12 FEBRUARY 2016 ? VOL 351 ISSUE 6274 645
SCIENCE https://www.wendangku.net/doc/9f6315344.html,
I L L U S T R A T I O N : S W I N B U R N E A S T R O N O M Y P R O D U C T I O N S
By Adrian Cho
L
ong ago, deep in space, two massive black holes—the ultrastrong gravita-tional ? elds left behind by gigantic stars that collapsed to in? nitesimal points—slowly drew together. The stellar ghosts spiraled ever closer, un-til, about 1.3 billion years ago, they whirled about each other at half the speed of light and ? nally merged. The collision sent a shudder through the universe: ripples in the fabric of space and time called gravita-tional waves. Five months ago, they washed past Earth. And, for the ? rst time, physicists detected the waves, ful? lling a 4-decade quest and opening new eyes on the heavens.
The discovery marks a triumph for the 1000 physicists with the Laser Interfero-
meter Gravitational-Wave Observatory (LIGO), a pair of gigantic instruments in
Hanford, Washington, and Livingston,
Louisiana. Rumors of the detection had circulated for months. But as Science went
to press, the LIGO team planned to make it of cial on 11 February in a press confer-ence in Washington, D.C. “We did it!” says
David Reitze, a physicist and LIGO execu-tive director at the California Institute of Technology (Caltech) in Pasadena. “All the rumors swirling around out there got most of it right.”
Albert Einstein predicted the existence
of gravitational waves 100 years ago, but
directly detecting them required mind- c erent distances along c, and the crashing IN DEPTH
PHYSICS
Triumph for gravitational wave hunt
Observation made with newly detectable radiation clinches case for black holes
Published by AAAS
o n F e b r u a r y 17, 2016
D o w n l o a d e d f r o m
NEWS|IN DEPTH
646 12 FEBRUARY 2016 ? VOL 351 ISSUE https://www.wendangku.net/doc/9f6315344.html, SCIENCE I L L U S T R A T I O N : V . A L T O U N I A N / S C I E N C E
ington is the right timing for a light-speed wave zipping across both detectors.
The signal exceeds the “?ve-sigma” standard of statistical signi?cance that physicists use to claim a discovery, LIGO researchers report in a paper scheduled to be published in Physical Review Letters to coincide with the press conference. It’s so strong it can be seen in the raw data, says Gabriela González, a physicist at Louisiana State University, Baton Rouge, and spokes-person for the LIGO scienti? c collaboration. “If you ? lter the data, the signal is obvious to the eye,” she says.
Comparison with computer simulations reveals that the wave came from two ob-jects 29 and 36 times as massive as the sun spiraling to within 210 kilometers of each other before merging. Only a black hole—which is made of pure gravitational energy and gets its mass through Einstein’s famous equation E=mc2—can pack so much mass into so little space, says Bruce Allen, a LIGO member at the Max Planck Insti-tute for Gravitational Physics in Hanover, Germany. The observation provides the ? rst evidence for black holes that does not depend on watching hot gas or stars swirl around them at far greater distances. “Be-fore, you could argue in principle whether or not black holes exist,” Allen says. “Now you can’t.”
The collision produced an astounding, invisible explosion. Modeling shows that the ? nal black hole totals 62 solar masses—3 solar masses less than the sum of the ini-tial black holes. The missing mass vanished in gravitational radiation—a conversion of mass to energy that makes an atomic bomb look like a spark. “For a tenth of a second [the collision] shines brighter than all of the stars in all the galaxies,” Allen says. “But only in gravitational waves.”
For 5 months, LIGO physicists struggled to keep a lid on their pupating discovery. Ordinarily, most team members would not have known whether the signal was real. LIGO regularly salts its data readings with secret false signals called “blind injections” to test the equipment and keep research-ers on their toes. But on 14 September 2015, that blind injection system was not running. Physicists had only recently completed a 5-year, $205 million upgrade of the machines, and several systems—including the injection system—were still of ine as the team wound up a preliminary “engineering run.” As a result, the whole collaboration knew that the observation was likely real. “I was convinced that day,” González says.
