a cosmic ray creates a muon in the earth's upper atmosphere that travels towards the earth at 0.998c. the muon exists for 3.16 ms when measured by an observer on earth. during its lifetime: a) how far does an observer on earth say the muon has traveled? b) find the lifetime of the muon as measured by an observer at rest with respect to the muon. c) how far does the muon say that it has traveled relative to earth?