By Emily Conover

The LIGO Laboratory operates two detector sites, one near Hanford in eastern Washington, and another near Livingston, Louisiana. This photo shows the Livingston detector site. The detector arm stretching off in the distance is 4 km long.
Photo: Caltech/MIT/LIGO Laboratory

The LIGO Laboratory operates two detector sites, one near Hanford WA, and another near Livingston, LA. This photo shows the Livingston detector site. The detector arm stretching off in the distance is 4 km long.

LIGO waves charts
Photo: Adapted from PRL 116, 061102 (2016)

Nearly simultaneous signals at the LIGO Observatories clinch the case for gravitational waves. Right side shows the two signals shifted and superimposed.

In the culmination of a decades-long quest, physicists have directly detected the minuscule ripples in spacetime known as gravitational waves. Predicted one hundred years ago as part of Einstein’s general theory of relativity, gravitational waves stretch and squeeze space itself. Such waves are generated by some of the most violent cataclysms in the universe, like the exploding stars known as supernovae, or pairs of neutron stars or black holes coalescing into one.

In a paper published in Physical Review Letters on February 11, the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo collaborations announced the detection of just such a black hole merger — knocking out two scientific firsts at once: the first direct detection of gravitational waves and the first observation of the merger of so-called binary black holes. The detection heralds a new era of astronomy — using gravitational waves to “listen in” on the universe (see related article-International News)

In the early morning hours of September 14, 2015 — during an engineering run just days before official data-taking started — a strong signal, consistent with merging black holes, appeared nearly simultaneously in LIGO’s two observatories, located in Hanford, Washington and Livingston, Louisiana.

That observation has left scientists stunned. “My jaw dropped,” says Emanuele Berti of the University of Mississippi, who was not involved in the experiment. “The significance of the detection is so high that it’s extremely unlikely that this is not a binary black hole signal. ... Never would I have thought in my life that I would see a signal that clear so early.”

“I’m totally beside myself,” says Clifford Will of the University of Florida, Gainesville, who was not involved in the experiment. “It’s tremendously exciting. I actually was shown the paper a couple of weeks ago, and I’m still excited two weeks later.”

Each LIGO observatory boasts a pair of four-kilometer-long arms arranged in an L to form an enormous interferometer. In the absence of gravitational waves, the light from a laser travels the same distance along each arm, and the beams from the two arms interfere destructively when they meet at the arms’ intersection, so that no light reaches a detector that monitors the beam.

But when a gravitational wave passes through the observatory, it will ever-so-slightly lengthen one arm and shorten the other, preventing the full cancellation of the two beams, letting light through to the detector and producing a signal. LIGO is designed to catch length differences a billionth the size of an atom.

LIGO’s two separate observatories help to rule out spurious signals from the local environment, which can be caused by events as innocuous as a truck rumbling by, or ocean waves crashing on the shore. Gravitational wave signals should appear in both detectors, nearly simultaneously.

And that is just what happened. Both observatories recorded a signal (see graph below) consistent with predictions for a black hole merger. In such an event, two black holes rapidly spiral closer and closer together, until they meet to form a single black hole, which then undergoes “ringdown” — in analogy to a bell ringing after being struck with a hammer. According to predictions, the process should produce a telltale “chirp” signal of increasing frequency. That is exactly what LIGO saw, and the LIGO team is extremely confident that it is the real deal: They expect an event like this to appear as a false alarm only once every 203,000 years.

“It was amazing; this was a gift of nature. It was not just black holes but it was a signal we could see by eye,” LIGO spokesperson Gabriela González of Louisiana State University said at a press conference in Washington, DC on February 11, noting that the signal was strong enough to stick out obviously above the noise.

Some LIGO team members say the signal initially struck them as too good to be true. And since LIGO’s process includes “blind injections” — test signals planted in the data that only a few collaboration members know about — it well could have been. “I thought it had to be an injection; it was so beautiful,” says John Veitch of the University of Birmingham. But collaboration leaders quickly confirmed that it was not a drill.

