Quantum Gravity Condensates

Dear Colleagues,

The problem of quantum gravity remains unsolved as of today, more than 90 years after the big change in the landscape of theoretical physics in the last century. A further conceptual leap forward is expected to be brought about by the completion of this task, applying the probabilistic quantum description of phenomena to space and time themselves. Generally, in background-independent approaches to quantum gravity, the space–time structure (in both its geometric and topological properties) is dynamically generated from some basic building blocks. The study of the origin and emergence of continuum space–time from these microscopic and fundamentally discrete degrees of freedom represents one of the most relevant paths of research to further test any nonperturbabtive approach.

A new perspective has recently appeared based on the hypothesis that space-time is a sort of condensate of microscopic building blocks and that general relativity emerges as the description of its geometry in a similar way as hydrodynamics and thermodynamics emerge as the macroscopic description of microscopic degrees of freedom. This point of view is gaining support. Results in this direction come from discrete approaches to quantum gravity, e.g., group field theory, where the construction of quantum gravity condensate states allows to describe the effective dynamics of continuum homogeneous quantum geometries; and from results in string theory dualities and in the AdS/CFT correspondence, where an holographic notion of entanglement for quantum field theories with bulk holographic duals has been used to study the emergence of the thermal properties of the dual spacetime. Insights about the possible emergent nature of gravity and geometry are also provided by condensed matter-inspired quantum gravity models and analogue gravity models. All the above results support the idea of general relativity as the hydrodynamics of pre-geometric space–time building blocks and of space–time as a condensate of these elementary constituents. Spacetime and its continuum symmetry would be emergent concepts, valid at macroscopic scales, whose emergence is the result of a collective dynamical process (a phase transition) of the fundamental degrees of freedom.

Moreover, in the last ten years, quantum information theory has constantly gained importance in investigating the fundamental nature of space, time, and matter. The information-theoretic interpretation of the notion of entanglement has provided new tools to investigate the pre-geometric quantum texture of spacetime in several background independent approaches to quantum gravity.

This Special Issue is intended to collect contributions from scientists working on the fields of quantum gravity and information theory, with the aim to enhance and exploit a cross-fertilization of ideas and techniques of crucial importance towards a quantum description of the gravitational field.

Dr. Daniele Pranzetti
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Symmetry is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

• Emergent spacetime
• Symmetry
• Group field theory
• AdS/CFT correspondence
• Holography and entanglement entropy
• Black hole thermodynamics
• Graviton Bose–Einstein condensate
• Analogue models of gravity
• Quantum information metric and gravity dual
• Tensor networks