Hi guys, just wanted to ask a question related to muon experiments (and all other which can be presented this way) to get some things clearer. Ok, here goes: We have lots of muons travelling towards Earth. Their mean lifetime, measured in lab conditions (at rest) is 2.2µs. Their concentrations (flux) have been measured at different altitudes, and their speed (0.99c) has been measured near the Earth surface. By comparing their concentrations at an altitude of 15km and at sea level, it has been shown that many more survive than expected, considering their speed and their mean lifetime. SR calculation follows: Note: I took delta_x (change of height) to be negative, because it's decreasing, but this is a matter of choice. Speed of light is c=299792458m/s Speed of muon is -0.99c = -296794533,4 m/s Lorentz factor is then: gamma = 7.08881205 EARTH's frame delta_x: -15km = -15000m (height decreased by 15km) delta_t: delta_x/v = 50.54µs It takes 50µs for the muon to travel 15km. v=0.99c. MUON's frame delta_t': gamma*(delta_t-(v*delta_x)/(c*c)) = 7.13µs delta_x': gamma*(delta_x-v*delta_t) = 0m (in this frame, muon is stationary) distance to Earth at t'=0: delta_x/gamma = -2116m It takes 7µs for the muon to travel 2km. v=0.99c. Ok, so far everyhing is as Relativity predicts. Now the strange part. What if we started with the fact that it takes 7µs for the muon to travel 2km at that speed and want to find out delta_t in Earth's frame? Let's say that muon is stationary and Earth is travelling towards the muon. MUON's frame delta_x: -2.116km = -2116m delta_t: delta_x/v = 7.13µs Now we are in muon's frame, and want to find out the time and distance Earth needs to travel in Earth's frame. We should get 50µs, distance of 0m, but we should be able to calculate muon's distance also. Using exactly the same reasoning as when we started, we get: EARTH's frame delta_t': gamma*(delta_t-(v*delta_x)/(c*c)) = 1.01µs delta_x': gamma*(delta_x-v*delta_t) = 0m (in this frame, Earth is stationary) distance to muon at t'=0: delta_x/gamma = -298.5m Shouldn't we be able to get our starting results (50µs, 0m, -15km)?