Two black holes, each with accretion disks, are illustrated here just before they collide. With the new announcement of GW190521, we discovered the heaviest mass black holes ever detected in gravitational waves, crossing the 100 solar mass threshold and revealing our first intermediate mass black hole. (MARK MYERS, ARC CENTRE OF EXCELLENCE FOR GRAVITATIONAL WAVE DISCOVERY (OZGRAV))

Why 28 + 47 = 72, Not 75, For Black Holes

Even addition has to play by different rules for black holes.

How do you add 28 and 47 together? This simple math question helps us highlight the many different ways that people conceptualize numbers in their heads. Some of us break down 28 and 47 into 20 + 8 and 40 + 7, and then go from there. Equivalently, you can view them as 30–2 and 50–3, and then combine those results. Another approach is to split them into 25 + 3 and 50–3, with many other possible, and equivalent, approaches. As long as your methods are sound and you get the right answer, that 28 + 47 = 75, there isn’t really a wrong way to do it.

But for certain physical objects obeying the law of gravity, addition isn’t always so simple. If you merged a 28 solar mass black hole with a 47 solar mass black hole, the black hole you wind up with, at the end, would be 72 solar masses, not 75. In fact, for any two black holes that you merge together, you wind up with less mass than you started with. This isn’t due to a flaw in our math, but rather something very special about how gravity works. Here’s why merging black holes always lose mass.

When a black hole and a companion star orbit one another, the star’s motion will change over time owing to the gravitational influence of the black hole, while matter from the star can accrete onto the black hole, resulting in X-ray and radio emissions. If another black hole is orbiting instead, gravitational radiation will dominate. (JINGCHUAN YU/BEIJING PLANETARIUM/2019)