Hello, I read this: Einstein's Special Theory of Relativity is valid for systems that are not accelerating. on this site: http://csep10.phys.utk.edu/astr162/lect/cosmology/gravity.html Honestly I cannot understand that. I have read the famous Gravitation by Wheeler and "Classical theory of fields" by Landau, and I know some of the motivations for going to GR from SR. I can imagine that strong gravitational fields or strong accelerations are a limit to the theory. Or even that gravitation doesn't fit well in the STR. But is that not a bit exagerated to say that: Einstein's Special Theory of Relativity is valid for systems that are not accelerating. What are the facts? Thanks for your ideas.