Still, LIGO physicists had to rule out every alternative, including the possibility that the reading was a malicious hoax. “We spent about a month looking at the ways
that somebody could spoof a signal,” Reitze
says, before deciding it was impossible. For
González, making the checks “was a heavy
responsibility,” she says. “This was the ? rst
detection of gravitational waves, so there
was no room for a mistake.”
Proving that gravitational waves exist
may not be LIGO’s most important legacy,
as there has been compelling indirect evi-
dence for them. In 1974, U.S. astronomers
Russell Hulse and Joseph Taylor discovered
a pair of radio-emitting neutron stars called
pulsars orbiting each other. By timing the
pulsars, Taylor and colleague Joel Weisberg
demonstrated that they are very slowly spi-
raling toward each other—as they should if
they’re radiating gravitational waves.
It is the prospect of the science that
might be done with gravitational waves
that really excites physicists. For example,
says Kamionkowski, the theorist at Johns
Hopkins, the ?rst LIGO result shows the
power of such radiation to reveal unseen
astrophysical objects like the two ill-fated
black holes. “This opens a new window on
this vast population of stellar remnants that
we know are out there but of which we have
seen only a tiny fraction,” he says.
The observation also paves the way for
testing general relativity as never before,
Kamionkowski says. Until now, physicists
have studied gravity only in conditions
where the force is relatively weak. By study-
ing gravitational waves, they can now explore
extreme conditions in which the energy in an
object’s gravitational ? eld accounts for most
or all of its mass—the realm of strong gravity
so far explored by theorists alone.
With the black hole merger, general rela-
tivity has passed the ?rst such test, says
Rainer Weiss, a physicist at the Massachu-
setts Institute of Technology (MIT) in Cam-
bridge, who came up with the original idea
for LIGO. “The things you calculate from
Einstein’s theory look exactly like the sig-
nal,” he says. “To me, that’s a miracle.”
The detection of gravitational waves
marks the culmination of a decades-long
quest that began in 1972, when Weiss wrote
a paper outlining the basic design of LIGO.
In 1979, the N ational Science Foundation
funded research and development work at
both MIT and Caltech, and LIGO construc-
tion began in 1994. The $272 million instru-
ments started taking data in 2001, although
it was not until the upgrade that physicists
expected a signal.
If LIGO’s discovery merits a Nobel Prize,
who should receive it? Scientists say Weiss
is a shoo-in, but he demurs. “I don’t like to
think of it,” he says. “If it wins a Nobel Prize,
it shouldn’t be for the detection of gravita-
tional waves. Hulse and Taylor did that.”
Many researchers say other worthy recipi-
ents would include Ronald Drever, the ? rst
director of the project at Caltech who made
key contributions to LIGO’s design, and Kip
Thorne, the Caltech theorist who champi-
oned the project. Thorne also objects. “The
people who really deserve the credit are the
experimenters who pulled this of , starting
with Rai and Ron,” he says.
Meanwhile, other detections may come
quickly. LIGO researchers are still analyzing
data from their ? rst observing run with their
upgraded detectors, which ended 12 Janu-
ary, and they plan to start taking data again
in July. A team in Italy hopes to turn on its
rebuilt VIRGO detector—an interferometer
with 3-kilometer arms—later this year. Phys-
icists eagerly await the next wave.
■
Catching a wave
Input
port
Dark
port
Detection
Light bounces back
and forth in the
4-kilometer arms of a
LIGO interferometer.
When a wave makes
the arms unequal in
length, light leaks out
the interferometer's
"dark port," revealing
the wave.
Space travel
Zipping along at light speed, a wave stretches space
in one direction and squeezes in the perpendicular
direction, then reverses the distortions.
Birth
As Einstein calculated, a whirling barbell-shaped
mass, such as two black holes spiraling together,
radiates ripples in spacetime: gravitational waves.
Stretching
Squeezing
Direction
of wave
S
Laser
Mirror
Published by AAAS