Originally Posted by homealone

its to do with the equivalence of mass & energy & their relation to the speed of light, the classic E=mc² (Energy equals mass times the speed of light squared) applies to a stationary object, it gets a little more complicated if the object is moving...........

It's the main reason why, imo, the concept of some kind of 'warping' of the space/time continuum is necessary to travel any useful distance.