Unless Einstein is wrong, the black hole is determined by three properties: mass, rotation, and electric charge. The charge of a black hole should be almost zero, as the material captured by the black hole is electrically neutral. The mass of a black hole determines the size of its event horizon and can be measured in several ways, from the brightness of the material around it to the orbital motion of nearby stars. The spin of a black hole is much more difficult to study.
The rotation of a black hole is basically its rotation. Just as stars and planets rotate, so do black holes. The difference is that black holes do not have a physical surface like stars and planets. The spin of the black hole, like the table, is a property in space-time. Spin determines how the space around a black hole is distorted. To measure the rotation of a black hole, you need to study how matter holds close to it.
The rotation measurement of some supermassive black holes was measured. With a few active black holes, we can study the X-rays emitted by their accretion discs. The X-ray light from the disk receives an amplification of the energy from the rotation and by measuring this thrust we can determine the rotation. Another way is to make a direct image of the black hole, as we did with the one in the center of the M87. The ring of light we see is brighter than the side that turns toward us.
But we don’t know the rotation of the nearest supermassive black hole, the one in our own galaxy. Our black hole is not very active and is much smaller than the one in M87. We cannot measure its rotation by observing light near it. But a new paper in Astrophysical Journal Letters claims that there is another way to measure spin.
Their method uses a property known as sliding frames. When a table rotates, it slightly rotates the space around it. We know it’s real because we’ve measured the drag effect of the Earth’s rotation. The rotation of a black hole creates the same kind of sliding of the frame and by measuring it we can determine the rotation of the black hole. We can’t put a probe in orbit around a black hole the way we did with Earth, but we can use the next best thing.
Hundreds of stars orbit the black hole at the center of our galaxy. About forty of them, known as S-stars, have orbits close to the black hole. Over time, their orbits shift from the sliding effect of the frames. If we can measure these displacements, we can measure the spin – the greater the rotation, the greater the displacement of the orbit.
In this new work, the team studied the orbits of S-stars and found no change in the sliding of the frame. Given how well we know the orbits of these stars, we know that the black hole at the center of our galaxy must rotate slowly. The team determined that its rotation can be no more than 0.1 on a scale from 0 to 1, which means that it rotates less than 10% of the maximum possible rotation for a black hole. In contrast, the rotation of the black hole of the M87 is at least 0.4.
Reference: Fragione, Giacomo and Abraham Loeb. “Upper limit of SgrA’s rotation * Based on stellar orbits in its vicinity.” Astrophysical Journal Letters 901.2 (2020): L32.
Reference: Nemmen, Rodrigo. “Rotation of M87 *.” Astrophysical Journal Letters 880.2 (2019): L26.