| Muons are radioactive particles which decay into an electron and 2 neutrinos with an average lifetime measured when they are at rest of 2 microseconds (2x10-6 seconds). In this amount of time, light can travel at most 600 meters. Muons are sometimes produced in the upper atmosphere by cosmic rays (protons) that bombard our atmosphere from space. When produced in this way, the muons are travelling at nearly (but less than) the speed of light but are observed nevertheless to travel much further than 600 meters. How can this be? (There are two frames of reference that are important here: The frame attached to the muon (the muon's frame) in which the muon is at rest and the frame attached to our earth-bound laboratory (the laboratory frame) from which we observe the moving muon.) |