Relativity question, photon and proton.... 1. The problem statement, all variables and given/known data Proton of gamma factor 10^12 and a photon set off from one side of our galaxy to the other take the distance to be 9.3x10^20m. What is the time interval that the photon precedes the proton? 2. Relevant equations \gamma = sqrt(c/2epsilon). (epsilon being c-v) 3. The attempt at a solution Worked out epsilon to be 1.5x10^(-18) Not sure where to go from there anyhelp/hints would be most appreciated.