relativistic mass is defined by the lorentz contraction equations (i.e time & legnth). but why should mass increase only for speeds very close to c? is there any theory that explains this? we know that rest mass is basically the resistance to acceleration and the higgs theory postulates that mass arises when a group of charges interacts with the higgs field. Is it possible that rest mass and relativistic mass are actually the same phenomenon? perhaps the question is not what sets mass at a certain value, but what stops mass being infinite.