DE3) at the `Fourteenth Marcel Grossman Meeting on General Relativity' are related to these challenges.Comment: 17 pages; references added. Matches published version in Int. J. Mod. Phys. D; Report on Parallel Session DE3 of MG1

Accessing the Thermal Mass above Suspended Ceilings via a Perimeter Gap: a CFD Study of Naturally Ventilated Spaces

There is a growing desire to reduce peak temperatures within non-domestic buildings by accessing the thermal mass of separating floors. These floors are typically formed of concrete and can store reasonable amounts of heat. Unfortunately, they are usually thermally isolated from the room below by a suspended ceiling. Therefore, some architects try to access the concrete by leaving a perimeter gap in the suspended ceiling in each room to allow airflow across the underside of the separating floor. For visual and acoustic reasons, there is the desire to make this gap as small as possible. Using computational fluid dynamics we examine the relationship between gap size and airflow above the suspended ceiling for naturally ventilated spaces. We show that although the precise details of the airflow depends on the size of the room, levels of incidental gains, ventilation rates and the location of heat sources, in all cases increasing the perimeter width within realistic bounds results in a linear increase in the mean tangential speed of airflow across the underside of the ceiling. This is common for both single sided and cross ventilated rooms and for both single and double raft designs; however, the double raft design performs significantly better.EPSR