This link is confusing me. http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/muon.html In this link, with non-relativistic method, half life is calculated as 21.8. But with relativistic method half life is calculated as 4.36. We calculate half life considering time dilation for muon in earth frame. And, we calculate half life considering length contraction for muon in muon frame. We get same value using both method as 4.36. So, the question is: Would length contraction and time dilation both not exist for muon? Would we not include both factor in equation to calculate half life? Why are we using length contraction and time dilation one by one to calculate half life?