Especially those most curious about astrophysics already know that black holes are regions of very high density in space. Regions so dense that they create a gravitational field so strong that nothing can escape, not even light.
Note that we say that a black hole is a region , not “something”. This choice of words is very important and says a lot about what black holes really are.
Contrary to what many think, a black hole is not simply a singularity surrounded by an event horizon. Maybe it was, at the beginning of the 20th century with the theory of black holes still static. However, a black hole has a series of complex and interesting internal structures and has angular momentum — they rotate because the star that gave rise to them also rotated and angular momentum is conserved. We will see this in detail.
So… let’s learn more about black holes?
Who invented black holes?
The idea of black holes was first proposed in the late 18th century by English geologist John Michell and French mathematician and astronomer Pierre-Simon Laplace. Michell and Laplace independently suggested that there could be objects in space with gravity so strong that nothing, not even light, could escape them. However, at that time, the idea was purely theoretical and had no observational evidence to support it.
Karl Schwarzschild was a German physicist and astronomer who made important contributions to the theory of black holes. In 1916, just a few months after Albert Einstein published his theory of general relativity, Schwarzschild used the new theory to derive a mathematical solution that described the gravitational field around a spherically symmetric object. This solution, now known as the Schwarzschild solution , is still used today to describe spacetime around non-rotating black holes.
Schwarzschild’s solution showed that if an object had enough mass and collapsed in on itself, it could form a region of space where the gravitational pull was so strong that its escape velocity would exceed the speed of light. This region of space, now known as the event horizon , marks the boundary of what we now call a black hole. Schwarzschild also calculated the radius of the event horizon, which we now call the Schwarzschild radius (
The term “black hole” was coined in 1967 by American physicist John Wheeler, who played a key role in developing our modern understanding of these objects. Since then, many observations and studies have provided strong evidence for the existence of black holes, and they are now widely accepted as a fundamental aspect of astrophysics and cosmology.
Schwarzschild’s work was an important milestone in the development of our understanding of black holes, and his solution provided the basis for much of the subsequent research into these objects. The Schwarzschild radius and the Schwarzschild solution are still important concepts in the study of black holes today and are named in honor of Karl Schwarzschild’s contributions to the field.
What’s inside a black hole?
Until then, everything is great! Einstein presented his theory of general relativity in 1915 and within a few months Karl Schwarzschild found the first exact solution for a point mass (the same as a spherical, non-rotating black hole). The next step in modeling this problem more realistically was to consider what would happen if the black hole also had angular momentum, rather than just mass — which wasn’t solved until New Zealand mathematician Roy Kerr found the exact solution in 1963 in which became known as the Kerr metric.
This solution revealed that, rather than a single event horizon with a point singularity, a black hole contains an inner and outer event horizon , as well as an inner and outer ergosphere , in addition to a ring-like singularity of substantial radius.
A test particle approaching the ergosphere in the retrograde direction is forced to change its direction of motion.
The ergosphere is a region around a rotating black hole where spacetime is dragged along with the black hole’s rotation. It is a consequence of spacetime drag due to the immense rotational energy of the black hole.
NOTE: Later work on the ergosphere and the Kerr black hole was developed by Roger Penrose, winner of the 2020 Nobel Prize in Physics.
There are some fundamental and important differences between the Schwarzschild solution, which is more naive and simple, and the Kerr solution, which is more realistic and complex. In no particular order, here are some fascinating contrasts:
- Instead of a single solution for where the event horizon is, a rotating black hole has two mathematical solutions: an inner and outer event horizon.
- Just outside the outer event horizon, there is a place known as the ergosphere, where spacetime itself is dragged along at a rotational speed equal to the speed of light, and falling particles experience enormous accelerations.
- There is a maximum ratio of angular momentum to mass that is allowed; if there is too much angular momentum, the black hole will radiate that energy (via gravitational radiation) until it falls below this threshold.
And, perhaps most fascinating and one of the facts that science books don’t show: the singularity at the center of the black hole is not an infinitesimal point, but rather a one-dimensional ring, where the radius of the ring is determined by the mass and angular momentum of the black hole. Black Hole.
“Ah, this is all theory!” It will be?
Now that we have finally observed the event horizon of a black hole for the first time in 2019, M87* , due to the incredible success of the Event Horizon Telescope (EHT), scientists have been able to compare their observations with theoretical predictions.
By running a variety of simulations detailing what the signals would be for black holes with various masses, spins, orientations, and streams of accreting matter, they were able to find the best fit for what they saw. Although there are some substantial uncertainties, the black hole at the center of M87 appears to be:
- Spinning at 94% of its maximum speed;
- With a one-dimensional ring singularity with a diameter of ~118 AU (larger than Pluto’s orbit);
- With its axis of rotation pointing away from Earth by ~17°;
All observations are consistent with a Kerr black hole (which is preferred over a static Schwarzschild black hole).
How are black holes formed?
First of all, it is important to divide black holes into two main, very distinct classifications:
- Stellar-mass black holes ; It is
- Supermassive black holes .
(Intermediate-mass black holes are still one of the mysteries of astrophysics)
Stellar-mass black holes originate from the evolution process of massive stars, typically those at least 12 times the mass of our Sun. Stars go through several stages of nuclear fusion , where hydrogen is converted into helium and heavier elements . This melting process creates an internal pressure that neutralizes the gravitational force — hydrostatic equilibrium .
