Question: Muons created in the upper atmosphere are sometimes able to reach the earth's surface. Imagine that one such muon travels the 60km from the upper atmosphere to the ground (in the earth's frame) in one muon half-life of 1.52us (in the muon's frame). How thick is that part of the earth's atmosphere from the muon's creation point to the ground in the muon's frame? Attempted solution: Assuming the muon travels at 0.99c, in on half life cycle in it's own frame it would travel 0.99c*1.52us. This yields d=451 meters. That seems a little too easy for my liking, is it correct? My physics class basically got a crash course in relativity, that is 10 chapters in two weeks and we did very few examples so this seems foreign to me.