Except in the trivial sense, diffeomorphism invariance is destroyed by the introduction of such a frame of
reference. Often, an attempt is made to partially restore it by demanding invariance under all permutations of
the labeling of the world-lines of the fibration. When a coordinate system is adapted to the fibration, this is
described as three-dimensional diffeomorphism invariance or invariance under spatial diffeomorphisms, and the
distinction is sometimes elided. “What are the quanta of the gravitational field? Or, since the gravitational field
is the same entity as spacetime, what are the quanta of space?” (Rovelli, 2004, p. 18; see also Section 4.1,
pp. 145–153).