MATH & TECHNOLOGY
New imaging system works
like a periscope mirror
ATOM & COSMOS
Japan will join gravitational wave hunt
Underground detector will look for cosmic ripples in a new way
BY MARIA TEMMING
With a new computer program, a photographer can take a picture of something
that’s not even in frame.
The system analyzes light reflected off
matte surfaces, such as walls, to discern
out-of-sight images, similar to the way a
periscope mirror reveals what’s around
a corner. Whereas other techniques for
spotting out-of-sight objects require
expensive, specialist optical equipment
(SN: 1/9/16, p. 15), the new program can
render a rough, full-color reconstruction
of a hidden scene using a single snapshot
from an ordinary digital camera.
“Looking way beyond what the camera can see could be very, very useful” for
BY EMILY CONOVER
In the quest for better gravitational wave
detectors, scientists are going cold.
An up-and-coming detector called
KAGRA aims to spot spacetime ripples
by harnessing advanced technological
twists: chilling key components to temperatures hovering just above absolute
zero and placing the ultrasensitive setup
in an enormous underground cavern.
Scientists with KAGRA, located in
Kamioka, Japan, now have results from
their first ultrafrigid tests. Those experiments suggest that the detector should
be operational later this year, the team
reports January 14 at arXiv.org.
KAGRA will join similar observato-
ries in the search for gravitational waves,
minute undulations stirred up by violent
events like the collisions of black holes.
The Advanced Laser Interferometer
Gravitational-Wave Observatory, LIGO,
Sankaranarayanan, a computational
photography researcher at Carnegie
Mellon University in Pittsburgh who was
not involved in the work. This technol-
ogy, described in the Jan. 24 Nature, may
also help police monitor buildings from
the outside during hostage situations
or first responders scout out collapsed
buildings after disasters.
Electrical engineer Vivek Goyal and
colleagues at Boston University tested
has two detectors located in Hanford,
Wash., and Livingston, La. Another observatory, Virgo, is located near Pisa, Italy.
Those detectors sit above ground and
don’t use the cooling technique, making
KAGRA the first of its kind.
KAGRA consists of two 3-kilometer-
long arms arranged in an “L” shape.
Within each arm, laser light bounces
back and forth between two mirrors
located at both ends. The light acts like a
measuring stick, capturing tiny changes
in the length of each arm, which can be
caused by a passing gravitational wave
stretching and squeezing spacetime.
Because gravitational wave detectors
measure length changes tinier than a pro-
ton’s diameter, minuscule effects like the
jiggling of molecules on the mirrors’ sur-
faces can interfere with measurements.
Cooling the mirrors to about 20 kelvins
(–253° Celsius) limits that jiggling.
the system by displaying images on an
LCD monitor facing a wall. In between
the two, the team placed an object that
blocked some of the light emanating from
the monitor, casting shadows on the wall.
(The team used a rectangular panel, but
an object of any size or shape would do.)
Both the light that reaches the wall and
the shadows created by the intervening
object contain clues about the LCD display. To picture why, imagine a screen
displaying two white circles side by side
against a black backdrop. Someone looking at the smear of light that these circles
In the new tests, performed in spring
2018, researchers cooled only one of
the four mirrors in KAGRA’s arms, says
KAGRA leader Takaaki Kajita of the
University of Tokyo. When the detector starts up for real, the others will be
Having the detector underground also
helps keep the mirrors from vibrating
due to activity on Earth’s surface. LIGO
is so sensitive that it can be affected by
rumbling trucks, a stiff breeze or even
wildlife (SN Online: 4/18/18).
Building underground and going cold
required years of effort from KAGRA’s
researchers. “They’ve taken on these
two great challenges, which are both
important to the long-term future of the
field,” says LIGO spokesperson David
Shoemaker of MIT. In the future, even
more advanced gravitational wave detectors could build on KAGRA’s techniques.
For now, adding KAGRA to the existing observatories should help scientists
improve studies of where gravitational
wiggles come from. Once scientists
detect a gravitational wave signal, they
By analyzing a multicolored jumble of light and shadow on a wall (middle), a computer program can
create a rough reconstruction (right) of an image displayed on an out-of-frame LCD screen (left).