The researchers estimate that the black holes’ masses were 36 and 29 times the mass of the Sun, and pegged them at a distance of 1.3 billion light years from Earth. When these two behemoths combined, their coalescence was so intense that it radiated away 3 solar masses worth of energy in gravitational waves, leaving behind a black hole 62 times the mass of the Sun.

“There’s really no doubt that this is a real detection, a real signal,” says Will. “They do a very careful job of worrying about any kind of issues that might have fooled them.”

The discovery of the system and its merger is significant in itself, affirming the power of gravitational waves to unlock new secrets of the cosmos. The result shows that binary black holes can form and merge — something predicted but never before seen.

Additionally, the merging black holes are more massive than most “stellar mass” black holes, and also much smaller than the supermassive black holes found at the centers of galaxies, which can have masses billions of times that of the Sun. But there is a no-man’s-land between the two groups, with the stellar mass black holes topping out around 15 or 20 times the mass of the Sun, says Berti. The result shows that more massive black holes of this size indeed exist.

“The motivation wasn’t just to detect gravitational waves and go home, but the potential to create a completely new science,” says Barry Barish of Caltech, a member of the LIGO collaboration and 2011 APS president. “This is a completely different way to look at the sky.”

The researchers also set a bound on the mass of the graviton — the hypothetical particle that transmits the gravitational interaction — and put general relativity through its paces by performing consistency tests, which it passed handily.

LIGO detected a second, less significant event, which was also compatible with a binary black hole merger. But, “My feeling is that it’s not part of the story,” says Barish. “It doesn’t measure to be statistically probable enough that we should talk about it.” The false alarm rate for an event like this is once every 2.3 years.

The discovery came on the heels of a $200-million upgrade to the experiment, called Advanced LIGO, intended to boost its chances of finding the elusive signals. During LIGO’s previous run, from 2002 to 2010, the collaboration came up empty-handed. Currently, the detector’s sensitivity to binary neutron star mergers is improved over its previous incarnation by a factor of three to five, says Barish. Eventually, the sensitivity will reach a factor-of-ten improvement — increasing the rate of binary neutron star mergers LIGO can detect by a factor of 1000, by effectively allowing LIGO to peer further out in space.

Several planned gravitational wave observatories, including Advanced Virgo in Italy, will soon form a network of detectors along with LIGO, allowing physicists to more accurately pinpoint sources on the sky, and point telescopes in the direction of candidates to look for corresponding electromagnetic signals. “We can start seeing the universe and listening to it at the same time,” says Chiara Mingarelli of Caltech.

In fact, LIGO and Virgo are already working as a team and collaborating on data analysis, so members of the Virgo collaboration were also listed as authors on the paper.

Although the result is the first direct detection of gravitational waves, physicists have long been confident in their existence, persuaded by indirect evidence gleaned from long-term observation of a pulsar — a rapidly rotating neutron star that appears to pulse regularly — in a binary system. Over decades, analysis of the pulses’ timing revealed a slow but steady loss of energy, at just the rate expected for the emission of gravitational waves. The 1974 discovery of this binary pulsar earned its discoverers, Russell Hulse and Joseph Taylor, the 1993 Nobel Prize in physics. For years, physicists have speculated that the first direct detection of gravitational waves will be Nobel-worthy too.

With the paper covering only a few weeks of operation, and months more of data already in the can, it might not take long for new signals to appear. “They’ll find more stuff,” says Virginia Trimble of the University of California, Irvine. “The next rumor, of course, is events two and three.”

There’s plenty to come from LIGO, said Kip Thorne of Caltech during the press conference. “It’s really fantastic; we are going to have a huge richness of gravitational wave signals.” As for what the new data will bring, “I think we can be rather sure that we will see big surprises,” he said.

For more on the discovery read the Physics Viewpoint by Emanuele Berti and the Physical Review Letters paper.

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