College Physics 10th Edition by Raymond Serway,Chris Vuille

Exercise 12

The cosmic rays of highest energy are protons that have kinetic energy on the order of 10 13 MeV. (a) From the point of view of the proton, how many kilometers across is the galaxy (b) How long would it take a proton of this energy to travel across the Milky Way galaxy, having a diameter ~ 10 5 light-years, as measured in the proton's frame