Clocks and Relationalism in the Thermal Time Hypothesis
  The Thermal Time Hypotheis (TTH) has been proposed as a general method for identifying a time variable from within background-free theories which do not come equipped with a pre-defined clock variable. Here, we explore some implications of the TTH in an entirely relational context by constructing a protocol for the creation of "thermal clocks" from components of a large but finite quantum mechanical system. The protocol applies locally, in the sense that we do not attempt to construct a single clock describing the evolution of the entire system, but instead we construct clocks which describe the evolution of each subsystem of interest. We find that a consistency condition required for the evolution of our clocks is operationally equivalent to the general relativistic Tolman-Ehrenfest relation for thermal equilibrium in a static gravitational field but without the assumption of gravity or a metric field of any kind. 
Nicolas C. Menicucci, S. Jay Olson, Gerard J. Milburn
gr-qc