  Characteristics of observers in relativity theory are critically examined.
For field measurements in Minkowski spacetime, the Bohr-Rosenfeld principle
implies that the connection between actual (i.e., noninertial) and inertial
observers must be nonlocal. Nonlocal electrodynamics of non-uniformly rotating
observers is discussed and the consequences of this theory for the phenomenon
of spin-rotation coupling are briefly explored.
