Artist’s illustration of two merging neutron stars. The rippling spacetime grid represents gravitational waves emitted from the collision, while the narrow beams are the jets of gamma rays that shoot out just seconds after the gravitational waves (detected as a gamma-ray burst by astronomers). Image credit: NSF / LIGO / Sonoma State University / A. Simonnet.

LIGO’s Greatest Discovery Almost Didn’t Happen

The neutron star-neutron star merger was initially only seen in 1 detector out of 3. Here’s how scientists didn’t let it slip away.

On August 17th, 2017, a gravitational wave event unlike any other showed up in one of the LIGO detectors: at Hanford, WA. Just a few days prior, the first black hole-black hole merger with all three detectors running — LIGO Livingston, LIGO Hanford, and Virgo — was detected. This time, a new event was recorded, but instead of having 1–2 seconds of data, the significance lasted over a minute. With a false alarm probability of only one-in-300 billion (3 × 10^–12), an alert went out to everyone on the team. But LIGO Livingston, which had come through every time before, showed nothing. Without a signal in all detectors, there was no “event” to declare. Without confirmation, this would merely go down as a false alarm.

The Omega Scan of the LIGO Hanford data, giving the first gravitational wave signal arising from a neutron star-neutron star merger. Image credit: B.P. Abbott et al., PRL 119, 161101 (2017).

Fortunately for us, scientists are passionate about what they do, and don’t simply leave the results up to computers or automated algorithms. Two minutes after the alert went out, what’s known as an “omega scan” came back, showing a new kind of…