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Surfing space-time — gravitational waves

John Healy

Asymmetric gravitational interactions between two bodies produce gravitational waves.1 Such perturbations in space-time are so small that only the most massive interactions have been detected by LIGO, which uses laser interferometry and is “the largest and most ambitious project ever funded by the NSF.”

While there was speculation about gravitational waves prior to Einstein’s theory of general relativity, his prediction energized the quest to detect them.

Wiki: On February 11, 2016, the LIGO Scientific Collaboration and Virgo Collaboration teams announced that they had made the first observation of gravitational waves, originating from a pair of merging black holes using the Advanced LIGO detectors. Since the initial announcement LIGO has confirmed two more (and one potential) detections of gravitational wave events.

Like the discovery of the Higgs boson, there’s a lot of mathematics and digital processing required to analyze experimental data to confirm any detections. Like the LHC, LIGO is a huge collaborative effort, with online data sets available for review and analysis.

Which gets me to this guest post by Ian Harry, postdoctoral physicist at the Max Planck Institute for Gravitational Physics, Potsdam-Golm, on Sean Carroll’s blog regarding the recent detection of signals from inspiralling black holes. As noted above, data analysis in these experiments is complex; so, peer reviews witness the dynamic process of science in discovering new knowledge.2 Here’s Harry’s overview (highlights are mine).

In this article I will go into some detail to try to refute the claims of Creswell et al. Let me start though by trying to give a brief overview. In Creswell et al. the authors take LIGO data made available through the LIGO Open Science Data from the Hanford and Livingston observatories and perform a simple Fourier analysis on that data. They find the noise to be correlated as a function of frequency. They also perform a time-domain analysis and claim that there are correlations between the noise in the two observatories, which is present after removing the GW150914 signal from the data. These results are used to cast doubt on the reliability of the GW150914 observation. There are a number of reasons why this conclusion is incorrect: 1. The frequency-domain correlations they are seeing arise from the way they do their FFT on the filtered data. We have managed to demonstrate the same effect with simulated Gaussian noise. 2. LIGO analyses use whitened data when searching for compact binary mergers such as GW150914. When repeating the analysis of Creswell et al. on whitened data these effects are completely absent. 3. Our 5-sigma significance comes from a procedure of repeatedly time-shifting the data, which is not invalidated if correlations of the type described in Creswell et al. are present.

Despite data near the experimental limits of accuracy at the time, in 1919 the detection of the bending of light — as predicted by Einstein’s theory of general relativity — was greeted with public fanfare.3 While the process of science embraces skepticism, discoveries in today’s cultural din face skepticism toward science itself. Skilled science communicators and public outreach are needed more than ever.

 

[1] Space.com, June 16, 2017, “‘We Don’t Planet’ Episode 8: Gravitational Waves.”

Even then, the confirmed gravitational waves barely nudged the sensitive LIGO apparatus by less than the width of an atom.

[2] This is a point which Otto repeatedly makes in his book.

The process of science is designed to cull out reliable knowledge — no matter who does the investigating or reports on the outcome … trimming away all those subjective forms of bias reporters think we can never escape until we are left with knowledge that is provisionally objective in the stories we tell about reality. While it may not be possible to attain total objectivity, approaching it is what science is all about, and the reliable knowledge it produces is responsible for every advance in the modern world. — Otto, Shawn Lawrence (2016-06-07). The War on Science: Who’s Waging It, Why It Matters, What We Can Do About It (Kindle Locations 562-567). Milkweed Editions. Kindle Edition.

[3] And further work in the 20th century confirmed the deflection of light. Such is the process of science. Today gravitational lensing is well documented in astronomical images.

One thought on “Surfing space-time — gravitational waves”

  1. Here’s a link to a Quanta Magazine article “Strange Noise in Gravitational-Wave Data Sparks Debate” regarding recent LIGO data.

    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.

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