ATOM & COSMOS Physicists detect gravitational waves News LIGO experiment’s discovery opens new window to the cosmos BY ANDREW GRANT each other and coalesced. If Isaac Newton of mass is packed into small spaces and WASHINGTON — Tremors in the cos- had been right about gravity, then the moving very quickly. The colliding black mic fabric of space and time have finally mass of the two black holes would have holes certainly qualify. Their tremen- been detected, opening a new avenue for exerted an invisible force that pulled the dous mass was packed into spheres about exploring the universe. objects together. But general relativity 150 kilometers in diameter. By the time The historic discovery of those trem- maintains that those black holes merged the black holes experienced their final ors, known as gravitational waves, because their mass indented the fabric unifying plunge, they were circling each comes almost exactly a century after of space and time (SN: 10/17/15, p. 16). As other at about half the speed of light. Albert Einstein first posited their exis- the black holes drew near in a deepening On September 14 at 4:50 a.m. Eastern tence. Researchers with the Advanced pit of spacetime, they also churned up time, the gravity waves emitted by the Laser Interferometer Gravitational- that fabric, emitting gravitational radia- black holes during their last fractions of Wave Observatory, or Advanced LIGO, tion (or gravity waves, as scientists often a second of independence encountered announced the seminal detection call them). Unlike more familiar kinds of the two L-shaped LIGO detectors. February 11 at a news conference and in waves, these gravitational ripples don’t LIGO’s detectors in Hanford, Wash., a paper in Physical Review Letters. The travel “through” space; they are vibra- and Livingston, La., newly reactivated gravitational swell originated more than tions of spacetime itself, propagating out- after five years of upgrades, each con- 750 million light-years away, where the ward in all directions at the speed of light. sist of a powerful laser that splits into high-speed dance of two converging black Nearly every instance of an object two perpendicular, 4-kilometer-long holes shook the very foundation upon accelerating generates gravity waves — beams (see Page 22). When the gravita- which planets, stars and galaxies reside. you produce feeble ones getting out of tional waters of spacetime are calm, the “It’s the first time the universe has bed in the morning. Advanced LIGO is beams recombine at the junction and spoken to us through gravitational fine-tuned to home in on more detect- cancel each other out — the troughs of waves,” LIGO laboratory executive able (and scientifically relevant) fare: one beam’s 1,064-nanometer waves of director David Reitze said. waves emitted from regions where a lot laser light completely negate the crests The discovery immediately becomes of the second beam’s waves. Predicted LIGO Hanford a likely candidate for a Nobel Prize, and But the gravitational disturbance from ) 1.0 21 not just because it ties a neat bow around – 0.5 the black hole pair distorted spacetime, decades of evidence supporting a major 0 slightly squeezing one arm of the detector prediction of Einstein’s 1915 general –0.5 while stretching the other (SN: 1/8/00, theory of relativity. “Gravitational waves (10 Strain –1.0 p. 26). When the beams recombined, the allow us to look at the universe not just light no longer matched up perfectly. The with light but with gravity,” says astro- Predicted LIGO Livingston detectors sensed that crest missed trough physicist Shane Larson of Northwestern ) 1.0 by the tiniest of distances, about a thou- 21 University in Evanston, Ill. Gravitational – 0.5 sandth the diameter of a proton. 0 waves can expose the gory details of black The LIGO facilities registered the sig- –0.5 Strain (10 Strain nal just 7 milliseconds apart, indicating a holes and other extreme phenomena that –1.0 can’t be obtained with traditional tele- light-speed pulse from deep space rather scopes. With this discovery, the era of LIGO Hanford (shifted) LIGO Livingston than a slower-moving vibration from an gravitational wave astronomy has begun. ) 1.0 underground quake or a big rig rumbling 21 The detection occurred September – 0.5 along the highway. Physicists used the 14, 2015, four days before the official 0 combined measurements to estimate a start of observations for the newly –0.5 distance of 750 million to 1.8 billion light- upgraded LIGO. Striking gold so quickly (10 Strain –1.0 years to the black holes, with 1.3 billion raises hopes for an impending flurry of 0.30 0.35 0.40 0.45 light-years as the best estimate. At least sightings. Time (seconds) one more detector, preferably two, would The fleeting burst of waves arrived Clear