On Decoherence in Quantum Gravity

It was previously argued that the phenomenon of quantum gravitational decoherence described by the Wheeler-DeWitt equation is responsible for the emergence of the arrow of time. Here we show that the characteristic spatio-temporal scales of quantum gravitational decoherence are typically logarithmically larger than a characteristic curvature radius $R^{-1/2}$ of the background space-time with a factor under the logarithm proportional to $M_{P}^{2}/R$. This largeness is a direct consequence of the fact that gravity is a non-renormalizable theory, as the corresponding effective field theory is nearly decoupled from matter degrees of freedom in the physical limit $M_{P}\to\infty$. Therefore, as such, quantum gravitational decoherence is too ineffective to guarantee the emergence of the arrow of time at scales of physical interest. We argue that the emergence of the arrow of time is directly related to the nature and properties of physical observer.