A giant experiment that cracked a 100-year mystery of physics just came back online — and it’s more powerful than ever

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ligo nsf
The
LIGO instrument in Hanford, Washington.


LIGO
Laboratory/NSF



In February 2016, physicists declared the century-long search for
gravitational waves
was over
.

Einstein predicted the existence of such ripples in the fabric of
spacetime in 1915, but he doubted their weak signatures could be
detected.

But more than 1,000 researchers used the Laser Interferometer
Gravitational-Wave Observatory (LIGO) — two giant, L-shaped
detectors — to record such waves in September 2015 emanating from
the cataclysmic merger of two black holes. They also found
another signature in
December 2015
.

However, LIGO was shut down in January 2016 and has been offline
ever since — until Wednesday morning at 11 a.m. ET, according to

a Caltech University press release
.

“LIGO’s scientific and operational staff have been working hard
for the past year and are enthusiastic to restart round-the-clock
observations,” Joe
Giaime
, an astrophysicist at Louisiana State University and
LIGO member, said in the release.

The reason for the shutdown? Technicians were upgrading LIGO over
10 months to make it even more sensitive to gravitational waves,
pushing open the doors to
a bizarre and powerful new form of astronomy
.

“We may not immediately publish [a study], but there’s a good
chance we’re going to see more black hole collisions this year,”
Imre Bartos, a
physicist at Columbia University and LIGO, told Business Insider
in September while the upgrade was underway.

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 currently a combination of two different yet nearly
identical instruments that work together. (More could be added
later to its network.)

The two L-shaped detectors — each with 2.5-mile-long arms — are
separated by more than 2,200 miles. One is at the Hanford Site in
Washington (where Cold War-era nuclear weapons production went
down) and the other is in Livingston, Virginia.

Together, the detectors hunted for gravitational waves from 2002
to 2010 without any luck, until a new-and-improved “advanced” and
upgraded LIGO came online in 2015.

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 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. It can be disturbed by
the vibration of trucks driving on nearby roads, or even a slight
breeze.

Which is why there are two LIGO instruments: If they detect a
signal occurring at exactly the same time, it’s incredibly likely
that a huge gravitational wave is passing by and through Earth.

The events that cause these ripples in space must be unimaginably
powerful; the two confirmed events detected by LIGO so far are
both thought to be merging black holes.

Such collisions 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.

Pushing for more sensitivity

Still, such events are relatively rare and their signatures are
extraordinarily weak. 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
.

To upgrade LIGO’s sensitivity, researchers looked at their years
of operation and made “improvements to lasers, electronics, and
optics,” according to the release.

The 10-month upgrade increased the frequency range at which the
Livingston-based LIGO detector can
“hear” gravitational wave signatures
, plus it reduced some of
the laser-light scattering (which can interfere with
measurements). It’s said to be 25% more sensitive now.

Meanwhile, workers gave a boost to the power of the Hanford LIGO
detector’s laser-interferometer. They also upgraded its
vibration-reducing equipment

Together, the more-sensitive detectors should be capable of more
frequently detecting black hole-merger events, plus those farther
away than the current limit of 1.4 billions light-years.

Vicky
Kalogera
, an astrophysicist at Northwestern University and
LIGO, previously told Business Insider that the experiment could
detect 10 or more new gravitational waves after the upgrades —
and possibly up to 100 a year later on, with the help of another
experiment called
Advanced Virgo
.

“This has opened a new window to what we can detect in the
universe,” Bartos said in September. “We can detect this, we can
now see gravitational waves. But the real exciting things are
what we discover with these gravitational waves.”

Kelly Dickerson and Sarah Kramer contributed to this
post.

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