February 11, 2016 -
The
following is adapted from a press release issued today by MIT and
Caltech, which jointly operate the Laser Interferometer
Gravitational-Wave Observatory (LIGO) with funding from the National
Science Foundation.
For the first time, scientists have observed ripples in the fabric of
spacetime called gravitational waves, arriving at the 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 Sept. 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 three 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=mc2. This energy is emitted as a final
strong burst of gravitational waves. It is these gravitational waves
that LIGO has observed.
LIGO was originally proposed as a means of detecting these
gravitational waves in the 1980s by Rainer Weiss, emeritus professor of
physics, from MIT; Kip Thorne, Caltech’s Richard P. Feynman Professor of
Theoretical Physics, emeritus; and Ronald Drever, emeritus professor of
physics, also from Caltech.
“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,” Thorne says.
“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,” Weiss says.
“Caltech thrives on posing fundamental questions and inventing new
instruments to answer them,” says Caltech president Thomas Rosenbaum,
the Sonja and William Davidow Presidential Chair and professor of
physics. “LIGO represents an exhilarating example of how this approach
can transform our knowledge of the universe. We are proud to partner
with NSF and MIT and our other scientific collaborators to lead this
decades-long effort.”
“The LIGO team has uncovered fresh news about the building blocks of
the universe, and they have opened a whole new field of inquiry,” adds
MIT president L. Rafael Reif. “The discovery we celebrate today embodies
the paradox of fundamental science: that it is painstaking, rigorous,
and slow — and electrifying, revolutionary, and catalytic. Without basic
science, our best guess never gets any better, and ‘innovation’ is
tinkering around the edges. With the advance of basic science, society
advances, too. We are tremendously proud of the thousands of
researchers, across three generations, who made this impossible dream
come true.”
“Our observation of gravitational waves accomplishes an ambitious
goal set out over five 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.
“This discovery is just the beginning,” says Fiona Harrison, the
Benjamin M. Rosen Professor of Physics and holder of the Kent and Joyce
Kresa Leadership Chair of the Division of Physics, Mathematics and
Astronomy at Caltech. “Over the next years, LIGO will be putting general
relativity to its most stringent tests ever, it will be discovering new
sources of gravitational waves, and we will be using telescopes on the
ground and in space to search for light emitted by these catastrophic
events.”
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 the Earth.
LIGO research is carried out by the LIGO Scientific Collaboration
(LSC), a group of more than 1,000 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.
The discovery was made possible by the enhanced capabilities of
Advanced LIGO, a major upgrade that increases the sensitivity of the
instruments compared with 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 U.S.
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 at 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
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.”
“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, and delighted
Advanced LIGO delivered its groundbreaking detection so quickly.”
At each observatory, the 2.5-mile-long L-shaped LIGO interferometer
uses laser light split into two beams that travel back and forth down
the arms (4-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
(10-19 meter) can be detected.
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.
A network of detectors will significantly help to localize the
sources. The Virgo detector will be the first to join later this year.
The LIGO Laboratory also 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.
Virgo research is carried out by the Virgo Collaboration, consisting
of more than 250 physicists and engineers belonging to 19 different
European research groups: six from Centre National de la Recherche
Scientifique (CNRS) in France; eight from the Istituto Nazionale di
Fisica Nucleare (INFN) in Italy; two 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.
