LIGO in the United States and Virgo in Pisa, Italy, received and detected the gravitational wave caused by the collision of two neutron stars. This is the first gravitational wave not caused by the merger of two black holes. It is located 130 million light-years away. Immediately after the gravitational wave, 2 seconds later, came the powerful gamma-ray burst GRB 170817A. The paths of the gravitational waves and the arriving gamma-ray photons were only 2 seconds apart! (out of 130 million years of travel). This is a fundamental validation of the theory of General Relativity, as it demonstrates that light waves and gravitational waves travel not only at the same speed, but also along the exact same path through space-time, being subject to the same slowing caused by space dilation. This allows us to reject a series of alternative theories that proposed radical modifications to Relativity to explain dark matter phenomena (theories that attempted to explain observations without introducing dark matter and instead by modifying the Theory of Relativity).
