Do not let the event's horizon steal the show, keep it for the ergosphere

The gravitational influence of a black hole is a twisted thing that has many parts. We all know the horizon of events because of its wonderful ability to capture "even the light" in its envelope, while keeping everything locked in the darkest possible as long as the black hole stays in. life. But beyond the horizon of events, there is another region with equally exceptional capabilities – if not more – that distorts the perception of reality in its own unique way.

Since both abilities are activated by gravity, let's start there.

The gravitational force is actually an effect that objects seem to feel because of the shape of the space-time continuum. All objects move on the surface of the continuum and, when the surface is folded, an observer sees the object moving as on a curve. These deformations are caused by massive bodies: the heavier a body is, the more it bends the continuum around it. For the observer, it seems that the heavy body causes the orbit of the lightest object.

The Moon revolves around the Earth because the mass of the Earth has bent the space-time continuum around it. In fact, the Moon simply moves on the surface of the continuum and seems to revolve around the Earth because of the shape of the continuum in this region. Credit: Mysid / Wikimedia Commons, CC BY-SA 3.0

Depending on the mass of the deformed body, this effect can be felt over great distances. For example, Pluto revolves around the Sun at an average distance of 5.9 billion km. Thus, the average orbit of Pluto indicates the deformation that an object as heavy as Pluto is a victim of the Sun (and other planets as well as the Kuiper belt) at this distance. According to the laws of Newtonian gravitation, the strength of the force diminishes by the square of the distance. So, if the force between two bodies is X at a distance of Y, it will be X / 4 at a distance of 2Y (assuming that the gravitational constant is the same at Y and at 2Y). However, the force never drops to zero unless the objects are infinitely far from each other.

Now, if Pluto wanted (for a fantastic reason) to leave his orbit, he would have to move at a certain speed to escape. Say it was the Death Star instead of Pluto and the Death Star had thrusters. He would have to shoot these thrusters to accelerate to a point where his speed exceeds the limit at which the Sun can hold Pluto by its gravity.

The basic configuration is the same when it is a black hole, but the numbers are more extreme. When you look at a black hole, you actually see its horizon of events. The gravitational attraction of the black hole itself emanates from a point in its center called singularity. This singularity distorts the space-time continuum in unimaginable ways, even as it becomes more and more imaginable as you move away from the center.

The event horizon is the distance at which the continuum is distorted so that you have to move faster than at the speed of light to escape it, that is to say that if you were caught on the horizon of events, even if you were moving exactly the speed of light will keep you only on the horizon of the event and will not let you slip into it. ;space. (In other words, it would allow us to calculate the speed of light in a given universe using the basic rules of gravitational physics and the size of black holes in this universe.)

This is also the reason why the horizon of events is what you see when you see a black hole: it is a literal horizon of events. Events occurring on one side can not be seen on the other because the light that carries the information you "see" can neither pass through nor return it. This should in turn raise the question of whether there is an area of ​​space around the black hole where its attractive effects can be felt but does not delineate the "points of no return". The answer is yes; this is called the ergosphere.

The name itself takes a very utilitarian look at the idea – that this is the area of ​​space from which you can extract work from the black hole – but it's true. The ergosphere is the region in which the space-time continuum has been deformed by the black hole to such an extent that you can get in and out if you traveled fast enough (but less than at speed light). However, even if the effects of the black hole, of the singularity on the event horizon, are completely distorted and the event horizon itself constitutes an important – albeit arbitrary – boundary, the effects of the black hole in the ergosphere remain hallucinating.

This is partly due to an effect of turning black holes called frame slip. Imagine yourself (an immortal elf) watching Pluto orbiting the Sun from Mercury, through a fixed window between the orbits of Neptune and Pluto. If you continue to look out the window, you will see Pluto pass once every 248 years. In addition to the fantastic elements, this scenario is also physically possible because the window is virtually motionless. The part of the space-time continuum on which it rests, so to speak, is not in motion itself because of the rotation of the Sun. That is, there is a negligible number of streaks in the image.

