Mass and gauge invariance: Holography for the Karch-Randall model

We argue that the Karch-Randall compactification is holographically dual to a 4D conformal field theory coupled to gravity on anti--de Sitter space. Using this interpretation we recover the mass spectrum of the model. In particular, we find no massless spin-2 states. By giving a purely 4D interpretation to the compactification we make clear that it represents the first example of a local 4D field theory in which general covariance does not imply the existence of a massless graviton. We also discuss some variations of the Karch-Randall model discussed in the literature, and we examine whether its properties are generic to all conformal field theory.