We compute a gravitational on-shell action of a finite, spherically symmetric causal diamond in (d + 2)-dimensional Minkowski spacetime, finding it is proportional to the area of the bifurcate horizon \( {A}_{\mathcal{B}} \) . We then identify the on-shell action with the saddle point of the Euclidean gravitational path integral, which is naturally interpreted as a partition function. This partition function is thermal with respect to a modular Hamiltonian K. Consequently, we determine, from the on-shell action using standard thermodynamic identities, both the mean and variance of the modular Hamiltonian, finding 〈K〉 = 〈(∆K)2〉 = \( \frac{A_{\mathcal{B}}}{4{G}_N} \) . Finally, we show that modular fluctuations give rise to fluctuations in the geometry, and compute the associated phase shift of massless particles traversing the diamond under such fluctuations.
