After some forum searching, I concluded that the question I'm confused about was too hard to find (or not yet posted at all), so I'll be so bold as to post it myself. Apologies if this has already been treated exhaustively! I'm not sure where to post this exactly, as it ties in with both classical physics and relativity - at least, I think it does... The main question is really: can you consider a reference frame at rest w.r.t. a non-rotating massive object to be an inertial frame? I'd say you can't, as you would experience an acceleration in that frame and, according to the equivalence principle, you hence might as well be in an accelerating frame, which would be non-inertial - and as such you would be equating an inertial frame to a non-inertial frame, which would seem nonsensical. Others have said that you can very well treat the frame at rest w.r.t. a massive object as an inertial frame, when you just treat the gravitational force as a real force instead of an artefact of an accelerating reference frame. This might not seem like a problem at all (but merely like two alternative interpretations), but I think there is a definitive difference once you start considering the problem of a charge in a gravitational field (which has been talked about on these forums already I believe), and whether or not it radiates - as opposed to a uniformly accelerating charge. So, what do you people think? At rest w.r.t. a massive object, are you in an inertial reference frame? Why, or why not? Thanks in advance for your thoughts.