A thought occurred to me when reading the Encarta article on relativity: Now, it occurs to me that the velocity of the electron, being relative to the "original" reference frame of the radioactive sample, can also be considered to be the (relative) velocity of the radioactive sample, the laboratory, the earth, the solar system, etc. - if one only considers things from the electron's frame of reference. So, one would expect to see a corresponding, velocity-induced mass increase in the original sample, the laboratory, the earth, solar system, etc. - because of the relative velocity of those things from the point of view of the electron's frame of reference. Right?