Feb 10, 2016 · Scientists say they heard the faint chirp of two black holes colliding a billion light-years away, fulfilling Einstein’s general theory of relativity.
About a hundred years ago, Einstein predicted the existence of gravitational waves, but until now, they were undetectable.
By DENNIS OVERBYE, JONATHAN CORUM and JASON
DRAKEFORD on
Publish Date February 11, 2016.
Photo by Artist's rendering/Simulating eXtreme Spacetimes.
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A
team of physicists who can now count themselves as astronomers
announced on Thursday that they had heard and recorded the sound of two
black holes colliding a billion light-years away, a fleeting chirp that
fulfilled the last prophecy of Einstein’s general theory of relativity.
That
faint rising tone, physicists say, is the first direct evidence of
gravitational waves, the ripples in the fabric of space-time that
Einstein predicted a century ago. And it is a ringing (pun intended)
confirmation of the nature of black holes, the bottomless gravitational
pits from which not even light can escape, which were the most
foreboding (and unwelcome) part of his theory.
More
generally, it means that scientists have finally tapped into the
deepest register of physical reality, where the weirdest and wildest
implications of Einstein’s universe become manifest.
Conveyed
by these gravitational waves, an energy 50 times greater than that of
all the stars in the universe put together vibrated a pair of L-shaped
antennas in Washington State and Louisiana known as LIGO on Sept. 14.
Audio
What Two Black Holes Colliding Sounds Like 0:12
Play
These chirps are gravitational waves
converted to audible sounds. The faint thump matches the gravitational
waves’ frequencies. The louder chirp is a higher frequency better suited
to human ears. LIGO
“We
are all over the moon and back,” said Gabriela González of Louisiana
State University, a spokeswoman for the LIGO Scientific Collaboration,
short for Laser Interferometer Gravitational-Wave Observatory. “Einstein
would be very happy, I think.”
Members
of the LIGO group, a worldwide team of scientists, along with
scientists from a European team known as the Virgo Collaboration,
published a report in Physical Review Letters on Thursday with more than
1,000 authors.
“I
think this will be one of the major breakthroughs in physics for a long
time,” said Szabolcs Marka, a Columbia University professor who is one
of the LIGO scientists.
“Everything
else in astronomy is like the eye,” he said, referring to the panoply
of telescopes that have given stargazers access to more and more of the
electromagnetic spectrum and the ability to peer deeper and deeper into
space and time. “Finally, astronomy grew ears. We never had ears
before.”
Long-Awaited Triumph
The
discovery is a great triumph for three physicists — Kip Thorne of the
California Institute of Technology, Rainer Weiss of the Massachusetts
Institute of Technology and Ronald Drever, formerly of Caltech and now
retired in Scotland — who bet their careers on the dream of measuring
the most ineffable of Einstein’s notions.
“Until
now, we scientists have only seen warped space-time when it’s calm,”
Dr. Thorne said in an email. “It’s as though we had only seen the
ocean’s surface on a calm day but had never seen it roiled in a storm,
with crashing waves.”
Photo
Important players in the LIGO
project, from left to right: Kip Thorne of the California Institute of
Technology, France A. Córdova of the National Science Foundation, Rainer
Weiss of the Massachusetts Institute of Technology, David Reitze of
Caltech and Gabriela González of Louisiana State University.Credit
Lexey Swall for The New York Times
The
black holes that LIGO observed created a storm “in which the flow of
time speeded, then slowed, then speeded,” he said. “A storm with space
bending this way, then that.”
The chirp is also sweet vindication for the National Science Foundation,
which spent about $1.1 billion over more than 40 years to build a new
hotline to nature, facing down criticism that sources of gravitational
waves were not plentiful or loud enough to justify the cost.
“It’s
been decades, through a lot of different technological innovations,”
France Córdova, the foundation’s director, said in an interview,
recalling how, in the early years, the foundation’s advisory board had
“really scratched their heads on this one.”
Word
of LIGO’s success was met by hosannas in the scientific community,
albeit with the requisite admonishments of the need for confirmation or
replication.
“I
was freaking out,” said Janna Levin, a theorist at Barnard College at
Columbia who was not part of LIGO but was granted an early look at the
results for her warts-and-all book about the project, “Black Hole
Blues,” to be published this spring.
