Black holes have become one of the most important objects in contemporary physics research. They serve as a benchmark for the foundational theoretical framework of general relativity, a testing ground for prevailing ideas in astrophysics, and a tool for probing the evolution of the universe. In this essay, we will explore the central ideas in the history of black hole physics, beginning with Michell’s concept of
We humans are a curious species by nature. We have looked up to the skies to know when to plant our crops by analyzing the patterns of the constellations, to study the motion of planets and the Sun, or simply for the pure admiration of a starry night. This fascination has been documented since antiquity, in every culture, regardless of the region of the globe. It took centuries to realize that there was much more than what our eyes, or even our basic telescopes, could see.
Among the plethora of objects in our universe, such as planets, galaxies, nebulas, cosmic rays, and supernovas, to name a few, there also exists something that essentially tries to hide itself.
Black holes (BHs) are some of the most complex and intriguing objects in our universe. They have been depicted in pop culture as methods for traveling vast distances in space: it was portrayed as the main character in Nolan’s film
Surely, the structure of a BH is not that simple. A general static BH, i.e., one that is not spinning, has three main characteristics: a singularity of infinite density where general relativity (GR) breaks down—a problem we hope to resolve with a full theory of quantum gravity, which is yet to be discovered; an event horizon, which splits the BH into two regions and is the point of no return—once crossed, there is no way to come back, and not even light can escape, rendering the inner structure black to any outside observer; a photon sphere just outside the event horizon where light has unstable orbits; and the innermost stable circular orbit (ISCO) region, the last region where stable orbits are possible around it.
However, a real astrophysical BH should be spinning and not held static. There are two main differences that come with this: first, the singularity is no longer a point—it gets stretched due to the spinning effect, turning it into a ring; secondly, a new region appears where spacetime itself rotates with the BH, called the ergosphere. In this region, it is impossible to keep any particle at rest; everything must co-rotate with it—even if we try to go against it.
Today, it is almost impossible to think that BHs do not exist, although it is easy to forget that it took centuries to come to that realization. In Historical Background, we will see how well-known figures in science, such as Laplace, Newton, Einstein, and Hawking, from as early as the late 1700s to the 1900s, contributed to building the theoretical knowledge required to understand these objects.
However, developing the theoretical framework to study BHs is one thing; developing the tools to prove their existence is another. In Observational Milestones, we will uncover how, in the mid-1900s, the scientific community began to be convinced that the only explanation for certain observations was the existence of BHs. However, the definitive proof took a bit longer to be found. It was only with the advent of gravitational-wave astronomy that we finally proved BHs to be a reality, and even more astonishingly, in 2019, we captured the first image of one.
BH physics is far from complete; it remains a highly active area of research, both theoretically and observationally. In Future Directions in Black Hole Physics, we will conclude with a discussion of some theoretical challenges still open to debate and the ongoing efforts to enhance the observation of one of the most interesting objects lying in the cosmos.
In the eighteenth century, physics was still in its infancy regarding the debate on the nature of light. While Christiaan Huygens argued that light is similar to how we currently understand sound waves, a wave of pressure flowing through a medium called the
In the meantime, Newton also published his three tomes on “The Mathematical Principles of Natural Philosophy” in 1687, giving birth to what is now known as classical mechanics.
That is, the force of gravity
In his first definition of the first book, Newton develops the idea of mass and density:
“The quantity of matter is the measure of the same, arising from its density and bulk conjunctly. Thus air of double density, in double space, is quadruple in quantity; (…) The same thing is to be understood of snow and fine dust of powders that are condensed by compression or liquefaction; and of all bodies that are by any causes whatever differently condensed. (…) It is this quantity that I mean hereafter everywhere under the name of body or mass.”
That is, for Newton, everything has mass. Everybody, particle, or
Amid the debate about the nature of light and the (fairly) recent formulation of Newton’s theory of gravity, the geologist and physicist John Michell wrote a private letter to his colleague Henry Cavendish in 1784:
“If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface a greater velocity than that of light, and consequently, assuming light to be attracted by the same force in proportion to its vis inertiae as other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity. This assumes that gravity influences light in the same way as massive objects.”
