Let's say I have a spherically-uniform black-body radiator. It is losing energy, and therefore some of its mass, at a particular rate. From the frame of reference of the radiator, it has no momentum, but it has a changing amount of energy. From its frame of reference, the pressure on the radiator yields no net force upon it. However, from another frame of reference, and in this case let's choose an independent inertial frame of reference, the radiation would have a net contribution to the momentum due to the relativistic Doppler effect. So there would be a net force on the radiator from this frame of reference. However, according to the radiator's frame of reference, the pressure is uniform so it shouldn't be subject to acceleration. Is then the change of mass of the spherically-uniform radiator as it travels through space at some velocity responsible for producing a "force" on the radiator devoid of any acceleration of the radiator itself?