Gravitational Affairs: LIGO's Little Black Box
For the last several weeks, there have been whispers of excitement surrounding possible new results from the Laser Interferometer Gravitational Observatory (LIGO), a huge collaboration that has been searching space for gravitational waves since 2002.
Just last month, LIGO scientists reported the best limits yet on gravitational waves arising from massive black hole binary systems. Had the first evidence for this key prediction of general relativity finally been observed?
Well, this week the LIGO/VIRGO collaboration meeting took place in Southern California, and I’m sorry to report that the answer is no. But it’s not because the physicists analyzing the data made mistakes.
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It turns out that the little blip in the signal picked up by the researchers was a red herring — a fake signal planted quite deliberately by the LIGO Internal Affairs division, to test the “data analysis pipeline” and ensure it’s working as planned.
Yep — LIGO has its own little black box, to guard against the potential for human bias. See, everyone on the collaboration really, really wants to be the first to detect gravitational waves. Scientists are just as prone to wishful thinking, and seeing what they want or expect to see, as any other human on the planet. Schemes like this one help ensure that bias hasn’t crept into the analysis process. So in that sense, the LIGO team passed with flying colors.
Gravitational waves are incredibly faint, so how does this all work? LIGO is essentially a giant interferometer. There is a very large mirror hung in such a way as to form an arm, with two more mirrors hung perpendicular to it to form an L-shape when viewed from above. Scientists then pass laser light through a beam splitter, thereby dividing the beam between those two arms, and let the light bounce back and forth a few times before returning to the beam splitter.
LIGO has three such detectors, since it needs to operate at least two detectors at the same time as a control, so they don’t get false positives. A passing gravity wave will cause ripples in space time, which in turn will change the distance measured by a light beam; the amount of light falling on the strategically placed photodetector will vary slightly in response. The resulting signal will tell scientists how the light hitting the photodector changes over time. LIGO scientists liken the instrument to “a microphone that converts gravitational waves into electrical signals.”
LIGO hasn’t reached its full sensitivity yet; that will happen once the planned upgrades for Advanced LIGO are complete. So nobody was really expecting to detect a bona fide signal. Still, the data had to be analyzed, just to be sure.
Among the many young scientists charged with this thankless task is Amber Stuver, who, in a rare first-person account of science actually being done, wrote about the spotting of a “candidate event” over at Living LIGO:
This looked just like what we would expect from a neutron star-black hole binary (pair) system orbiting into each other (to make a bigger resulting black hole). But one of the things that every scientist needs to learn is not to get overexcited and declare this to be the first direct detection of gravitational waves without making sure that this isn’t a false alarm. There are 2 ways this could be a false alarm: 1: There is something in the environment that just so happened to make a coincident signal in all of the detectors (LIGO in Louisiana, LIGO in Washington state and Virgo in Italy) or 2: This could be a blind injection (test).
There was still the tantalizing possibility that this signal — which the team dubbed “Big Dog” since it was detected near the Canis Major constellation (“We think we’re cute,” Stuver joked) — was the real deal. Somehow the entire collaboration of 800 or so kept this possible signal under wraps for six months, until this past week’s big unveiling. “My entire career has been dedicated to the effort of detecting gravitational waves,” Stuver writes. “If the ‘Big Dog’ had been real, this would have been a fulfillment of [that goal].”
So, was Stuver disappointed in the null result? Perhaps a little, although she certainly wasn’t surprised; a fake signal had been a very real possibility all along. Nor was all the hard work she put in wasted effort; she claims it “has been a priceless exercise for the collaboration…. We have developed skills for when we do make the first detection with Advanced Ligo.”
Now the LIGO Internal Affairs division can rest easy, knowing their international team is prepared for finding the real deal. And they also know their scientists can keep a secret.