Board Thread:General Discussion/@comment-9154802-20180509212825/@comment-29564364-20180515110950

Ah, I see the source of our misunderstanding. You mistook the definition of observer in relativity for the colloquial definition. In the context of relativity, an inertial observer is an observer that is at rest with respect to an inertial reference frame, which is a reference frame that drifts in gravity-free space without undergoing rotation or acceleration. Basically, think of an inertial frame of reference as the image recorded by a camera floating through space at a constant speed while the observer is the cameraman holding it.