Muons decay into other particles obeying an exponential decay law. In the frame in which they are at rest, the number of muons, N, at a time, t, is given by N = N_0*e^-t/τ where N_0 is the number of muons at t = 0 and τ is the mean lifetime (τ = 2.2 μs).
a) If muons are produced at a height of 4.0 km in our atmosphere and are heading toward the ground at a speed of 0.93c, what fraction will survive to reach the ground? [answer: 9.1%]

b) What fraction would reach the ground if you used only classical physics to do this problem?