By Paul I. Jefremov and Volker Perlick.
Among all known solutions to Einstein’s vacuum field equation the (Taub-)NUT metric is a particularly intriguing one. It is that metric that owing to its counter-intuitive features was once called by Charles Misner “a counter-example to almost anything”. In what follows we give a brief introduction to the NUT black holes, discuss what makes them interesting for a researcher and speculate on how they could be detected should they exist in nature.
Volker Perlick and Pavel (Paul) Ionovič Jefremov from the Gravitational Theory group at the University of Bremen in Germany. Volker is a Privatdozent and his research interests are in classical relativity, (standard and non-standard) electrodynamics and Finsler geometry. He is an amateur astronomer and plays the piano with great enthusiasm and poor skills. Paul got his diploma in Physics at the National Research Nuclear University MEPhI in Moscow, 2014. Now he is a PhD Student in the Erasmus Mundus Joint Doctorate IRAP Programme at the University of Bremen. Beyond the scientific topics in physics his interests include philosophy in general, philosophy of science, Eastern and ancient philosophy, religion, political and social theories and last but not the least organic farming.
The NUT (Newman–Unti–Tamburino) metric was obtained by Newman, Unti and Tamburino (hence its name) in 1963. It describes a black hole which, in addition to the mass parameter (gravito-electric charge) known from the Schwarzschild solution, depends on a “gravito-magnetic charge”, also known as NUT parameter. If the NUT metric is analytically extended, on the other side of the horizon it becomes isometric to a vacuum solution of Einstein’s field equations found by Abraham Taub already in 1951. However, for an observer who is prudent enough to stay outside the black hole, the Taub part is irrelevant.
At first sight, the existence of the NUT metric seems to violate the uniqueness (“no-hair”) theorem of black holes according to which a non-spinning uncharged black hole is uniquely characterised by its mass. Actually, there is no contradiction because Continue reading
By Jishnu Bhattacharyya, Mattia Colombo and Thomas Sotiriou.
Black holes are perhaps the most fascinating predictions of General Relativity (GR). Yet, their very existence (conventionally) hinges on Special Relativity (SR), or more precisely on local Lorentz symmetry. This symmetry is the local manifestation of the causal structure of GR and it dictates that the speed of light is finite and the maximal speed attainable. Accepting also that light gravitates, one can then intuitively arrive at the conclusion that black holes should exist — as John Michell already did in 1783!
One can reverse the argument: does accepting that black holes exist, as astronomical observations and the recent gravitational wave direct detections strongly suggest, imply that Lorentz symmetry is an exact symmetry of nature? In other words, is this ground breaking prediction of GR the ultimate vindication of SR?
Jishnu Bhattacharyya, Mattia Colombo and Thomas Sotiriou from the School of Mathematical Sciences, University of Nottingham.
These questions might seem ill-posed if one sees GR simply as a generalisation of SR to non-inertial observers. On the same footing, one might consider questioning Lorentz symmetry as a step backwards altogether. Yet, there is an alternative perspective. GR taught us that our theories should be expressible in a covariant language and that there is a dynamical metric that is responsible for the gravitational interaction. Universality of free fall implies that Continue reading
by Carlos Herdeiro and Eugen Radu, Guest Editors of Focus Issue: Hairy Black Holes.
Carlos A. R. Herdeiro (left) got his PhD from Cambridge University (U.K.) in 2002. He is currently an assistant professor at Aveiro University, Portugal, and an FCT principal researcher. He is also the founder and coordinator of the Gravitation group at Aveiro University (gravitation.web.ua.pt). Eugen Radu (right) got his PhD from Freiburg University (Germany) in 2002. He is currently an FCT principal researcher at Aveiro University (Portugal).
One of the most recognizable statements about black holes is that they have “no-hair”. Close inspection, however, shows that this is a belief rather than a mathematically proven theorem. Moreover, decades of research on this topic have shown that, depending on what one precisely means, this statement may be simply wrong. That is, as solutions of Einstein’s equations, in a generic context, black holes are not necessarily “bald”. Then, less ambitious, but perhaps more relevant questions are: “Can astrophysical black holes have hair?” and “Can we test the existence of black hole hair with present and future astrophysical observations?”.
