Gravitational waves that back Einstein’s theory detected again

A computer simulation showed the collision of two black holes. Caltech/MIT/LIGO Laboratory/Handout via Reuters

The scientists who made a global splash in February by revealing that they had detected gravitational waves -- invisible ripples in space-time that Einstein first predicted a century ago -- kept something under their hat. They didn’t just do it once, they did it twice.

Even as they announced the detection of a wave that had traveled more than 1 billion light-years toward earth from the cataclysmic collision of two black holes, they were vetting a second signal from a similar collision in another distant corner of space -- this wave even fainter.

Those two gravitational waves, rippling toward earth at the speed of light, registered at the twin detectors known as LIGO -- massive, finely tuned “listening devices” first conceived at MIT and located in remote Louisiana and Washington locales -- 14 weeks apart.

And though the waves kept right on moving past the earth in less than the blink of an eye, unseen and undetectable except to LIGO, they carried messages for the researchers from two violent collisions, long ago and across the universe.

“It’s a wondrous thing,” said David Shoemaker, who leads the MIT lab that helped build the detectors, in an interview coinciding with the publication of the new findings Wednesday. “Three months apart, 1.4 billion years ago, these two events happened at two different places in the sky.”

The finding does more than prove the first was no fluke. As the detectors become more sensitive, and as more like them get built, researchers expect to detect waves from many more black hole collisions and from other previously unobserved cataclysms, like the collision of neutron stars, the researchers said.

“With this, we can tell you now, the era of gravitational-wave astronomy has begun,” said Gabriela González, a Lousiana State University professor and the spokeswoman for the LIGO (which stands for Laser Interferometer Gravitational-wave Observatory) scientific collaboration, in a press conference announcing the findings.

The twin detectors and the 1,000-scientist coalition, led by the California Institute of Technology and MIT and funded by the National Science Foundation, grew out of a classroom “thought experiment” first proposed by MIT physics professor Rainer Weiss half a century ago.

At the time, many prominent scientists doubted whether gravitational waves were even real -- and not just a quirk of Einstein’s relativity theories -- let alone whether anyone could ever design an instrument sensitive enough to detect them.

But Weiss, who still works six days a week on LIGO as an emeritus professor, became fascinated by the possibility of splitting and bouncing laser beams back and forth many times between mirrors hanging freely at opposite ends of vacuum tubes, inside miles-long tunnels. He was convinced that if they could perfect the device, they could register the nearly infinitesimal way that a passing gravitational wave jostled the mirrors and squeezed and stretched the paths traveled by the laser beams -- and he helped form the scientific coalition and will the observatories into existence.

Decades in the making, the LIGO detectors had not quite reached their designed sensitivity when the first wave struck last Sept. 14. Though it squeezed and stretched their 2.5-mile arms by just 1/1,000th of a proton’s width, that signal was so clear and strong that it played as a chirp on control-room speakers and danced distinctively on the real-time graphs that had been fed until then by the constant wiggles of the mirrors based on rumbles here on earth.

As researchers were parsing that signal for a paper and preparing to announce “the chirp heard round the universe,” the next wave hit late on Christmas Day at the two observatories -- officially, 3:38 a.m. on Dec. 26 in the international standard known as Coordinated Universal Time.

“Einstein’s Christmas present,” Rick Fienberg, a spokesman for the American Astronomical Society dubbed it on Wednesday, during the webcast press conference at the society’s 228th meeting.

This wave came from the final, dramatic collision of two black holes that had been circling each other -- one more than 14.2 times as massive as our sun, the other 7.5 times -- and that slammed together to form an even bigger, spinning black hole 20.8 times as massive as our sun, while releasing the remaining energy in the form of gravitational waves.

That light-speed signal reached the twin detectors on opposite sides of North America barely a millisecond apart, but it was so faint -- unlike the first -- that it was only seen and heard by computers, extracting the signal from massive reams of data.

So if the credit for the first finding goes to the “instrumentalists” who designed and built LIGO, Shoemaker said, then this one should go to the “relativists who interpret Einstein’s equations, the numerical relativists who solve the equations when they’re too difficult to solve by hand, and the data analysts who write up the code that allows you to pull these signals out” from amid all the local noise continually jostling the mirrors and moving the laser beams.

With a third observatory known as VIRGO slated to begin running soon in Italy, and with others slated for Japan and India, the researchers will be able to better “triangulate” the locations in the universe where incoming gravitational-waves originated. That means they will be able to tell traditional astronomers where to point their telescopes, in the hopes of catching a glimpse of the events -- turning a coordinated eye and ear onto the universe.

“We’re literally standing at the dawn of a new era in astronomy and physics,” Fienberg said.