 Furthermore, there are conceptual reasons why both Einstein and many contemporary physicists do not add a subscript “\(o\)” to \(m\) when denoting rest-mass. For example, Taylor and Wheeler argue that the phrase “rest-mass” engenders potential confusion as it might lead the reader to ask: What happens to the rest-mass when the object is moving? Answer: nothing (Taylor and Wheeler 1992, p. 251). The rest-mass of an object is an invariant quantity in special relativity; it has the same value for all inertial observers. Taylor and Wheeler quip: “In reality mass is mass is mass” (Taylor and Wheeler 1992, p. 251). From their perspective, which is quite standard now in physics textbooks and articles, there is no need to prefix the term “mass” with “rest,” because there is no other kind of mass worth speaking about in special relativity (see  Section 1.6).  