## All the mysteries about black holes, from A to Z

From A to Z
A black hole is a region of space with such a density and concentration of mass that it generates a gravitational field from which nothing, not even light, can escape.

(Black hole) Binary. System formed by two black holes orbiting around each other. Its existence, although it had been theorized long before, was not confirmed until 2016, along with that of gravitational waves; in fact, binary holes are the largest source of the latter in the universe.

Gravitational collapse. It is the collapse or implosion of a stellar body due to the effect of its own gravity. This is the best known mechanism by which black holes are formed, although there are also other ways in which these bodies can arise.

Medium density. Black holes are made up of a point where the mass is concentrated (the singularity) and a delimited area in which the action of gravity prevents anything that goes in from leaving. Therefore, the greater the mass of a black hole, the greater this area, as the gravitational effect is greater; although that mass is concentrated in the same volume. As a consequence, in especially large (supermassive) black holes, the average density can be very low, even less than that of water, if the mass exceeds at least a billion times that of the Sun, due to the amplitude that reaches the radius of the hole in these cases.

Space time. Modern physics, thanks mainly to Einstein’s theory of special relativity, considers space and time as two inevitably linked concepts, as a four-dimensional continuum (the three observable physics and one time dimension). According to this theory, moreover, these four dimensions are relative to the state of motion of the observer. This model is essential to understand some effects of black holes, such as the event horizon or the microlensing effect.

Black hole merger. Two black holes in a binary system can collide and merge. When this happens, the resulting hole has a radius similar to the sum of those of the two holes, and it emits a gravitational wave signal that we can detect.

Gravity. In the conception of general relativity, gravity is a deformation of the geometry of space-time as a result of the mass of bodies. This idea is what explains the effects of black holes on space-time. Roughly speaking, the huge amount of mass concentrated at the center of a black hole (singularity) acts as a gravitational well, warping space in such a way that the event horizon “grows” inside (only for an observer inside) faster than light, which is why not even light can get out of it.

Event horizon. An event horizon is a boundary in space-time beyond which events occurring on one side cannot affect an observer on the other. In the case of a black hole, the event horizon is located around it, marking the area from which the warping of space-time is so extreme that nothing that crosses it can come out again, not even rays of light. light; This is why black holes are so hard to see (because they don’t emit light) and they get their name.

Cosmic inflation. According to this theory, in the first moments of the universe, a very rapid expansion process took place, due to the action of a hypothetical field called the inflaton. This would explain two fundamental phenomena of the universe: the low curvature of space-time and its homogeneity and isotropy on a large scale (that is, it is similar in all directions). It is in this phase that the theoretical origin of primordial black holes is located, whose existence has not yet been confirmed. Thus, these would have been formed by the gravitational collapse of the plasma that made up the early universe in the gravitational wells that would have generated a specific mode of expansion.

(Angular momentum) J. J is the mathematical sign of the magnitude known as angular momentum, which is defined in a rotating object as the resistance of that object to the variation of angular velocity. According to the hairless theorem, the only three quantities needed to define the state of a black hole are its mass, its electric charge, and its angular momentum, which will define it as rotated.

(Black hole of) Kerr. This type of hole is also called a rotating black hole, as opposed to a Schwarzschild black hole (static black hole). One of the peculiarities of this type of hole is that, when they have enough mass, they deform the space-time around them in such a way that they create a zone around them in which, although you can stay in it or leave it without being drawn into the singularity, it is impossible to remain static. The most interesting thing, however, is that these holes, at least in theory, would allow time travel thanks precisely to this effect; and that, in a certain way (coinciding with the direction and speed of rotation) it is possible to enter and leave them in another place in space-time avoiding the singularity.

Light. Light is simply a segment of the electromagnetic radiation spectrum that we can perceive thanks to our eyes. However, it does have one key feature, and that is its speed in a vacuum. This number, which in physics is represented by the letter c, is a universal constant that, for a number of physical reasons, is a kind of ‘speed limit of the universe’, since an object cannot move faster than c for space. However, space can expand or deform faster than c; which explains phenomena such as event horizons.

