I saw a thread that asked the same basic question as I'm asking, but the explanation was beyond my current knowledge. Please consider answering my question as if you were being interviewed for a Discovery Channel special and had to make it comprehensible for a general audience. Thanks! For me, the most confusing concept in relativity, so far, is if there really is only one version of an object and accelleration can really add mass to that object and any object can be declared to be accellerating in relation to another object, how do objects avoid having a whole range of masses and thereby cause a range of gravitational fields to distort spacetime around them? I thought about observing a neutron star that was just under the limit of density to collapse into a black hole being pushed over that limit to observers who had a relative motion near the speed of light relative to the neutron star.