Our universe is filled with vast mysteries that are yet to be fully understood and completely grasped by its residents. One such beautiful mystery is that of black holes. A black hole is defined as a region of spacetime where gravity is so strong that nothing, not even light, can escape its clutches. These gravitational anomalies are formed when stars that run out of their fuel collapse under their own weight due to the strong effect of gravity. These mysterious bodies absorb matter, turning into a smooth, featureless singularity having only the most basic of properties: Mass, Charge, and Spin
You read it right! Black holes have the property of spin. We know that stars spin around their own axis and will tend to continue to keep spinning because of the law of conservation of angular momentum. However, as stars constantly keep losing mass, they start spinning faster and faster. So as the core of the star collapses, the momentum makes it spin at tremendous speeds till it collapses into a black hole that continues spinning.
Spinning black holes, like their conventional, static counterparts, contain a point where all their mass is concentrated. This special point is known as a ringularity, a ring without any thickness and surface that spins at approximately 90% of the speed of light. This high-speed rotation of the ringularity distorts the entire space-time and creates a region known as the ergosphere, where the black hole forces its own rotational energy. This makes it practically impossible for any object to exit the ergosphere as it would require the object to travel at the speed of light to just stay still.
So, what makes these black holes so different from their non-spinning counterparts? Well, in the far future it may actually be possible to steal energy from these astronomical wonders.
This was theorized by British Physicist, Roger Penrose. He stated that a lump of matter entering the ergosphere can be split into 2 different parts. The momentum of the two pieces of matter when they separate can be arranged so that one piece escapes from the black hole (it "escapes to infinity"), whilst the other falls past the event horizon into the black hole. With careful arrangement, the escaping piece of matter can be made to have greater mass-energy than the original piece of matter, which would mean that the piece falling in has negative mass-energy. Although momentum is conserved, the effect is that more energy can be extracted than was originally provided. This difference in energy being provided by the black hole itself.
This entire process allows the escaping piece to exit with much higher mass-energy than it entered with. In addition, it shortens the size of the black hole and slightly reduces its angular spin. Conversely, we can increase the spin of the black hole by inserting particles that give their entire angular momentum. However, to create such a structure itself would take enormous amounts of time, resources, and energy.
To conclude, while it is far from certain whether we will ever be able to reach this point in the near future, if we manage to overcome this challenge, the only major limitation that will be left is our own imagination.
-Sagnik Majumder