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.

Lucy Reading-Ikkanda/Quanta Magazine

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.