Gravitational waves detected 100 years after Einstein's prediction
fabric of spacetime called gravitational waves, arriving at
Earth from a cataclysmic event in the distant universe. This
confirms a major prediction of Albert Einstein's 1915 general
theory of relativity and opens an unprecedented new
window onto the cosmos.
Gravitational waves carry information about their dramatic
origins and about the nature of gravity that cannot
otherwise be obtained. Physicists have concluded that the
detected gravitational waves were produced during the final
fraction of a second of the merger of two black holes to
produce a single, more massive spinning black hole. This
collision of two black holes had been predicted but never
observed.
The gravitational waves were detected on September 14,
2015 at 5:51 a.m. Eastern Daylight Time (09:51 UTC) by
both of the twin Laser Interferometer Gravitational-wave
Observatory (LIGO) detectors, located in Livingston,
Louisiana, and Hanford, Washington, USA. The LIGO
Observatories are funded by the National Science
Foundation (NSF), and were conceived, built, and are
operated by Caltech and MIT. The discovery, accepted for
publication in the journal Physical Review Letters, was made
by the LIGO Scientific Collaboration (which includes the
GEO Collaboration and the Australian Consortium for
Interferometric Gravitational Astronomy) and the Virgo
Collaboration using data from the two LIGO detectors.
Based on the observed signals, LIGO scientists estimate
that the black holes for this event were about 29 and 36
times the mass of the sun, and the event took place 1.3
billion years ago. About 3 times the mass of the sun was
converted into gravitational waves in a fraction of a
second -- with a peak power output about 50 times that
of the whole visible universe. By looking at the time of
arrival of the signals -- the detector in Livingston recorded
the event 7 milliseconds before the detector in Hanford --
scientists can say that the source was located in the
Southern Hemisphere.
According to general relativity, a pair of black holes orbiting
around each other lose energy through the emission of
gravitational waves, causing them to gradually approach
each other over billions of years, and then much more
quickly in the final minutes. During the final fraction of a
second, the two black holes collide into each other at
nearly one-half the speed of light and form a single more
massive black hole, converting a portion of the combined
black holes' mass to energy, according to Einstein's formula
E=mc . This energy is emitted as a final strong burst of
gravitational waves. It is these gravitational waves that
LIGO has observed.
The existence of gravitational waves was first
demonstrated in the 1970s and 80s by Joseph Taylor, Jr.,
and colleagues. Taylor and Russell Hulse discovered in 1974
a binary system composed of a pulsar in orbit around a
neutron star. Taylor and Joel M. Weisberg in 1982 found
that the orbit of the pulsar was slowly shrinking over time
because of the release of energy in the form of
gravitational waves. For discovering the pulsar and showing
that it would make possible this particular gravitational wave
measurement, Hulse and Taylor were awarded the Nobel
Prize in Physics in 1993.
The new LIGO discovery is the first observation of
gravitational waves themselves, made by measuring the tiny
disturbances the waves make to space and time as they
pass through Earth.
"Our observation of gravitational waves accomplishes an
ambitious goal set out over 5 decades ago to directly detect
this elusive phenomenon and better understand the
universe, and, fittingly, fulfills Einstein's legacy on the 100th
anniversary of his general theory of relativity," says
Caltech's David H. Reitze, executive director of the LIGO
Laboratory.
The discovery was made possible by the enhanced
capabilities of Advanced LIGO, a major upgrade that
increases the sensitivity of the instruments compared to the
first generation LIGO detectors, enabling a large increase
in the volume of the universe probed -- and the discovery
of gravitational waves during its first observation run. The
US National Science Foundation leads in financial support
for Advanced LIGO. Funding organizations in Germany
(Max Planck Society), the U.K. (Science and Technology
Facilities Council, STFC) and Australia (Australian Research
Council) also have made significant commitments to the
project. Several of the key technologies that made
Advanced LIGO so much more sensitive have been
developed and tested by the German UK GEO
collaboration. Significant computer resources have been
contributed by the AEI Hannover Atlas Cluster, the LIGO
Laboratory, Syracuse University, and the University of
Wisconsin- Milwaukee. Several universities designed, built,
and tested key components for Advanced LIGO: The
Australian National University, the University of Adelaide,
the University of Florida, Stanford University, Columbia
University of the City of New York, and Louisiana State
University.
"In 1992, when LIGO's initial funding was approved, it
represented the biggest investment the NSF had ever
made," says France Córdova, NSF director. "It was a big
risk. But the National Science Foundation is the agency that
takes these kinds of risks. We support fundamental science
and engineering at a point in the road to discovery where
that path is anything but clear. We fund trailblazers. It's
why the U.S. continues to be a global leader in advancing
knowledge."
