Traces of quantum fuzziness on the black hole shadow and particle deflection in the multi-fractional theory of gravity

Abstract: In this paper, we investigate the properties of black holes within the framework of multi-fractional theories of gravity, focusing on the effects of q-derivatives and weighted derivatives. These modifications, which introduce scale-dependent spacetime geometries, alter black hole solutions in intriguing ways. Within these frameworks, we analyze two key observable phenomena - black hole shadows and particle deflection angle in the weak field limit - using both analytical techniques and observational data from the Event Horizon Telescope (EHT) for M87* and Sgr A*. The study from $q$-derivative formalism reveals that the multi-scale length $\ell_$ influences the size of the black hole shadow in two ways, and modifies the weak deflection angle. Constraints on $\ell_$ are derived from the EHT observations, showing significant deviations from standard Schwarzschild black hole predictions, which range from $10^{9}$ to $10^{10}$ orders of magnitude. Additionally, the weak deflection angle is computed using the non-asymptotic generalization of the Gauss-Bonnet theorem, revealing the effects of finite-distance and multi-scale parameters. Using the Sun for Solar System test, the constraints for $\ell_*$ range from $10^{8}$ to $10^{9}$ orders of magnitude. Results from the weighted derivative formalism generates a dS/AdS-like behavior, where smaller deviations are found in the strong field regime than in the weak field regime. The results suggest that while these effects are subtle, they provide a potential observational signature of quantum gravity effects. The findings presented here contribute to the broader effort of testing alternative theories of gravity through black hole observations, offering a new perspective on the quantum structure of spacetime at cosmological and astrophysical scales.