"Long before it's in the papers"
February 11, 2016


Ripples in spacetime detected after long search

Feb. 11, 2016
Courtesy of LIGO
and World Science staff

For the first time, sci­en­tists say they have de­tected rip­ples in the fab­ric of space­time called gravita­t­ional waves, gen­er­at­ed by a col­li­sion of black holes. 

The find­ing con­firms a ma­jor pre­dic­tion of Al­bert Ein­stein’s 1915 gen­er­al the­o­ry of rel­a­ti­vity and opens an un­prec­e­dent­ed new win­dow on­to the cos­mos, ac­cord­ing to phys­i­cists.

Artist's conception of black holes about to merge. (Courtesy LIGO)

Gravita­t­ional waves car­ry in­forma­t­ion about their dra­mat­ic ori­gins and about the na­ture of gra­vity that can’t oth­er­wise be ob­tained. 

Phys­i­cists have con­clud­ed that the gra­vi­ta­t­ion­al waves arose dur­ing the fi­nal in­stant of the merg­er of two black holes to pro­duce one big­ger, spin­ning black hole. 

A black hole is an object so compact that its grav­it­ation­al force be­comes over­whelm­ing and draws in any­thing near­by, including light. 

The gravita­t­ional waves were de­tected on Sept. 14 at 5:51 a.m. East­ern Day­light Time by both of the twin de­tec­tors of the La­ser In­ter­fer­om­eter Gravita­t­ional-wave Ob­serv­a­to­ry, or LIGO, in Liv­ing­ston, Lou­i­si­ana, and Han­ford, Wash­ing­ton, USA. 

The find­ing, ac­cept­ed for pub­lica­t­ion in the jour­nal Phys­i­cal Re­view Let­ters, was made by the LIGO Sci­en­tif­ic Col­la­bora­t­ion and the Rome-based Vir­go Col­la­bora­t­ion. 

Sci­en­tists with the proj­ect es­ti­mate that the black holes “weighed”—or more ac­cur­ate­ly, had mass—the equiv­a­lent of about 29 and 36 Suns. And esti­mates indi­cate the event took place 1.3 bil­lion years ago, al­though the sig­nals only reached us re­cent­ly. 

Be­cause, as Ein­stein pro­posed, mass and en­er­gy are in­ter­change­able, about three Suns’ worth of ma­te­ri­al were con­vert­ed in­to gravita­t­ional waves dur­ing the event, ac­cord­ing to the au­thors. 

By look­ing at the time of ar­ri­val of the sig­nals—the de­tec­tor in Liv­ing­ston recorded the event sev­en thou­sandths of a sec­ond be­fore the de­tec­tor in Han­ford—sci­en­tists de­ter­mined that the source was lo­cat­ed in the south­ern sky.

Ac­cord­ing to gen­er­al rel­a­ti­vity, a pair of black holes or­bit­ing around each oth­er lose en­er­gy by emitting 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 crash at nearly half the speed of light and form one more mas­sive black hole. The event con­verts a part of the com­bined black holes’ mass, or “weight,” to en­er­gy, ac­cord­ing to Ein­stein’s for­mu­la E=mc2. This en­er­gy ra­diat­es out as a fi­nal strong burst of gravita­t­ional waves.

Jo­seph Tay­lor, Jr. and col­leagues are credited with de­mon­strat­ing gravita­t­ional waves ex­ist, in the 1970s and 1980s. Tay­lor and Rus­sell Hulse found in 1974 a double sys­tem com­posed of a pul­sar, a type of star, in or­bit around a neu­tron star. Tay­lor and Jo­el M. Weis­berg in 1982 found that the pul­sar’s orbit was slowly shrink­ing be­cause en­er­gy was leaking as gra­vi­ta­t­ional waves. Hulse and Tay­lor won the No­bel Prize in Phys­ics in 1993 for their work.

The new find­ing 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. This “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,” said Cal­tech’s Da­vid H. Re­itze, ex­ec­u­tive di­rector of the LIGO Lab­o­r­a­to­ry.

The U.S. Na­t­ional Sci­ence Founda­t­ion-funded ob­ser­va­to­ry was con­ceived, built, and is op­er­ated by the Cal­i­for­nia In­sti­tute of Tech­nol­o­gy and the Mas­sa­chu­setts In­sti­tute of Tech­nol­o­gy. 

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For the first time, scientists say they have detected ripples in the fabric of spacetime called gravitational waves, generated by a collision of two black holes. The finding confirms a major prediction of Albert Einstein’s 1915 general theory of relativity and opens an unprecedented new window onto the cosmos, according to physicists. Gravitational waves carry information about their dramatic origins and about the nature of gravity that can’t otherwise be obtained. Physicists have concluded that the gravitational waves were produced during the final fraction of a second of the merger of black holes to produce a single, more massive spinning black hole. The gravitational waves were detected on Sept. 14 at 5:51 a.m. Eastern Daylight Time by both of the twin detectors of the Laser Interferometer Gravitational-wave Observatory, or LIGO, in Livingston, Louisiana, and Hanford, Washington, USA. The finding, accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration and the Rome-based Virgo Collaboration. The U.S. National Science Foundation-funded observatories were conceived, built, and are operated by the California Institute of Technology and the Massachusetts Institute of Technology. Scientists with the project estimate that the black holes involved “weighed” the equivalent of about 29 and 36 suns, and the event took place 1.3 billion years ago, although the signals only reached us recently. Because, as Einstein proposed, mass and energy are interchangeable, about three suns’ worth of material were converted into gravitational waves during the event, according to the authors. By looking at the time of arrival of the signals—the detector in Livingston recorded the event seven thousandths of a second before the detector in Hanford—scientists determined that the source was located in the southern sky. 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. The existence of gravitational waves was first demonstrated in the 1970s and 1980s by Joseph Taylor, Jr. and colleagues. Taylor and Russell Hulse found 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 won the Nobel Prize in Physics in 1993. The new finding 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. “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,” said Caltech’s David H. Reitze, executive director of the LIGO Laboratory.