Let's say there's a massive and uniform sphere of radius, r, and there's a box in its gravitational field and the sphere is so massive that the box is accelerated to a velocity near the speed of light; could the classical coordinate system still be used or is the Schwarzschild coordinate system needed? Furthermore say there's an observers further from the gravitational field than the box. Is the affect of time dilation on different aspects of the box negotiable from the observers perspective and what is the radial coordinate of the observer? Wikipedia says that it is analogous to the classical distance; could the classical distance and classical coordinate system be used instead where the classical coordinate system is a three dimensional version of Cartesian coordinates? I've researched this but do not understand. Also is it suitible to you use classical physics to describe the acceleration of the box?