Author Archives: BenSheard_Publisher

New focus issue: Advanced interferometric gravitational wave detectors

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Peter Shawhan and Marie-Anne Bizouard

Peter Shawhan is an Associate Professor of Physics at the University of Maryland, USA. His primary research area is in the analysis of data from gravitational wave detectors and connections with astrophysical events.
Marie-Anne Bizouard is a research fellow at CNRS, Laboratoire de l’Accélérateur Linéaire, Orsay, France. She is an experimental physicist working on gravitational wave searches with ground based interferometric detectors.

The quest to detect gravitational waves directly has seen great advances over the past five decades, with the earlier resonant “bar” detectors being surpassed in sensitivity by large laser interferometers in the last decade.  The first generation of interferometric detectors proved the viability of the approach, progressively improving sensing and control techniques and running up against the fundamental limitations of their designs.  Along the way, many searches for gravitational wave signals were carried out and published, but none achieved the milestone of detecting a clear gravitational-wave signal.

All of that is about to change.  The lessons learned from the first full-scale interferometric detectors fed into the design of advanced detectors which are now being constructed and commissioned and will soon begin collecting data.  Higher laser power, Continue reading →

Memory at a distance

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Jeff Winicour

The author, overshadowed by nature in the Muir Woods on the California coast.
Jeff Winicour is a Professor of Physics and Astronomy at the University of Pittsburgh

Can the supertranslation symmetry of radiating spacetimes affect angular momentum loss?

That was the question on my mind when I went to a workshop at Berkeley, California last winter. I knew that the supertranslations were a global aspect of the gravitational memory effect, which produces a net displacement between particles after passage of a gravitational wave. What I didn’t know, and learned from David Garfinkle at Berkeley, was that there was an electromagnetic analog of radiation memory, which produces a momentum kick on test charges after passage of a wave. Surprisingly, this result has apparently gone unnoticed in Continue reading →

New focus issue: Entanglement and quantum gravity

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Eugenio Bianchi and Carlo Rovelli

Eugenio Bianchi (left) is an assistant professor at Pennsylvania State University and Carlo Rovelli (right) is a professor at Aix-Marseille University at the Centre de Physique Theorique de Luminy

Quantum gravity alone is not the only major theoretical open problem in fundamental physics: gravity, quantum theory and thermodynamics form a triple, whose full interconnections we have definitely not yet understood. As soon as quantum effects appear in a curved spacetime, thermal aspects appear to be unavoidable. Combining thermodynamics and (full) gravity might turn out to be even more crucial than understanding the quantum aspects of the gravitational field alone. In recent years, it has become increasingly clear that entanglement entropy is a central ingredient for the synthesis we are seeking. Continue reading →

The new CQG Highlights of 2013-14

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Ben Sheard

Ben Sheard is the publishing editor of Classical and Quantum Gravity

It is my pleasure to present the CQG Highlights of 2013-14. The Highlights articles are chosen by the Editorial Board as a selection of some of the best work published in the journal, based on criteria of interest, significance and novelty.

The articles span the whole of CQG’s subject scope and include focus issue articles and topical reviews in addition to regular papers. All of the Highlights articles are free to download until the end of 2015.

As part of the promotion of the Highlights we produce an annual Highlights brochure which contains further information about journal activity including forthcoming special issues and prize Continue reading →

New hair on black rings

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Simon Ross

Simon Ross is a professor in the department of mathematical sciences at Durham University, and a member of the university’s Centre for Particle Theory.

Relating different charge densities gives black rings with non-trivial profiles with smooth horizons.

There is a rich space of solutions in five-dimensional supergravity, including smooth horizonless supertube solutions and black ring solutions. Supertubes can have arbitrary profiles, and varying charge densities along the profile, but previously-known black ring solutions required a constant charge density along the ring to have a smooth horizon.

