this thought was inspired by the recent news about FTL neutrinos. of course i suspect their instruments are broken and that no such thing happened, but regardless, the question stands: it's easy to talk about the rest frame of classical objects where the notion of "trajectory" applies and velocity has an unambiguous meaning. but neutrinos and mesons are not little tennis balls zipping about. how does one make sense of boosting into the rest frame of a neutrino, when the "momentum" of such entities is defined by pointwise measurements and not a time derivative of a trajectory? or how does one make sense of the proper time of a meson, and their longer-than-expected observed lifetimes as evidence for relativistic time dilation? a meson is a quantum entity, not a ball! it does not follow a classical trajectory whose time derivative yields its momentum, and hence cannot be interpreted as "moving with velocity v" wrt an earthbound laboratory. i can't see how the relativistic notion of velocity and boosting into a rest frame can possibly apply to a meson, or neutrino, or any atomic entity.