Take the 2-minute tour ×  I understand that you can (in
principle) measure whether "free particles" (no forces)
experience accelerations in order to tell whether a frame is
inertial. But fundamentally, what determines which frames are
inertial (i.e. what principle selects in which frames free
particles will not appear to accelerate)? I've been told that
the cosmic microwaves determine the ultimate rest frame of the
universe, but that doesn't make sense to me, since one can still
ask why that frame is an inertial frame.  Also, I understand
that there are no real inertial frames in general relativity,
but it seems like there certainly exists approximate inertial
frames and we can ask why those frames are approximately
inertial and not others. For example, in the frame of a person
riding a merry go round, free particles appear to greatly
accelerate; while in the frame of someone standing next to the
merry go round there are no such great accelerations. Why does
the guy (or gal) on the merry go round see free par