signal The LIGO detectors registered have been needed to triangulate the pre- on Earth long after two black holes, one nearly identical signals (top and middle) almost cise location of the black holes in the sky. simultaneously as gravity waves from a black about 36 times the mass of the sun and hole collision passed by the Earth. The signals While the black hole rendezvous was the other roughly 29, spiraled toward closely match predictions. millions of years in the making, only the LIGO 6 SCIENCE NEWS | March 5, 2016 final two-tenths of a second produced The observatory achieved what its pre- Gravity waves gravity waves with the requisite intensity decessor, which ran from 2001 to 2010, from a black hole collision and frequency for detection by Advanced could not because of an upgrade that came from LIGO. Those two-tenths of a second told enhanced sensitivity by at least a factor about 1.3 bil- Large Magellanic Cloud quite a story. At first, the black holes were of three. Increased sensitivity translates lion light-years away, probably in circling each other about 17 times a sec- to identifying more distant objects: If the direction of the Small Magellanic Cloud ond; by the end, it was 75. The gravity the search area of first-generation LIGO Magellanic clouds. wave frequency and intensity reached a included all the space that could fit within peak, and then the black holes merged. a baseball, Advanced LIGO could spot they can spot neutron star and Combining the wave measurements everything inside a basketball. Advanced black hole collisions even farther away. with computer simulations, the scien- LIGO’s range extends up to 5 billion light- The observatory should be back up and tists determined that a pair of 36- and years in all directions for merging objects running by late summer, says LIGO chief 29-solar-mass black holes had become about 100 times the mass of the sun, proj- detector scientist Peter Fritschel. one 62-solar-mass beast. The missing ect leader David Shoemaker of MIT says. Later this year, European partners of mass had been transformed into energy That extended reach, plus a boost in sen- the LIGO collaboration plan to restart and carried away as gravity waves. The sitivity at the wave frequencies associated their revamped gravity wave observa- power output during that mass-energy with black holes, enabled the detection. tory, Advanced VIRGO, near Pisa, Italy, conversion was 50 times greater than that This ability to examine black holes providing a crucial third ultrasensitive of all the stars in the universe combined. and other influential dark objects with- detector for pinpointing gravity wave The observed LIGO signal matches out actually “seeing” them with light has sources. Similar detectors are in the what physicists expected from a black scientists excited about the gravitational works for Japan and India. hole merger almost perfectly. Ingrid wave era. Black holes gobble up some LIGO was designed to spot waves in Stairs, an astrophysicist at the Univer- matter and launch the rest away in pow- the sweet spot for converging black holes sity of British Columbia in Vancouver erful jets, scattering atoms within and and neutron stars, with a frequency rang- who was not involved with LIGO, says she between galaxies; pairs of neutron stars, ing from tens of hertz to several thou- and colleagues were “bowled over by how also targets of Advanced LIGO, may ulti- sand. But just as scientists use radio and beautiful it was.” Translated into sound, mately trigger gamma-ray bursts, among gamma-ray telescopes to probe different the signal resembled a rumbling fol- the brightest and most energetic explo- frequencies of light, physicists are build- lowed by a chirp. “It stood out like a sore sions known in the universe. ing detectors sensitive to a range of grav- thumb,” says Rainer Weiss, one of the pri- Yet while the influence of these cos- ity wave frequencies. The eLISA mission, mary architects of LIGO. The 83-year-old mic troublemakers is sometimes visible consisting of three satellites, will hunt physicist had visited Livingston just days with traditional telescopes, the objects for waves with frequencies under 1 hertz before and almost shut down the detec- themselves are not. Gravity waves offer when it launches in the 2030s. The sat- tor to fix some minor problems. Had he a direct probe, and as a bonus they don’t ellite trio should be able to resolve black done so, “we would have missed it.” get impeded by gas, dust and other cos- holes from the early universe and ones Despite the seeming no-doubt signal, mic absorbers as light does. “It opens up millions of times the mass of the sun. LIGO researchers conducted a series of a new window into astronomy that we The LIGO result is distinct from the rigorous statistical tests.