The Global Network of Detectors
Virgo does not operate in the same way as optical telescopes that point to a direction in the sky: it can detect GWs coming from almost any direction. To localize a source of gravitational waves a triangulation process is necessary: this means that we need at least three detectors placed at different positions on Earth. This procedure is very similar to that used by seismologists to determine where earthquakes originate.
Having several detectors in operation also helps to disentangle real gravitational signals from spurious ones, triggered by various noise fluctuations. These spurious signals are likely induced by local events occurring at the site of a detector. So, it is unlikely that similar noises get detected at the same time in detectors separated by large distances (hundreds of thousands of kilometers) on the planet’s surface.
Such a network has been set up for gravitational wave detection since 2007. Besides Virgo, there are the two 4-km long interferometers of the Laser Interferometer Gravitational Observatory (LIGO) located in the United States near Livingston, Louisiana and Hanford, Washington State. There is also a fourth interferometer belonging to this network, GEO600, which operates in Germany near Hannover and which is 0.6 km in arm length. The scientists working at these four detectors collaborate and have been sharing their knowledge about the different technologies. Furthermore, this scientific collaboration has been extended so that data from the detectors are shared and analyzed in a common framework since 2007.Since 2007 Virgo, LIGO and GEO600 have formed the first worldwide network of interferometers in history and have observed the sky, looking for gravitational waves. This collaboration continues in the era of the Advanced Virgo and Advanced LIGO where scientific data are shared and analyzed jointly. In 2019, the Japanese Kamioka Gravitational Wave Detector (KAGRA) has joined the network, which is named the LVK Collaboration; KAGRA will participate in the observing period O4.