Holo-ween

We argue that given holographic CFT$_1$ in some state with a dual spacetime geometry M, and given some other holographic CFT$_2$, we can find states of CFT$_2$ whose dual geometries closely approximate arbitrarily large causal patches of M, provided that CFT$_1$ and CFT$_2$ can be non-trivially coupled at an interface. Our CFT$_2$ states are "dressed up as" states of CFT$_1$: they are obtained from the original CFT$_1$ state by a regularized quench operator defined using a Euclidean path-integral with an interface CFT$_1$ CFT$_2$ and CFT$_1$. Our results are consistent with the idea that the precise microscopic degrees of freedom and Hamiltonian of a holographic CFT are only important in fixing the asymptotic behavior of a dual spacetime, while the interior spacetime of a region spacelike separated from a boundary time slice is determined by more universal properties (such as entanglement structure) of the quantum state at this time slice. Our picture requires that low-energy gravitational theories related to CFTs that can be non-trivially coupled at an interface are part of the same non-perturbative theory of quantum gravity.