Robert Garisto, the editor of Physical Review Letters, said he had gotten goose bumps while reading the LIGO paper.
Elusive Disturbances
When
Einstein announced his theory in 1915, he rewrote the rules for space
and time that had prevailed for more than 200 years, since the time of
Newton, stipulating a static and fixed framework for the universe.
Instead, Einstein said, matter and energy distort the geometry of the
universe in the way a heavy sleeper causes a mattress to sag, producing
the effect we call gravity.
Photo
A pair of L-shaped antennas,
known as LIGO, in Hanford, Wash., left, and Livingston, La., detected
the gravitational waves on Sept. 14.Credit
Caltech-M.I.T.-LIGO Lab
A
disturbance in the cosmos could cause space-time to stretch, collapse
and even jiggle, like a mattress shaking when that sleeper rolls over,
producing ripples of gravity: gravitational waves.
Einstein
was not quite sure about these waves. In 1916, he told Karl
Schwarzschild, the discoverer of black holes, that gravitational waves
did not exist, then said they did. In 1936, he and his assistant Nathan
Rosen set out to publish a paper debunking the idea before doing the
same flip-flop again.
According
to the equations physicists have settled on, gravitational waves would
compress space in one direction and stretch it in another as they
traveled outward.
In 1969, Joseph Weber,
a physicist at the University of Maryland, made headlines when he
claimed to have detected gravitational waves using a six-foot-long
aluminum cylinder as an antenna. Waves of the right frequency would make
the cylinder ring like a tuning fork, he said.
Others
could not duplicate his result, but few doubted that gravitational
waves were real. Dr. Weber’s experiment inspired a generation of
scientists to look harder for Einsteinian marks on the universe.
In
1978, the radio astronomers Joseph H. Taylor Jr. and Russell A. Hulse,
then at the University of Massachusetts Amherst, discovered a pair of
neutron stars, superdense remnants of dead stars, orbiting each other.
One of them was a pulsar, emitting a periodic beam of electromagnetic
radiation. By timing its pulses, the astronomers determined that the
stars were losing energy and falling closer together at precisely the
rate that would be expected if they were radiating gravitational waves.
Another group of astronomers who go by the name Bicep made headlines in 2014
when they claimed to have detected gravitational waves from the
beginning of the Big Bang, using a telescope at the South Pole. They
later acknowledged that their observations had probably been contaminated by interstellar stardust.
Photo
An engineer upgrading a
component of the LIGO system. The team had barely finished calibrating
its equipment when a loud signal was first detected.Credit
Caltech-M.I.T.-LIGO Lab
A Quixotic Project
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.)
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.
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.
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.
Predicted by Einstein’s general theory of relativity 100
years ago, gravitational waves have been directly detected for the first
time. LIGO, the Laser Interferometer Gravitational-Wave Observatory,
heard black holes colliding.
LIGO Hanford
Each arm is
2.5 miles long
LIGO Livingston
TWO BLACK HOLES
About 1.2 billion years ago in a distant
galaxy, a pair of black holes circled each other. The larger black hole
was 36 times the mass of our sun, and the smaller one 29 times.
COLLISION
The intense gravity accelerated the black
holes to half the speed of light, pulling them closer and carving
distortions in space and time. In a fraction of a second, the pair
collided and merged into an irregular shape.
RING DOWN
The unstable blob smoothed into a sphere, a
process called ring down. Three solar masses worth of energy were
vaporized in a storm of gravitational waves, distorting space and time
and leaving a new black hole 62 times the mass of the sun.
GRAVITATIONAL WAVES
The invisible waves rippled outward at the
speed of light. But waves fade with distance, and when they finally
reached Earth, the distortions were too small to be measured above the
heat, noise and other vibrations of our planet.
DETECTION
LIGO is a pair of L-shaped observatories
1,900 miles apart. Ultra-pure mirrors at the ends of each arm are
isolated from vibrations. Passing gravitational waves push and pull the
arms, changing the length of tunnels by less than the width of a proton.
A CHIRP
On Sept. 14, LIGO’s detectors measured their
first vibrations from a gravitational wave. Translated to sound, it was a
short chirp, the billion-year-old echo of the collision of those two
black holes.
