Dr.
Thorne of Caltech and Dr. Weiss of M.I.T. first met in 1975, Dr. Weiss
said, when they had to share a hotel room during a meeting in
Washington. Dr. Thorne was already a renowned black-hole theorist, but
he was looking for new experimental territory to conquer. They stayed up
all night talking about how to test general relativity and debating how
best to search for gravitational waves.
Dr.
Thorne then recruited Dr. Drever, a gifted experimentalist from the
University of Glasgow, to start a gravitational wave program at Caltech.
Dr. Drever wanted to use light — laser beams bouncing between precisely
positioned mirrors — to detect the squeeze and stretch of a passing
wave.
Dr.
Weiss tried to mount a similar effort at M.I.T., also using the laser
approach, but at the time, black holes were not in fashion there.
(Things are better now, he said.)
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The
technological odds were against both efforts. The researchers
calculated that a typical gravitational wave from out in space would
change the distance between a pair of mirrors by an almost imperceptible
amount: one part in a billion trillion. Dr. Weiss recalled that when he
explained the experiment to his potential funders at the National
Science Foundation, “everybody thought we were out of our minds.”
In
1984, to the annoyance of Dr. Drever and the relief of Dr. Weiss, the
National Science Foundation ordered the two teams to merge. Dr. Thorne
found himself in the dual roles of evangelist for the field of
gravitational waves and broker for experimental disagreements.
Progress was slow until the three physicists were replaced in 1987 by a single director as part of the price of going forward.
The
first version of the experiment, known as Initial LIGO, started in 2000
and ran for 10 years, mostly to show that it could work on the scale
needed. There are two detectors: one in Hanford, Wash., the other in
Livingston, La. Hunters once shot up the outside of one of the antenna
arms in Louisiana, and a truck crashed into one of the arms in Hanford.
In neither case was the experiment damaged.
Over
the last five years, the entire system was rebuilt to increase its
sensitivity to the point where the team could realistically expect to
hear something.
LIGO’s
antennas are L-shaped, with perpendicular arms 2.5 miles long. Inside
each arm, cocooned in layers of steel and concrete, runs the world’s
largest bottle of nothing, a vacuum chamber a couple of feet wide
containing 2.5 million gallons of empty space. At the end of each arm
are mirrors hanging by glass threads, isolated from the bumps and
shrieks of the environment better than any Rolls-Royce ever conceived.
Thus
coddled, the lasers in the present incarnation, known as Advanced LIGO,
can detect changes in the length of one of those arms as small as one
ten-thousandth the diameter of a proton — a subatomic particle too small
to be seen by even the most powerful microscopes — as a gravitational
wave sweeps through.
An Earthling’s Guide to Black Holes
Welcome to the place of no return — a region in space
where the gravitational pull is so strong that not even light can escape
it. This is a black hole.
Even
with such extreme sensitivity, only the most massive and violent events
out there would be loud enough to make the detectors ring. LIGO was
designed to catch collisions of neutron stars, which can produce the
violent flashes known as gamma ray bursts.
As
they got closer together, these neutron stars would swing around faster
and faster, hundreds of times a second, vibrating space-time geometry
with a rising tone that would be audible in LIGO’s vacuum-tube “sweet
spot.”
Black
holes, the even-more-extreme remains of dead stars, could be expected
to do the same, but nobody knew if they existed in pairs or how often
they might collide. If they did, however, the waves from the collision
would be far louder and lower pitched than those from neutron stars.
Dr.
Thorne and others long thought these would be the first waves to be
heard by LIGO. But even he did not expect it would happen so quickly.
‘It Was Waving Hello’
On
Sept. 14, the system had barely finished being calibrated and was in
what is called an engineering run at 4 a.m. when a loud signal came
through at the Livingston site. “Data was streaming, and then ‘bam,’ ”
recalled David Reitze, a Caltech professor who is the director of the
LIGO Laboratory, the group that built and runs the detectors.
Seven
milliseconds later, the signal hit the Hanford site. LIGO scientists
later determined that the likelihood of such signals landing
simultaneously by pure chance was vanishingly small. Nobody was awake,
but computers tagged the event.
Graphic
What Is General Relativity?
Einstein presented his general theory of relativity 100 years ago this month.
Dr.
Reitze was on a plane to Louisiana the next day. Dr. Weiss, on vacation
in Maine, found out when he checked in by computer that morning. “It
was waving hello,” he said. “It was amazing. The signal was so big, I
didn’t believe it.”
The
frequency of the chirp was too low for neutron stars, the physicists
knew. Detailed analysis of its form told a tale of Brobdingnagian
activities in a far corner of the universe: the last waltz of a pair of
black holes shockingly larger than astrophysicists had been expecting.
One
of them was 36 times as massive as the sun, the other 29. As they
approached the end, at half the speed of light, they were circling each
other 250 times a second.
And
then the ringing stopped as the two holes coalesced into a single black
hole, a trapdoor in space with the equivalent mass of 62 suns. All in a
fifth of a second, Earth time.
Dr.
Weiss said you could reproduce the chirp by running your fingernails
across the keys of a piano from the low end to middle C.
Lost
in the transformation was three solar masses’ worth of energy,
vaporized into gravitational waves in an unseen and barely felt
apocalypse. As visible light, that energy would be equivalent to a
billion trillion suns.
And yet it moved the LIGO mirrors only four one-thousandths of the diameter of a proton.
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