The Milky Way’s black hole is spinning as fast as it can

Pick any object in the universe and it’s probably spinning. Asteroids roll off end, planets and moons spin on their axes, and even black holes spin. And for anything that spins, there is a maximum speed at which it can spin. The black hole in our galaxy is spinning at nearly full speed.

For objects like Earth, the maximum rotation speed is defined by its surface gravity. The weight we feel when we stand on the ground is not just due to the earth’s gravitational pull. Gravity pulls us toward the center of the universe, but Earth’s rotation also tends to pull us outward. This “centrifugal” force is very small, but it means that your weight at the equator is slightly less than it is at the North or South Pole.

With our 24 hour day, the difference in weight between the equator and the pole is only 0.3%. But 10 hours of Saturn day means that the difference is 19%. So much so that Saturn tilts slightly outwards at its equator. Now imagine a planet rotating so fast that this difference is 100%. At that point, the planet’s gravitational pull and its centrifugal force at the equator are neutralized. If the world were to turn faster. It would have separated probably at a slower spin rate, but this is clearly the maximum spin rate.

For black holes, things are a little different. Black holes are not objects with a physical surface. They are not made of materials that can be separated. But they still have a maximum rotation speed. Black holes are defined by their extreme gravity, which distorts the space and time around them. The event horizon of a black hole defines the point of no return for nearby objects, but it is not a physical surface.

The rotation of a black hole is not defined by the rotation of physical mass, but by the rotation of spacetime around the black hole. When objects like Earth rotate, they twist the space around them very little. This is an effect known as frame dragging. The rotation of a black hole is defined by this frame cache effect. Black holes spin without physical rotation of matter, just a complex 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 rotation of a black hole is measured by a quantity called Awhere A It must be between zero and one if the black hole is spinless, a = 0, and a = 1 if it is at its maximum spin.

This brings us to a new study of 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. Due to the framing of spacetime near a black hole, the spectrum of light from nearby matter is distorted. By observing the intensity of light at different wavelengths, the team was able to estimate the amount of rotation. What they found was that A Our black hole’s value is between 0.84 and 0.96, which means it’s spinning incredibly fast. In the upper range of the estimated rotation, it spins at almost maximum speed. This is even higher than the black hole spin parameter in M87, where A It is estimated between 0.89 and 0.91.

Reference: Daly, Ruth A., et al. “New Black Hole Spin Values ​​for Sagittarius A* Obtained by the Output Method.” Monthly Notices of the Royal Astronomical Society (2023): stad3228.