  Here we calculate the effects of astrophysical gravitational waves (GWs) on
the travel times of proton bunch test masses in circular particle accelerators.
We show that a high-precision proton bunch time-tagging detector could turn a
circular particle accelerator facility into a GW observatory sensitive to
millihertz (mHz) GWs. We comment on sources of noise and the technological
feasibility of ultrafast single photon detectors by conducting a case study of
the Large Hadron Collider (LHC) at CERN.