Recently, we discovered a new kind of supersymmetric horizonless object which generalizes the supertube, which we dubbed the magnetube. They carry coordinated electric charge densities with Continue reading →

Three-dimensional massive gravity and AdS/CFT

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Alasdair Routh and Wout Merbis

Alasdair Routh (left) is a Ph.D. student in the Department of Applied Mathematics and Theoretical Physics at the University of Cambridge. Wout Merbis (right) is a Ph.D. student at the Centre for Theoretical Physics of the University of Groningen.

Einstein’s gravitational field equations, which relate the geometry of spacetime to the matter in it, can also be applied to a spacetime of three dimensions (3D) but in this case the matter completely determines the geometry, so there is no “room” for gravitational waves: gravitons in the quantum theory. However, in 3D there is a simple extension of Einstein’s second-order equations to the third-order equations of “Topologically-Massive Gravity” (TMG), which propagates a single massive spin-2 mode; i.e. a massive graviton.

In the context of asymptotically anti-de Sitter (AdS) space times, both 3D Einstein gravity and TMG are potentially semi-classical approximations to some consistent 3D quantum gravity theory defined, via the AdS/CFT correspondence, in terms of a 2D conformal field theory (CFT). However, Continue reading →

Attempting to quantize geometry

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Jan Ambjørn is professor of theoretical high energy physics at the Niels Bohr Institute, University of Copenhagen and at IMAPP, Radboud University.

Jan Ambjørn is professor of theoretical high energy physics at the Niels Bohr Institute, University of Copenhagen and at IMAPP, Radboud University.

The Standard Model of particle physics is a quantum theory. It is born quantum. The observations of the weak and the strong interactions were from the beginning linked to quantum phenomena. For gravity the situation is different. Because the gravitational coupling constant is so small compared to coupling constants in the Standard Model, any observations of quantum aspects of gravity have been ruled out so far. Here we will assume that gravity is a quantum theory. However, quantizing gravity has so far turned out to be difficult. That Continue reading →

Symmetry operators

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Thomas Bäckdahl

Thomas Bäckdahl is a Post-Doctoral Research Assistant in the School of Mathematics at the University of Edinburgh.

Conserved quantities, for example energy and momentum, play a fundamental role in the analysis of dynamics of particles and fields. For field equations, one manifestation of conserved quantities in a broad sense is the existence of symmetry operators, i.e. linear differential operators which take solutions to solutions. A well-known example of a symmetry operator for the scalar wave equation is provided by the Lie derivative along a Killing vector field.

It is important to note that other kinds of objects Continue reading →

Testing the weak equivalence principle with atom interferometry in space

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Clifford Will

Clifford Will is the Editor-in-Chief of Classical and Quantum Gravity

The Weak Equivalence Principle (WEP) is one of the three pillars that support all metric theories of gravity, and testing it to high precision has occupied experimentalists for over 100 years. Although many successful tests have been performed, there is still room for new experiments (see this recent CQG focus issue on tests of WEP).

This paper describes in detail a concept called STE-QUEST for testing WEP in space. What makes this different from other space experiments, such as MICROSCOPE, due for launch in 2016, and STEP, still only a design concept, is that those experiments use macroscopic bodies, while STE-QUEST will use fundamentally quantum-mechanical systems: Bose-Einstein condensates of rubidium isotopes. Using atom Continue reading →

A unified description of the second order cosmological density contrast

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In this paper the authors introduce a new way of expressing the relativistic density contrast of matter perturbations in four commonly used gauges, both at first and second orders.

Julien Larena

Dr Julien Larena is a senior lecturer in the Department of Mathematics at Rhodes University, South Africa. His research is centred on relativistic corrections to cosmology, tests of the Copernican principle, and the backreaction issue in cosmology.

This new method is very interesting, since it provides a unified treatment of the density contrast in the various gauges, thus allowing a straightforward comparison of results obtained by other authors in different gauges. This should be useful when computing non-trivial effects such as the properties Continue reading →