Scientists around the world celebrated the first confirmed detection of gravitational waves several years ago, a discovery that resulted in Nobel prizes for Rainer Weiss, Kip Thorne, and Barry Barish in 2017. Since then, The American LIGO and Italian Virgo instruments have spotted many more waves from the collision of pairs of black holes and neutron stars. Now, scientists believe they’ve spotted a black hole gobbling up a neutron star for the first time ever.
Gravitational waves were predicted in general relativity, but no one had been able to verify their existence before LIGO came online. While these ripples in spacetime come from cataclysmic events like colliding black holes, the waves are extremely faint. That’s why LIGO (Laser Interferometer Gravitational-Wave Observatory) and Virgo use a technique called laser interferometry. They bounce lasers off of reflectors at the end of long tubes — they’re 2.4 miles (4 kilometers) in the case of LIGO. The beams cancel each other out if they bounce back unaltered, but gravitational waves cause tiny fluctuations that produce a signal. LIGO can detect movements as small as a ten-thousandth the charge diameter of a proton.
It was clear from the start that the LIGO-Virgo detection on August 14th would be an interesting one. At first, it looked like the objects were in the so-called “mass gap,” meaning they were too small to be black holes and too large to be neutron stars. However, human analysis eventually confirmed the event, known as S190814bv, was almost certainly the collision of a black hole and neutron star.
This is the last type of gravity wave collision scientists have been looking for, having already seen several signals from pairs of black holes and neutron stars. It’s not a sure thing yet, although the initial analysis says S190814bv is a neutron star-black hole merger with greater than 99 percent confidence. The larger of the two objects is well within black hole territory, but the smaller one is a bit south of three solar masses. Between one and two solar masses, and the object is a neutron star. Above three, it may be a black hole.
Scientists are anxious to spot a neutron star colliding with a black hole because it could reveal much about these super-dense star remnants. The gravitational waves and any electromagnetic component of S190814bv could narrow down the dimensions of the neutron star, which would be a hugely important discovery in nuclear physics. We have only a basic understanding of neutron stars — their structure pushes even the best current models of particle physics to the breaking point. This distant gravitational event could be the first step in improving that understanding.