The Nobel prizewinning LIGO collaboration has published a paper describing in more detail than ever before how it analyses gravitational wave signals, partly in response to an investigation by听91av. But some physicists still say LIGO鈥檚 work contains errors.
Almost no one doubts that gravitational waves exist. They are a prediction of general relativity, a听highly successful physics theory. When the Laser Interferometer Gravitational-Wave Observatory (LIGO) first announced it had detected one in 2016 it was cause for celebration but not surprise.
But a team of researchers based at the Niels Bohr Institute in Copenhagen, Denmark, has since questioned whether LIGO鈥檚 signal analysis is reliable.
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LIGO鈥檚 detectors aim to spot space itself being rhythmically squeezed and compressed. They do this by firing lasers along tubes roughly 4 kilometres long and checking how the distance they travel changes. However, these changes are minuscule and the detectors pick up random noise, such as weak seismic tremors, as听well as gravitational waves.
To minimise the risk of noise creating false alarms, LIGO initially used two detectors situated 3000 kilometres apart. The noise experienced by each should be entirely different.
However, the Danish group claimed to have found similarities in the noise seen by both detectors when they observed that first gravitational wave. This suggested LIGO鈥檚 signal processing hadn鈥檛 been done properly, the team said.
A 91av investigation previously reported on听all this and exposed more irregularities in the presentation of LIGO鈥檚 data. This听prompted the LIGO collaboration to promise a full justification of its techniques, which has now been published.
The LIGO collaboration, which has been augmented by a third detector in Italy, is confident in its methods, the paper says. It says that 鈥渢here are no anomalous or unexpected correlations to be seen鈥�. It also suggests that the Danish analysis is flawed. 鈥淭he听Danish group neglected to听implement basic steps in the analysis,鈥� says Patrick Brady at the听University of Wisconsin-Milwaukee, a LIGO spokesperson.
That includes 鈥渨indowing鈥� the听data, which isolates particular wave frequencies for analysis. However, the Danish researchers still insist that windowing is a mistake as it skews the signal and听renders further analysis unreliable. 鈥淭he data windowing techniques that LIGO has adopted are known to distort phases,鈥� says Andrew Jackson, spokesperson for the Danish group.
Bad vibrations
Jackson and his colleagues also object to LIGO鈥檚 use of 鈥渨hitened鈥� data. This practice involves reducing the level of prominent frequencies in the signal, which arise because of the vibration of听wires that suspend the laser-guiding mirrors in the detector. The Danish group believes this also creates distortions.
It can鈥檛 prove this, the group says, because LIGO hasn鈥檛 released enough raw data. However, LIGO鈥檚听new paper points out that four independent groups have performed analyses of the available data, and their outcomes support LIGO鈥檚 conclusions.
Among those external scientists are John Moffatt and Martin Green at the Perimeter Institute for Theoretical Physics in Canada. They disagree with the Danish group鈥檚 analysis. 鈥淚 remain convinced that their analysis and听conclusions are not correct,鈥� says Green.
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