I have a biased opinion, I admit it, but one of the film images and one of the most iconic astronomical observations in history is that of a black hole “seen closely.” Specifically, I refer to the fiction images of Gargantúa and the Royals of the Supermasive Black Hole of M87, a gigantic galaxy that we have there next to it, just 53 million light years. M87 houses a monster that concentrates the mass of about seven billion soles. But it is that it does so in a volume of similar size to the distance at which the Voyager nave 1 of us is right now, about 140 times the size of the orbit of the earth around the sun, or less of the average tenth century from the distance that separates us from the closest star. That is, the same volume that houses a couple of stars here (the sun and next Centauri), the supermassive black hole of M87 concentrates the equivalent of several thousand. And it is not the largest black hole.
And how do we know that there is a black hole? What do these monsters look? Specifically, how are you seen in short distances, if we were in front of him of Bruces? To understand it, we will present the problem with several assumptions.
First, let’s see what would happen if the black hole was completely alone in the universe. We must have that the most popular definition of a black hole is that not even the light can escape its gravitational attraction, that is, the light has not enough speed to escape. In the case of a mass like the earth, nothing can leave it if it does not go to more than 11 kilometers per second (about 40,000 kilometers per hour, which is a unit that we understand better). In light, with its speed of 30,0000 kilometers per second, it does not give it to evade the gravitational effect of a solitary black hole. So we should not see anything, I would not emit light, it would be literally invisible.
But maybe this is not true. I say perhaps because it is not proven, but the so -called Hawking radiation would be a way in which a black hole would emit light. That radiation would be the result of quantum fluctuations just on the horizon of events, which would be the surface where the exhaust speed is equal to that of light. In a quantum fluctuation for a moment you can create a particle and an antiparticle. And it turns out that the antiparticle of one is another photon. If two photons are created by a quantum fluctuation, in normal conditions they would be annihilated and we would not find out. But if the pair of photons is created, one is slightly within the horizon of events and another outside, the first one will not be able to escape and the second yes. So ! At the expense of its mass. They would not be so invisible, although Hawking calculations determine that the probability of that phenomenon is so small that they would emit an extremely weak light, too much to detect it with our current instruments. Even so, it is so important that we are looking for her.
Let’s continue adding assumptions, which in physics quickly add complexity to the problem. Actually, in our article today we are discussing what we would see from a black hole, so we are already assuming that it is not alone in the universe, at least it would have us in front. And of us (assuming that we are not at zero degrees Kelvin, we are alive). The most robust definition of a black hole is that the space-time curves so that nothing can accelerate to have an open (or exhaust) trajectory, everything that comes too close will fall into the black hole. But if we put ourselves at an adequate distance, some of the photons that leave us can travel through the space curved by the black hole and, again, if we are in the right place, they could turn around the black hole (always beyond the horizon of events, surpass it is the “death” of the photon) and return to us. So we could see ourselves, as in a mirror! Our image would be quite distorted, but we would know that there is a black hole. In fact, that mirror would be special, because if we put ourselves in the right place and without even looking directly at the black hole, photons that leave our neck could see their career so curved that they could reach us in the eyes after turning around the black hole. It would be a mirror that would allow us to see our neck, there is nothing.
A black hole is, then, like a mirror, but also like a magnifying glass. Because if there are things (planets, stars, galaxies) behind the black hole, close or distant, with greater detail and/or distorted, depending on its distance and relative position with respect to the black hole. It is exactly like a magnifying glass, which can focus the sunlight at a point or create a rare image (a caustic, is called, a word not well known).
Let’s continue complicating the problem. The new case we propose is actually the most common way to detect black holes. If the black hole has material around, it is distributed on a disc, which is a circular structure, with the black hole in the middle, and very flat, hence its name. In that disc, the material is heated at large temperatures. In the case of them, and as if it were a large incandescent bulb, it can shine like a whole galaxy. We are talking about gas within an area of a few tens of light days, that is, much less than the size of the solar system to its confines, in what is known as the Oort cloud, which is several hundreds of light days (the Voyager 1 that we mention is almost one day light). The material of the disc, when emitting light, loses energy, so in the end the black hole can swallow it, hence the structure is known as a accretion disk, using another word not well known in Spanish, but that the SAR defines as “growth by addition of matter.”
But let’s go back to our question of what. In this last casuistry, when there is material around, we should see an album emitting light. But that is only true if we observe the black hole from above, from the same axis around which the disc is broken. If we observe the album from one side, with a certain angle, we do not see how one would expect. Make the test of looking at a CD, or better a vinyl, which has its black little hole in the middle such as astronomical accretion discs. If they observe it in perpendicular, they see it circular, if they observe it from a certain angle they see an ellipse, and if they observe it from the plane of the disc, they will see a very fine segment. But the analogy is not worth us for cosmic monsters. Because the spacetime curvature they produce implies that the trajectory of the light of the part of the disc that is beyond the black hole, and that in principle traveled in perpendicular to the disk, is distorted and revolves around the black hole finally traveling towards us. That light seems to us that it comes from the area over the black hole, but comes from the back of the disc. As the disc is circular, in the end like a comb above the black hole. And the same goes underneath: the rays of light from the bottom of the area of the disc that is behind the Negreo hole, rays that in principle traveled down, curve and reach us as if coming from.
The image of Gargantúa in Interstellar It shows it perfectly, it is based on our best models of the emission of an accretion disc around a black hole. I want to emphasize that I am using present in this paragraph, not subjunctive, and we have seen this effect! Both for the supermassive black hole of the M87 galaxy and for the Milky Way.
The vision of a black hole from close has more tremendously curious peculiarities, but I stay without space. I only finally name the light ring that forms around, very close to the horizon of events and in an internal area than the limits of the accretion disc, where photons can acquire almost stable circular orbits around the monster, such as, in what is known as the photon sphere. Some of those photons at the end escape and leave us the vision of a ring. Look for the image of Gargantúa, that ring is seen in the movie. If we were in that ring, it is just where we could see the neck, and having closed an argumentative circle in this article, such as the infinite curvature of the black hole in its center, I leave it.
