On the formation of shocks for quasilinear wave equations

The paper is devoted to the study of shock formation of the 3-dimensional quasilinear wave equation \begin{equation}\label{Main Equation} - \big(1+3G^{\prime\prime}(0) (\partial_t\phi)^2\big)\partial^2_t \phi +\Delta\phi=0,\tag{\textbf{$\star$}} \end{equation} where $G^{\prime\prime}(0)$ is a non-zero constant. We will exhibit a family of smooth initial data and show that the foliation of the incoming characteristic hypersurfaces collapses. Similar to 1-dimensional conservational laws, we refer this specific type breakdown of smooth solutions as shock formation. Since $(\star)$ satisfies the classical null condition, it admits global smooth solutions for small data. Therefore, we will work with large data (in energy norm). Moreover, no symmetry condition is imposed on the initial datum. We emphasize the geometric perspectives of shock formations in the proof. More specifically, the key idea is to study the interplay between the following two objects: (1) the energy estimates of the linearized equations of $(\star)$; (2) the differential geometry of the Lorentzian metric $g=-\dfrac{1}{\left(1+3G^{\prime\prime}(0) (\partial_t\phi)^2\right)} d t^2+dx_1^2+dx_2^2+dx_3^2$. Indeed, the study of the characteristic hypersurfaces (implies shock formation) is the study of the null hypersurfaces of $g$. The techniques in the proof are inspired by the work \cite{Ch-Shocks} in which the formation of shocks for $3$-dimensional relativistic compressible Euler equations with small initial data is established. We also use the short pulse method which is introduced in the study of formation of black holes in general relativity in \cite{Ch-BlackHoles} and generalized in \cite{K-R-09}.