48 Chapt. 5 Newton’s Laws and Dynamics I frames not accelerating with respect to the primary set are also part of the special class. We actually can observationally determine the primary set by measurements of distant objects in the universe and/or by measurements of the cosmic microwave background radiation (CMB). Sets of references frames attached to physical objects like galaxies, stars, and planets (e.g., the Earth) are accelerated frames with respect to the special class since those objects rotate and/or orbit, and so are not exactly in the special class. But they may approximately belong to the special class depending on the context of the system one is considering. The Earth for many purposes— but not all purposes—is approximately in the special class. The fact that Newton’s laws must be referenced to the special class of frames and not physical bodies (except as approximations or with inertial forces included) shows that space has a physical nature beyond simple geometry. The nature of space’s physical nature is still actually not fully elucidated: full elucidation may
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