Spinning Black Holes:
Black Holes are really mysterious and amazing things. But something more mysterious and amazing is a spinning Black Hole. With non-point singularity and particle emission, they are just something that sounds like it shouldn’t be possible. A rotating black hole is a black hole that possesses angular momentum. In particular, it rotates about one of its axes of symmetry.
Laws of Motion:
The First Law of Motion states that something that is still, wants to stay still, while something in motion wants to stay in motion.
In an inertial frame of reference, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.
Likewise, something that is rotating, wants to keep rotating at the same velocity, and if it’s size is decreased, it’s angular velocity increases, as it’s circumference is decreased.
Implementation of the First Law:
Every star rotates upon its axis. Upon its death to form a Black Hole, it becomes smaller, so it’s angular velocity increases. This is required to prevent the violation of the First Law. Therefore, a Black Hole must be spinning, very very rapidly.
But this conclusion has a major flaw. The singularity is a point, and a point cannot rotate. If it rotates it is no longer a point, as points simply cannot spin.
This makes things interesting at the singularity. Where does all that spin energy go? Well, a simple hypothesis is that the singularity no longer stays a point, it becomes a ring, aka, a ringularity. A ringularity is not a point, it is a ring with zero surface area. But it solves the spin paradox, as rings, even with zero surface area, can spin.
The Merry-Go-Round:
Black Holes spin extremely rapidly. Just like a Ballerina, or an Ice-skater pulling his/her arms in (i.e., becoming smaller), the star starts spinning more and more rapidly while transforming into a Black Hole. This causes the spin factor of the Black Holes to reach astronomical speeds. A Black Hole, ASASSN-14li, is spinning at 50% the speed of light, or nearly 149,896,229 m/s. It completes its rotation every 2 minutes, compared to 24 hours taken by our earth. But things get weirder…
Non-Black Black Holes?
The very definition of Black Holes is that it is black, a dark void in space emitting absolutely nothing, not even light. This makes Black Holes hard to find and observe. But spinning Black Holes are weird. They bend our very understanding of Black Holes. They can actually emit light and other particles, making them observable.
Star-food and acceleration disks:
When a star comes near a supermassive Black Hole, it does not end well for it. As it approaches, one side of the star experiences a much stronger gravitational pull ripping the star to shreds. The matter starts orbiting the black hole in a disk-like form, known as an acceleration disk.
This is called a Tidal Disruption Event, aka TDE. The matter spins rapidly, the Black Hole’s spin contributing to it, and it starts to heat up, radiating visible light and other waves in flashes. This can actually be measured from the earth and the Black Hole’s spin and mass can be precisely calculated.
Naked Singularities: A view into the depths
Any massive object has an ISCO, an Innermost Stable Circular Orbit. It is the closest orbit to the object a body can have without being pulled in and crashing with our massive object. Black Holes too have an ISCO.
The ISCO of a black hole can be brought closer by increasing the spin. The faster the black hole spins, the closer its ISCO is.
This however leads to another interesting problem, at par with the weirdness of spinning Black Holes. The spin of a Black Hole can be increased to such an amount, that the ISCO will be smaller than the Event Horizon.
That means objects can have stable orbits even inside the event horizon! Now that should not be possible. It implies that light can escape from the black hole and we can see into the singularity!
This makes many scientists uncomfortable. Therefore there is a maximum amount of spin a Black Hole can have. Now theoretically, there is no obstacle preventing Black Holes from spinning at higher speeds than the max. It is just we haven’t seen one yet. But still, the fact that Naked Singularities may be possible is fascinating.
The highest speed a Black Hole has ever been found to be spinning is >98% of the maximum. It was found in the Black Hole GRS 1915+105 in 2006.
The Ergosphere:
In my article, Introduction to Black Holes, I talked about the Ergosphere, a component of spinning Black Holes which I had not discussed there.
The Ergosphere is a region located outside the event horizon of a spinning Black Hole. The outer boundary touches the event horizon on its northern and southern poles. It is possible to enter this region and then leave again, but it won't be a pleasant experience as in the ergosphere, you have to move faster than light just to stay still, as the black hole transfers its rotational energy to everything in the ergosphere. The Ergosphere is like a whirlpool, everything rotating and you have to travel quite fast to stay still.
The Speculation: Free energy?
The greatest reason for all the fuss with the Ergosphere is that it is theoretically possible to extract free energy from it.
The procedure is not complex at all. As mentioned above, the Ergosphere is like a whirlpool. Just like in a whirlpool, we can use the rotational energy to our advantage.
We simply need to launch a rocket into the Ergosphere. If done correctly, the rocket will return with a higher speed than it left with. The Black Hole will lose a bit of its rotational energy in the process, but it can be easily restored by feeding it all our waste, as Black Holes are not picky eaters at all.
Conclusion:
If Black Holes were weird, spinning Black Holes are even weirder. It is possible to find them, observe them, look into their singularity, get rid of all our waste, and get free energy. The world is weird.
-Rak Laptudirm
