Category Archives: Author Insights

Author Insights are short informal articles written by authors of high quality CQG papers for the broad community served by the journal.

Noisy surface charges on gravitational wave detector optics

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Paul Campsie

Paul Campsie completed his Ph.D. in the Institute for Gravitational Research at the University of Glasgow. He now works as a Product & Test Engineer for Freescale Semiconductor.

A direct measurement of the fluctuating force noise created by surface charge on dielectrics

It has been known that future interferometric gravitational wave detectors could have their low frequency sensitivity limited by excess surface charges on the detector optics. Though it is suspected that the limiting effects of this noise source have been observed in initial detectors, this was never directly verified because there was no measurement of the charge on the optic.

In our recent CQG article we present a direct measurement of the fluctuating force noise created by excess surface charges (charging noise) on a dielectric. This measurement is 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 →

Isolated systems are asymptotically… flat

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Martin Reiris

Martin Reiris is Junior Scientist at the Max Planck Institute for Gravitational Physics (Golm-Germany) since 2009. He received the PHD in pure math in 2005 at SUNYSB, and held a Moore Instructor in Math at MIT from 2006 to 2009. His interests are mainly in geometry and the mathematical structure of general relativity

In the extraordinary manuscript The Foundation of the Generalised Theory of Relativity, printed in 1916 in Annalen der Physics, Einstein begins addressing what he calls an epistemological defect of classical mechanics, (as well as of special relativity), whose dignity he attributes to E Mach. He imagined two bodies, A and B, made of the same fluid material and sufficiently separated from each other that none of the properties of one could be attributed to the existence of the other. Observers at rest in one body, he continues, see the other body rotating at a constant angular velocity, yet these same observers measure a perfect round surface in one case and an ellipsoid of rotation in the other case. It is then asked: “Why is this difference between the two bodies?”. 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 →

Black hole voyeurism

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Alexander Smith and Robert Mann

Alexander (right) is a PhD student in the department of Physics & Astronomy at the University of Waterloo. Robert (left), a past-president of the Canadian Association of Physicists, is a senior professor in the department of Physics & Astronomy and the department of Applied Mathematics at the University of Waterloo and an affiliate at the Perimeter Institute.

Could a quantum detector peek inside a black hole?

It has long been known that the thermal radiation emitted by a black hole can be detected by a particle detector, and even today the details of this process are an active area of research. But are such detectors sensitive to the interior structure of black holes? From a classical perspective, conventional wisdom would suggest not: the topological censorship theorem relegates all isolated topological structures (such as wormholes, topological knots, etc) to be hidden behind a horizon and thus inaccessible to observers by classical probes. But Continue reading →

Asymptotic flatness and quantum geometry

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Sandipan Sengupta

Sandipan Sengupta is a post-doctoral fellow at the Raman Research Institute in Bangalore, India

From the perspective of quantum gravity, the spacetime is smooth only in an effective sense, and is expected to exhibit a discrete structure at suitably small length scales. Within the gauge theoretic formulation of gravity, there are certain kinematical states which provide an elegant realization of such a scenario. These are known as the spin-network states, and are used extensively in certain quantization approaches, e.g. Loop Quantum Gravity (LQG). However, since these states correspond to a spatially discrete quantum geometry, they cannnot be used to capture the notion of a classical spacetime continuum. This leads to a serious obstacle towards a quantization of Continue reading →

Homogeneous cosmological model from a discrete matter distribution

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Mikolaj Korzynski

Mikolaj Korzynski is an Assistant Professor at the Center of Theoretical Physics
of the Polish Academy of Sciences, Warsaw

How does a homogeneous FLRW metric arise from a cosmological model with black holes as the only source of gravitational field?

In astrophysical applications of general relativity we often need to apply the Einstein’s field equations to situations where the matter distribution, and consequently also the metric tensor, has a complicated form with relatively smooth large scale behavior and a  complicated structure on smaller scales. The problems of this kind are usually approached in the following way: instead of solving the equations directly we apply them to an idealized metric with the small-scale structure removed by Continue reading →

Melvin magnetic cosmologies

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Magnetic fields are ubiquitous in the universe – observed on scales ranging from stellar, through galactic and beyond – and are key to the physics of dramatic astrophysical objects such as pulsars and active galactic nuclei. Meanwhile, the origin of large-scale magnetic fields is still a topic of great debate in the cosmological literature.

Our recent CQG article presents a new family of exact solutions to the Einstein-Maxwell equations for cosmological magnetic fields. These solutions are both inhomogeneous and anisotropic, with the magnetic field having nontrivial dependence on Continue reading →

Probing the notion of gravitational entropy in inhomogeneous cosmologies

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Roberto Sussman

Dr Roberto A Sussman is a senior researcher in Theoretical Cosmology at the Institute for Nuclear Sciences (ICN) of the National University of Mexico (UNAM).

One of the long standing open problems in General Relativity is to find a self-consistent theoretically robust definition of a classical “gravitational” entropy, which is distinct (though possibly connected) to the entropy of the field sources (hydrodynamical or non-collisional) and to holographic and black hole entropies. Current research has produced two main classical gravitational entropy proposals: one by Clifton, Ellis and Tavakol, based on an effective construction from the “free” gravitational field associated to the Bell-Robinson tensor (the CET proposal), the other, by Hosoya and Buchert, is based on the Kullback-Leibler functional of Information Theory (the HB proposal).

Continue reading →

First occurrence of a double layer in a gravity theory found

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José Senovilla

Jose M. M. Senovilla is a full Professor of Theoretical Physics at the University of the Basque Country (UPV/EHU) in Bilbao, Spain

Gravitational double layers turn out to be feasible in quadratic theories of gravity. New physics arises.

Double layers (DL) may be found in several disciplines: in biology separating two different forms of matter, in chemistry as interfaces between different phases (liquid and solid), or in physics when two laminar parallel shells with opposite electric charges are found next to each other. DL are especially important in plasma and cellular physics, representing abrupt drops in the electric potential by which the cell, or plasma, “protects” itself from the environment.

However, gravitational DL were nowhere to be found in gravitational physics, until now. Continue reading →