Wormholes can fix black holes

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

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

Insights from the Boundary: black holes in a magnetic universe

Hari Kunduri

Hari Kunduri

Following from the seminal work of Dain, a great deal is now known concerning geometric inequalities relating the area, charge, and angular momentum of axisymmetric black hole horizons in (possibly dynamical) spacetimes.  A key feature of these results is that they are quasi-local: they depend on spacetime only near the horizon itself and so are not sensitive to the asymptotic behaviour of the geometry.

For Einstein-Maxwell theory the celebrated uniqueness theorems tell us under certain conditions, that the Kerr-Newman (KN) family of solutions are the only stationary, axisymmetric and asymptotically flat black hole spacetimes. These are the model geometries that originally motivated the inequalities. However if we relax the condition of asymptotic flatness there are many other families of black hole solutions. While in general these will not contain event horizons (whose standard definitions require flat or AdS asymptotics) they still contain singularities and Killing horizons. In this paper we focussed Continue reading

Perfect accordance of the gravitational and the electromagnetic field in 3D

Maro Cvitan (assistant professor at the University of Zagreb) (left), Predrag Dominis Prester (associate professor at the University of Rijeka) (centre) and Ivica Smolić (assistant professor at the University of Zagreb) (right)

Maro Cvitan (assistant professor at the University of Zagreb) (left), Predrag Dominis Prester (associate professor at the University of Rijeka) (centre) and Ivica Smolić (assistant professor at the University of Zagreb) (right)

Does a physical field have to share the symmetries of the ambient spacetime?

Open a typical textbook on classical electrodynamics and you will find numerous examples of wrinkled and twisted electromagnetic fields, in a sheer contrast with the maximally symmetric Minkowski spacetime they inhabit. These, however, are the weak fields which do not “bend” the spacetime “fabric”. Once we allow the field to interact with the spacetime geometry via gravitational field equations, the symmetry constraints become much more stringent. When the answer to the opening question is affirmative we say that the field inherits the spacetime symmetries. Symmetry inheritance is not only used as a convenient assumption in a choice of the ansatz, but is also an important ingredient of Continue reading

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

Accreting onto almost Kerr-de Sitter black holes

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

Understanding blobs of spacetime

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

*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

The return of Newton-Cartan geometry

Jelle Hartong

Jelle Hartong is a postdoctoral researcher at the Université Libre de Bruxelles. His research concerns the foundations and applications of various non-AdS holographies and non-relativistic gravity.

Non-relativistic field theories defined on Newton-Cartan Geometry and its extension called Torsional Newton-Cartan Geometry, have (re-)appeared in recent studies of non-AdS holography and condensed matter physics.

Relativistic, Poincaré invariant, field theories are defined on Minkowski space-time. This flat background can be turned into a curved geometry by coupling the theory to a Lorentzian metric as one does when adding matter to Einstein’s theory of gravity. There are many areas of physics, notably Continue reading