  The equivalence principle is the conceptual basis for general relativity. In
contrast Mach's principle, although said to have been influential on Einstein
in his formulation of general relativity, has not been shown to be central to
the structure of general relativity. In this essay we suggest that the quantum
effects of Hawking and Unruh radiation are a manifestation of a {\it thermal}
Mach's principle, where the local thermodynamic properties of the system are
determined by the non-local structure of the quantum fields which determine the
vacuum of a given spacetime. By comparing Hawking and Unruh temperatures for
the same local acceleration we find a violation of the Einstein elevator
version of the equivalence principle, which vanishes in the limit that the
horizon is approached.