In this letter, the idea of a body sufficiently compact to trap light within it was first conveyed. Michell coined the term
Independently of Michell, about a decade later in 1796, Pierre-Simon Laplace evaluated the minimum radius needed for a body of mass
Solving the previous equation for the velocity
which is known as the
This equation specifies the minimum radius for a body of mass
Despite the progress made on this concept, Newton’s interpretation of light became less popular over the decades, and the massless wave interpretation of light gained strength. Since Newtonian gravity relies on the masses of two different bodies, the idea of
Nineteenth century witnessed tremendous progress in fundamental physics. James Clerk Maxwell released his two-volume treatise on electromagnetism, summarizing the recent developments on the subject while adding his own ideas.
In 1905, during Albert Einstein’s
setting the foundation for our current understanding of gravity. His theory was no longer fixed to a particular reference frame. We now have a general theory of relativity.
The left-hand side of the EFE in (5), known as Einstein’s tensor, contains information about the geometry of spacetime, i.e., how it is curved. The right-hand side, the stress-energy (or energy-momentum) tensor, encloses the information about the mass/energy
“Spacetime tells matter how to move; matter tells spacetime how to curve.”
Despite the deceptive simplicity of
One year. Actually, less than a year. In January of 1916, Karl Schwarzschild, amid the First World War, found the first exact solution to the EFE.
with spherical symmetry, and a body that is static, not evolving in time and thus not depending on the time coordinate either. This led to what is now known as Schwarzschild’s metric:
The term
Schwarzschild’s solution presented two main issues, at
Up to this point, Schwarzschild’s solution was considered nothing more than a mathematical solution to the EFE, with no real application or object to be described by it. However, Oppenheimer and Snyder showed in 1939 that if a sufficiently heavy star collapses after exhausting any source of nuclear energy in its core, the star will emit its mass as radiation, and what remains after the gravitational collapse process can be described by Schwarzschild’s solution.
In 1958, Finkelstein realized that by using a different coordinate system than Schwarzschild’s, anything that fell past
Around the middle of the twentieth century, GR became a mature theory, well-accepted in mainstream physics and widely studied for its implications. There was a pivotal 10-year period between the 1960s and 1970s during which previous ideas underwent significant development and became central topics of debate in astrophysics and GR.
Until that time, several other analytical solutions to Einstein’s equations had been obtained. However, none of them described a rotating solution, despite it being well-known that everything in the universe exhibits rotational motion. But it was with Roy Kerr’s solution in 1963, where he generalized the static Schwarzschild metric to a rotating body.
In 1965, Roger Penrose proved that singularities are formed in a gravitational collapse under general conditions.
It was finally in 1967, during a lecture at the American Association for the Advancement of Science, that John Wheeler coined the term
Whatever falls inside the event horizon is lost within it, and no information about the nature of the matter can be retrieved. For example, we cannot determine whether a BH with 10 solar masses is composed of matter or antimatter. Israel demonstrated this in 1967,
In 1975 and 1976, Stephen Hawking showed that by examining quantum effects in curved spacetime—specifically how particles are created and annihilated near the event horizon of a BH—an outside observer would see particles being emitted from the BH itself. He concluded that this particle emission would cause the BH to lose mass, leading to its gradual evaporation through what is now known as
In just over a decade, we finally found a suitable name for the “gravitationally completely collapsed object”; we discovered that BHs are described by only three parameters; they must contain a singularity; Kerr’s metric was crucial in our understanding of real, astrophysical BHs; and we realized that BHs are not completely “black” due to Hawking radiation. With the theory now well-developed and well-established, the next step was to seek out these objects.
In a survey conducted in 1965 during two rocket flights, eight new sources of cosmic X-rays were detected.