This CQG focus issue brings together a set of papers describing models in which black holes do have “hair”, as well as observational efforts that have the potential to assess if this is (or not) the case for astrophysical black hole candidates. This collection of research papers is by no means a faithful and complete description of all possible alternatives to the Kerr paradigm in the literature. Rather, the selected papers focus on Continue reading
One of the authors, Ian Jubb, discussing a pair of trousers with his colleagues at Imperial College London. Ian Jubb is currently the PhD student of Fay Dowker in the Theoretical Physics group at Imperial College London.
by Ian Jubb and Michel Buck.
Did you know that Quantum Gravity literally sets pants on fire?
Your pants are not just a nifty garment, they are also a perfect example of a space undergoing a process known as topology change. Take a space that initially consists of two separate circles. If they were to meet and merge into a single circle, the topology of the space would have changed. The trousers allow us to visualise each stage of this process, with cross sections higher up the trouser leg corresponding to later times in the process (if we hold the trousers upside-down, we get the reverse process, corresponding to a single circle splitting into two circles). Instead of viewing this process as the space changing in time, Einstein would tell us to view the trousers in their entirety, as one whole spacetime — the trousers spacetime.
But why should we care about spaces that can ‘split’ and ’attach’ like this? It turns out that there are good reasons to believe that Continue reading
Timothy J. Walton occupies some quantum state between a physicist and a mathematician, having obtained his PhD from the physics department at Lancaster University in 2008 but now masquerading as a lecturer in mathematics at the University of Bolton.
by Timothy J. Walton.
Applying techniques from classical electrodynamics to generate new gravitational wave perturbations
I must begin with a confession: I don’t view myself as a gravitational physicist. Despite my PhD at Lancaster University involving a formulation of relativistic elasticity and an awful lot of differential geometry, my research thus far has been within the realm of classical and quantum electrodynamics. But it was precisely within that domain, along one particular avenue of investigation, where the first seeds of an idea were sown. Following my earlier work on a class of exact finite energy, spatially compact solutions to the vacuum source-free Maxwell equations – pulsed electromagnetic waves – describing single cycle pulses of laser light , together with Shin Goto at Kyoto University in Japan and my former PhD supervisor Robin Tucker at Lancaster University, a new question arose: “do pulsed gravitational waves exist?’’
As I recall, this question was posed and began to take root during one of the regular meetings I have with Robin. Within my institution, I am fortunate enough Continue reading
Written by Madhavan Varadarajan
The author’s research group busy at work. Madhavan Varadarajan is a Professor at the Raman Research Institute in Bangalore, India.
At the Planck scale of 10−33cm, where the very notion of classical spacetime ceases to exist due to large quantum fluctuations of spacetime geometry, can meaning be given to the notion of “causality”? We are interested in this question in the context of Loop Quantum Gravity (LQG).
The basic quantum states of LQG are labelled by graphs. Each such state describes discrete one dimensional excitations of spatial geometry along the edges of its graph label. These ‘graphical’ states provide the Continue reading
Written by Michael Coughlin
The future of gravitational-wave astronomy after the first detection
Michael Coughlin is currently a post-doctoral fellow at Harvard University with Prof. Christopher Stubbs. In September 2016, he successfully defended his physics PhD at Harvard, titled “Gravitational-wave astronomy in the LSST era”. He began researching gravitational waves with LIGO over eight years ago as a college freshman at Carleton College in Northfield, MN and it was very exciting for him to be part of LIGO’s historical confirmation in February 2016. At Harvard, he added the Large Synoptic Survey Telescope (LSST), Pan-STARRS, and ATLAS to his research areas, including designing and building a prototype calibration system, which he nicknamed “CaBumP”.
Since LIGO announced the detection of gravitational waves from binary black hole mergers in its first observing run [1-2], the most common question I have received is “What was it like to be part of such a historic scientific discovery?” The second most common question has been: “So what happens now?” The answer is a lot of stuff! Here I’ll focus on three main goals:
- Using LIGO to detect other sources of gravitational-waves
- Improving the gravitational-wave detectors in order to probe farther into the cosmos
- Electromagnetic follow-up of gravitational-wave events with telescopes to get a more complete picture
What else does nature have in store for us?
The detection of gravitational waves from binary black hole mergers has been incredibly exciting, and we look forward to the detection of more such systems. Of course, there are many other sources (pulsars, supernovae, binary neutron stars, etc.) that we hope to detect as well. As a member of the group in LIGO searching for a stochastic background of gravitational waves, I am particularly interested in the processes that could create such a signal. This includes backgrounds from compact binary coalescences, pulsars, magnetars, or core-collapse supernovae. A cosmological background (such as from inflation!) could be generated by various physical processes in the early universe. In particular, with the recent discovery of binary black-hole mergers, there is a really good chance of observing a stochastic gravitational-wave background from these systems .
