Pick any object in the universe, and it’s probably rotating. Asteroids roll end over end, planets and moons spin on their axes, and even black holes spin.
For everything that rotates, there is a maximum limit to which it can rotate. The black hole in our galaxy rotates at approximately this maximum rate.
For objects such as the Earth, the maximum rate of rotation is determined by their surface gravity. The weight we feel while standing on the Earth is not only due to the Earth’s gravity.
Gravity pulls us toward the center of our universe, but Earth’s rotation also tends to fling us away from Earth. This “centrifugal” force is very small, but it means that your weight at the equator is slightly less than at the North or South Pole.
In a 24-hour day, 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 bends slightly outward at the equator.
Now imagine a planet rotating so fast that the difference is 100%. At that point, the planet’s gravitational and centrifugal force at the equator will disappear.
If the world spins faster, it will fall apart. It would probably fly away at a slower spin rate, but that’s clearly the maximum spin rate.
For black holes, things are a little different. Black holes are not objects with a physical surface. It is not made of a material that can fly off. But they still have a maximum turnover rate.
Black holes are characterized by their enormous gravity, which distorts space and time around them. The black hole’s event horizon represents the point of no return for nearby objects, however It is not a physical surface.
The black hole’s rotation is also determined not by the rotation of physical mass, but by the warping of spacetime around the black hole. When objects like the Earth rotate, they twist space around themselves very slightly. This is an effect known as Pull the frame.
The black hole’s rotation is determined by the effect of the frame’s drag. Black holes rotate without the physical rotation of matter. Just a warped 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, which must be between zero and one. If the black hole has no spin, then a = 0, and if it has maximum spin, then a = 1.
This brings us to a new study about the rotation 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.
Because the space-time frame is pulled close to the black hole, the spectra of light coming from matter near it are distorted. By monitoring the intensity of light at different wavelengths, the team was able to estimate the amount of rotation.
What they found is that our black hole has a value between 0.84 and 0.96, which means it is spinning incredibly fast. In the upper range of rated rotation, it will rotate at approximately the maximum rate.
This is higher than the black hole rotation coefficient in M87, which is estimated to be between 0.89 and 0.91.
“Devoted student. Bacon advocate. Beer scholar. Troublemaker. Falls down a lot. Typical coffee enthusiast.”