We investigate the physics of quantum reference frames. Specifically, we
study several simple scenarios involving a small number of quantum particles,
whereby we promote one of these particles to the role of a quantum observer and
ask what is the description of the rest of the system, as seen by this
observer? We highlight the interesting aspects of such questions by presenting
a number of apparent paradoxes. By unravelling these paradoxes we get a better
understanding of the physics of quantum reference frames.Comment: 11 pages, 4 figures. v2: Published versio
