Internal-external-dynamics decoupling in canonical general relativity

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Gerhard Schäfer

Gerhard Schäfer is a retired professor at the University of Jena. His main scientific interests are equations of motion in general
relativity and their applications in astronomy and astrophysics.

Research on general-relativistic equations of motion based on Hamiltonian or canonical frameworks is not quite a main-stream doing; likely because of the all-over covariance of the theory and canonical is just not covariant but rather quite the opposite. Covariance under spacetime coordinate transformations makes the theory a spacetime-local one with its local scalars, vectors and tensors, the canonical picture on the other side is at home in the phase space of the dynamics which combines position and momentum variables. Crucial object-changing operations in spacetime are covariant derivatives, crucial ones in phase space are Poisson brackets.

What is the benefit of performing research in general relativity within a canonical framework? Let us concentrate on gravitating systems living in asymptotically flat spacetimes. Then there exist global quantities — energy, linear momentum, angular momentum, Lorentz-boost vector — which are nicely conserved. If those quantities are calculated within Continue reading →

Spectral analysis in resonant interferometry: following the traces of thermal deformation

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How researchers from the LIGO scientific collaboration use signals generated from higher-order mode resonances to glean crucial information about the thermal state of their interferometers.

Chris Mueller

Chris Mueller received his Ph.D. in physics with Guido Mueller at the University of Florida and has since moved to industry.

Imagine for a moment that you’ve accepted the challenge of trying to make the first direct detection of gravitational waves. To achieve such a daunting task you’ll need to devise an instrument capable of measuring a change in length of just 10-19 m over a distance of several km. At these length scales everything matters; the ground is vibrating, air molecules are buzzing around, and the molecules which make up the test masses of your detector are quivering. This challenge is precisely Continue reading →

How wild can a static spacetime get?

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Stationary spacetimes—sounds fairly simple, unchanging.  Static—even more boring.  But are they?

Steve (Stacey) Harris

Steve (also known as Stacey) Harris is a mathematical relativist in the Department of Mathematics at Saint Louis University, mostly working on global structures such as causal boundary, and a notorious campus gadfly and rebel, being active in AAUP and fanatical about shared governance. Off campus there’s hiking and playing flute in concert band.

Consider an experiment of emitting a photon along a closed path—closed either due to a constraining light-tube or due to a topological closure in the spacetime—and finding the time till the photon returns to the starting point.  (Our naive expectation is for the time to be the same as the length of the path, if our clocks and measuring rods are in geometric units, set to show speed of light is unity.)  Now turn around and emit a photon along the same path but in the reverse direction—does it take the same time to Continue reading →

Can’t solve an equation….bypass it !!!

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Radouane Gannouji

Radouane Gannouji is an associate professor at the Department of Physics, PUCV.

In general relativity, to understand how the spacetimes behave in presence of a given form of matter, we have to solve the Einstein field equations, which in general, are a set of 10 very complicated coupled nonlinear second order partial differential equations that describes the fundamental interaction of gravitation as a result of spacetime being curved by matter and energy. Once we solve these set of field equations we get the metric of the spacetime that describes all the general important physical features of the spacetime, for example Continue reading →

The return of Newton-Cartan geometry

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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 →

Designing curved blocks of quantum space-time…Or how to build quantum geometry from curved tetrahedra in loop quantum gravity

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Etera Livine

Etera Livine is a CNRS researcher and focuses especially on mathematical aspects of loop quantum gravity and spinfoam path integral models. Etera and Christoph both work on quantum gravity at the Laboratoire de Physique de l’Ecole Normale Supérieure de Lyon (LPENSL) in France.

Among the various approaches to the quantum gravity challenge, loop quantum gravity proposes a framework for a canonical quantization of general relativity, describing how the 3d geometry evolves in time. It does not require a priori extra dimensions or supersymmetry. It defines spin network states for the quantum geometry directly at the Planck scale, with a discrete spectra of areas and volumes, and computes their transition amplitudes by path integrals inspired from topological field theory, called spinfoam models. This framework is mathematically rigorous but Continue reading →

New realizations of quantum geometry

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Bianca Dittrich is faculty researcher at Perimeter Institute, previously she has been at the Max Planck Institute for Gravitational Physics and at Utrecht University.

Bianca Dittrich is faculty researcher at Perimeter Institute, previously she has been at the Max Planck Institute for Gravitational Physics and at Utrecht University.

The main lesson taught by Einstein’s theory of gravity is that the gravitational field has a geometrical nature, while that of quantum theory is that fields are quantized and come with fundamental excitations. This suggest a realization of quantum gravity in terms of quantum geometry.

In loop quantum gravity, this idea is realized explicitly, and observables encoding the intrinsic and extrinsic data of a spatial geometry are represented as quantum operators on a Hilbert space. This allows to discuss rigorously the quantum properties of geometrical operators measuring the area or Continue reading →

Black-hole superradiance and the hunt for dark matter

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Paulo Pani

Paolo Pani is a Marie Curie Fellow at Sapienza University of Rome and FCT Researcher at Instituto Superior Tecnico in Lisbon. His research interests include black holes, foundations of General Relativity and relativistic astrophysics. He is co-author of the book “Superradiance” (Springer-Verlag), now in press.

Little is known about dark matter, despite the numerous searches for its constituents. Fortunately, everything falls in the same way, so possible imprints of dark matter can be found in gravitational fields. In particular, if ultralight bosons exist in nature, they would make spinning black holes unstable. How does such instability evolve in realistic scenarios? And what can it teach us about the existence of dark matter?

In our recent CQG paper, we take the first step to address these questions by studying how a light scalar field grows near Continue reading →

Interview with Patricia Schmidt, winner of the 2015 GPG thesis prize

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Patricia Schmidt

Patricia Schmidt is a Postdoctoral Scholar in TAPIR at Caltech

What was the most interesting thing that happened during your PhD?

The most interesting thing was that literally every day, you would do something new. Even looking at the same set of equations again and again you would get new ideas and new insights.

Were there any big surprises?

The biggest surprise was that it actually worked out! We didn’t have a path set out to reach this result when we started, so who would have thought when Mark and I set out that we would actually have exactly Continue reading →

Book Review: On the topology and future stability of the universe, by Hans Ringström

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Mihalis Dafermos

Mihalis Dafermos is Professor of Mathematics at Princeton University

This impressive new book is first and foremost an original and thought-provoking contribution to the study of cosmology in research monograph form, in the best tradition of the kind of deep mathematical work which has played a crucial role in the development of the subject. At the same time, the book doubles as a dependable introduction and  reference for several foundational results in the analysis of the Einstein equations and relativistic kinetic theory which are hard to find elsewhere but which form the basis of so much current (and hopefully, future!) work.

Both these roles are most welcome.

Let me first discuss what this Continue reading →