About Lorentz invariance in a discrete quantum setting

TL;DR

This paper challenges the misconception that Lorentz invariance conflicts with discrete spacetime by demonstrating, through a quantum gravity-inspired model, that Lorentz invariance can coexist with a quantum of geometry, with implications for quantum gravity and DSR.

Contribution

The paper introduces a toy model inspired by Loop Quantum Gravity showing Lorentz invariance is compatible with discrete geometric spectra and explores the structure of DSR in this context.

Findings

Quantum length states transform into superpositions under boosts.

Discrete spectra of geometric operators are compatible with Lorentz invariance.

The structure of DSR supports Lorentz invariance with quantum geometry.

Abstract

A common misconception is that Lorentz invariance is inconsistent with a discrete spacetime structure and a minimal length: under Lorentz contraction, a Planck length ruler would be seen as smaller by a boosted observer. We argue that in the context of quantum gravity, the distance between two points becomes an operator and show through a toy model, inspired by Loop Quantum Gravity, that the notion of a quantum of geometry and of discrete spectra of geometric operators, is not inconsistent with Lorentz invariance. The main feature of the model is that a state of definite length for a given observer turns into a superposition of eigenstates of the length operator when seen by a boosted observer. More generally, we discuss the issue of actually measuring distances taking into account the limitations imposed by quantum gravity considerations and we analyze the notion of distance and the phenomenon of Lorentz contraction in the framework of ``deformed (or doubly) special relativity'' (DSR), which tentatively provides an effective description of quantum gravity around a flat background. In order to do this we study the Hilbert space structure of DSR, and study various quantum geometric operators acting on it and analyze their spectral properties. We also discuss the notion of spacetime point in DSR in terms of coherent states. We show how the way Lorentz invariance is preserved in this context is analogous to that in the toy model.