As a massive star consumes its nuclear fuel, it eventually uses up its hydrogen and other elements in the core. Fusion reactions cease, and without internal pressure, gravity takes over, causing the star’s core to collapse, triggering a cataclysmic explosion known as a supernova . The outer layers of the star are ejected into space, releasing a huge amount of energy. What remains is a highly dense core called a stellar remnant . If the stellar remnant has a mass above a certain limit, known as the Tolman-Oppenheimer-Volkoff limit, the force of gravity becomes so intense that it overcomes all other forces. The collapse undergoes a chain reaction: gravity collapses the core further, which increases its density, which creates an even greater gravitational force, collapsing it further, which creates an even greater force, and…
Anyway, you get the idea.
As the core continually collapses, it becomes increasingly compact and its density approaches ever closer to infinity, at a point known as the singularity . An event horizon forms around this singularity, which is the boundary beyond which nothing can escape, not even light. This marks the formation of a black hole.
What about supermassive black holes?
Black holes found at the center of galaxies, with millions (or billions) of solar masses, known as supermassive black holes , have origins that are not yet fully understood. However, there are several important theories about its formation:
- Accretion and growth: One possibility is that supermassive black holes formed from the accretion of large amounts of gas and matter in the early Universe. As matter falls into a dense region, such as the center of a galaxy, it can form an accretion disk — a rotating disk of gas and dust around the black hole. Over time, the black hole’s gravitational pull causes it to grow in mass, becoming a supermassive black hole.
- Direct collapse: Another hypothesis suggests that supermassive black holes may have formed through the direct collapse of massive gas clouds in the early Universe. Such clouds, with sufficient mass and density, would collapse directly into a black hole without forming a star first. This process could explain the rapid formation of extremely massive black holes.
- Stellar evolution and mergers: Supermassive black holes may have formed through the growth and mergers of smaller black holes that formed through the collapse of massive stars. Over billions of years, these smaller black holes can merge due to gravitational interactions, leading to the formation of a supermassive black hole.
How can we detect a black hole?
In some cases, astrophysicists can indirectly detect the presence of a black hole by studying the motion of nearby stars. If a black hole exists in a galactic cluster or center, it can gravitationally influence surrounding stars, causing their orbits to exhibit distinct patterns or high speeds. When this black hole is in a binary system with a companion star, it can pull matter from the companion, forming an accretion disk. The black hole’s intense gravitational pull causes the disk to heat up and emit X-rays. Astrophysicists use X-ray space telescopes, such as the Chandra Observatory (NASA), to detect and study these high-energy emissions.
Black holes can have profound effects on their surroundings. They can generate powerful jets of particles and emit high-energy radiation. By detecting and studying these emissions across the electromagnetic spectrum, astronomers can infer the presence of a black hole.
Modern space observatories and missions, such as the James Webb Space Telescope (JWST) and the European Space Agency’s Advanced Telescope for High-Energy Astrophysics (ATHENA ) , will offer enhanced capabilities for studying of black holes, potentially providing new insights into their properties and detection.
Finally, astrophysicists can also use the effect of gravitational lensing. Microlensing is a phenomenon in which the gravity of a foreground object, such as a black hole, bends and magnifies the light from a background star. By observing these temporary glows, astrophysicists can infer the presence of a black hole.
Extra: Can the Earth be sucked into a black hole?
Ah, one of the biggest astrophysical urban legends propagated on social media…
No, the Earth is not at risk of being “sucked in” by the supermassive black hole at the center of our galaxy, known as Sagittarius A* (Sgr A*) . Sgr A* is a supermassive black hole with a mass about 4.3 million times that of our Sun. Sgr A*’s gravitational influence extends over a vast region, but its gravitational pull on distant objects like Earth does not is significant enough to cause any concern.
The Earth orbits around the Sun, which is part of the Milky Way. Although Sgr A* is located at the center of our galaxy, it is estimated to be about 26,000 light-years away from us. The gravitational forces that govern the movement of planets in the Solar System are dominated by the Sun’s gravitational pull, and the influence of Sgr A* on Earth’s orbit is negligible.
The fact is that the Solar System does not orbit Sagittarius A* directly, but rather the entire mass of stars and gases that make up the inner part of the arms and the central bulge of the Milky Way (in addition to the dark matter) which, if added together, has much more mass than Sagittarius A* alone.
Furthermore, black holes do not have a magical or all-encompassing “suction” force. Its gravitational pull is intense only within a certain proximity, particularly near its event horizon. As long as an object, including Earth, remains well outside a black hole’s event horizon, it can continue in its stable orbit without being pulled inward.
- EMPARAN, R.; REALL, HS Black Holes in Higher Dimensions. Living Reviews in Relativity , vol. 11, 2008. DOI: 10.12942/lrr-2008-6.
- HOD, S.; PIRAN, T. The Inner Structure of Black Holes. General Relativity and Gravitation , vol. 30, no. 11, 1998. DOI: 10.1023/A:1026654519980.
- KERR, R. Spinning Black Holes . University of Canterbury, Chistchurch. Available at: https://www.youtube.com/watch?v=nypav68tq8Q&t=48m5s.
- TAYLOR, E.; WHEELER, JA; BERTSCHINGER, E. Exploring Black Holes . Spinning Black Hole. 2. ed., 2006.
- VISSER, M. The Kerr spacetime: A brief introduction . 2008. arXiv:0706.0622.