At the time, the notion of Cyg XR-1 being a BH was considered implausible:
“We suggest that multiple-star models of the Cyg XR-1 system must be considered further before it can be concluded that Cyg XR-1 is a black hole.”
This was stated in reference
Still, in that same year, Stephen Hawking and Kip Thorne made a bet on whether Cyg XR-1 was a BH or not, with Hawking betting against the idea but hoping to lose.
Cyg XR-1 became one of the most researched astronomical objects, and even today, we do not have direct evidence of it being a BH. Despite this, it is widely accepted within the scientific community that Cyg XR-1 is a 21.1 solar mass BH
…and Hawking conceded the bet to Thorne back in 1990.
In the early days of radio astronomy, in 1933, Karl Jansky discovered a source of high-frequency electromagnetic waves coming from a fixed position in the constellation Sagittarius,
Within this region, in 1974 Balick and Brown observed a substructure of “very bright radio waves” emanating from a compact object that could have been a quasar.
Although it was evident since the 1980s that Sgr A* was a BH, it was only in 1994 that Reinhard Genzel
However, after ten years of high-resolution astrometric imaging, in 2002, a team led by Genzel ruled out any other possible explanation for Sgr A*.
In 1992, two sites were chosen in the United States—Hanford and Livingston—to build an upscaled version of the Michelson-Morley experiment. The goal was very clear: to find the ripples of spacetime in the form of gravitational waves. Both observatories were completed in 1999, and the Laser Interferometer Gravitational-Wave Observatory (LIGO) was successfully established.
In parallel, in 1997, the construction of the Franco- Italian Virgo interferometer in Cascina, Italy, began, being inaugurated in 2003. The design is essentially the same as LIGO’s; however, while LIGO’s interferometers were each 4 km long, Virgo’s were 3 km.
LIGO began operations in 2002, and in 2007, it joined forces with Virgo, creating the LIGO-Virgo Collaboration (LVC) with three observatories working as a single machine. This initial run ended in 2010; even though it placed important limits on gravitational-wave sources, no detection of any kind was found. It would take five more years of significant improvements to LIGO’s observatories to finally have another chance at detecting these waves
Two days later, on September 14, a new window on the universe was opened. The signal of a binary system of two BHs, one with 36 and the other with 29 solar masses forming a 62 solar mass BH remnant was detected.
“This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.”
The LVC collaboration was joined by the Japanese Kamioka Gravitational Wave Detector (KAGRA) in 2021, and they are currently running the fourth observation run (O4). Together, they have found 90 mergers of binary BH systems, 2 neutron star-BH systems, and 2 binary neutron star coalescences (
Despite a BH being essentially black, it should be found in an environment clustered with other stars, gases, and accretion matter revolving around it. Even though the BH itself could never be directly seen, the region outside its event horizon interacting with the medium around it must be visible.
In 2009, the Event Horizon Telescope (EHT) project began. It is composed of several radio telescopes spread around the world, working together as a single observatory. To put it simply: an effective telescope the size of Earth. In the beginning, the EHT collaboration had four main goals: i) test GR in the strong field regime; ii) check the existence of an event horizon; iii) assess the plausibility of estimating the BH spin by observing orbits near the event horizon; iv) attempt to explain how BHs accrete matter and create powerful jets. And they were not shy about what it could accomplish:
“direct imaging of black holes can be achieved within the next decade.”
In one of the largest and most massive galaxies in our local universe, in the constellation of Virgo, a rather strong source of radio-frequency energy was detected in 1949 from M87.
It was then, in 2017, that the EHT collaboration gathered the first set of data
From there, the EHT collaboration continued to reveal images of BHs around the universe. In 2021, they released the reconstructed image of Centaurus A’s jet emission;
We could say that BH physics is experiencing its second golden age, this time on the observational side. However, many questions remain unresolved in the theoretical domain as well. GR is a very well-tested theory that continues to provide us with a deeper understanding of the universe as a whole. Despite this, it is clear that GR is not our final theory of gravity.
