Light and gravitational waves don’t arrive simultaneously

There’s an important rule in relativity that — as far as we know — all objects must obey. If you have no rest mass as you travel through the vacuum of space, you absolutely are compelled to travel exactly at the speed of light. This is exactly true for all massless particles, like photons and gluons, approximately true for particles whose mass is tiny compared to their kinetic energy, like neutrinos, and should also be exactly true for gravitational waves. Even if gravity isn’t inherently quantum in nature, the speed of gravity should be exactly equal to the speed of light. At least, that’s a necessity if our current laws of physics are correct. And yet, when we saw the first neutron star-neutron star merger in both gravitational waves and with light, the gravitational waves arrived first by a substantial, measurable margin: by almost 2 seconds. What’s the explanation? Even though the signal originated from 130 million light-years away, the distance shouldn’t matter; if the signals were generated at the same time, and they travel at the same speed and follow the same path, then they should’ve arrived at the same time, too. <a href="https://medium.com/starts-with-a-bang/light-and-gravitational-waves-dont-arrive-simultaneously-edf80c8bad15">Learn More</a>