Some events in the universe are so cataclysmic the ripples in space-time can spread across billion so flight years. The existence of these “gravitational waves” was the last major prediction in Einstein’s general relativity, and scientists confirmed it in 2016 thanks to the Laser Interferometer Gravitational-Wave Observatory (LIGO) project. After a series of instrument upgrades, LIGO is coming back online April 1 to search for fainter gravitational waves around the universe.
When LIGO began operations, it consisted of two facilities, one in Washington state and the other in Louisiana. The European Gravitational Observatory added a third station in Italy in 2017, which is known as the Virgo detector. During the downtime, the US-based facilities got new hardware that should increase sensitivity by about 40 percent. The Italian facility is now twice as powerful as it was last year.
LIGO uses a technique called laser interferometry to detect gravitational waves. It bounces lasers off reflectors at the end of two 4-kilometer tubes in an L shape. Detectors monitor the laser’s return for evidence of movement from gravitational waves. If there’s no perturbation in the mirrors, the light returns unchanged, and the beams cancel each other out. If a gravitational wave causes even tiny changes in the system, the waves won’t cancel out.
New detectors in LIGO can reduce quantum-scale uncertainty in photons through a process known as “squeezing.” By shifting uncertainty in the photons, scientists can sacrifice the certainty of amplitude to get more accuracy on timing, and timing is the important aspect when looking for gravitational waves. The LIGO team swapped five of the eight mirrors for better performing versions, too.
In 2016, scientists working at LIGO confirmed the first detection of gravitational waves from the collision of two black holes 1.3 billion light years away. That discovery won Rainer Weiss, Kip Thorne, and Barry Barish the Nobel Prize in physics the following year. Since then, LIGO has spotted nine more gravitational wave sources from black hole mergers and one caused by colliding neutron stars.
When LIGO-Virgo stars running again next month, scientists expect they’ll be able to detect neutron star collisions about 550 million light years away, which is 190 million light years more distant than before. We may even finally spot a merger of a neutron star with a black hole.