The physics experiments carried out at the LIGO are strengthening our knowledge of not only gravitational waves but also relativity, astrophysics, cosmology, particle physics, and nuclear physics.
At the Hanford site, USA, Laser Interferometer Gravitational-Wave Observatory (LIGO) is built to carry out Physics experiment. The main objective of this project is the detection of cosmic gravitational waves. The project was granted in the year of 1990. The first installation (initial LIGO: iLIGO) was completed by 2002. This observatory was operated till the year of 2010. Advanced LIGO (aLIGO) was installed and in use since the year of 2015.
LIGO is a ‘L’ shaped observatory. It has 4 km vacuum steel tubes perpendicular to each other. These tubes are isolated from the world. A beam is split into two out of phase, i.e. 90 degrees apart, beams. These two beams travel through the tubes. Then these beams are reflected back from mirrors present at the end of these tubes.
If the beams cover the same distance, these two similar but out of phase waves cancel each other when they meet again at the splitter. Thus, photodetector present at the back of the beam splitter does not detect any light. If the distance travelled by both beams differs due to disturbance in the space-time, light is not cancelled completely, due to the difference in the phases at the intersection point. The escaped light is detected by the photodetector. This is the basic working principle of the observatory like 'interferometer'.
How gravitational waves are detected?
Gravitational waves disrupt the space as they travel in space-time. This causes the space to stretch in the direction of propagation of the wave, and thus compacting it in the perpendicular direction. Now, when beam travels in the observatory, due to gravitational waves, space is disturbed in both tubes. As paths are perpendicular to each other, the distance covered by both beams is not equal. Thus, the photodetector senses the light escaped after the interference of reflected beams that are not cancelled completely.
LIGO first detected the gravitational waves in September 2015. Recently, on April 25, 2019, the LIGO and the VIRGO: an interferometer situated in Italy, detected the neutron star smash up. The LIGO and the VIRGO interferometers are teamed up to share, analyse and publish their data jointly.
The physics experiments carried out at the LIGO are strengthening our knowledge of not only gravitational waves but also relativity, astrophysics, cosmology, particle physics, and nuclear physics. The observatory can precisely identify the change of less than a ten-thousandth the diameter of a proton. It is the most complex and sophisticated engineering creation we have witnessed till date!