Choose any object in the universe; It’s probably turning. Asteroids crash into each other, planets and moons spin around their axes, and even black holes spin. And everything that rotates has a maximum speed at which it can rotate. The black hole in our galaxy is spinning at almost maximum speed.

For objects like the Earth, the maximum spin rate is determined by surface gravity. The weight we feel when we stand on the Earth is not only due to the Earth’s gravitational force. Gravity pulls us towards the center of our earth, but the Earth’s rotation also tends to push us outward from the Earth. This “centrifugal” force is small, but it does mean that your weight at the equator is slightly less than at the north or south pole.

In today’s 24-hour conditions, the weight difference between the equator and the pole is only 0.3%. But Saturn’s 10-hour day means the difference is 19%. So much so that Saturn curves slightly outward at the equator. Now imagine a planet rotating so fast that the difference is 100%. At this point, the planet’s gravity and centrifugal force at the equator will disappear.

If the world rotated faster, it would fly away. It will probably explode at an even lower spin rate, but this is clearly the maximum spin rate. The situation is slightly different in black holes. Black holes are not objects with physical surfaces. They are not made of material that can blow away. But they still have maximum rotation speed.

Black holes are defined by their immense gravity that distorts 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 physical surface. The rotation of a black hole is also determined not by the rotation of physical mass, but by the bending of space-time around the black hole. When objects such as the Earth rotate, they bend the space around them by a very small amount. This is an effect known as frame dragging.

The rotation of the black hole is determined by this frame drag effect. Black holes rotate without physical rotation of matter, only a distorted space-time structure. This means that there is an upper limit to this rotation due to the inherent properties of space and time.

In Einstein’s equations of general relativity, the spin of a black hole is measured by a quantity known as a; where A must be between zero and one. If the black hole has no spin, A = 0, and if it has maximum spin, A = 1.

This brings us to a new study of the spin of the supermassive black hole in our galaxy. The team looked at radio and X-ray observations to estimate the black hole’s spin. Due to the drag of space-time near a black hole, the spectrum of light from nearby material is distorted. The team was able to estimate the amount of rotation by observing the intensity of light at different wavelengths.

They found that the A value for our black hole is between 0.84 and 0.96, which means that the black hole is spinning incredibly fast. It will spin at nearly maximum speed in the upper range of the calculated spin. This is even higher than the spin parameter of the black hole in M87, where A is estimated to be between 0.89 and 0.91. Source