 2016

Source paper: www.whatdotheyknow.com/request/oxford_physics_past_papers_2/response/2340696/attach/4/Physics%20Papers%202015.pdf?cookie_passthrough=1

Solutions use natural units $\hslash =c=1$ and Minkowski metric $\eta =\mathrm{diag}(1,-1,-1,-1)$. The d'Alembert operator is $\square ={\partial }_{\mu }{\partial }^{\mu }$. All four questions are covered, including the optional fourth question.