(November 22, 2011 at 8:48 am)IATIA Wrote:(November 22, 2011 at 7:57 am)little_monkey Wrote: Particles, massive or massless, don't age.A subatomic particle known as a pion has a lifespan of 000000026 seconds.
Yes, that is the time it would take for a pion to decay (btw, there's a decimal point missing), usually into a muon and a muon neutrino -- there are other modes of decay but less likely to occur. However it doesn't refer to its "age". There are no "young" pions or "old" pions. By some principles in QFT, all pions are indistinguishable, similarly for all the elementary particles of the Standard Model such as electrons, quarks, neutrino, etc...