# On quantum propagation in Smolin's weak coupling limit of 4d Euclidean Gravity

## TL;DR

This paper investigates quantum propagation in a specific LQG model, demonstrating that certain constraint modifications enable robust propagation of spin network states, addressing earlier concerns about propagation difficulties in LQG.

## Contribution

It shows that by slightly modifying quantum constraint actions, physical states can encode propagation, countering previous beliefs about LQG's limitations in this aspect.

## Key findings

- Physical states encode propagation with modified constraints
- Propagation involves merging, separating, and entangling vertices
- Constraints based on LQG methods can support vigorous propagation

## Abstract

Two desireable properties of a quantum dynamics for Loop Quantum Gravity (LQG) are that its generators provide an anomaly free representation of the classical constraint algebra and that physical states which lie in the kernel of these generators encode propagation. A physical state in LQG is expected to be a sum over graphical $SU(2)$ spin network states. By propagation we mean that a quantum perturbation at one vertex of a spin network state propagates to vertices which are `many links away' thus yielding a new spin network state which is related to the old one by this propagation. A physical state encodes propagation if its spin network summands are related by propagation. Here we study propagation in an LQG quantization of Smolin's weak coupling limit of Euclidean Gravity based on graphical $U(1)^3$ `charge' network states. Building on our earlier work on anomaly free quantum constraint actions for this system, we analyse the extent to which physical states encode propagation. In particular, we show that a slight modification of the constraint actions constructed in our previous work leads to physical states which encode robust propagation. Under appropriate conditions, this propagation merges, seperates and entangles vertices of charge network states. The `electric' diffeomorphism constraints introduced in prevous work play a key role in our considerations. The main import of our work is that there are choices of quantum constraint constructions through LQG methods which are consistent with vigorous propagation thus providing a counterpoint to Smolin's early observations on the difficulties of propagation in the context of LQG type operator constructions. Whether the choices considered in this work are physically appropriate is an open question worthy of further study.