At the beginning of next week Jennifer Sanders and I will be representing the CQG editorial team at the Loops’ 17 conference at the University of Warsaw in Poland.

# Tag Archives: Loop Quantum Gravity

# Want to crush a singularity? First make it strong and then …

*by Parampreet Singh.*

**Einstein’s theory of classical general relativity breaks down when spacetime curvature **

**becomes extremely large near the singularities. To answer the fundamental questions **

**about the origin of our Universe or what happens at the central singularity of the black holes ****thus lies beyond the validity of Einstein’s theory. Our research deals with discovering the framework which guarantees resolution of singularities.
**

It has been long expected that quantum gravitational effects tame the classical singularities leading to insights on the above questions. A final theory of quantum gravity is not yet there but the underlying techniques can be used to understand whether quantum gravitational effects resolve cosmological and black hole singularities. Our goal is Continue reading

# Propagation in the absence of classical spacetime

*Written by Madhavan Varadarajan
*

At the Planck scale of 10^{−33}cm, 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

# 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?*

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

# Book review: Covariant Loop Quantum Gravity, an elementary introduction to quantum gravity and spinfoam theory

**Review of “Covariant Loop Quantum Gravity, an elementary introduction to quantum gravity and spinfoam theory” by Carlo Rovelli and Francesca Vidotto**

One of the central problems of contemporary physics is finding a theory that allows for describing the quantum behavior of the gravitational field. This book is a remarkable update on one of the most promising approaches for the treatment of this problem: loop quantum gravity. It places special emphasis on the covariant techniques, which provide with a definition of the path integral, an approach known as spin foams. It is a field that has undergone quite a bit of development in the last two decades. The book gives an overview of this area, discussing a series of results that are presented with great clarity. Both students and established researchers will benefit from the book, which provides a dependable introduction and reference material for further studies. Only a basic knowledge of general relativity, quantum mechanics and quantum field theory is assumed. The conceptual aspects and key ideas are discussed in the main body of the book and Continue reading

# Book Review: The Singular Universe and the Reality of Time

**Review of “The Singular Universe and the Reality of Time” by Roberto Mangabeira Unger and Lee Smolin**

*The Singular Universe* is effectively two separate books held together by some common ideas. Roberto Mangabeira Unger is a philosopher, social and legal theorist and politician who helped to bring about democracy in Brazil and has twice been appointed as its Minister of Strategic Affairs (in 2007 and 2015). According to *Wikipedia* (current entry), “his work begins from the premise that no natural social, political or economic arrangements underlie individual or social activity.” A similar spirit informs his approach to cosmology. Lee Smolin is of course well known as one of the creators of loop quantum gravity and as the author of several popular-science books. For brevity, I shall refer to the authors as RMU and LS. The book is over 500 pages in length. The first part, by RMU, is more than twice the length of LS’s and could have been shortened without loss of essential content. There is a final 20-page section detailing differences of view, which are substantial in some cases because RMU advocates a much greater break with the conventional approach to science than LS.

The two authors are agreed that a new ‘historical’ approach to cosmology is needed. For RMU, the mere fact that the universe has been shown to have a history is enough to indicate that the methods hitherto used to study the universe must be radically modified. LS argues for a new approach because of our failures to understand the history and properties of the universe as so far discovered. He points out that, Continue reading

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

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

# Towards quantum asymptotic flatness

It is of great physical interest to construct a canonical quantization of asymptotically flat spacetimes. The classical phase space variables are subject to delicate boundary conditions at spatial infinity and the first challenge is to construct a quantum kinematics which carries an imprint of these boundary conditions.

This work is one of a series of papers which seeks to construct such a Continue reading

# Continuous symmetries in discrete space?

Violating Lorentz symmetries can be dangerous. A new model assesses this threat in loop quantum gravity.

Discrete space is attractive in quantization attempts of gravity, but it implies the great danger of violating Lorentz transformations. Any approach must show that modifications of symmetries by discrete space are tame enough for predictions to be consistent with continuum low-energy physics. In the CQG paper, Sandipan Sengupta constructs an encouraging model using Continue reading

# Asymptotic flatness and quantum geometry

From the perspective of quantum gravity, the spacetime is smooth only in an effective sense, and is expected to exhibit a discrete structure at suitably small length scales. Within the gauge theoretic formulation of gravity, there are certain kinematical states which provide an elegant realization of such a scenario. These are known as the spin-network states, and are used extensively in certain quantization approaches, e.g. Loop Quantum Gravity (LQG). However, since these states correspond to a spatially discrete quantum geometry, they cannnot be used to capture the notion of a classical spacetime continuum. This leads to a serious obstacle towards a quantization of Continue reading