Gravitational-Wave Detector LIGO Is Back

After three years of upgrades, the gravitational-wave detector known as LIGO, or Laser Interferometer Gravitational-Wave Observatory, has resumed searching for colliding black holes and other cosmic cataclysms. 'The improvements should allow the facility to pick up signals from colliding black holes every two to three days, compared with once a week or so during its previous run in 2019-20,' reports Nature. From the report: The Virgo detector near Pisa, Italy, which has undergone its own $9-million upgrade, was meant to join in, but technical issues are forcing its team to extend its shutdown and perform further maintenance. 'Our expectation is we'll be able to restart by the end of summer or early autumn,' says Virgo spokesperson Gianluca Gemme, a physicist at Italy's National Institute for Nuclear Physics in Genoa.

KAGRA, a gravitational-wave detector located under Mount Ikenoyama, Japan, is also restarting on 24 May. Its technology, although more advanced -- it was inaugurated in 2020 -- is being fine-tuned, and its sensitivity is still lower than LIGO's was in 2015. Principal investigator Takaaki Kajita, a Nobel Prize-winning physicist at the University of Tokyo, says that KAGRA will join LIGO's run for a month and then shut down again for another period of commissioning. At that point, the team will cool the interferometer's four main mirrors to 20 kelvin, Kajita says -- a feature that sets KAGRA apart from the other detectors that will serve as the model for next-generation observatories.

In upgrades carried out before the 2019-20 run, LIGO and Virgo tackled some of this noise with a technique called light squeezing. This approach deals with inherent noise caused by the fact that light is made of individual particles: when the beams arrive at the sensor, each individual photon can arrive slightly too early or too late, which means that the laser waves don't overlap and cancel out perfectly even in the absence of gravitational waves. 'It's like dropping a bucket of BBs [lead pellets]: it's going to make a loud hiss, but they all hit randomly,' physicist Lee McCuller explained while showing a prototype of the LIGO interferometers at the Massachusetts Institute of Technology (MIT) in Cambridge. Light squeezing injects an auxiliary laser beam into the interferometer that reduces that effect. 'Its photons arrive more regularly, with less noise,' said McCuller, who is now at the California Institute of Technology in Pasadena.

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