Blind Injection: The Crucial Part of LIGO Operations


By Emily LevesqueUniversity of Washington

The LIGO—Laser Interferometer Gravitational-Wave Observatory—detectors had been tested for years as part of the advanced upgrades, and on September 14, 2015, the detectors were in a phase with a few auxiliary systems still offline. One of those systems, known as the blind injector, had proven to be a crucial part of LIGO operations.

Artist’s impression of a neutron star
In 2017, LIGO detected gravitational waves emitted by two merging neutron stars. (Image: ESO/L. Calçada/M. Kornmesser/Public domain)

The Unusual Task of Blind Injection

Early on in the project’s development, a handful of team members had been given an unusual task: they were asked to occasionally inject fake signals, designed to look like real gravitational waves, into the data coming from the detectors.

These injections wound up testing the noise of detectors as well as the noise of the people that made up the LIGO collaboration. If a detector failed to catch one of these signals, it was a clear sign that the technology still needed work. If a signal was detected, the challenge became how the team responded. 

LIGO team members knew that faked signals could be injected into the data, but they also knew that LIGO might, at any moment, capture the real thing, and the work of the blind injection team was, true to its name, hidden from the rest of the collaboration.

LIGO noise curve
The blind injection was used to test LIGO sensitivity to noise. (Image: Stefan L. Danilishin, Farid Ya. Khalili, Haixing/Public domain)

Because of this, any signal that appeared in the data had to be treated as real. Team members would analyze the signal’s data, work through the math and physics to determine what might have produced it, and even draft a research paper announcing the result. 

The work would culminate in a final team-wide meeting where the blind injection team would be called forth to report on their work. Then, and only then, would the LIGO scientists find out if they had been working on an injected signal or a real detection.

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…And They Were Finally Detected

The blind injection system had worked incredibly well during the earlier phases of the project, testing the detectors with occasional fake chirps simulating colliding black holes and keeping the team on their toes. So far, every chirp that had been detected by LIGO and met with excitement by the team of collaborators had come from the blind injector. 

However, on the morning of September 14, 2015, the blind injector was off. Team members had been tinkering with the system’s calibration just the night before, preparing for the first official observations of LIGO’s advanced stage, but had left it off as part of LIGO’s final engineering tests. This meant that, when an immense chirp from two merging black holes appeared in the data, several LIGO scientists immediately knew that it might be the real thing.

Numerical simulations of the gravitational waves
Emitting gravitational waves are the direct result of two black holes merging together. (Image: NASA/Public domain)

The data were still carefully scrutinized. The team even considered the possibility of what they called a malicious injection, a faked signal secretly inserted by someone outside the blind injection team or even someone outside the collaboration. But, eventually, months of careful analyses and tests were complete. The September 14th chirp was real. LIGO had detected gravitational waves.

The discovery by the combined efforts of thousands of scientific heroes working on one of the largest and most complicated physics experiments in history. Decades of determination and dedication and groundbreaking engineering work by this team had finally succeeded, ushering in a new era of astronomy.

More Detections Yet to Come

Since that first discovery in 2015, LIGO has continued to successfully detect gravitational waves, finding more merging black holes and even picking up the first signs of other rarer phenomena. In August of 2017, LIGO made another groundbreaking discovery, detecting the longer chirp of two merging neutron stars. That chirp appeared in the Louisiana detector first, and in the Washington detector three milliseconds later, revealing that it had come from somewhere in the Southern Hemisphere.

One point seven seconds later, a space telescope detected a telltale quick burst of gamma rays from the same part of the sky, something that hadn’t been possible with previous discoveries since no light is emitted by colliding black holes. Astronomers had previously predicted that merging neutron stars could produce something called a kilonova, an extremely bright and brief flash of light observable across the entire electromagnetic spectrum and that this gamma-ray burst was the earliest signature of such an event.

Immediately, observers all over the world scrambled to point over 70 of the planet’s best telescopes at the patch of sky where the neutron star merger had happened, and they soon spotted the light of the kilonova in a nearby galaxy. This meant that, for the first time, astronomers could combine electromagnetic light and gravitational waves from a single event, gaining a new and rich combination of observations for studying stars, space, and gravity.

Common Questions about Blind Injection, the Crucial Part of LIGO Operations

Q: What was the unusual task during testing and operating LIGO?

In the early stages of the project’s development, a few team members were asked to occasionally inject spurious signals, just like real gravitational waves, into the data coming from the detectors. This task which was proven to be an essential part of LIGO’s operation, is called blind injection.

Q: What was the purpose of using blind injection?

The purpose of using blind injections was to test the noise of the people who worked around the LIGO and the noise of the detectors. If the detectors failed to catch one of the fake signals, it meant that the detectors needed more development. And if the signals were detected, then the challenge was all about how the team members responded to them.

Q: What happened on the morning of September 14, 2015?

The night before September 14th, the team members were mending the system’s calibration and preparing it for its first official observations, so they had left the blind injector off as part of the final engineering tests. On the morning of September 14, 2015, the detectors received signals. Later, analyses showed that LIGO had detected the real gravitational waves

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