 According to Baierlein, “symmetry alone requires that the atom \([B]\) remain at rest in Alice’s frame” (1991, p. 170). It also follows directly from this that since \(B\) remains at rest in \(K'\), the velocity of \(B\) does not change in \(K\) after the emission of light. However, as we will see shortly, the momentum of \(B\) does change in both \(K\) and \(K'\) because the light it emits carries away momentum. If we assume the classical definition for the momentum of \(B\) as the product of its mass \(m\) and its velocity \(v\), and \(v\) does not change, it follows that in order for the law of conservation of momentum to be satisfied, the mass (i.e., rest-mass) of \(B\) must change. 