Author Archives: Jennifer Sanders

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About Jennifer Sanders

Jennifer is Editor for a range of astrophysics, measurement and instrumentation journals. She has worked in scientific publishing at IOP Publishing since completing her PhD in Photon Physics at The University of Manchester in 2012.

Book review: Covariant Loop Quantum Gravity, an elementary introduction to quantum gravity and spinfoam theory

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Rodolfo Gambini is Professor of Physics at Universidad de la República, Montevideo Uruguay

Rodolfo Gambini is Professor of Physics at Universidad de la República, Montevideo Uruguay

Review of “Covariant Loop Quantum Gravity, an elementary introduction to quantum gravity and spinfoam theory” by Carlo Rovelli and Francesca Vidotto

One of the central problems of contemporary physics is finding a theory that allows for describing the quantum behavior of the gravitational field. This book is a remarkable update on one of the most promising approaches for the treatment of this problem: loop quantum gravity. It places special emphasis on the covariant techniques, which provide with a definition of the path integral, an approach known as spin foams. It is a field that has undergone quite a bit of development in the last two decades. The book gives an overview of this area, discussing a series of results that are presented with great clarity. Both students and established researchers will benefit from the book, which provides a dependable introduction and reference material for further studies. Only a basic knowledge of general relativity, quantum mechanics and quantum field theory is assumed. The conceptual aspects and key ideas are discussed in the main body of the book and Continue reading →

Wormholes can fix black holes

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Diego Rubiera-Garcia and Gonzalo Olmo

Diego Rubiera-Garcia (left, Lisbon University) and Gonzalo J. Olmo (right, University of Valencia – CSIC) after crossing a wormhole that connects Europe with the beaches of the Brazilian Northeast.

According to Einstein’s theory of general relativity (GR), black holes are ferocious beasts able to swallow and destroy everything within their reach. Their strong gravitational pull deforms the space-time causal structure in such a way that nothing can get out of them once their event horizon is crossed. The fate of those incautious observers curious enough to cross this border is to suffer a painful spaghettification process due to the strong tidal forces before being destroyed at the center of the black hole.

Antonio Sanchez-Puente

Antonio Sanchez-Puente (University of Valencia – CSIC) enjoying a sunny day in Valencia after submitting yet another postdoc application.

For a theoretical physicist, the suffering of observers is admissible (one might even consider it part of an experimentalist’s job) but their total destruction is not. The destruction of observers (and light signals) is determined by the fact that the affine parameter of their word-line (its geodesic) stops at the center of the black hole. Their clocks no longer tick and, therefore, there is no way for them to exchange or acquire new information. This implies the breakdown of the predictability of the laws of physics because physical measurements are no longer possible at that point. For this reason, when a space-time has incomplete geodesics — word-lines whose affine parameter does not cover the whole real line — we say that it is singular.

In order to overcome the conceptual problems raised by singularities, a careful analysis of what causes the destruction of observers is necessary. Our intuition may get satisfied by blaming the enormous tidal forces near the center, but the problem is much subtler. This is precisely what we explore in our paper. Continue reading →

Book Review: The Springer Handbook of Spacetime

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David Garfinkle

David Garfinkle is Professor of Physics at Oakland University. His research is in numerical relativity: the use of computer simulations to study the properties of strong gravitational fields.

Review of “The Springer Handbook of Spacetime” edited by Abhay Ashtekar and Vesselin Petkov

The word “Handbook” in the title is something of a misnomer: it is perhaps better to think of this book as a collection of mini review articles on various topics in relativity.  The best way to use the book is to think of a topic in relativity about which you would say “I wish I knew and understood more about X, but I don’t have the time to read a review article about X, nor the expertise to understand a typical review article on the subject.”  Then look in the book to see if there is a chapter on X, and if so, read it.  (Then repeat the process for each X).  Each mini review article comprises a chapter and the chapters are organized in sections that reflect a particular aspect of relativity.

The first two sections, Introduction to Spacetime Structure and Foundational Issues concentrate mostly on the basic properties of spacetime and on philosophical issues connected with special and general relativity.  I found these sections Continue reading →

Taking Newton into space

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The test mass retroreflector

Schematic of M R Feldman et al‘s  proposed experiment. The test mass retroreflector, exhibiting harmonic motion within the tunnel of the larger layered sphere, is represented by the filled black circle on the left. Determinations of the round trip light-time from the host spacecraft (on the right) using an onboard ranging system provide measurements of the period of the oscillator.

Newton’s gravitational constant, G, is crucial for fundamental physics: it governs how much spacetime curves for a given mass, is essential for metrology, and might give clues to a deeper understanding of quantum gravity. However, G continues to present unexpected issues in need of resolution. Determinations over the last thirty years have yielded inconsistencies between experiments significantly greater than their reported individual uncertainties, oddly with possible periodic behavior. To push forward, the National Science Foundation (NSF) has recently called for new “high-risk/high-impact” proposals to produce a step-change improvement in measurements (NSF 16-520).

In response, we propose taking advantage of the classic gravity train mechanism by Continue reading →

Insight: Some one loop gravitational interactions in string theory

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Anirban Basu

Anirban Basu is a researcher at Harish-Chandra Research Institute, Allahabad
India

String theory yields ultraviolet finite scattering amplitudes in theories of gravity coupled to matter. While the matter content of the theory is dependent on the compactification, the presence of gravity in the spectrum is universal. Hence, this is drastically different from the high energy behavior of conventional quantum field theories of point like excitations because such amplitudes are generically ultraviolet divergent. While in quantum field theory the ultraviolet divergences arise from short distance effects which manifest themselves as divergences arising from high momentum modes in loop integrals in various Feynman diagrams, these divergences are absent in string theory where analogous loop integrals involve an integration over the fundamental domain of the moduli space of two dimensional Riemann surfaces which is the Euclidean worldsheet of the string propagating in the background spacetime. The fundamental domain precisely excludes the regions of moduli space which yield the ultraviolet divergences in quantum field theory. The ultraviolet finiteness of string theory makes it, among other reasons, particularly attractive in the quest for a theory of quantum gravity. On the other hand, there are infrared divergences that arise from the boundaries of moduli space in calculating string amplitudes which reproduce expectations from quantum field theory, which must be the case as string theory must reproduce field theory at large distances. Hence, their cancellation proceeds as in field theory.

However, calculating these loop amplitudes in perturbative string theory is not an entirely trivial exercise. In the absence of Ramond–Ramond backgrounds, tree level amplitudes have been calculated in superstring theory. The one loop amplitudes, which are more complicated, have also been Continue reading →