## In search of quantum gravity

26-07-2024

**In the project implemented by dr Mehdi Assanioussi, who received funding from the National Science Center under the SONATA programme, the key challenge will be understanding gravity. Can it be reconciled with nuclear research, where quantum physics has determined the way of understanding phenomena and interactions for over a hundred years? How does gravity behave in extreme conditions, such as black holes or the initial moments of the universe? How does a quantum field theory in fixed spacetime follow from a quantum theory of gravity coupled to matter fields?**

The project „Loop quantum gravity with matter fields: continuum limit and emergent phenomenology” touches one of the main challenges of modern theoretical physics, which is the reconciliation of general relativity, the classical theory of the gravitational interaction, and the principles of quantum mechanics. This longstanding problem consists of seeking a quantum theory of gravity, which allows a consistent coupling of gravity to the Standard Model matter fields and provides a complete description of systems where gravity reaches critical scales and quantum effects cannot be ignored, for instance within black holes and at the early evolution phase of the Universe. If such theory is obtained, it would undoubtedly revolutionize our understanding of fundamental physics. This line of research is known as the field of quantum gravity.

„There are several approaches which attempt to resolve the problem of quantum gravity, one of which is the loop quantum gravity program and that is the subject of the present project.” – dr Assanioussi explained. „Loop quantum gravity follows the conceptual ideas of a quantization of general relativity. It provides a common framework where gravity and the Standard Model matter fields are quantized in a background independent setting.”

There has been many developments in this program and the resulting theory has very solid mathematical foundations. However, several issues remain unresolved, in particular the ambiguities in the implementation of the dynamics, the construction of the physical states and observables, and the recovery of the continuum limit of the theory. The issue of the continuum limit is the most significant problem faced in the theory. It can roughly be phrased in the question of how quantum field theory on a fixed spacetime arises from the quantum theory of gravity coupled to matter fields. Fundamentally, this question is not much different from asking how to recover the theory of fluid dynamics from quantum mechanics.

„There are two main pillars to this project.” – dr Assanioussi describes. „The first pillar concerns the investigation of the semi-classical and dynamical properties of certain coherent states in the context the loop quantum gravity, and the evaluation of possible connections between such coherent states and the emergence of an effective dynamics within a coarse-graining process. The objective will be to use these results to construct a renormalization procedure for the loop quantum theory of gravity coupled to matter fields. The second pillar concerns the study of physical symmetries of spacetime and matter fields in covariant gauge theories, and the establishment of their role in the quantum theory of gravity coupled to matter fields as well as within a potential renormalization scheme, to ultimately develop effective models from the quantum gravity theory and derive their phenomenological consequences.” – project leader concludes.

Indeed, the plan is ambitious, and it touches on extremely sophisticated issues that require extensive knowledge and experience in mathematics and physics. Even many physicists have problems understanding all the technicalities and subtleties. That’s why we have even more respect for dr Assanioussi and his knowledge. Congratulations on the financing and we keep our fingers crossed for these extremely interesting reflections on the principles of the functioning of the world around us.

Krzysztof Petelczyc