I was wondering, are mass and energy specific to a particular reference frame, as well as things like time? Lets say for example, there's two observers, one in motion (A) and another stationary (B) standing along side my path of direction. If I were in accelerated motion, observer B would witness my mass increase, right? So how would that reverse, would observer A witness no increase in his own mass and instead, it would appear as if observer B's mass had increased? Or decrease perhaps?