The cosmic rays of highest energy are protons that have kinetic energy on the order of 1013 MeV.
(a) Find how long would it take a proton of this energy to travel across the Milky Way galaxy, as measured in the proton's frame? (The Milky Way has a diameter of ~ 105 light-years as measured in its rest frame.)
s
(b) From the point of view of the proton, determine how many kilometers across is the galaxy?
km