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LIGO Hanford
The beamlines for the LIGO detector site at Hanford stretch out across the desert terrain of southeastern Washington. Each arm of the L-shaped detector is 2.5 miles long. (Credit: LIGO)

HANFORD, Wash. – Physicists here had to keep mum for nearly five months about the gravitational-wave breakthrough they helped make, but they made up for their silence today with superlatives.

“I looked at it, and it was so beautiful, so clean,” said Fred Raab, director of Hanford’s Laser Interferometer Gravitational-Wave Observatory.

Raab and other scientists at LIGO Hanford learned of the discovery back on the morning of Sept. 14, when they opened an email from colleagues in Germany with the subject line “Interesting Event.”

Image: Fred Raab
Fred Raab, director of LIGO Hanford, is all smiles as he discusses the gravitational-wave discovery. (GeekWire photo by John Stang)

Analysts at the Albert Einstein Institute noticed that readings from the Hanford facility, and from a sister observatory in Livingston, La., fit the pattern for the merger of two massive black holes. That was exactly the kind of signal the LIGO team had been seeking for more than a decade.

“There were a lot of high fives going around,” Greg Mendell, a member of the LIGO science team, recalled at a Hanford news briefing.

After going through months’ worth of verification and analysis, the results were finally published today in Physical Review Letters, amid worldwide fanfare. The readings marked the first-ever direct detection of gravitational waves, the first-ever observation of a black hole merger, and the best confirmation to date for Albert Einstein’s century-old general theory of relativity.

The detailed analysis determined that the two black holes were about 29 times and 36 times the mass of the sun. Three suns’ worth of that mass was converted into energy in the form of gravitational waves. For a mere 20 milliseconds, the power output was 50 times greater than the power of all the stars of the universe put together.

Scientists couldn’t determine exactly where in the sky the black holes crashed, but they knew it was in the southern celestial hemisphere because the Livingston detector picked up the signal 0.007 seconds before Hanford did.

Mendell said his colleagues in Livingston “like to stick it to us that they saw it first.”

Then Raab piped up: “We like to say we saw it better.” And in fact, the highs and lows of the wave pattern, known as the “ringdown,” look sharper in Hanford’s chart.

LIGO data
These graphs compare the data from the LIGO’s Hanford and Livingston detectors with what would be expected from a black hole merger. (Credit: LIGO)

Caltech’s Jenne Driggers, a member of the LIGO team, marveled at the fact that the signal was sent out from the merging black hole about 1.3 billion years ago. “That’s back when single-cell organisms were the only life on Earth,” she said. “This is phenomenally old. … This is the most powerful event ever witnessed by a human being. It is enormous and exotic and amazing.”

The discovery had to be kept secret while researchers readied their paper for publication. Rumors slipped out, but Raab refused to comment publicly. It took him two weeks to decide to tell his wife, and that’s only because he couldn’t hide his phone conversations from her.

“Yes, I had to swear her to secrecy,” he said.

Raab said he saw today’s announcement as “a big curtain coming down on Act One of a very long play.” The paper in Physical Review Letters covers only the first month of observations, and LIGO’s instruments are in the midst of an upgrade to make them even more sensitive to gravitational waves.

Meanwhile, there are already rumors that LIGO registered more readings that aren’t yet ready to be reported.

So what does Raab say to that? “Read my lips,” he said. Then he shut his mouth.

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