Just last month, we told you about a small group of Danish physicists who were casting doubt on the original gravitational wave signal detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO), saying it was an "illusion." The researchers alleged that the collaboration mistook patterns in the noise for a signal. Now Quanta is reporting that two independent analyses have been completed that confirm that detection. This should lay any doubts about the momentous discovery to rest.
“We see no justification for lingering doubts about the discovery of gravitational waves,” the authors of one of the papers, Martin Green and John Moffat of the Perimeter Institute for Theoretical Physics, told Quanta. That paper appeared in Physics Letters B in September. A second paper by Alex Nielsen of the Max Planck Institute for Gravitational Physics in Hannover, Germany, and three coauthors, was posted to the physics preprint site arXiv.org last month and is under review by the Journal of Cosmology and Astroparticle Physics.
“The Copenhagen group refuse to accept that they may be wrong. In fact, they are wrong.”
But some drama still remains. Andrew Jackson, group spokesman for the skeptical physicists at the Niels Bohr Institute in Copenhagen, Denmark, is refusing to accept the results of the two independent groups' analyses. Quanta's Natalie Wolchover writes:
In an email, Jackson called Green and Moffat’s paper, which was published in Physics Letters B in September, “absolute rubbish.” When asked to elaborate, he appeared to wrongly characterize their argument and didn’t address the most important issues they raised about his team’s work. Jackson also dismissed the second set of findings by [Nielsen et al]. “We are in the process of writing a response to this latest paper,” Jackson wrote, so “I will not explain where they (once again) made their mistakes.”
“The Copenhagen group refuse to accept that they may be wrong,” Moffat said. “In fact, they are wrong.”
The fundamental issue was how the original analysis dealt with the inevitable noise in the data. The signal is extremely faint (on the order of a billionth of a billionth the diameter of an atom); that's why you need such sensitive detectors to pick them up at all. Each instrument is so sensitive that it also picks up small ambient vibrations, like a rumbling freight train or natural thermal vibrations in the detectors themselves. So the LIGO collaboration goes to great lengths to shield its instruments and minimize noise in its data.
A question of noise
Jackson began raising doubts soon after the first direct detection was announced on February 11, 2016. (The signals were detected several months earlier in September 2015.) LIGO detects gravitational waves via laser interferometry, using high-powered lasers to measure tiny changes in the distance between two objects positioned kilometers apart. (LIGO has detectors in Hanford, Washington, and in Livingston, Louisiana. A third detector in Italy, Advanced VIRGO, came online in 2016.)
Gravitational waves travel at the speed of light, but since the detectors are so far apart, there should be a slight time lag and a small adjustment in amplitude to account for the curvature of the Earth. The signals will be correlated in the data. The noise, however, should not be correlated. The Danish team concluded from its analysis that the noise was correlated at both the Hanford and Livingston detectors, potentially contaminating the data. For Jackson, this called into question LIGO's ability to sufficiently distinguish between signal and noise and, hence, the validity of the detection itself.
According to LIGO spokesperson David Shoemaker (MIT), the collaboration worked closely with Jackson's group over the last two years to improve their understanding of LIGO's methods. An October article in New Scientist (which LIGO Executive Director David Reitze denounced as "very biased and sensational") brought Jackson's claims back into the spotlight.
Bolstering the case for the validity of LIGO's detection is the number of additional detections it has made since then, most notably last year's binary neutron star merger, supported by a simultaneous gamma-ray burst and signals in the rest of the electromagnetic spectrum. And earlier this month, physicists from the LIGO and Virgo collaboration reported four previously unannounced detections of gravitational waves from merging black holes, bringing the total number of detections to 11. All four are part of the first official catalog of gravitational wave events (called the Gravitational Wave Transient Catalog, or GWTC-1), listing all events detected to date.
It's worth noting that even those physicists who went on the record with New Scientist in support of the Danish team's minority opinion still thought the LIGO results would hold up. They emphasized the need for independent confirmation of LIGO's analysis. And now they have it.
Both new papers re-analyzed the LIGO data using different algorithms than those used by the LIGO collaboration and were able to identify the same gravitational signals that the collaboration found. Green and Moffat also noted several errors in how Jackson et al. handled their data, leading to the appearance of a noise correlation that isn't really there. "Our concern is that the calculation done by the Copenhagen group was contrived to get the result they wanted to get," Green told Quanta.
LIGO didn't escape criticism, either. Nielsen et al. think that some of the Jackson team's errors resulted from Figure 1 in LIGO's 2016 Physical Review Letters paper describing their discovery. Pieces of that figure were illustrative, something that was not made crystal clear at the time. LIGO's claims of detection weren't based on that plot but instead were based on a more rigorous analysis. Jackson et al., however, based their code on Figure 1's template waveform, leading to false correlations in the noise.
Jackson has accused LIGO scientists of "misconduct" by using that figure and of "violating one of the central canons of good scientific practice." And he stubbornly maintains that his team has found noise correlations in the data, that the new independent results are incorrect, and that LIGO's first gravitational wave detection in particular was probably a distant lightning strike or seismic vibration. He plans to publish a rebuttal soon.
For most physicists, however, LIGO has been vindicated. “Seeing those two non-Collaboration re-analyses does reaffirm my certainty that the detections [of gravitational waves] are genuine,” Shoemaker told Quanta, “and also is a reinforcement of our earlier perception of where the Jackson et al. paper has problems. That the Jackson et al. work has stimulated some additional independent investigations can be seen as a positive outcome, but I personally think it comes with a fully unnecessary cost of ‘drama.’”