One of the most remarkable experiments in history – a pair of giant machines that listen for ripples in spacetime called gravitational waves – will wake up from a half-year nap on Monday. And it will be about 40% stronger than before.
That experiment is called the Laser Interferometer Gravitational-Wave Observatory (LIGO); it consists of two giant, L-shaped detectors that together solved and a 100-year-old mystery posed by Albert Einstein.
In 1915, Einstein predicted the existence of ripples in the fabric of space. However, he did not think these gravitational waves would ever be detected – they seemed too weak to pick up all the noise and vibrations on Earth. For 1
Even as hundreds of scientists worked on LIGO from 2002 to 2015, they failed to "hear" any waves. This was despite predictions that collisions of two black holes should make gravitational waves at detectable levels.
But that 13-year slump ended in September 2015, when an upgraded "advanced" LIGO detected its first gravitational waves: signals from the merger of two black holes some 1.3 billion light-years away. The following December, the team detected a second collision event. By 2017, three researchers who helped conceive of LIGO earned a Nobel Prize in Physics.
Science has not been the same since. The global research team affiliated with LIGO has today made 11 detections of massive collisions in deep space. Gravitational-wave astronomy is still in its infancy, though, and the teams behind each observatory are constantly scheming to improve the sensitivities of their machines.
In fact, LIGO is about to wake up from its second major slumber, following a series of hardware upgrades. The scientific collaboration expects the devices to be 40% more sensitive than in the previous run, which ran from November 2016 to August 2017.
"We will certainly detect many more gravitational waves from the types of sources we have seen so far , "Peter Fritschel, LIGO's chief detector at MIT, said in a press release. "We're eager to see new events too, like a merger of a black hole and a neutron star."
This third run of the observatory is expected to last a full year.
Here's how LIGO works, according to an animation created by researchers behind the experiment, and how recent improvements made it even more sensitive.
How LIGO detects gravitational waves
The two L-shaped detectors – each with 2.5-mile-long arms – are separated by nearly 1,900 miles. One is at the Hanford Site in Washington (Cold War) and the other is in Livingston, Louisiana.
Together, the detectors hunted for gravity waves for years without any luck, until a new and improved "advanced" and upgraded LIGO came online in 2015.
One of the notable collisions it has observed since then, unceremoniously dubbed "GW170817" and announced in October 2017, came from two neutron stars smashing together. Astronomers who saw the signal alerted the telescopes around the globe to zero in on the event. The resulting "multi-messenger" observations suggested that the cataclysm spewed unfathomable amounts of silver, gold, platinum, and other freshly created elements on the Periodic Table into space.
Read more : Astronomers detected 100 Earths' worth of gold being forged in space
To make their observations, each LIGO detector shoots out a laser beam and split it in two. One beam is sent down and a 2.5-mile long tube, the other down an identical yet perpendicular tube.
The beams bounce off the mirrors and converge back near the beam splitter.
The light waves return at the same length and line up in such a way that they cancel each other out.
As a result, the light detector part of the instrument does not see any light.
But when a gravitational wave comes through, it warps spacetime – making one tube longer and the other shorter. This rhythmic stretching-and-squeezing distortion continues until the wave passes.
When this kind of interference occurs, the two waves of light are not equal when they return, so they do not line up and neutralize each other. That means the detector would record some flashes of light.
A physicist measuring these changes in brightness would thus be measuring and observing gravitational waves.
This setup is extraordinarily sensitive. When a wave passes, the arm's length changes by less than 1 / 10,000th of the width of a subatomic proton particle, according to LIGO.
This also means that a detector can be disturbed by the vibration of trucks driving on nearby roads or even a slight breeze.
That's why there are two LIGO instruments: If they detect a signal occurring at exactly the same time, it's likely that a huge gravitational wave is passing through and through Earth.
The events that cause these ripples in space must be unimaginably powerful. So far, LIGO has confirmed that it has been merging black holes. When two black holes go, the collision can instantly convert several suns' worth of mass into pure gravitational wave energy, which is why we can detect them on Earth from more than a billion miles away. The Caltech / MIT / LIGO: a gravitational-wave observatory The Strobner laser beams, better mirrors, and 'squeezed' light
Still, such events are relatively rare and their signatures are extremely weak.
LIGO's last upgrade took 10 months in 2016 and boosted its sensitivity by about 25%. The newest upgrade took six months, ends on April 1, and added a 40% increase in sensitivity over the last upgrade to LIGO.
That increased sensitivity should help scientists pinpoint the locations of neutron star collisions, for example, up to 550 million light-years away, or about 190 million light-years farther away than it could before.
That jump comes from doubling the power of each LIGO facility's lasers. Each machine also has five of eight upgraded mirrors, as well as new hardware to catch and reduce stray light. It can also now "squeeze" photons of light using clear principles of quantum physics.
"We had to break the fibers holding the mirrors and very carefully take out the optics and replace them," Calum Torrie, head of LIGO's mechanical-optical engineering department at Caltech, said in a press release. "It was an enormous engineering enterprise."
Vicky Kalogera, an astrophysicist at Northwestern University and LIGO, told Business Insider that the experiment could ultimately detect 100 collisions per year – that is, with the help of a third detector called Virgo, and a new facility called KAGRA being built in Japan, and other gravitational wave detectors.
"This has opened a new window to what we can detect in the universe," Imre Bartos, a physicist at Columbia University and LIGO, previously told Business Insider. "We can detect this, we can now see gravitational waves. But the real exciting things are what we find with these gravitational waves."
This is an updated version of a story published on November 30, 2016.