Does Gravity Fall Down? Evidence for Gravitational-wave Deflection along the Line of Sight to GW170817

We present a novel test of general relativity (GR): measuring the geometric component of the time delay due to gravitational lensing. GR predicts that photons and gravitational waves follow the same geodesic paths and thus experience the same geometric time delay. We show that for typical systems, the time delays are tens of seconds, and thus can dominate over astrophysical delays in the timing of photon emission. For the case of GW170817, we use a multi-plane lensing code to evaluate the time delay due to four massive halos along the line of sight. From literature mass and distance measurements of these halos, we establish at high confidence (significantly greater than $5\sigma $) that the gravitational waves of GW170817 underwent gravitational deflection to arrive within 1.7 s of the photons.