  The notion of "reference frame" is a central theoretical construct for
interpreting the physical implications of spacetime diffeomorphism invariance
in General Relativity. However, the alternative formulation of classical
General Relativity known as Shape Dynamics suggest that a subset of spacetime
diffeomorphisms - namely hypersurface deformations - are, in a certain sense,
dual to spatial conformal (or Weyl) invariance. Moreover, holographic
gauge/gravity dualities suggest that bulk spacetime diffeomorphism invariance
can be replaced by the properties of boundary CFTs. How can these new
frameworks be compatible with the traditional notion of reference frame so
fundamental to our interpretation of General Relativity? In this paper, we
address this question by investigating the classical case of maximally
symmetric spacetimes with a positive cosmological constant. We find that it is
possible to define a notion of "Shape Observer" that represents a conformal
reference frame that is dual to the notion of inertial reference frame in
spacetime. We then provide a precise dictionary relating the two notions. These
Shape Observers are holographic in the sense that they are defined on the
asymptotic conformal boundaries of spacetime but know about bulk physics. This
leads to a first principles derivation of an exact classical holographic
correspondence that can easily be generalized to more complicated situations
and may lead to insights regarding the interpretation of the conformal
invariance manifest in Shape Dynamics.
