The mu-meson is a heavy version of an electron and I had come to look at an even heavier version which played an essential role in determining G, the constant of gravitation.
What quantity, relative to the mu-mesons motion during its life time, would both observers agree upon, and what is its numerical value?
It is a mistake to think that what is really, non-inferentially, observed is only the vapour trail and that the presence of mu-mesons is only inferred.
A classic experiment on the time dilation phenomenon was performed by B. Rossi and D.B. Hall in 1941, using the mu-mesons produced by cosmic rays which enter the earth's atmosphere from outer space at relativistic speeds.
Discovered in 1935 by CarlAnderson, the mu-meson is now known to be a type of lepton, a particle similar to the ordinary electron, but more than two hundred times as heavy.