In the limit of shallow potential minima and weak spatial curvature, it
is permissible to switch back and forth between a Newtonian and a general
relativistic viewpoint, adopting whichever one is more convenient. I will
frequently adopt the Newtonian view of gravity in this book because, in
many contexts, it is mathematically simpler and conceptually more familiar.
The question of why it is possible to switch back and forth between the two
very different viewpoints of Newton and Einstein is an intriguing one, and
deserves closer investigation.