from the feel-the-noize dept.
Gravitational waves were the most controversial and difficult to verify prediction of Albert Einstein's Theory of General Relativity, so much so that at one point even Einstein himself thought that they might just be an artefact of the mathematics. It wasn't until the 1970s that careful observations of binary pulsar systems showed the indirect effects of gravitational waves, and not until 2016 that LIGO, an extremely sensitive instrument designed to detect gravitational waves directly, managed to detect the gravitational waves from the merger of two black holes. It has made two more gravitational wave detections since. However, a new analysis of the LIGO data by an independent team led by Prof. Andrew D. Jackson at the Niels Bohr Institute in Copenhagen has cast doubt on the detections, hinting that they might just be seeing patterns in the noise. Sabine Hossenfelder has an article on this:
A team of five researchers — James Creswell, Sebastian von Hausegger, Andrew D. Jackson, Hao Liu, and Pavel Naselsky — from the Niels Bohr Institute in Copenhagen, presented their own analysis of the openly available LIGO data. And, unlike the LIGO collaboration itself, they come to a disturbing conclusion: that these gravitational waves might not be signals at all, but rather patterns in the noise that have hoodwinked even the best scientists working on this puzzle.
The LIGO gravitational wave observatory consists of two experimental sites – one in Livingston, Louisiana and one in Hanford, Washington – each of which is a laser interferometer with arms that are several kilometers in length. Even for these super-sensitive detectors, however, gravitational waves are difficult to measure. The problem isn't so much the absolute weakness of the waves, the problem is that there are many other disturbances that also wiggle the interferometer. The challenge, thus, is to tell the signal from the noise.
[...] The Danish group found, however, that the noise at both detector sites — and puzzlingly, between the two supposedly independent detectors — is also correlated. And worse, the correlation time is similar to the time-lag between the recorded signals, for each of the three so-far confirmed events. According to Andrew Jackson, the leader of the Danish group,
"If the correlation properties of signal and the noise are similar, how is one to know precisely what is signal and what is noise?"
That's a really important realization. A correlation in the noise would not affect the individual signals at each of the sites. But in order to achieve a highly significant signal between the detectors, the LIGO collaboration takes into account how both signals are correlated. If this correlation were not reliable, because (for example) there was the possibility that noise correlations contaminated their data, the statistical significance of the detection would be reduced. In other words, what appears to be a signal might actually be caused merely by fluctuations. How much the statistical significance would be affected, however, the Danish researchers have not quantified.
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Scientists are claiming a stunning discovery in their quest to fully understand gravity. They have observed the warping of space-time generated by the collision of two black holes more than a billion light-years from Earth. The international team says the first detection of these gravitational waves will usher in a new era for astronomy.
It is the culmination of decades of searching and could ultimately offer a window on the Big Bang. The research, by the LIGO Collaboration, has been accepted for publication in the journal Physical Review Letters. The collaboration operates a number of labs around the world that fire lasers through long tunnels, trying to sense ripples in the fabric of space-time. Expected signals are extremely subtle, and disturb the machines, known as interferometers, by just fractions of the width of an atom. But the black hole merger was picked up by two widely separated LIGO facilities in the US.
The historic paper in question: Observation of Gravitational Waves from a Binary Black Hole Merger (open, DOI: 10.1103/PhysRevLett.116.061102)
Archived video of the press conference webcast will be available here.
NASA provided an infographic for their Astronomy Picture of the Day feature with details about the discovery.