the total distance a beam of light travels). Such pertur-
bations are inconceivably tiny, far smaller than the
diameter of a single proton—meaning that each facility
must somehow account for or suppress an almost
countless assortment of contaminating noises, from
the enormous seismic motions of earthquakes and
tides to the softer vibrations caused by airplanes over-
head, passing cars, nearby wildlife or even a mirror’s
jiggling atoms. Distinguishing between legitimate
gravitational-wave signals and noise-induced “glitches”
is an almost overwhelming task—and one that has con-
tributed to numerous false alarms mixed in with the
dozens of authentic detections collaboratively
announced to date by LIGO and Virgo.
Buried deep below its mountain, KAGRA will be the
first major laser interferometer built and operated
entirely underground, far from the cacophony of back-
ground noise at the terrestrial surface. It is also the
first to use cryogenically cooled mirrors—each a pol-
ished 23-kilogram cylinder of sapphire crystal—which
can dramatically reduce thermal vibrations and deliver
corresponding boosts in sensitivity. LIGO’s and Virgo’s
mirrors are kept at room temperature; KAGRA’s will be
maintained at a frigid 20 degrees above absolute zero.
Although these two advances could in principle
allow KAGRA to find fainter sources of gravitational
waves than LIGO or Virgo, they are not without draw-
backs: Mechanical coolers keep the laser-bathed mir-
rors cold but also introduce their own vibrational noise
into measurements, and water from rain and melting
snow regularly infiltrates KAGRA’s tunnels, forcing
workers to install plastic sheets to protect delicate
equipment. Even with protection, the moisture may
halt operations during the wettest times of year.
If all goes according to plan, KAGRA will not only
help make additional major discoveries but also dem-
onstrate the new technologies likely to be used by the
next generation of more advanced gravitational-wave
observatories around the globe.SHIELDING
VIBRATIONS
A technician squats beside the uppermost sec-
tion of a 14-meter-tall vibration isolation system
for one of KAGRA’s polished sapphire mirrors.
Such systems are necessary shields against
outside noises, allowing a passing gravitational
wave’s minuscule signature—a mirror’s shift
by a fraction of a thousandth of the width of
ENRICO SACCHETTI a proton—to be detected.© 2019 Scientific American