The Laser Interferometer Gravitational-Wave Observatory or LIGO is a large-scale experiment to detect cosmic gravitational waves. Unlike other observatories that detect electromagnetic waves, LIGO has no lenses. LIGO is an L-shaped vacuum tube with each arm about a few kilometres long. It has a beam splitter at the junction and carries mirrors at the ends.
A laser beam that bounces off the mirror at one end splits into two parts at the beam splitter. The beam returning from the other mirror interferes with the split part of the first reflected beam. The interference pattern changes when there is a gravitational wave. The method of detection is so sensitive that it can detect a change of less than one ten-thousandth the diameter of a proton!
However, seismic activities and even drilling in nearby areas introduce noise in the results. To overcome this, scientists have implemented several technologies. Now, scientists from the LIGO collaboration have introduced a method to improve the Advanced LIGO’s robustness to earthquakes.
They made use of SEISMON, an early alert system that uses the data from the global network of seismic observatories, along with a machine learning algorithm, to predict the arrival time and the surface wave amplitude of the expected seismic events. In the case of an incoming earthquake that can potentially cause significant ground motion, it alerts the LIGO detector, which switches on the seismic configuration mode.
A few tweaks to the digital filters, appropriate changes to control loops related to the internal seismic isolation system, and the Advanced LIGO is ready to be tested against the onslaught of earthquakes.
The scientists got the opportunity to test the system during a 6.3 magnitude earthquake event on January 27, 2020. The earthquake mode was engaged, and the interferometer stayed locked with the ground.
According to scientists, seismic models predict the local platform’s behavior, and the system is now capable of optimizing the performance of the detector during an earthquake.
“The earthquake mode contributes to the stability of the interferometers in extreme environmental conditions and significantly improves the data quality,” says T R Saravanan IUCAA, Pune.
Several Indian scientists are part of the LIGO Scientific Collaboration and will incorporate the new developments in the LIGO-India project to be operational in 2024.
Classical and Quantum Gravity, 37 (23): 235007 (2020)
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