During experimental runs of Dark SRF, two cavities (shown here) are bathed in liquid helium to keep them cold. One is held perfectly still, tuned to a specific frequency and kept as empty of photons as possible. The other cavity, which is intended to generate dark photons, has a special setup to adjust it to match the frequency of the first cavity in real time. The experiment aims to catch photons produced in the adjustable cavity as they make their ghostly journey, through metal walls, into the empty cavity. Photo: Reidar Hahn, Fermilab