There is a fundamental rule in relativity that all objects must follow: if you have no rest mass as you travel through space, you are obligated to move at the speed of light. This holds true for massless particles like photons and gluons, as well as for particles with tiny mass compared to their kinetic energy, such as neutrinos. Even gravitational waves, if gravity is not inherently quantum, should travel at the speed of light. However, when the first neutron star-neutron star merger was observed in both gravitational waves and light, the gravitational waves arrived almost 2 seconds earlier. This puzzling phenomenon has a compelling explanation based on our current understanding of the universe.
On August 17, 2017, the signal from an event 130 million light-years away finally reached Earth. Two neutron stars within the distant galaxy NGC 4993 were locked in a gravitational dance, orbiting each other at speeds approaching the speed of light. As they orbited, they caused distortions in space due to their mass and motion through curved space.
Whenever masses accelerate through curved space, they emit gravitational waves, which are invisible to traditional telescopes. These waves act as ripples in the fabric of spacetime and carry energy away from the system, causing the orbits to decay. Eventually, the two neutron stars began to inspiral, spiraling closer and closer together as gravitational waves dissipated their orbital energy.
At a critical moment, the two stars collided, and the gravitational wave signal abruptly ended. The ensuing phenomenon was a remarkable scientific discovery: the formation of a black hole. Just 1.7 seconds after the gravitational wave signal ceased, an enormous burst of gamma rays, a form of electromagnetic radiation, arrived. By combining the gravitational wave and electromagnetic data, scientists were able to locate the event precisely within NGC 4993.
Over the following weeks, light in various wavelengths arrived from the event, providing a detailed understanding of the aftermath of the neutron star merger. The fact that we observed this event 130 million years after it occurred showcases the immense timescales involved. Meanwhile, the gravitational waves and light traveled through space at the speed of gravity and light, respectively, until they reached Earth.
This observation provided a groundbreaking measurement of the speed of gravity, demonstrating that it is equal to the speed of light to within one part in a quadrillion. Prior to this, indirect constraints suggested that gravity and light were approximately equivalent in speed. The improvement in constraints by more than 12 orders of magnitude with a single observation represents an unprecedented leap in understanding.
Does this mean that the speed of gravity and the speed of light are not perfectly equal? Unlikely. If that were the case, different energies and wavelengths of light would travel at varying speeds, which contradicts our observations. It is more reasonable to consider alternative explanations for this observation. However, in physics, we must consider all possibilities before drawing definitive conclusions.
