Gravitational Waves: An Historical Perspective

On September 14, 2015, the Earth was hit by a very brief and extremely weak signal, which was the only trace of a catastrophic cosmic event that took place 1.3 billion light years from our planet. That tiny signal recounted the last whirling moments of the life of a binary system of black holes, before the two bodies—of masses 30 times greater than the mass of the Sun—merged into each other at speeds comparable to the speed of light. Captured by two special experimental devices—the interferometric detectors LIGO in the United States—the radiation of September 14 represents the first gravitational signal ever observed by man and the first confirmation that binary systems of black holes exist. Einstein had predicted the existence of gravitational waves as early as 1916. Nevertheless, their reality as physical entities—and not just mathematical solutions of Einstein’s field equations—were still the subject of theoretical discussions in the 1950s, when the first ideas of how to detect them started to develop. Since the first detection in September 2015, an extraordinary new field of cosmic investigation is born: gravitational wave astronomy.