  In standard quantum mechanics, reference frames are treated as abstract
entities. We can think of them as idealized, infinite-mass subsystems which
decouple from the rest of the system. In nature, however, all reference frames
are realized through finite-mass systems that are subject to the laws of
quantum mechanics and must be included in the dynamical evolution. A
fundamental physical theory should take this fact seriously. In this paper, we
further develop a symmetry-inspired approach to describe physics from the
perspective of quantum reference frames. We find a unifying framework allowing
us to systematically derive a broad class of perspective dependent descriptions
and the transformations between them. Working with a translational-invariant
toy model of three free particles, we discover that the introduction of
relative coordinates leads to a Hamiltonian structure with two non-commuting
constraints. This structure can be said to contain all observer-perspectives at
once, while the redundancies prevent an immediate operational interpretation.
We show that the operationally meaningful perspective dependent descriptions
are given by Darboux coordinates on the constraint surface and that reference
frame transformations correspond to reparametrizations of the constraint
surface. We conclude by constructing a quantum perspective neutral structure,
via which we can derive and change perspective dependent descriptions without
referring to the classical theory. In addition to the physical findings, this
work illuminates the interrelation of first and second class constrained
systems and their respective quantization procedures.