There are other sources that are likely to produce long-lived transients, including emission from rotational instabilities in proto-neutron stars and black-hole accretion disk instabilities. There is ongoing significant effort to improve Continue reading
Written by Samantha Usman, who is currently pursuing an MPhil at Cardiff University, UK under the supervision of Prof. Stephen Fairhurst. She graduated in May 2016 with a BS in Mathematics and Physics at Syracuse University. While at Syracuse, Usman worked with Prof. Duncan Brown on improving LIGO’s sensitivity to gravitational waves from binary star systems. In her spare time, Usman trains in Brazilian jiu jitsu and Muay Thai kickboxing and enjoys walks with her Australian Shepherd, Marble.
The discovery of gravitational waves from an undergraduate’s perspective
Author Samantha Usman training for competition in Brazilian jiu jitsu.
The first time I learned LIGO might have detected a gravitational wave, I was listening in on a conference call on September 16, 2015. Two days earlier, ripples in the fabric of space from massive black holes crashing into each other at half the speed of light had passed through the Earth. The LIGO detectors picked up these faint changes in the length of space, but they pick up all sorts of extra noise that you’d never expect; how could we be sure this was really a gravitational wave?
On September 16th, I was an undergraduate starting my senior year at Syracuse University. I’d been doing LIGO research with my advisor, Prof. Duncan Brown, for almost two and a half years. Since LIGO had yet to start an observing run, my research had been focused on testing improvements to the codes that we use to search for gravitational waves. I’d been told in those two and a half years that it would take a few years to get our detectors to design sensitivity and not to expect a detection until I was well into graduate school.
So when I sat in my boss’ office listening to a colleague in Germany say he thought we’d really seen something, I rolled my eyes and muttered, “There’s no way it’s real.” I was convinced people were Continue reading
Written by Jesper Møller Grimstrup, an independent danish theoretical physicist. He has collaborated with the mathematician Johannes Aastrup for more than a decade developing what they now call quantum holonomy theory. His present research is financed by an Indiegogo crowdfunding campaign (still open). Find more information on www.jespergrimstrup.org.
Could the laws of nature originate from a principle, that borders a triviality?
Does a final theory that cannot be explained by yet another, deeper theory, exist? What could such a theory possibly look like — and what might we learn from it?
Jesper Møller Grimstrup
These are the million dollar questions. Will the ladder of scientific explanations that take us from biology to chemistry and down through atomic, nuclear and particle physics, end somewhere? Will we one day reach a point where it is clear that it is no longer possible to dig deeper into the fabric of reality? Will we reach the bottom?
Together with the mathematician Johannes Aastrup I have developed a new approach to this question. Our theory — we call it quantum holonomy theory — is based on an elementary algebra, that essentially encodes how stuff is moved around in a three-dimensional space.
This algebra, which we call the quantum holonomy-diffeomorphism (QHD) algebra , is interesting for two reasons Continue reading
Written by Dr Georgios V Kraniotis, a theoretical physicist at the University of
Ioannina in the physics department.
Solving in closed form the Klein-Gordon-Fock equation on curved black hole spacetimes
A new exciting era in the exploration of spacetime
The investigation of the interaction of a scalar particle with the gravitational field is of importance in the attempts to construct quantum theories on curved spacetime backgrounds. The general relativistic form that models such interaction is the so called Klein-Gordon-Fock (KGF) wave equation named after its three independent inventors. The discovery of a Higgs-like scalar particle at CERN in conjuction with the recent spectacular observation of gravitational waves (GW) from the binary black hole mergers GW150914 and GW151226 by LIGO collaboration, adds a further impetus for probing the interaction of scalar degrees of freedom with the strong gravitational field of a black hole.
Kerr black hole perturbations and the separation of the Dirac’s equations was a central theme in the investigations of Teukolsky and Chandrasekhar .
All the above motivated our research recently published in CQG on the scalar charged massive field perturbations for the most general four dimensional curved spacetime background of a rotating, charged black hole, in the presence of the cosmological constant .
Where interesting physics meets profound mathematics
The KGF equation is the relativistic version of the Schrödinger equation and thus is one of the fundamental equations in physics.
In our recent CQG paper, we examined Continue reading