  Dimensionality plays a crucial role in long-range dipole-dipole interactions
(DDIs). We demonstrate that a resonant nanophotonic structure modifies the
apparent dimensionality in an interacting ensemble of emitters, as revealed by
population decay dynamics. Our measurements on a dense ensemble of interacting
quantum emitters in a resonant nanophotonic structure with long-range DDIs
reveal an effective dimensionality reduction to $\bar{d} = 2.20 (12)$, despite
the emitters being distributed in 3D. This contrasts the homogeneous
environment, where the apparent dimension is $\bar{d} = 3.00$. Our work
presents a promising avenue to manipulate dimensionality in an ensemble of
interacting emitters.
