Einstein's theory of special relativity is deeply connected with the notions of inertial reference frame, and of inertial observer; nevertheless, there is a special interest in the consideration of accelerated observers, even in a special-relativistic context. First, accelerated frames are historically the germ from which general relativity was born; second, there are special topics in relativistic theories (such as the now-famous Unruh effect and the problem of radiation reaction) where we gain physical insight by a subjective description made from the point of view of accelerated observers. In this thesis, we study the physics of accelerated observers from several points of view.
First, we identify the origin of the Unruh and Hawking effects in the classical principle of perspectival semantics, according to which some familiar notions defined in special-relativistic theories (such as particle and radiation) become blurred when they are transported to accelerated frames or to curved spacetimes. We support our claim by presenting a classical analogue of the Unruh effect based on the noninvariance of the notion of electromagnetic radiation when switching from inertial to accelerated observers. Second, we propose a general scheme to build an accelerated system of coordinates (Märzke--Wheeler coordinates) adapted to the motion of a generic accelerated observer. These coordinates reduce to Lorentz coordinates in a neighborhood of the observer's worldline, and they generally provide a smooth and consistent foliation of spacetime into spacelike surfaces. We employ Märzke--Wheeler coordinates to clarify the relativistic paradox of the twins, and we suggest that field quantization in these coordinates could solve a well-known inconsistency in the theory of the circular Unruh effect (quantization in rotating coordinates is inconsistent with the measurements of a rotating detector). Finally, we review the perennial debate (among relativists and philosophers of physics alike) on the conventionality of Einstein simultaneity in special relativity, and we find that the evidence for the nonconventionality of Einstein synchronization appears very compelling. We extend the discussion to accelerated observers in special relativity and we we make the case for the conventionality of Märzke--Wheeler simultaneity.
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