I-Love-Q Probes of Modied Gravity
By Toral Gupta, Barun Majumder, Kent Yagi, and Nicolás Yunes
Although General Relativity has passed all tests carried out so far with flying colors, probes of the extreme gravity regime, where the gravitational interaction is simultaneously strong, non-linear and highly dynamical, have only recently began. This is timely because attempts to reconcile general relativity with quantum mechanics, be it in the form of string theory or loop quantum gravity, and attempts to explain cosmological observations, be it in the early or late universe, may require modifications to Einstein’s general theory. New electromagnetic telescopes, like the Neutron Star Interior Composition Explorer, and gravitational wave detectors, like advanced LIGO and Virgo, can now provide the first detailed observations of the extreme gravity regime. These new telescopes herald the era of extreme experimental relativity, allowing for new stringent constraints of deviations from Einstein’s theory, or perhaps, if we are lucky, pointing to signals of departures.
Toral Gupta is a graduate student at Indian Institute of Technology Gandhinagar.
Barun Majumder is a research assistant at Wilfrid Laurier University.
Kent Yagi is an assistant professor at University of Virginia.
Nicolás Yunes is an associate professor at Montana State University.
From quantum gravity to early universe cosmology using group field theory condensates
By Marco de Cesare, Daniele Oriti, Andreas Pithis, and Mairi Sakellariadou
“If you can look into the seeds of spacetime,
And say which grain will grow and which will not,
Speak then to me.”
– adapted quote from William Shakespeare’s, Macbeth
When we try to describe the earliest stages of the expansion of our Universe, the current picture of spacetime and its geometry as given by Einstein’s theory of General Relativity (GR) breaks down due to the extreme physical conditions faced at the Big Bang. More specifically, theorems by Hawking and Penrose imply that the cosmos emerged from a spacetime singularity. The existence of a cosmological singularity represents a main obstacle in obtaining a complete and consistent picture of cosmic evolution. However, there are reasons to believe that quantum gravitational effects taking place at the smallest scale could lead to a resolution of such singularities. This would have a huge impact for our understanding of gravity at a microscopic level, and for Cosmology of the very early Universe.
Marco de Cesare is a PhD student at King’s College London, working under the supervision of Mairi Sakellariadou on the cosmological consequences of quantum gravity.
Andreas Pithis is a PhD student at King’s College London, UK. He is a frequent visitor of the MPI for Gravitational Physics and held a visiting graduate fellowship at Perimeter Institute for Theoretical Physics, Canada.
Mairi Sakellariadou is a professor of theoretical physics at King’s College London and a member of the LIGO Scientific Collaboration. She is also Chair of the Gravitational Physics Division of the European Physical Society.
Daniele Oriti is a senior researcher and group leader at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Potsdam.
by Adriana V. Araujo, Diego F. López and José G. Pereira
The Quest for Consistency in Spacetime Kinematics
Newton’s inception of the theory for the gravitational interaction in 1687 was a landmark for modern physics. In addition to explaining all known gravitational phenomena of that time, Newton’s gravitational theory was consistent with the kinematic rules of the Galilei group, known as Galilei relativity. Such consistency provided an atmosphere of intellectual comfort, which lasted for more than two centuries.
From left to right, José, Adriana and Diego. Click here to see the authors taking advantage of all dimensions of a space section of the universe.
By the mid nineteenth century, most secrets of the electric and magnetic fields were already unveiled. Those advancements culminated with the publication by Maxwell of a comprehensive treatise on the unification of electricity and magnetism, which became known as Maxwell’s theory. This theory brought to the scene the first inconsistency of our tale. In fact, it became immediately clear that the electromagnetic theory was inconsistent with the Galilei relativity: electromagnetism was claiming for a new relativity. In response to this claim, and with contributions from Lorentz and Poincaré, Einstein published in 1905 the basics of what is know today as Einstein special relativity. According to this theory, for velocities near the velocity of light, spacetime kinematics would no longer be ruled by Galilei, but by the Poincaré group. Most importantly, electromagnetism was consistent with Einstein special relativity! Mission accomplished? Not quite! Continue reading
by J. Brian Pitts.
J. Brian Pitts is a Senior Research Associate, Faculty of Philosophy, University of Cambridge.
Observables and the Problem of Time
Mixing gravity and quantum mechanics is hard. Many approaches start with a classical theory and apply the magic of quantization, so it is important to have the classical theory sorted out well first. But the “problem of time” in Hamiltonian General Relativity looms: change seems missing in the canonical formulation.
Are Hamiltonian and Lagrangian forms of a theory equivalent? It’s not so obvious for Maxwell’s electromagnetism or Einstein’s GR, for which the Legendre transformation from the Lagrangian to the Hamiltonian doesn’t exist. It was necessary to reinvent the Hamiltonian formalism: constrained Hamiltonian dynamics. Rosenfeld’s 1930 work was forgotten until after Dirac and (independently) Bergmann’s Syracuse group had reinvented the subject by 1950. Recently a commentary and translation were published by Salisbury and Sundermeyer.
As canonical quantum gravity grew in the 1950s, it seemed less crucial for 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
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
by Kirill Krasnov
Kirill Krasnov, Professor of Mathematical Physics, University of Nottingham. Pictured here visiting Newstead Abbey, Nottinghamshire
We seem to live in four space-time dimensions, and so should take the structures available in this number of dimensions seriously. One of these is chirality, see below for clarifications on my usage of this term. Related to chirality, there is a remarkable phenomenon occurring in General Relativity (GR) in four space-time dimensions. This phenomenon is so stunning that I would like to refer to it as the chiral miracle. It is well-known to experts. Still, even after almost 40 years after it had appeared in the literature, it has not become part of the background of all GR practitioners. I would like to use this CQG+ insight format to try to rectify this.
I start by reviewing the notion of chirality in four space-time dimensions. I then describe the “chiral miracle” that allows for chiral description(s) of gravity in Continue reading
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
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
Yvonne Choquet-Bruhat is a French mathematician and physicist, renowned for her pioneering work on the initial value problem of General Relativity. Her work was one of the “Milestones of General Relativity” featured in a recent CQG focus issue. She has been on the faculty of the University of Marseille, the University of Reims and the University Pierre-et-Marie-Curie in Paris. She was the first woman to be elected to the French Academy of Sciences and is a Grand Croix of the Legion of Honour of France. She is also an elected member of the American Academy of Arts and Sciences. This reminiscence of Einstein was presented at the conference “A Century of General Relativity” held in Berlin, 30 November to 5 December, 2015. This image of Yvonne Choquet-Bruhat has been obtained from Wikipedia where it was made available by Momotaro under a CC-BY-SA 2.0 license. It is included within this article on that basis and attributed to Oberwolfach Photo Collection.
I met Einstein in 1951 at the Institute for Advanced Study in Princeton. I was making there a postdoctoral stay, as assistant to the great mathematician Jean Leray, a part-time permanent professor at the IAS. I had defended a thesis on General Relativity under the official direction of André Lichnerowicz, but it was Jean Leray who had encouraged me to attack the problem of the existence of solutions of the Einstein equations taking given initial values, without assuming their analyticity. When I told Lichnerowicz about Leray’s suggestion, he said “it is too difficult for a beginner”. In fact it was not so difficult. In harmonic coordinates, called then “isotherm”, introduced by Lanczos, DeDonder and Georges Darmois, the Einstein equations in vacuum look like a system of quasidiagonal, quasilinear system of second order partial differential equations hyperbolic for a Lorentzian metric. I found by chance an article written in French by Continue reading