Gravitational microlensing. Einstein’s theory of general relativity predicted that the gravity of massive objects could bend the path of light reaching them. Because of this, it has been observed that a lens-like optical effect occurs around sufficiently large celestial bodies, bending light to the point where objects behind them can be observed. For a black hole, which emits no light and so can be very difficult to see, gravitational microlensing is an easy way to detect its presence.

Black hole starship. There is a theoretical idea, coming from science fiction literature, that an artificial black hole could be used to generate power that could propel an interstellar spacecraft. And, despite its currently fictitious status, some scientists have used this idea as a theoretical experiment, concluding that it could be feasible as technology and our knowledge of quantum physics advance, potentially becoming a method of power generation. efficient and safe.

Gravitational waves. Gravitational waves are disturbances in space-time produced by massive objects in motion. This phenomenon was predicted by Einstein in his theory of general relativity, and its existence was experimentally confirmed in 201. They are used to detect black holes, binary systems, neutron stars and other objects.

(Problem of) Final Parsec. In a binary system, two closely spaced black holes revolve around each other, getting closer and spinning faster and faster until they collide and merge, releasing strong gravitational waves in the process. However, before merging, two nearby holes have to lose energy, something that is not very clear how this happens (although it is believed to be due to interactions with nearby matter). The problem of the final parsec refers to this enigma, the parsec being a unit of measurement of distances on an astronomical scale that is equivalent to just over three light years.

Quasar. A quasar is an astronomical source of electromagnetic energy that originates when a massive black hole in the center of a galaxy begins to absorb nearby matter, which forms an accretion disk that rotates at high speed and emits a large amount of energy. in the form of radio waves, visible light, infrared rays, ultraviolet rays and X-rays.

Relativity. The theory of relativity includes both the so-called “general relativity” and “special relativity”, and was formulated by Einstein as an update of the Newtonian theoretical canon to integrate observations on electromagnetism, incompatible with Newtonian mechanics. At a very general level, he posits that both time and space are relative to the motion of the observer. Thus, it is essential to understand space-time as a continuum and to explain phenomena such as gravitational waves, space-time distortion, cosmic inflation or black holes themselves.

Gravitational singularity. A gravitational, or spatiotemporal, singularity is an area of space-time in which no physical magnitude of the gravitational fields can be defined; it would be, for example, a point in space-time with infinite mass and zero volume, like the one found at the center of a black hole.

Quantum theory of gravity. Although it has not yet been possible to formulate it, the quantum theory of gravity is the fundamental objective of the field of quantum gravity: a theory that allows unifying the quantum field theory (for which it is necessary to understand time as linear and not dynamic, as in classical physics) and the theory of general relativity, which, as we have mentioned, postulates space-time as a continuum relative to the observer. This theory would allow a better understanding of phenomena such as gravitational singularities, and consequently black holes or the Big Bang.

Universe. For physics, the universe is the total of time and space, as well as all the matter and energy it contains and the laws that govern it. As far as it has been verified, it consists of at least four dimensions (three spatial and one temporal) although some theories postulate more, such as string theory. In recent times, some theories have gained popularity that indicate the possibility of the existence of several universes that coexist, as is the case of the Multiverse Theory.

Milky Way. The Milky Way, our galaxy, has a large black hole at its center, like most of the ones we know, which is called Sagittarius A and consists of about 2.6 million solar masses. This hole is responsible for its shape and (spiral) motion, as matter is attracted by its gravity and pulled by the gravitational pull of its rotation.

Wheeler. John Archibald Wheeler was an American theoretical physicist born in 1911 and died in 2008. He made important contributions to different fields of physics, such as the theory of nuclear fission. He participated in the Manhattan project, and collaborated in the development of the hydrogen bomb. Wheeler also contributed to the field of relativity, coining terms such as “black hole” or “wormhole”.

(X-rays. This name refers to electromagnetic radiation with frequencies between 30 and 30,000 PHz, invisible to the human eye and capable of passing through opaque bodies, which is why it is used in medicine to take X-rays. They also constitute an important part of the energy emitted by quasars.