This simulation shows two stills from the merger of two massive black holes in a realistic, gas-rich environment. If the gas density is high enough, a black hole merger could produce an electromagnetic (light) signal: something that may have been seen in a spectacular 2019 event in both gravitational waves and optical light. (ESA)

Shocking New Observation: Merging Black Holes Really Can Emit Light

Light cannot escape from a black hole, no matter what. But when two black holes merge? They just might.

On September 14, 2015, history was made as the NSF’s twin LIGO detectors directly observed humanity’s first gravitational wave. From over a billion light-years away, two black holes of 36 and 29 solar masses each merged together, creating the ripples in spacetime that arrived on that fateful day. In an unexpected twist, NASA’s Fermi satellite from an unidentified location just 0.4 seconds later.

In the subsequent 5 years, LIGO has been upgraded and joined by Virgo, where some ~50 additional black hole-black hole mergers have been seen. In all those events, not a single one emitted gamma-rays, X-rays, radio waves, or any other gravitational wave signal. Until, that is, May 21, 2019, when . If true, it could cause us to rethink everything. Perhaps merging black holes do emit light, after all.

For the real black holes that exist or get created in our Universe, we can observe the radiation emitted by their surrounding matter, and the gravitational waves produced by the inspiral, merger, and ringdown phases. However, light can only be emitted from outside a black hole’s event horizon. (LIGO/CALTECH/MIT/SONOMA STATE (AURORE SIMONNET))