The
signal conformed precisely to the predictions of general relativity for
black holes as calculated in computer simulations, Dr. Reitze said.
Shortly
after the September event, LIGO recorded another, weaker signal that
was probably also from black holes, the team said. According to Dr.
Weiss, there were at least four detections during the first LIGO
observing run, which ended in January. The second run will begin this
summer. In the fall, another detector, Advanced Virgo, operated by the
European Gravitational Observatory in Italy, will start up. There are
hopes for more in the future, in India and Japan.
Looking Forward
Astronomers
now know that pairs of black holes do exist in the universe, and they
are rushing to explain how they got so big. According to Vicky Kalogera
of Northwestern University, there are two contenders right now: Earlier
in the universe, stars lacking elements heavier than helium could have
grown to galumphing sizes and then collapsed straight into black holes
without the fireworks of a supernova explosion, the method by which
other stars say goodbye. Or it could be that in the dense gatherings of
stars known as globular clusters, black holes sink to the center and
merge.
Michael
S. Turner, a cosmologist at the University of Chicago, noted that
astronomers had once referred to the search for gravitational waves as
an experiment, not an observatory. “LIGO has earned its ‘O,’ ” he said.
“That is, it will be an observatory, getting tens of events per year.”
Dr.
Turner added, “The loudest things in the gravity-wave sky are the most
exotic things in the universe: black holes, neutron stars and the early
universe.”
The future for the dark side looks bright.
“There
just have to be big, momentous surprises, which there always have been
when a new window is opened,” said Dr. Thorne, who is now retired from
LIGO.
Dr.
Drever, who has dementia and lives in a nursing home near Edinburgh, is
not able to enjoy the victory lap. “Ron’s creative genius was crucial
to LIGO’s future success and was the reason we brought him to Caltech,”
Dr. Thorne wrote in an email.
Dr.
Weiss, who is retired with emeritus status at M.I.T., said his life now
was more like that of a graduate student — that is to say, tinkering
and making things work. This tendency was almost the undoing of the LIGO
discovery. Only three days before the black hole chirp came in, Dr.
Weiss was at the Livingston site, he recalled, and was horrified to find
that the antenna readings were plagued by radio interference.
That
needs to be fixed, he told his colleagues, imploring them to delay the
engineering run. But they demurred, saying that everything was ready,
that it was too late to stop the program. Lucky for them.
“We would have missed that big event,” Dr. Weiss said.
LIGO HanfordEach arm is2.5 miles longLIGO LivingstonTWO BLACK HOLESAbout 1.2 billion years ago in a distant galaxy, a pair of black holes circled each other. The larger black hole was 36 times the mass of our sun, and the smaller one 29 times.COLLISIONThe intense gravity accelerated the black holes to half the speed of light, pulling them closer and carving distortions in space and time. In a fraction of a second, the pair collided and merged into an irregular shape.RING DOWNThe unstable blob smoothed into a sphere, a process called ring down. Three solar masses worth of energy were vaporized in a storm of gravitational waves, distorting space and time and leaving a new black hole 62 times the mass of the sun.GRAVITATIONAL WAVESThe invisible waves rippled outward at the speed of light. But waves fade with distance, and when they finally reached Earth, the distortions were too small to be measured above the heat, noise and other vibrations of our planet.DETECTIONLIGO is a pair of L-shaped observatories 1,900 miles apart. Ultra-pure mirrors at the ends of each arm are isolated from vibrations. Passing gravitational waves push and pull the arms, changing the length of tunnels by less than the width of a proton.A CHIRPOn Sept. 14, LIGO’s detectors measured their first vibrations from a gravitational wave. Translated to sound, it was a short chirp, the billion-year-old echo of the collision of those two black holes.The signal conformed precisely to the predictions of general relativity for black holes as calculated in computer simulations, Dr. Reitze said.Shortly after the September event, LIGO recorded another, weaker signal that was probably also from black holes, the team said. According to Dr. Weiss, there were at least four detections during the first LIGO observing run, which ended in January. The second run will begin this summer. In the fall, another detector, Advanced Virgo, operated by the European Gravitational Observatory in Italy, will start up. There are hopes for more in the future, in India and Japan.
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