Pick any object in the Universe, and it’s probably rotating. Asteroids fall to infinity, planets and moons spin on their axes, and even black holes spin. And for everything that rotates, there is a maximum rate at which it can rotate. The black hole in our galaxy rotates at about that high rate.
For objects like Earth, the maximum degree of rotation is defined by the force of gravity on the surface. The weight we feel while standing on Earth is not just because of Earth’s gravity. Gravity pulls us to the center of our planet, but Earth’s rotation tends to push us outwards from Earth. This “centrifugal” force is small, but it means that your weight at the equator is slightly less than it is north or south.
In our 24-hour day, the difference in mass between the equator and the pole is 0.3%. But Saturn’s 10-hour day means the difference is 19%. So much so that Saturn bows slightly outwards at the equator. Now imagine a planet rotating so fast that the difference was 100%. At that time, the planet’s gravity and its equatorial force would end. If only the world could spin faster. it would fly apart. It may fly apart at a slower spin rate, but this is clearly a higher spin rate.
For black holes, things are a little different. Black holes are not objects with visible faces. They are not made of materials that can fly apart. But they still have a high turnover rate. Black holes are defined by massive gravitational forces, which distort space and time around them. The event horizon of a black hole marks the point of no return for nearby objects, but it is not a visible point.
The rotation of the black hole is also not explained by the rotation of the mass, but rather by the twist of time around the black hole. When objects like the Earth rotate, they twist their surroundings very slowly. It’s an effect known as frame dragging. The spin of a black hole is explained by this drag effect. Black holes rotate without the physical spin of matter, just a warped structure of time. This means that there is an upper limit to this throw due to the inherent properties of space and time. In Einstein’s equations of general relativity, the rotation of a black hole is measured by a quantity known as athere a must be between zero and one. If the black hole has no spin, then a = 0, and if it has a high spin, then a = 1.
This brings us to a new study of the orbit of the supermassive black hole in our galaxy. The team looked at radio and X-ray observations of the black hole to estimate its rotation. Due to the drag of the time frame near the black hole, the reflection of light from objects near it is distorted. By looking at the intensity of light at various wavelengths, the team was able to estimate the amount of spin. What they get is that a our black hole’s number is between 0.84 and 0.96, which means it’s spinning very fast. At the upper range of the limited rotation, it will be rotating at approximately the maximum rate. This is much higher than the spin parameter of the black hole in M87, where a it is estimated between 0.89 and 0.91.
Reference: Daly, Ruth A., et al. “New Values of the Black Hole Spin in Sagittarius A* Obtained by the Exit Method.” Monthly notices of the Royal Astronomical Society (2023): stad3228.