Over a distance of 130 million light years, the gamma rays and gravitational waves emitted by merging neutron stars arrived offset by a mere 1.7 seconds, an incredible result! In the final moments of merging, two neutron stars don't merely emit gravitational waves, but a catastrophic explosion that echoes across the electromagnetic spectrum. The arrival time difference between light and gravitational waves enables us to learn a lot about the Universe. After all, matter is transparent to gravitational waves, but it interacts with light all the time! There’s no doubt that the first gamma rays from this neutron star-neutron star merger arrived after the gravitational waves did.