Light and gravity travel at the same speed, but don't arrive together

"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 the massless ripples in spacetime created by purely gravitational effects: gravitational waves."

"What's the explanation? the gravitational waves arrived first by a substantial, measurable margin: by almost 2 seconds. Even though the signal originated from such a great distance away, that 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."

Relativity mandates that massless phenomena propagate at the speed of light, so gravitational waves must travel at c like photons. The first neutron-star merger detected both in gravitational waves and electromagnetic radiation, at roughly 130 million light-years, produced gravitational waves that were detected about two seconds before the light. A simultaneous-emission-and-equal-travel scenario would yield coincident arrival times, so the measured offset initially posed a puzzle. Accumulated observations and analysis point to a source-based explanation: electromagnetic emission occurred later due to astrophysical processes at the source, while both signals propagated at the same speed.

Read at Big Think