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Kerr (spinning) Black Holes

By Oleksandr Koliakin


Black holes that are born when stars die are usually Kerr black holes. The reason behind this is that stars usually spin. However, smaller objects spin faster than big objects when the same amount of energy is applied. This is because the parts of a sphere that are closer to its center need to travel a smaller amount of distance to cover the same angle.





The image above is an example of 2 objects orbiting around a different object. The object closest to the more massive object should travel faster than the more distant object if they have the same mass, diameter, initial velocity, etc.

 

Because some black holes are born when extremely massive stars collapse, and the stars are spinning, the black hole also keeps their angular momentum (rotation/spin). However, since black holes are smaller than the stars that died to create them (that is, before objects don't start falling into the black hole), black holes spin a lot faster than the stars that died to create them. Black holes can spin so fast, that they can drag spacetime with them, creating a new region of spacetime: the ergosphere. The ergosphere surrounds the Kerr black hole like a squashed sphere, since the ergosphere is (for a lack of better word) smaller at the poles of the black hole.


Since the ergosphere drags spacetime with it, it can 'push' anything that falls into it to make the object travel around in circles inside the ergosphere, until it falls beyond the event horizon of the black hole. However, objects can usually leave the ergosphere if they have enough energy to do so. This could, in theory, be used to accelerate rockets travelling to the distant stars in our galaxy.


Sources:

  • "Black Hole Bomb" video by the Kurzgesagt YouTube channel

  • "SCIENCIA" (book)

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