$T\bar{T}$ flow as characteristic flows

We show that method of characteristics provides a powerful new point of view on $T\bar{T}$-and related deformations. Previously, the method of characteristics has been applied to $T\bar{T}$-deformation mainly to solve Burgers' equation, which governs the deformation of the \emph{quantum} spectrum. In the current work, we study \emph{classical} deformed quantities using this method and show that $T\bar{T}$ flow can be seen as a characteristic flow. Exploiting this point of view, we re-derive a number of important known results and obtain interesting new ones. We prove the equivalence between dynamical change of coordinates and the generalized light-cone gauge approaches to $T\bar{T}$-deformation. We find the deformed Lagrangians for a class of $T\bar{T}$-like deformations in higher dimensions and the $(T\bar{T})^{\alpha}$-deformation in 2d with generic $\alpha$, generalizing recent results in arXiv:2206.03415 and arXiv:2206.10515.