It was recently noted that different internal quantum reference frames (QRFs)
partition a system in different ways into subsystems, much like different
inertial observers in special relativity decompose spacetime in different ways
into space and time. Here we expand on this QRF relativity of subsystems and
elucidate that it is the source of all novel QRF dependent effects, just like
the relativity of simultaneity is the origin of all characteristic special
relativistic phenomena. We show that subsystem relativity, in fact, also arises
in special relativity with internal frames and, by implying the relativity of
simultaneity, constitutes a generalisation of it. Physical consequences of the
QRF relativity of subsystems, which we explore here systematically, and the
relativity of simultaneity may thus be seen in similar light. We focus on
investigating when and how subsystem correlations and entropies, interactions
and types of dynamics (open vs. closed), as well as quantum thermodynamical
processes change under QRF transformations. We show that thermal equilibrium is
generically QRF relative and find that, remarkably, QRF transformations not only can change a subsystem temperature, but even map positive into negative temperature states. We further examine how
non-equilibrium notions of heat and work exchange, as well as entropy
production and flow depend on the QRF. Along the way, we develop the first
study of how reduced subsystem states transform under QRF changes. Focusing on
physical insights, we restrict to ideal QRFs associated with finite abelian
groups. Besides being conducive to rigour, the ensuing finite-dimensional
setting is where quantum information-theoretic quantities and quantum
thermodynamics are best developed. We anticipate, however, that our results
extend qualitatively to more general groups and frames, and even to subsystems
in gauge theory and gravity.Comment: 49 pages + appendices, 12 figures. Comments welcom