I have always been confused by SR and GR in terms of what each observer observes of their own frame of reference. e.g. Does an observer moving at a high v/c measure length contraction in their onw F of R? I assume not since all apparatus would be similarly contracted. Nor do they measure time dilation in their own F of R ---only observe it in other frames? Can they confirm that their mass is increased by applying a force to an object in their F of R? If mass is increased and length contracted then if density were to not increase the width perpendicular to the direction of motion would need to increase? In short, this begs the metaphysical question of whether these changes are real or simply distorted observations on the part of an oberver in another F of R and the lorentz transforms provide the description. iunderstand that time dilation has real, permanent and measurable effects but if mass is increasing then what is the physical mechanism- a gain in no. of molecules? If length is contracting what forces are compressing it and/or increasing it width wise?