Yearly Archives: 2015

Some memories from meeting Einstein in 1951 – 1952

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Yvonne Choquet-Bruhat

Yvonne Choquet-Bruhat is a French mathematician and physicist, renowned for her pioneering work on the initial value problem of General Relativity. Her work was one of the “Milestones of General Relativity” featured in a recent CQG focus issue. She has been on the faculty of the University of Marseille, the University of Reims and the University Pierre-et-Marie-Curie in Paris. She was the first woman to be elected to the French Academy of Sciences and is a Grand Croix of the Legion of Honour of France. She is also an elected member of the American Academy of Arts and Sciences. This reminiscence of Einstein was presented at the conference “A Century of General Relativity” held in Berlin, 30 November to 5 December, 2015. This image of Yvonne Choquet-Bruhat has been obtained from Wikipedia where it was made available by Momotaro under a CC-BY-SA 2.0 license. It is included within this article on that basis and attributed to Oberwolfach Photo Collection. 

I met Einstein in 1951 at the Institute for Advanced Study in Princeton. I was making there a postdoctoral stay, as assistant to the great mathematician Jean Leray, a part-time permanent professor at the IAS. I had defended a thesis on General Relativity under the official direction of André Lichnerowicz, but it was Jean Leray who had encouraged me to attack the problem of the existence of solutions of the Einstein equations taking given initial values, without assuming their analyticity. When I told Lichnerowicz about Leray’s suggestion, he said “it is too difficult for a beginner”. In fact it was not so difficult. In harmonic coordinates, called then “isotherm”, introduced by Lanczos, DeDonder and Georges Darmois, the Einstein equations in vacuum look like a system of quasidiagonal, quasilinear system of second order partial differential equations hyperbolic for a Lorentzian metric. I found by chance an article written in French by Continue reading →

Accreting onto almost Kerr-de Sitter black holes

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Read the full article for free* in Classical and Quantum Gravity:
Bondi-type accretion in the Reissner-Nordström-(anti-)de Sitter spacetime
Filip Ficek 2015 Class. Quantum Grav. 32 235008
arXiv: 1509.07005
*until 30/12/15

Filip Ficek

Filip Ficek is a graduate student in Theoretical Physics at Jagiellonian University.

In spite of numerous investigations, accretion flows onto the Kerr black hole are still not fully understood, especially for radially dominated flows, where aside from a very specific case of an ultra-hard fluid, general solutions are not known. Some insight may be provided by considering a simpler problem instead, namely spherically symmetric, steady accretion in Reissner-Nordström spacetimes. It is well known that rotating Kerr black holes and charged Reissner-Nordström black holes feature similar horizon and causal structures. In fact, it is common to treat a Reissner-Nordström black hole as a toy model of an astrophysical black hole. If we also take into account the cosmological constant, we may suppose, that accretion solutions in Reissner-Nordström-(anti-)de Sitter spacetime will Continue reading →

General Relativity turns 100!

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Clifford M Will

Clifford Will is the Editor-in-Chief of Classical and Quantum Gravity, Distinguished Professor of Physics at the University of Florida, Chercheur Associé at the Institut d’Astrophysique de Paris, and James McDonnell Professor of Space Sciences Emeritus at Washington University in St. Louis.

November 4, 1915 was a Thursday. It was the day that Albert Einstein gave the first of a series of four weekly lectures to the Prussian Academy of Sciences in Berlin. His life was a mess. He was separated from his wife Mileva, who had moved to Zurich taking his sons with her. He was having an affair with his second cousin Elsa. He was working night and day, was barely eating, and was suffering from stomach pains. He had agreed to give these lectures to present his theory of gravity but he still didn’t have it. To make matters worse, David Hilbert was racing to find the field equations first, and Einstein feared he would be beaten. Yet by the third lecture, Continue reading →

Quantum fluctuations of geometry in a hot Universe

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Read the full article for free* in Classical and Quantum Gravity:
Quantum fluctuations of geometry in a hot Universe
Iwo Bialynicki-Birula 2015 Class. Quantum Grav. 32 215015
arXiv:1501.07405
*until 23/12/15

Quantum uncertainty relations force the components of the Riemann curvature tensor to fluctuate

Iwo Bialynicki-Birula

Iwo Bialynicki-Birula is a member of the Polish Academy of Sciences working at the Center for Theoretical Physics in Warsaw. His main interests are classical and quantum electrodynamics, quantum mechanics, and general relativity. He has published 4 books and 184 papers, 44 of them were coauthored by his wife Zofia, also a theoretical physicist. All his papers are available on his website.

My paper published in Classical and Quantum Gravity, extends the concept of zero-point field fluctuations from electromagnetism to gravity with the use of an uncommon but very convenient tool: the Wigner function.

Electromagnetism is perhaps the most studied and the best understood part of physics. We have an almost perfect theory of electromagnetic phenomena both at the classical and at the quantum level. In contrast, our understanding of gravitational phenomena is not satisfactory even at the classical level. However, there are similarities between electromagnetism and gravitation that may help in exploration of quantum gravity. These similarities Continue reading →

Building a practically perfect gyroscope

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Mac Keiser

Mac Keiser was the chief scientist of the Gravity Probe B program. He is now a Senior Research Scientist, Emeritus, at Stanford.

