The defining characteristic of a Kerr black hole is, quite simply, its spin.
This rotation creates a unique distortion in spacetime around it, almost like a swirling vortex.
Imagine throwing a pebble into a still pond. The ripples move outward in circles. Now, picture throwing that pebble into a rapidly spinning whirlpool. The ripples would become distorted and twisted, just like the effect of a Kerr black hole's spin on spacetime.
Despite the fact that it is impossible to directly observe a black hole, astronomers can observe its influence on its surroundings with powerful telescopes by indirect obervations.
sag a* is a kerr black hole
The black hole at the centre of our galaxy is a Kerr black hole candidate: Studies using radio and X-ray observations confirm our Milky Way's supermassive, Sagittarius A*, is spinning super fast, fitting the description of a Kerr black hole.
This rapid rotation warps the surrounding spacetime, giving it a stretched, oval shape.
Our Sag A* is spinning at close to the speed of light.
Frame Dragging.
Also known as the the Lense-Thirring Effect. The following are a few bullet points regarding this interesting topic if you are new to it:
In 1918, Joseph Lense and Hans Thirring predicted frame dragging.
Any object with mass warps space-time around itself, so is not unique to black holes.
If this object is spinning, it will induce a second distortion as it twists space-time around itself.
If the space around this first mass is being dragged, then objects in or near this space should also affected.
Frame-dragging is yet another prediction of General Relativity, which describes how the rotation of a massive object, such as a black hole or a planet, can twist and distort the fabric of spacetime around it.
Initially, this effect was mathematically predicted in 1918 by physicists Joseph Lense and Hans Thirring, who used Einstein's equations to show how a rotating mass can drag spacetime along with it, kind of like a rotating sphere distorts a flexible surface— or kind of like a dough mixer in a bowl of dough. As the mixer rotates, it pulls and twists the dough around it. Similarly, a rotating black hole pulls and twists the fabric of spacetime around it. This is an imperfect analogy, but it gives us a good idea of what we're looking at.
ergosphere.
The ergosphere is a region outside a rotating black hole where objects cannot remain in place; they are dragged around by the black hole’s rotation due to that frame-dragging effect. This is the region where space-time itself is twisted, making it impossible for any object to stay still. It’s situated outside the event horizon and is shaped like an oblate spheroid.
What we love about this
"For those new to the topic, our discussion of Kerr black holes offers a wonderful wee introduction and a segue into related topic as such below. It's a fascinating area of astrophysics with so much to explore and understand and to...ponder!," says Siobhan, our editor.
An Exotic singularity.
Another distinctive aspect of Kerr black holes is their singularity. Unlike the point singularity of simpler black holes, a Kerr black hole's singularity takes the form of a ring. Having such a characteristic opens up discussions about exotic spacetime structures such as wormholes.
A Ring Singularity
Unlike the point singularity in a non-rotating (Schwarzschild) black hole, the singularity in a Kerr black hole is a ring-shaped one. Observations suggests this is due to the rotation, which causes the collapsing matter to spread out and form a ring.
Causality Violation
After crossing the outer event horizon, one encounters a second horizon called the Cauchy horizon. Beyond this, the normal rules of cause and effect as we understand them break down due to the extreme warping of spacetime, allowing effects to precede their causes. As weird and farfetched as it sounds, the equations of general relativity predict that the deterministic nature of spacetime ceases to apply past this boundary. Hence the possibility of time travel.
Closed Timelike Curves (CTCs):
In the region inside the Cauchy horizon, the Kerr solution to Einstein's equations permits the existence of closed timelike curves (CTCs). These are paths through spacetime that loop back on themselves, theoretically allowing an object to return to its own past.
If such CTCs exist, they could enable time travel in a manner where an effect could precede its cause, violating the current understanding of causality.
Naked Singularity Hypothesis
If the rotation is too fast, it is theorized that the event horizon could disappear, exposing the singularity directly. This would violate the cosmic censorship conjecture, which posits that singularities should not be observable.
To address the potential problem posed by naked singularities, the Cosmic Censorship Conjecture has been proposed relative to the naked singularity hypothesis. In 1969, Roger Penrose proposed the Cosmic Censorship Conjecture, which explains the nature of singularities predicted by general relativity.
In other work, Penrose proved that if an object such as a star collapses under its own gravity to a certain point, it would inevitably form a singularity—a region in spacetime where gravitational forces are so intense that they distort spacetime to an infinite degree. This work built on and significantly extended the earlier work by Robert Oppenheimer and others on gravitational collapse.
This is old news, yes, but Penrose's Nobel Prize recognized this as one of the key contributions, which he received relatively recently in 2020.
Alongside Roger Penrose's contributions was the research on black holes conducted by Reinhard Genzel and Andrea Ghez. They received the other half of the Nobel Prize jointly for their discovery of a supermassive compact object at the centre of our galaxy. Ghez, along with her team, used the Keck Observatory in Hawaii to observe the motions of stars in the same region. Their precise measurements of stellar orbits around an invisible object at the Galactic Centre also pointed to the existence of a supermassive black hole, with a mass equivalent to about four million suns.
As you can see, Penrose has been very busy with black holes, including Kerr black holes, which we'll ponder next. This will be a good segue on to the next section, the Penrose Process.
Penrose Process
This process outlines a mechanism by which energy can be extracted from a rotating black hole, presenting a potential source for future energy resources.
"First and foremost, the theoretical understanding of the Penrose process is still in its early stages, and significant research needs to be done to fully understand its mechanics and potential applications. Black hole energy extraction would also require overcoming the immense gravitational and technical obstacles, which remain highly uncertain. Even so, it is still an interesting thing to ponder without necessarily endorsing their validity. Thus, SP is not endorsing this part, but rather engaging in thought for the sake of curiosity and interest. We have only a blurry image of a black hole, let alone a method of capturing its energy." - Siobhan Macrae (editor of SP & BSc Physics graduate)
Energy Extraction Mechanism
An object enters the ergosphere and splits into two parts.
One part falls into the black hole.
The other part escapes the ergosphere with more energy than the original object.
The escaping part gains energy at the expense of the black hole’s rotational energy.
The process reduces the angular momentum of the black hole.
Although theoretically possible, there are significant practical challenges to utilizing this process, including reaching a suitable black hole and converting the extracted energy into a usable form.
And the most obvious challenge is reaching a suitable black hole. The nearest known black holes are light-years away, far beyond our current space travel capabilities.
The process also requires operating in the ergosphere of a black hole, an environment with intense gravitational forces and radiation that would destroy any existing spacecraft or equipment.
Kerr Worm hole
As you scroll down, we get to even more theoretical implications of these rotaiting black holes.
A Kerr wormhole would theoretically connect two different points in spacetime, allowing for faster-than-light travel or shortcuts between distant regions of the universe.
Such a wormhole can be analogized to a whirlpool in a body of water to help visualize its structure and effects.
Just as a whirlpool has a central rotating region where water spirals downward, a Kerr wormhole has a central rotating region defined by its ring singularity, where spacetime is highly curved and twisted
In a whirlpool, the water surrounding the central vortex is pulled into the spin, creating a flowing boundary. Similarly, the ergosphere of a Kerr wormhole is a region outside the event horizon where spacetime itself is dragged along with the rotation of the black hole. Objects within the ergosphere cannot remain stationary and must move along with the rotating spacetime.
Thanks for reading this article on the introduction of rotating black holes. Feel free to leave a wee comment below.
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