To measure a black hole's ultimate spin, we have to go to space‎

Despite their depiction as massive monsters that simply suck in everything, including light, astronomers know black holes actually spin. And they spin really, really quickly. Determining just how quickly is key to understanding how they affect their immediate vicinity and the galaxies that surround them. A new paper by Tegan Thomas of the University of Virginia and her colleagues, available on the arXiv preprint server, has good news and bad news on that front. The bad news is that we currently can't determine how fast black holes are actually spinning. The good news is that, hopefully in the next few years, we will have a new tool that will allow us to do so.