The importance of being Melvin

The authors, Jennie Traschen and David Kastor, enjoy the wit and humor of Oscar Wilde.

The authors, Jennie Traschen and David Kastor, enjoy the wit and humor of Oscar Wilde. The image above has been obtained from the Wikimedia website, where it is stated to have been released into the public domain. It is included within this blog post on that basis.

Like Oscar Wilde’s famous 1895 play, our recent CQG article “Melvin Magnetic Fluxtube/Cosmology Correspondence,” features an intricate interplay of dual and concealed identities. While our paper lacks the biting wit of Wilde’s dialogue, e.g.

“I do not approve of anything that tampers with natural ignorance. Ignorance is like a delicate exotic fruit; touch it and the bloom is gone. The whole theory of modern education is radically unsound. Fortunately in England, at any rate, education produces no effect whatsoever,”

our revelations regarding true identity do play out on a more vast, indeed a cosmic stage.

Melvin’s solution to the Einstein-Maxwell equations describes a static bundle of magnetic flux-lines bound together by self-gravity. Originally discovered in 1963, it has a rich and influential history. In 1964, Thorne studied the stability of what he called “Melvin’s Magnetic Universe.” Its resistance to gravitational collapse was an important clue leading to the formulation of his well-known hoop conjecture. In 1975, Ernst showed that Continue reading

The gravitational Hamiltonian, first order action, Poincaré charges and surface terms

Read the full article for free* in Classical and Quantum Gravity:
The gravitational Hamiltonian, first order action, Poincaré charges and surface terms
Alejandro Corichi and Juan D Reyes 2015 Class. Quantum Grav. 32 195024

*until 18/11/15

Ever since Einstein and Hilbert were racing to complete the general theory of relativity, almost 100 years ago, having a variational principle for it was at the forefront of the theoretical efforts. An action and the variational principle accompanying it are the preferred ways to describe a physical theory. At the classical level, all the information one can possibly ask about a physical system is conveniently codified into a single scalar function S. Additionally, in covariant approaches to quantum mechanics, the action S provides, through the path integral, a fundamental link between the classical and quantum descriptions. Ideally, the Hamiltonian structure of the theory itself -the starting point for canonical quantization- may too be extracted from the same action. Continue reading

A quantum kinematics for asymptotically flat gravity

Miguel and Madhavan

Miguel Campiglia, a postdoc at the Raman Research Institute (RRI), enjoying a traditional south Indian dish: masala dosa.
Madhavan Varadarajan (professor at RRI) not enjoying traditional South American drink: mate.

Isolated gravitating systems are modelled by asymptotically
flat space-times with the classical gravitational field subject to intricate and detailed asymptotic behaviour. The question we are interested in is: Is there a notion of an isolated quantum gravitating system? Specifically, can the classical
asymptotic conditions be suitably incorporated in quantum theory? Our work analyses this issue in the broad context of the Loop Quantum Gravity (LQG) approach.

At first it may seem this cannot be possible: The fundamental excitations in LQG are Continue reading

Holographic entanglement obeys strong subadditivity

Aron Wall

Aron Wall is a member of the School of Natural Sciences at the Institute for Advanced Study. In his spare time he blogs at Undivided Looking. He was the 2013 recipient of the Bergmann-Wheeler thesis prize, which is sponsored by Classical and Quantum Gravity.

Gauge-gravity duality allows us to calculate properties of certain quantum field theories (QFT) from classical general relativity. One famous piece of this conjecture, due to Ryu and Takayanagi, relates the entanglement entropy in a QFT region to the area of a surface in the gravitational theory. In addition to being a clue about quantum gravity, this proposal is one of the few tools which allow us to calculate entanglement entropy analytically. Since the entanglement entropy is of increasing interest for field theory and condensed matter applications, it is important to check if the conjecture is true.

One important property of the entropy is strong subadditivity (SSA). This quantum inequality says that the sum of the entropies in two regions is always greater than the sum of the entropies of their union and intersection. My article uses proof Continue reading

New focus issue: Entanglement and quantum gravity

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

Attempting to quantize geometry

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

Discrete wave operator for causets

Joseph Samuel

Joseph Samuel is an Editorial Board member of Classical and Quantum Gravity and a Professor at the Raman Research Institute, Bangalore, India

This paper leads to a discrete action functional on causets.

Lisa Glaser presents some tidy results in the definition of the discrete d’Alembertian operator on a causet in any dimension.

The causet approach to quantum gravity was pioneered by Rafael Sorkin in the 1980s. It approximates the space-time continuum by a discrete structure–a set with a partial order. The causet approach is marked by its minimalist philosophy, capturing Lorentzian manifolds in a discrete net with just Continue reading

Non-orientability constrains couplings in 2+1 quantum gravity

Jorma Louko

Jorma Louko is an Associate Professor in Applied Mathematics at the School of Mathematical Sciences, University of Nottingham

2+1 gravity is topological even without spacetime orientability, and quantisable for selected couplings

General relativity in four and more spacetime dimensions has local dynamical degrees of  freedom, as manifested for example in gravitational waves. In three spacetime dimensions,  by contrast, Einstein’s equations preclude local dynamics but allow still dynamics in the  global properties. This makes (2+1)-dimensional general relativity a dynamically simple but geometrically interesting arena for quantising gravity. Continue reading

Quantum gravity on a Klein bottle

Figure 1a

(A) Klein bottle, or the non-orientable surface of genus 2; The fundamental polygon representation of the Klein bottle is shown in the inset.

Figure 1b

(B) The orientable double cover of the Klein bottle is the orientable surface of genus 1, or the toroid. Closed loops on the double cover that traverse the non-orientable boundary— red/blue line in (B)— wind around the non-orientable surface in panel (A) twice.

In this work we study a model of quantum gravity on two-dimensional, non-orientable manifolds, for example a Klein bottle. We find that for a simplified version of quantum gravity called U(1) BF theory, a generalization of U(1) Chern-Simons theory, the fact that the manifold is non-orientable induces severe constraints on the values allowed for the coupling constant appearing in the action; in fact it can only take values of ½, 1, or 2. This comes about because the coupling constant appears in the commutation relation (or uncertainty relation) for the fields, and because the fields in the effective gauge theory must be consistent with the discrete symmetry groups for homeomorphisms on manifold. These discrete symmetry groups include the large gauge transformation group, the holonomy group, and the mapping class group. Continue reading