The singularity within BHs reveals one of the limitations of GR. Singularities cannot exist in reality; it seems curious, to say the least, to wonder how such a region would be created due to the gravitational collapse of a star, for example. Explaining what a singularity would look like is another struggle in itself. Singularities are in the extreme-field regime, where it is impossible to ignore gravity, but the region is also small enough for quantum mechanics to come into play. The unfortunate news is that despite several different attempts to resolve this issue,
Information cannot be lost. This is not only a plausible idea but a foundational consequence of quantum mechanics. However, whatever enters a BH is lost forever; we cannot retrieve any information from whatever went inside. Recall that a BH does not distinguish between matter and antimatter, for example, and Hawking radiation emits thermal energy only, which also does not carry any other information about the BH itself. This is what Hawking called the
On the observational side, we have just discovered a new way to investigate the universe using gravitational waves. The KAGRA observatory is still operating at around 10% of its maximum efficiency,
The Laser Interferometer Space Antenna (LISA), the first space-based interferometer, will be far more sensitive than any ground-based gravitational-wave observatory. Led by the European Space Agency (ESA) in collaboration with NASA, LISA will consist of three spacecraft in a triangular formation, separated by 2.5 million kilometers, using laser beams to detect minute changes in distance caused by passing GWs. Operating in space and free from terrestrial noise, LISA will observe sources of GWs inaccessible to ground-based detectors, including those from BHs of various masses, such as intermediate-mass and supermassive ones, shedding light on their formation, evolution, and mergers. The mission will also detect GWs emitted shortly after the Big Bang, offering new insights into the early universe’s structure and dynamics. The LISA mission is scheduled to launch in 2035.
A new generation of radio telescopes, the Square Kilometre Array (SKA), currently under construction in South Africa and Australia, will have an effective area of 1 km2. It is set to achieve its first light in 2028.
BHs are far from being mere objects of imagination or just pop culture phenomena. They are not only a central part of contemporary physics but also one of the biggest puzzles that allow us to further understand our universe. We have progressed from not understanding what light even was, to literally observing the light emitted from a BH’s accretion disk; from struggling to interpret preposterous observations, to measuring almost 100 BHs using gravitational waves; from mathematically being unable to cross the event horizon, to pondering how to resolve the singularities within them. It has taken us about 300 years to reach our current understanding, and it seems almost impossible to predict what discoveries the next few decades will bring as we continue to satiate our curiosity about the universe.
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Edison Santos – Conceptualization, Writing – original draft, review and editing
Edison Santos
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Interestingly, assuming Copenhagen’s interpretation of quantum mechanics, both theories can be seen as correct.
This equation will make a rather surprising comeback when we develop the idea of a BH in the context of GR.
Although unbeknownst to him at the time, Planck sets the framework to understand what light is at the fundamental level.
Since the famous equation E = mc2 had already been discovered, we can view mass as one type of energy.
The name “event horizon” was coined before Finkelstein’s discovery in a different context. Rindler was developing the ideas of horizons (particle and event horizon) in the context of the Friedmann–Lemaître–Robertson– Walker metric.
Penrose was awarded the Nobel Prize in 2020 “for the discovery that black hole formation is a robust prediction of the general theory of relativity.” Unfortunately, Stephen Hawking passed away two years earlier, in 2018.
He was comparing the situation to a room full of bald-pated people who had one characteristic in common, but no differences in hair length, style, or color for individual variations.
At least this is not such an unusual claim since about 10% of all stars are part of a ternary system.
Reinhard Genzel, Andrea Ghez, and Roger Penrose were awarded the Nobel Prize in 2020 “for the discovery of a supermassive compact object at the center of our galaxy.”
Virgo would complete its upgrade only in 2017.
The two initial masses of the BHs do not add up because 3 solar masses were irradiated as gravitational waves.
M87 is the entire galaxy, while M87* is the supermassive BH.