A gravity wave detector works by having mass which gets influenced by gravity. But this influence is a two-way street: the photon feels the detector's gravity a

A gravity wave detector works by having mass which gets influenced by gravity. But this influence is a two-way street: the photon feels the detector's gravity at exactly the same time as the detector feels the photon's gravity. If the detector is sensitive enough to distinguish the location of photons, it must be very close to the photons and/or have a huge mass! So it's not surprising that its gravitational field can influence the state of the photon.