# Setting space on fire

by Yasaman K. Yazdi and Niayesh Afshordi.

Niayesh Afshordi and Yasaman Yazdi discover that firewalls have consequences. Yasaman K. Yazdi is a PhD candidate at the University of Waterloo and the Perimeter Institute for Theoretical Physics. Niayesh Afshordi is an associate professor at the University of Waterloo and the Perimeter Institute for Theoretical Physics.

Thought experiments highlight the edge of our understanding of our theories.  Sometimes, however, we can get so caught up in heated debates about the solution to a thought experiment, that we may forget that we are talking about physical objects, and that an actual experiment or observation may give the answer.  In this Insight we discuss a proposed solution to the black hole information puzzle, and a possible observational signal that might confirm it.

The black hole information puzzle and a potential solution

The black hole information loss problem is a decades old problem that highlights the tensions between some of the pillars of modern theoretical physics. It has evolved from being Continue reading

# Propagation in the absence of classical spacetime

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

# OK, so what happens now?

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:

1. Using LIGO to detect other sources of gravitational-waves
2. Improving the gravitational-wave detectors in order to probe farther into the cosmos
3. 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 [3].

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

# “There’s no way it’s real”

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

# The possible emptiness of a final theory

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 [1], is interesting for two reasons Continue reading

# CQG+ Insight: The problem of perturbative charged massive scalar field in the Kerr-Newman-(anti) de Sitter black hole background

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

Dr Georgios V Kraniotis (University of Ioannina)

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 [1].

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 $\Lambda$ [2].

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

# CQG+ Insight: More Classical Charges for Black Holes

Written by Geoffrey Compère, a Research Associate at the Université Libre de Bruxelles. He has contributed to the theory of asymptotic symmetries, the techniques of solution generation in supergravity, the Kerr/CFT correspondence and is generally interested in gravity, black hole physics and string theory.

# Why mass and angular momentum might not be enough to characterize a stationary black hole

Geoffrey Compère having family time in the park Le
Cinquantenaire in Brussels.

“Black hole have no hair.” This famous quote originates from John Wheeler in the sixties. In other words, a stationary black hole in general relativity is only characterized by its mass and angular momentum. This is because multipole moments of the gravitational field are sources for gravitational waves which radiate the multipoles away and only the last two conserved quantities, mass and angular momentum, remain. That’s the standard story.

Now, besides gravitational waves, general relativity contains another physical phenomenon which does not exist in Newtonian theory: the memory effect. It was discovered beyond the Iron Curtain by Zeldovich and Polnarev in the seventies and rediscovered in the western world and further extended by Christodoulou in the nineties. While gravitational waves lead to spacetime oscillations, the memory effect leads to a finite permanent displacement of test observers in spacetime. The effect exists for any value of the cosmological constant but in asymptotically flat spacetimes, it can be understood in terms of an asymptotic diffeomorphism known as a BMS supertranslation.

In order to understand that, let’s go back to the sixties where the radiative properties of sources were explored in general relativity; it was found by Bondi, van der Burg, Mezner and Sachs that there is a fundamental ambiguity in the coordinate frame at null infinity. Most expected that Continue reading

# CQG+ Insight: Spacetime near an extreme black hole

Written by James Lucietti, a Lecturer in Mathematical Physics in the School of Mathematics at the University of Edinburgh; and Carmen Li, previously a graduate student in the School of Mathematics at the University of Edinburgh and now a postdoc in the Institute of Theoretical Physics at the University of Warsaw.

# How many extreme black holes are there with a given throat geometry?

James Lucietti, University of Edinburgh

The classification of equilibrium black hole states is a major open problem in higher dimensional general relativity. Besides being of intrinsic interest, it has numerous applications in modern approaches to quantum gravity and high energy physics. Two key questions to be answered are: What are the possible topologies and symmetries of a black hole spacetime? What is the ‘moduli’ space of black hole solutions with a given topology and symmetry? For vacuum gravity in four spacetime dimensions, these questions are answered by the celebrated no-hair theorem which reveals a surprisingly simple answer: the Kerr solution is the only possibility. However, since Emparan and Reall’s discovery of the black ring — an asymptotically flat five dimensional black hole with ‘doughnut’ topology — it has become clear that there is a far richer set of black hole solutions to the higher dimensional Einstein equations.

Carmen Li, University of Warsaw, at the top of Ben Nevis in the UK.

Over the last decade, a number of general results have been derived which Continue reading

# CQG+ Insight: Spectral Cauchy Characteristic Extraction of strain, news and gravitational radiation flux

Written by Casey Handmer, a postdoctoral scholar at the California Institute of Technology. Bela Szilagyi is a researcher at NASA’s Jet Propulsion Laboratory. Jeffrey Winicour is a professor at the University of Pittsburg. Find out more on their group website at www.black-holes.org.

Casey Handmer (Caltech), Bela Szilagyi (JPL) and Jeffrey Winicour (Pittsburg) reprise their former stance discussing asymptotically time-like inertial scri+ foliations, now with even better CGI. Image credit: Photo manipulation by Annie Handmer, background image by SXS Collaboration: Andy Bohn et al 2015 Class. Quantum Grav. 32 065002.

Gravitational waves were detected in 2015. GW150914 wiggled LIGO’s mirrors and shook the whole world, except perhaps Stockholm. The opening paragraph of our previous CQG+ article was rendered obsolete:

“Colliding black holes create powerful ripples in spacetime. Of this we are certain. Directly detecting these ripples, or gravitational waves, is one of the hardest unsolved problems in physics.”

That article presented the evolution algorithm that simulates gravitational waves from compact object binaries in computational general relativity. The evolution algorithm is the powerful engine that drives the present work: extraction of all radiative energy-momentum flux in addition to the usual strain and gravitational news.

How did this come about? At APS April 2014, my coauthor Bela and I were approached by Jeffrey Winicour: Bela’s doctoral advisor and, we learned, a referee of our first paper. He excitably described how we could use our evolution algorithm to compute the gravitational wave flux. I schemed to co-opt all other possible referees in the same way.

What is the flux? The ten Poincaré symmetries of asymptotically flat spacetime generate respective conserved Noether momenta: linear momentum, angular momentum, energy, and three boost momenta corresponding to Lorentz transforms. Supertranslations, a possible solution to the Firewall Paradox, also generate momenta that are calculated using this method.

Along the way we discovered a number of surprises. Did you know that spherical foliations of future null infinity in inertial coordinates are actually asymptotically time-like? Read more in our CQG paper.

Spectral Cauchy characteristic extraction of strain, news and gravitational radiation flux
Casey J Handmer et al 2016 Class. Quantum Grav. 33 225007

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