LIGO research is carried out by the LIGO Scientific
Collaboration (LSC), a group of more than 1000 scientists
from universities around the United States and in 14 other
countries. More than 90 universities and research institutes
in the LSC develop detector technology and analyze data;
approximately 250 students are strong contributing
members of the collaboration. The LSC detector network
includes the LIGO interferometers and the GEO600
detector. The GEO team includes scientists at the Max
Planck Institute for Gravitational Physics (Albert Einstein
Institute, AEI), Leibniz Universität Hannover, along with
partners at the University of Glasgow, Cardiff University,
the University of Birmingham, other universities in the
United Kingdom, and the University of the Balearic Islands
in Spain.
"This detection is the beginning of a new era: The field of
gravitational wave astronomy is now a reality," says
Gabriela González, LSC spokesperson and professor of
physics and astronomy at Louisiana State University.
LIGO was originally proposed as a means of detecting
these gravitational waves in the 1980s by Rainer Weiss,
professor of physics, emeritus, from MIT; Kip Thorne,
Caltech's Richard P. Feynman Professor of Theoretical
Physics, emeritus; and Ronald Drever, professor of physics,
emeritus, also from Caltech.
"The description of this observation is beautifully described in
the Einstein theory of general relativity formulated 100
years ago and comprises the first test of the theory in
strong gravitation. It would have been wonderful to watch
Einstein's face had we been able to tell him," says Weiss.
"With this discovery, we humans are embarking on a
marvelous new quest: the quest to explore the warped side
of the universe -- objects and phenomena that are made
from warped spacetime. Colliding black holes and
gravitational waves are our first beautiful examples," says
Thorne.
Virgo research is carried out by the Virgo Collaboration,
consisting of more than 250 physicists and engineers
belonging to 19 different European research groups: 6 from
Centre National de la Recherche Scientifique (CNRS) in
France; 8 from the Istituto Nazionale di Fisica Nucleare
(INFN) in Italy; 2 in The Netherlands with Nikhef; the
Wigner RCP in Hungary; the POLGRAW group in Poland;
and the European Gravitational Observatory (EGO), the
laboratory hosting the Virgo detector near Pisa in Italy.
Fulvio Ricci, Virgo Spokesperson, notes that, "This is a
significant milestone for physics, but more importantly merely
the start of many new and exciting astrophysical
discoveries to come with LIGO and Virgo."
Bruce Allen, managing director of the Max Planck Institute
for Gravitational Physics (Albert Einstein Institute), adds,
"Einstein thought gravitational waves were too weak to
detect, and didn't believe in black holes. But I don't think
he'd have minded being wrong!"
"The Advanced LIGO detectors are a tour de force of
science and technology, made possible by a truly exceptional
international team of technicians, engineers, and scientists,"
says David Shoemaker of MIT, the project leader for
Advanced LIGO. "We are very proud that we finished this
NSF-funded project on time and on budget."
At each observatory, the two-and-a-half-mile (4-km) long
L-shaped LIGO interferometer uses laser light split into
two beams that travel back and forth down the arms
(four-foot diameter tubes kept under a near-perfect
vacuum). The beams are used to monitor the distance
between mirrors precisely positioned at the ends of the
arms. According to Einstein's theory, the distance between
the mirrors will change by an infinitesimal amount when a
gravitational wave passes by the detector. A change in the
lengths of the arms smaller than one-ten-thousandth the
diameter of a proton (10 meter) can be detected.
"To make this fantastic milestone possible took a global
collaboration of scientists -- laser and suspension
technology developed for our GEO600 detector was used
to help make Advanced LIGO the most sophisticated
gravitational wave detector ever created," says Sheila
Rowan, professor of physics and astronomy at the
University of Glasgow.
Independent and widely separated observatories are
necessary to determine the direction of the event causing
the gravitational waves, and also to verify that the signals
come from space and are not from some other local
phenomenon.
Toward this end, the LIGO Laboratory is working closely
with scientists in India at the Inter-University Centre for
Astronomy and Astrophysics, the Raja Ramanna Centre for
Advanced Technology, and the Institute for Plasma to
establish a third Advanced LIGO detector on the Indian
subcontinent. Awaiting approval by the government of India,
it could be operational early in the next decade. The
additional detector will greatly improve the ability of the
global detector network to localize gravitational-wave
sources.
"Hopefully this first observation will accelerate the
construction of a global network of detectors to enable
accurate source location in the era of multi-messenger
astronomy," says David McClelland, professor of physics
and director of the Centre for Gravitational Physics at the
Australian National University.
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