But that would not be possible in the ergosphere of a rotating black hole. Suppose you are just above the horizon of events, looking through a window in the distance, an object orbiting the black hole at the inner edge of the ergosphere. Dragging the frame would absolutely prevent the window from being motionless, as well as you and the object. This is due to the fact that the black hole produces a gravitational attraction – that is to say a prodigious distortion of the continuum – is such that it not only deforms the continuum but also drives it when it turns, in the direction of its rotation, very pronounced.

Because of this frame drag, everything sitting on this part of the continuum also seems to be moved even though it has no velocity. in that direction to begin. It's as if you're watching your friend walking from west to east on a boat traveling at the speed of light: for all intents and purposes, she might as well walk east-west! This is the reason why a rotating black hole forces the object that tilts towards the ergosphere of the black hole in the opposite direction to seem to swing and move in the direction of its rotation.

Note the use of 'appear': the object will not be forced to change its direction in the direction of rotation of the black hole. However, the changing layout of space-time in the area, associated with light from the object and the viewer, will make it look as well.

If, under the effect of a certain constraint, an object insists on appearing stationary in the ergosphere, it can, but there is a problem. If it is in the ergosphere but above the event horizon, the object has no choice but to be moved by the framework. But just as the horizon of events is the surface on which you will travel for eternity if you travel at the speed of light, the ergosphere also has a surface on which you can avoid slipping by the frame if you you moved at the speed of light. This is simply called the ergosurface.

This picture is one of the slides of a presentation prepared by Professor Chris Reynolds, UMD.

(Anecdote: It is possible to explain the effects of gravity off the ergonomic surface using Newtonian physics, but inside you will need theories of relativity.)

The location of the two envelopes – the event horizon and the ergonomic surface – is determined by the speed of light. Their shapes are also determined by common factors: black hole mass and kinetic moment. However, they are not affected in the same way. For example, a non-rotating black hole will have a spherical event horizon, but a rotating black hole will have an oblate event horizon. On the other hand, a non-rotating black hole will not have an ergonomic surface, while a rotating black hole will have nothing between an oblique surface and an ergonomic pumpkin-shaped surface.

Credit: Yukterez / Wikimedia Commons, CC BY-SA 4.0

These are just a few of the reasons why the shadow of the black hole in the center of the M87 galaxy looked exactly like the image made by the Horizon Events Telescope (EHT). Aside from the way it was obtained (with techniques such as VLBI), the image contains many distortions that come from the black hole itself, so its interpretation is not a mere activity.

The EHT has recorded and studied only the radiation that could come off the black hole, many materials accumulating beyond this point and falling into the hole. In summary, we therefore examine the hot and magnetized matter, all of their radiation and the Doppler effects exerted on them, the effects of the ergosphere dragging them, then the shadow of the event horizon.

The shadow (black) and the horizon of events and the ergosphere (white) of a black hole turning from left to right. At a = 0, the black does not turn and at a = 1, it turns to the maximum. Credit: Yukterez / Wikimedia Commons, CC BY-SA 4.0

The idea of ​​being able to extract work from the ergosphere, thus giving the region its current name, can be attributed to some examples that different scientists have presented over the years. The three best-known examples are the Penrose mechanism, Hawking radiation, and the Blandford-Znajek process. The case of Hawking radiation is the easiest to explain (only because the popular press has done it enough times to be able to access it immediately), but its understanding also gives insight into Penrose's alternative.

The void of deep space is not a true vacuum: it contains a certain energy, notably the electromagnetic energy of distant stars, often transformed into a particle-antiparticle pair. That is, these particles are condensations of energy that appear and come back in the form of energy (here is a more detailed and accessible basic document) in a very short time. It is possible that this process will also occur near black holes simply because it is possible. And when that happens, something strange follows.

