The equations of motion for a classical field $\phi$ can be obtained using the Lagrange: $$ \frac{\partial \mathcal{L}}{\partial \phi} - \partial_\mu \bigg ( \frac{\partial \mathcal{L}}{\partial(\partial_\mu \phi)} \bigg )=0 \tag{1}$$ A simple Lagrangian: $$ \mathcal{L} = \frac{1}{2} \partial_\mu...

Carlo Rovelli is interviewed in this article (The illusion of time): Alongside and inspired by his work in quantum gravity, Rovelli puts forward the idea of 'physics without time' This stems from the fact that some equations of quantum gravity (such as the Wheeler–DeWitt equation, which assigns ...