LIGO experiment reports first detection of spacetime vibrations, opening new window to the cosmos
SWEET SUCCESS For the first time, physicists have directly observed gravitational waves, caused by two black holes colliding (illustrated here).
Editor's note: This story will be updated throughout the day.
WASHINGTON — Tremors in the cosmic fabric of space and time have finally been detected, opening a new avenue for exploring the universe.
The historic discovery of those tremors, known as gravitational waves, comes almost exactly a century after Albert Einstein first posited their existence. Researchers with the Advanced Laser Interferometer Gravitational-Wave Observatory, or Advanced LIGO, announced the seminal detection February 11 at a news conference and in a paper in Physical Review Letters. The gravitational swell originated more than 750 million light-years away, where the high-speed dance of two converging black holes shook the very foundation upon which planets, stars and galaxies reside.
"It's the first time the universe has spoken to us through gravitational waves," LIGO laboratory executive director David Reitze said at a press conference February 11. "As we open a new window on astronomy, we may see things we never saw before."
The discovery immediately becomes a likely candidate for a Nobel Prize, and not just because it ties a neat bow around decades of evidence supporting a major prediction of Einstein’s 1915 general theory of relativity. “Gravitational waves allow us to look at the universe not just with light but with gravity,” says Shane Larson, an astrophysicist at Northwestern University in Evanston, Ill. Gravitational waves can expose the gory details of black holes, stellar corpses and other extreme phenomena that can’t be obtained by traditional telescopes. With this discovery, the era of gravitational wave astronomy has begun.
The detection occurred on September 14, 2015, four days before the official start of observations for the newly upgraded observatory. Striking gold so quickly raises hopes for an impending flurry of sightings.
The fleeting burst of waves arrived on Earth long after two black holes, one about 36 times the mass of the sun and the other roughly 29, spiraled toward each other and coalesced. If Isaac Newton had been right about gravity, then the mass of the two black holes would have exerted an invisible force that pulled the objects together. But general relativity maintains that those black holes merged because their mass indented the fabric of space and time (SN: 10/17/15, p. 16). As the black holes drew near in a deepening pit of spacetime, they also churned up that fabric, emitting gravitational radiation (or gravity waves, as scientists often call them). Unlike more familiar kinds of waves, these gravitational ripples don’t travel “through” space; they are vibrations of spacetime itself, propagating outward in all directions at the speed of light.
LIGO’s detectors in Hanford, Wash., and Livingston, La., newly reactivated after five years of upgrades, each consist of a powerful laser that splits into two perpendicular, 4-kilometer-long beams. When the gravitational waters of spacetime are calm, the beams recombine at the junction and cancel each other out — the troughs of one beam’s 1,064-nanometer waves of laser light completely negate the crests of the second beam’s waves.
But the gravitational disturbance from the black hole pair distorted spacetime, slightly squeezing one arm of the detector while stretching the other (SN: 1/8/00, p. 26). When the beams recombined, the light no longer matched up perfectly. The detectors sensed that crest missed trough by the tiniest of distances, much less than the diameter of a proton. Both LIGO facilities registered the signal at almost exactly the same time, indicating a light-speed pulse from deep space rather than a slower-moving vibration from an underground quake or a big rig rumbling along the highway.
LIGO’s announcement falls between two very relevant centennials: Einstein’s introduction of general relativity (November 1915) and his prediction of gravitational waves (June 1916, though he had to fix the math two years later). Russell Hulse and Joseph Taylor Jr. won the 1993 Nobel Prize in physics for deducing gravity wave emission based on the motion of a stellar corpse called a neutron star and a closely orbiting companion. Now Advanced LIGO has sealed the deal with the first direct measurement.
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