Purpose of the "closed box" It's often stated that GR follows from the observation that an experimenter inside a closed box in freefall could not distinguish between the box being in that circumstance, and the box being in open space away from any gravitational field. In each case, objects inside the box exhibit inertial motion. But what if the box had glass walls and the experimenter looked outside? Then it would be obvious -- other free-falling objects outside the box would not appear to exhibit inertial motion. So what has happened to our ideas of being at rest or constant relative motion? Do they make any sense any more? Does the idea of inertial motion make any sense in anything but a local sense? How big can the box be and what is it trying to hide? The curvature of space? Is the experiment then telling us, in some sense, that space is locally flat? Could we somehow define a spatial coordinate system with the box at the origin, with respect to which even distant free-falling objects exhibit inertial motion? I don't see how. Fire two bullets in the same direction at different speeds in space and they will not follow the same path. Tune your coordinates to make one of their paths straight, and the other will appear to accelerate. Is that where the idea of spacetime curvature comes in? Is there some way to picture what this curvature looks like, for example around a field created by a point mass? Or is this the point where you can go no further without a mathematical understanding?