Strange Noise in Gravitational-Wave Data Sparks Debate

In February 2016, the leaders of the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced that they had successfully detected gravitational waves, subtle ripples in the fabric of space-time that had been stirred up by the collision of two black holes. The team held a press conference in Washington to announce the landmark findings.

They also released their data.

Now a team of independent physicists has sifted through this data, only to find what they describe as strange correlations that shouldn’t be there. The team, led by Andrew Jackson, a physicist at the Niels Bohr Institute in Copenhagen, claims that the troublesome signal could be significant enough to call the entire discovery into question. The potential effects of the unexplained correlations “could range from a minor modification of the extracted wave form to a total rejection of LIGO’s claimed [gravitational wave] discovery,” wrote Jackson in an email to Quanta. LIGO representatives say there may well be some unexplained correlations, but that they should not affect the team’s conclusions.

On June 13, 2017, Jackson and four co-authors published their criticism on the scientific preprint site arxiv.org. The paper generated considerable interest, prompting Ian Harry, a researcher at the Max Planck Institute for Gravitational Physics in Potsdam-Golm and a member of the LIGO Scientific Collaboration, to publish a public rebuttal five days later. Harry argued, in effect, that the independent team missed some subtleties in their data analysis, and that he couldn’t reproduce the claimed correlations. Jackson’s team then replied that they had found errors in Harry’s code, and that their argument stood. In an email to Quanta, Harry responded that he had corrected the typo in his code even before Jackson’s team published, and that in any case the error did not affect his analysis.

The technical issues at stake here have to do with the extreme difficulty of the measurements that LIGO attempts to make.

Gravitational waves are exceedingly faint, so to catch them LIGO was built with the ability to measure a change in distance just one-ten-thousandth the width of a proton. Lots of little bumps and vibrations can mimic a gravitational-wave signal, so LIGO uses two observatories, 3,000 kilometers apart, which operate synchronously, each double-checking the other’s observations. The noise at each detector should be completely uncorrelated — a jackhammer going off in the town near one detector won’t show up as noise in the other. Yet if a gravitational wave swoops through, it should create a similar signal in both instruments nearly simultaneously.

The main claim of Jackson’s team is that there appears to be correlated noise in the detectors at the time of the gravitational-wave signal. This might mean that, at worst, the gravitational-wave signal might not have been a true signal at all, but just louder noise.

A far more likely scenario is that the correlations in the noise, if real, point to something else. Perhaps the LIGO team subtracted the gravitational-wave signal from the raw data in such a way that it left a little correlated noise behind. Or perhaps there’s a small amount of correlation in the noise that caused the LIGO scientists to misinterpret their gravitational-wave signal. Vicky Kalogera, an astrophysicist at Northwestern University and a member of the LIGO team, said that the correlated noise, if significant, could cause a bias in the result that could “tell us potentially wrong information about the black holes” that created the gravitational waves.

But not everyone believes that the correlations are real. Harry, in his rebuttal, points out that Jackson’s team could have misused a common data-processing technique called the Fourier transform. The Fourier transform separates a data signal into a collection of simpler waveforms. The error, Harry writes, has to do with the technical assumption that the input data signal be “cyclical,” repeating itself without any breaks or discontinuities. For example, a cyclical sound wave would be the repetition of a sound clip without a pop in between each repetition. A signal that isn’t cyclical cannot be analyzed through the Fourier transform without introducing subtle errors. Otherwise, the so-called Gibbs phenomenon distorts the input signal’s frequencies, thus decreasing the accuracy of the ensuing analysis.

Since real-life data is almost never cyclical, anyone doing Fourier analysis must first execute an array of cleanup jobs on the raw data. “It looks like some of the results [of Jackson’s team] had to do with not pre-filtering the data before taking the Fourier transform,” said David Shoemaker, a physicist at the Massachusetts Institute of Technology and spokesperson for the LIGO Scientific Collaboration, echoing Harry’s public analysis.

Jackson, who declined to be interviewed for this article, writing in an email that “public polemics tend to harden positions and do not advance the desired end,” disputes this characterization. “We are aware of these issues. We neither agree with nor accept Harry’s views,” he wrote. Jackson’s four co-authors did not respond to Quanta’s requests for comments.

For now, confidence is high in LIGO’s conclusions. “The only persons qualified to analyze this paper are in the LIGO Scientific Collaboration,” said Robert Wagoner, a theoretical physicist at Stanford University who is not affiliated with LIGO. “They are the only ones who have had access to the raw data.” Steinn Sigurðsson, an astrophysicist at Pennsylvania State University who is also not affiliated with either team, agrees. “For now, I’d definitely go with the LIGO people,” he said. “It is very rare for outsiders to find major errors in a large collaboration.”

Nevertheless, “it’s going to take longer than people would like” to get these issues resolved, said Sigurðsson. “It’s going to take months.”

The LIGO team later reported that they had found gravitational waves from a second black-hole merger, then a third. Jackson and his colleagues have not yet published any analysis of these events.

What of the controversy, then? “There is no drama here,” Kalogera said. “It’s science as usual. … Healthy, positive communication is very much welcome amongst scientists.”

This article was reprinted on Wired.com and in Spanish at Investigacionyciencia.es.