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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

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