Now I don't pretend to be anything but a layman so 'm not sure about this but here goes anyway.........the theory contends that a body's apparent mass increases as it accelerates on account of an increase in its energy of motion. Is this energy thermal energy? If so what is its source? I read in Einsteins paper that if you approached a star at lightspeed the frequency of light would be infinite (from the star). Does this mean QED that your temperature, absorbed energy and thus apparent mass would all become very great (ie infinite)? Would this account for the inertia which prevents further acceleration? Would the direction of travel then be irrelevant, since high speed motion would foreshorten everything ahead of the traveller? I've puzzled over this matter of 'energy of motion' a lot but never seem to have found a reference to it that made any sense to me. Am I just a dud at science? Surely not altogether? Better barking up the wrong tree than at the moon I suppose..................