After three years of improvements, the Laser Interferometry Gravitational Wave Observatory, commonly known as LIGO, is back in action. This giant of science, capable of measuring gravitational waves – subtle ripples in space itself that travel through the universe – has been sitting idle while undergoing exciting upgrades.

Unlike light waves, gravitational waves are barely hindered by the galaxies, stars, gas, and dust that fill the universe. This means that by measuring gravitational waves, astrophysicists can look straight into the heart of some of the most spectacular phenomena in the universe. By detecting more events that create gravitational waves, there will be more opportunities for astronomers to also observe the light produced by those same events. Viewing an event through multiple channels of information, an approach called multi-messenger astronomy, provides astronomers with rare and coveted opportunities to learn about physics far beyond the scope of any laboratory test.

According to Einstein’s theory of general relativity, mass and energy warp the shape of space and time. Spacetime bending determines how objects move relative to each other, which people experience as gravity. Gravitational waves are created when massive objects like black holes or neutron stars merge with each other, producing sudden and large changes in space. The process of warping and bending space sends ripples through the universe like a ripple across a still pond. These waves travel in all directions from a disturbance, bending space tiny as they go and slightly changing the distance between objects in their path.

Although astronomical events that produce gravitational waves involve some of the most massive objects in the universe, the stretching and contraction of space is infinitesimally small. A strong gravitational wave passing through the Milky Way can change the diameter of the entire galaxy by just one meter. LIGO: the most accurate observatory ever built Around the year 2000, scientists at the California Institute of Technology, the Massachusetts Institute of Technology, and other universities around the world finished building what is essentially the most accurate ruler ever created: the LIGO observatory. LIGO consists of two separate observatories, one located in Hanford, Washington, and the other in Livingston, Louisiana. Each observatory is shaped like a giant L with two four kilometer long arms that extend from the center of the facility at 90 degrees to each other.

To measure gravitational waves, the researchers shine a laser from the center of the facility to the base of the L. There, the laser is split so that a beam travels down each arm, reflects off a mirror, and returns to the base. If a gravitational wave passes through the arms while the laser is shining, the two beams will return to center at slightly different times. By measuring this difference, physicists can discern that a gravitational wave passed through the facility. LIGO started operating in the early 2000s, but it wasn’t sensitive enough to detect gravitational waves. Therefore, in 2010, the LIGO team temporarily shut down the facility for improvements to increase sensitivity. The improved version of LIGO began collecting data in 2015 and almost immediately detected gravitational waves produced by the merger of two black holes. Since 2015, LIGO has completed three observation runs. The first, the O1 race, lasted about four months; the second, O2, about nine months; and the third, O3, lasted 11 months before the COVID-19 pandemic forced the facility to close. Since the O2 race, LIGO has been observing in conjunction with an Italian observatory called Virgo.

Scientists have been working on many technological improvements. One particularly promising upgrade involved adding a 1,000-foot (300-meter) optical cavity to improve a technique called squeezing. Squeezing allows scientists to reduce detector noise by using the quantum properties of light. With this improvement, the LIGO team should be able to detect much weaker gravitational waves than before.