1. The problem statement, all variables and given/known data http://www.maths.ox.ac.uk/system/files/private/active/0/b07.2_c7.209.pdf [Broken] Q1 (Also in Q2, what is the definition of 4 momentum for a massless particle?) 3. The attempt at a solution I can do the entire question if I first derive the Lorentz transformation but I suspect that as I can't do the question directly in its intended way I don't really understand what I am doing. I have done ii) using Bondi's k-factor but not sure this is right as I haven't considered the path of the light from B to M2. I have just drawn a spacetime diagram with R' the inertial frame and drawn a photon leaving R' origin at time t, reaching B at t'=kt (when the light gets back to B from M2) and getting back to R' origin at k^2t=kt' and solving. I am just getting into a mess trying to do iii) though and absolutlely no idea how to do iv) without Lorentz transformation. Thanks!