5-5. The mean lifetime of tau (z ) leptons is 2.9 x 10- s in their own rest frame. If a particular + particle has this rest frame lifetime, and the shortest distance that can be resolved in a particular detector in the laboratory is about 1 mm = 10-$ m, how fast must this r travel for us to see it? By the time it decays, how far would the detector have moved in the rest frame of the r