Spherical, electrostatically-supported gyroscopes have been used for navigation for more than 40 years but their accuracy is far from what was needed to measure the effects predicted by general relativity. Fortunately, the accuracy could be significantly improved by reducing the forces required to support the gyroscope, improving the sphericity of the rotor, and increasing the gyroscope spin speed. Operating in a satellite reduces the support forces by a factor of 107, and using one of the gyroscopes as a drag-free sensor brings a further reduction of nearly 104. Improving the sphericity of the rotor further reduces the support-dependent torques but leads to the additional questions about how to spin-up and measure the spin-axis orientation of the rotor. Using a superconducting readout that Continue reading →

Gravity Probe B, the simplest data analysis ever

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John Conklin

John W. Conklin is assistant professor in the University of Florida’s Department of Mechanical and Aerospace Engineering. Photo: Aurora M Agüero

What could be simpler than fitting a straight line to a set of data? The slope of the line is the science result for NASA’s Gravity Probe B experiment, a landmark satellite test of two predictions of general relativity. Prior to launch, the data analysis for GP-B was thought to be straight forward. After precise calibration of the measured data (not as easy as fitting a straight line), the resulting signal should change at a constant rate over time. Our goal was to measure this rate of change, hopefully with an accuracy of 1%.

In the fall of 2005, as the year-long science mission was concluding, the GP-B science team began producing plots of the science data collected from the Earth-orbiting probe.  The figure shows one such plot. The slope of the best-fit straight line to these data is the magnitude of the elusive frame dragging effect, the main science goal of Gravity Probe B. Fear began to set it in. Where was the straight line? Continue reading →

Classifying noise transients in advanced gravitational-wave detectors

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Read the full article in Classical and Quantum Gravity (Open Access):
Classification methods for noise transients in advanced gravitational-wave detectors   Jade Powell, Daniele Trifirò, Elena Cuoco, Ik Siong Heng and Marco Cavaglià 2015 Class. Quantum Grav. 32 215012
arXiv:1505.01299

Jade Powell

Jade Powell is a PhD student at the University of Glasgow

A careful analysis of detector noise is necessary to determine whether a real gravitational-wave signal exists in the data of Advanced LIGO and Virgo. Instrumental and environmental disturbances can produce non-astrophysical triggers in science data, so called “glitches”. These glitches may reduce the duty cycle of the interferometers, and they could lead to a false detection if they occur simultaneously in multiple detectors. In the initial science runs of LIGO and Virgo a glitch could be classified by looking at an image of its time series waveform or spectrogram. This proved to be a slow and inefficient method for characterising a large number of glitches. To solve this problem the detector characterization team proposed a challenge for the fast automatic classification of Continue reading →

The spin limit for cosmological black holes

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Read the full article in Classical and Quantum Gravity (Open Access):
The area-angular momentum inequality for black holes in cosmological spacetimes
María Eugenia Gabach Clément, Martín Reiris and Walter Simon 2015 Class. Quantum Grav. 32 145006
arXiv:1501.07243

In colloquial terms, the main achievement of our recent CQG article is simple to state: We have proven that the angular momentum J of an axially symmetric black hole (the Noether current) with surface area A satisfies the bound.equation Walter CQG+ post

Here \Lambda is the cosmological constant –  a standard ingredient in Einstein’s Continue reading →

CQG Highlights now live

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Adam Day

Adam Day is the Publisher of Classical and Quantum Gravity and CQG+.  Follow him on Twitter and Facebook.

I am delighted to announce the CQG Highlights of 2014-2015.  The collection is free to read until 31 December 2016.

How are the Highlights selected?

Each summer, the Editorial Board of Classical and Quantum Gravity convene in London for the journal’s annual Editorial Board meeting. In advance of the meeting, Board members submit proposed Highlights published in CQG in the previous 12 months. That list is discussed in the meeting: some papers are added to the list, others removed. Ultimately, the Highlights are approved by the Editor-in-Chief.

It’s a thorough process and the result is perhaps the best recognized collection in gravitational physics.

I would like to congratulate every author whose work was selected to be a Highlight this year and invite all authors to publish with CQG. A paper submitted today could be a Highlight by this time next year.

CQG LettersDon’t forget that you can now publish Letters in CQG. CQG Letters are required to be high quality, so they stand a good chance of being selected as Highlights.  CQG Letters will also feature in promotions here at CQG+. Submit your CQG Letter today.

Understanding blobs of spacetime

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Read the full article for free* in Classical and Quantum Gravity:
Spacetime condensation in (2+1)-dimensional CDT from a Hořava-Lifshitz minisuperspace model
Dario Benedetti and Joe Henson 2015 Class. Quantum Grav. 32 215007
arXiv:1410.0845
*until 25/11/15

Can we explain the condensation of spacetime seen in numerical simulations of Causal Dynamical Triangulations?

Dario Benedetti

Dario Benedetti is a CNRS researcher at the Laboratoire de Physique Theorique at Orsay, France. His work focuses on various approaches to quantum gravity including CDT and asymptotic safety.

In the search of a quantum theory of gravity, it is not often that we are faced with the challenge of explaining some novel physical phenomenon: experiments are notoriously lacking, and theoretical questions usually involve clarifying the features of the different approaches, or the paradoxes of established theories. One of the most exciting aspects of Causal Dynamical Triangulations (CDT) is that numerical studies can produce unexpected results, which must then be explained, much like in mainstream statistical mechanics research.

Our paper, published in Classical and Quantum Gravity, is concerned with providing such an explanation Continue reading →