An experiment that solved a 100-year-old mystery posed by Einstein is about to turn back on - and it's more powerful than ever
- Colliding black holes and neutron stars create ripples in spacetime, called gravitational waves. These were "heard" for the first time in September 2015.
- On Monday, a pair of gravitational-wave detectors called LIGO will turn back on after 6 months of downtime and upgrades.
- To boost its power, the experiment will now work with a sister machine in Italy called Virgo.
- Physicists expect the next period of searching for colliding black holes to last a year and be 40% more sensitive than before.
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 a 100-year-old mystery posed by Albert Einstein.
In 1915, Einstein predicted the existence of ripples in the fabric of space However, he didn't think these gravitational waves would ever be detected - they seemed too weak to pick up amid all the noise and vibrations on Earth. For 100 years, it seemed Einstein was right.
Even as hundreds of scientists worked on LIGO from 2002 into 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 hasn't 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 lasted from November 2016 through August 2017.
"We will surely detect many more gravitational waves from the types of sources we've seen so far," Peter Fritschel, LIGO's chief detector scientist at MIT, said in a press release. "We're eager to see new events too, such as 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
LIGO is actually two different yet nearly identical instruments that work together.
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 (where Cold War-era nuclear weapons production happened) and the other is in Livingston, Louisiana.
Together, the detectors hunted for gravitational 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 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.
To make their observations, each LIGO detector shoots out a laser beam and splits it in two. One beam is sent down a 2.5-mile long tube, the other down an identical yet perpendicular tube.
The beams bounce off mirrors and converge back near the beam splitter.
The light waves return at equal length, and line up in such a way that they cancel each other out.
As a result, the light detector part of the instrument doesn't 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 happens, the two waves of light aren't equal lengths when they return, so they don't line up and neutralize each other. That means the detector would record some flashes of light.
A physicist measuring those changes in brightness would thus be measuring and observing gravitational waves.
This setup is extraordinarily sensitive. When a wave passes by, the arm's length changes by less than 1/10,000th of the width of a subatomic proton particle, according to LIGO.
That also means 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 by and through Earth.
The events that cause these ripples in space must be unimaginably powerful. So far, LIGO has confirmed it detected merging black holes. When two black holes merge, 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.
Strogner laser beams, better mirrors, and 'squeezed' light
Still, such events are relatively rare and their signatures are extraordinarily 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 40% increase in sensitivity on top of 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 got five of eight mirrors upgraded, as well as new hardware to catch and reduce stray light. It can also now "squeeze" photons of light (to clear up noisy data) using 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, the head of LIGO's mechanical-optical engineering department at Caltech, said in a press release. "It was an enormous engineering undertaking."
Vicky Kalogera, an astrophysicist at Northwestern University and LIGO, previously told Business Insider that the experiment could ultimately detect 100 collisions per year - that is, with the help of a third detector called Virgo, 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 discover with these gravitational waves."
This is an updated version of a story published on November 30, 2016.
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