Hi... Please can anyone explain me why we can talk about force in special relativity? I thought that special relativity is valid only in inertial reference frames and the force has to product acceleration. And so the reference frame is not inertial and the principle of relativity doesn't have to be valid. And so the formula F=ma doesn't have to be true in this refernce frame. What's wrong with this thinking? My understanding of inertial frames? Thank you very much for explanation.