If such a pair of particles appears just above the event horizon, any of them could fall into the dark and the other be rejected in the ergosphere. . This thrust occurs due to the law of conservation of the momentum, and the energy carried by the pushed particle will be very small, transformed from the mass of the black hole. For a distant observer, it seems that the black hole has just emitted a particle and lose some of its mass to do it. Stephen Hawking and Jacob Bekenstein first predicted this phenomenon since 1974. When this process is repeated, a black hole could have lost all its mass and evaporate completely into nothingness.

The British mathematician physicist Roger Penrose proposed a somewhat similar idea that was relatively more practical (and that was used in the film Interstellar also in 1969. As explained by Suvrat Raju, theoretical physicist at ICTS Bangalore: Let's say that an object – like a rock – is projected into the ergosphere. When it gets closer to the event horizon, it is caused by a deliberate mechanism to split into two pieces, so that a piece falls in the event horizon in the direction opposite to the rotation of the black hole. As a result, the other would be accelerated in his journey into the ergosphere by a "kick" from the black hole.

If orchestrated properly, the struck coin can emerge from the ergosphere with more energy than it had received – energy provided by the black hole by converting some of its mass . Scientists have already determined that the average energy gain achievable in each Penrose mechanism attempt would be about 21%.

"In conventional processes, you can never reduce the surface of the black hole, but the Penrose process can reduce its mass," he explained. "The fantasy of science fiction is that a sufficiently advanced civilization could use rotating black holes for waste disposal and even get energy through the Penrose process."

The Blandford-Znajek process is less brutal and more … involved. Suppose that a star is too close to a black hole and is shredded into fragments that fall into orbit around the event horizon. The friction between these bits warms them to a very high temperature, pushing them into a state of plasma material. These bits also contain electric and magnetic fields, and the electric and magnetic field lines go through them even as they whirl around the monster and fall closer and closer.

At this point, let me quote the following educational material from Daniel Nagasawa, Stanford University, 2011:

In essence, the black hole acts as a massive conductor that rotates in a very large magnetic field produced by the accretion disk, where a voltage is induced between the poles of the black hole and its equator. The end result is that the power is dissipated by slowing down the rotation of the black hole …

To extract energy in this scenario, one of the ways – as proposed by the user CapnTrippy on Everything2 – is to build a superconductor in orbit around the poles of the black hole, so as to intercept and take away some current flowing from the equator to the poles. let it settle in the plasma in the ergosphere. With respect to the black hole itself, this electrical energy has two sources: its rotation energy and that soaked by the plasma. Since a black hole can carry up to 29% of its total rotational energy mass, it is also the maximum possible energy that can be extracted during this process. It's not great, but it's still fantastic, because black holes are often heavy enough to provide energy for years. According to Nagasawa,

… for a 10⁸ Black solar mass hole with a magnetic field of 1 T, the power generated is about 2.7 × 10³⁸ W. In perspective, the annual energy consumption in the world is estimated at … 5 × 10²⁰ J. The example presented here produces more energy in one second than the whole globe consumes in one year. Although this statement is bold, this is just one example in which not all energy produced is extractable as usable energy. However, at this point, even a system that is less than 10^-15 % of efficiency would be enough to provide enough energy to power the world for a full year.

The Blandford-Znajek process remains an active research topic to date. Part of this is fortunately for a reason that has little to do with Earth's food: relativistic throws. They are extremely powerful and narrow beams of radiation that move almost at the speed of light observed by astronomers in space. Astrophysicists believe that the Blandford-Znajek process and the Penrose mechanism together can explain how they formed and were fired from supermassive rotating black hole poles and traveled billions of kilometers. In fact, it is thought that galaxy CGCG 049-033, located 680 million light-years away from Earth, would house a black hole weighing 2 billion solar masses that would propel its planes 1.5 million light-years in the space.

So, the next time you read information about black holes, do not let the event horizon steal the show (even literally). Actions and dramas also occur above its surface, where things are still visible while behaving in a strange way, where a gallery of plasma, energy fields and a moving continuum expose the works of attraction from this black hole to the total vision of the universe. Do not forget that what you see is not what you get.

This article has been published for the first time on the author's blog and has been republished here with some modifications.