Black Holes

SUMMARY: Black holes were implied by Einstein’s general relativity and have challenged physicists’ theories since.

A black hole is a finite region of space during a period of time (called space-time) subject to a singularity caused by a large concentration of mass in its interior. This massive object generates a gravitational field so powerful that atoms are compacted in super-high densities, which in turn increases the gravitational pull. A singularity is created in space because no particle of matter, not even light photons, can escape from that region. Hence the name: a black hole is an invisible region because it does not reflect any light (all light is absorbed). Many aspects of black holes can be described and studied using algebraic and geometric concepts, but the existence of black holes is still under debate. For example, Australian mathematician Stephen Crothers argues that black holes are inconsistent with general relativity and critiques the mathematics used by others to demonstrate their existence. It is believed that black holes originate when stars run out of gas needed to maintain their temperature, causing a decrease in volume. As volume decreases, the proximity of particles increases the gravitational pull in a positive feedback loop; as particles get closer, the gravitational force keeps increasing. This compaction process continues until a singularity, called the “event horizon,” is created. The event horizon is defined as a boundary in space and time beyond which events cannot affect an outside observer. The event horizon separates the black hole region from the rest of the universe and is the boundary of space from which no particle can leave, including light.

The singularity caused by a black hole is considered as a curvature in space-time. This curvature is explored by Albert Einstein’s general relativity theory, which predicted the existence of black holes—though Einstein himself did not believe in them. In the 1970s, Stephen Hawking, George Ellis, and Roger Penrose proved several important theorems on the occurrence and geometry of black holes. Previously, in 1963, Roy Kerr had shown that black holes in a space-time have an almost-spherical geometry determined by three parameters: their mass, their total electric charge, and angular momentum.

It is believed that at the center of most galaxies, including the Milky Way, there are supermassive black holes. The existence of black holes is supported by astronomical observations, in particular through the emission of X-rays. Some black hole candidates have been identified experimentally using observations and data. There are different types of black holes, such as rotating black holes and stationary black holes, and these are described by using various metrics in physics and differential geometry.

In January 2017, NASA announced that it was undertaking a new mission to study supermassive black holes. It planned to launch three telescopes equipped with cameras to monitor black hole activity; these telescopes were launched in 2021, marking the first time a space agency launched astronomical equipment into space with the specific intention of studying black holes. In 2020 NASA researchers also earned a Nobel Prize for proving that the center of the Milky Way contained a supermassive black hole.

In subsequent years, aided by new technology such as the James Webb Space Telescope (JWST), NASA researchers continued to study black holes and deepen scientific understanding of these bodies. Other space agencies around the world also undertook similar efforts; for example, the Indian space agency launched an astronomical space observatory in 2024 to study black holes, making India the second country (after the US) to do so.

Origins of Human Awareness of Black Holes

The concept of a body so dense that even light could not escape was described in a paper submitted in 1783 to the Royal Society by an English geologist named John Michell. By then, Isaac Newton’s theory of gravitation and the concept of escape velocity were well known. Michell computed that a body with a radius 500 times that of the sun and the same density, would, on its surface, have an escape velocity equal to that of light and would therefore be invisible.

In 1796, the French mathematician Pierre-Simon Laplace explained in the first two editions of his book Exposition du Système du Monde the same idea; however, the concept that light was a wave without mass and therefore unaffected by gravitation was prevalent in the nineteenth century, and Laplace discarded the idea in later editions.

In 1915, Einstein developed his general relativity theory, and showed that light was influenced by the gravitational interaction. A few months later, Karl Schwarzschild found a solution to Einstein’s equations, where a heavy body would absorb the light. We now know that the Schwarzschild radius is the radius of the event horizon of a black hole that will not turn, but this was not well understood at the time. Schwarzschild himself thought it was just a mathematical solution, not physical. In 1930, Subrahmanyan Chandrasekhar showed that any star with a critical mass (now known as the Chandrasekhar limit) and that does not emit radiation would collapse under its own gravity. However, Arthur Eddington opposed the idea that the star would reach a size zero, implying a naked singularity of matter; instead, the black hole should have something that will inevitably put a stop to collapse, an idea adopted by most scientists.

In 1939, Robert Oppenheimer predicted that a massive star could suffer a gravitational collapse and therefore black holes might be formed in nature. This theory did not receive much attention until the 1960s because after World War II he was more interested in what was happening at the atomic scale.

In 1967, Stephen Hawking and Roger Penrose proved that black holes are solutions to Einstein’s equations and that in certain cases the creation of a black hole is the inevitable consequence of a star aging. The black hole idea gained force with the scientific and experimental advances that led to the discovery of pulsars. Soon after, in 1969, John Wheeler coined the term “black hole” during a meeting of cosmologists in New York, to designate what was formerly called “star in gravitational collapse completely.”

The Entropy of Black Holes

The mathematical tools used to model black holes use fundamental laws of physics, particularly relativity and thermodynamics. According to initial theories by Stephen Hawking, black holes violate the second law of thermodynamics (the entropy, or disorder, of isolated systems tend to increase over time), which led to speculations about travel in space-time wormholes (tunnels that would allow time travel or fast travel over very long distances). Hawking has recanted his original theory and has admitted that the entropy of the matter is kept inside a black hole. According to Hawking, despite the physical impossibility of escape from a black hole, it may end up evaporating by constant leakage of x-ray energy that escapes the event horizon, called Hawking radiation. According to this model, black holes have intrinsic gravitational entropy, which implies that gravity introduces an additional level of unpredictability over the quantum uncertainty. It appears, based on the current theoretical and experimental capacity, as if nature took decisions by chance or, more generally, far from precise laws.

The hypothesis that a black hole contains entropy and, furthermore, it is finite, required to be consistent with such holes emitting thermal radiation, at first seems contradictory. The explanation is that the radiation escapes the black hole in such a way that an external observer knows only the mass, angular momentum, and electric charge. This means that all combinations or configurations of radiation of particles having energy, angular momentum, and electric charge are equally likely. Physicists such as Jacob D. Bekenstein have been linked to black hole entropy and information theory.

Firewall Debate

In 2012, four researchers published a theory that sparked a national debate about what would happen to a person if he or she were to fall into a black hole. The theory was especially controversial because it violated Einstein's equivalence principle, which has served as the foundation of general relativity and the theory of gravity. According to their proposition, rather than passing through the event horizon uneventfully as previously believed, a person would encounter a wall of fire at this boundary that would instantly incinerate him or her. Scientists have continued to debate this idea due to its challenges of basic laws of quantum mechanics and general relativity.

Black Hole Information Paradox

Another mystery associated with black holes and the theory of general relativity that scientists have continued to struggle with is the idea that as matter inside a black hole evaporates and disappears, all of the information about what is inside of it must disappear as well. Known as the black hole information paradox, this destruction of information would contradict the law of quantum mechanics that states that information is never lost.

In August 2015, at a conference in Sweden, Hawking announced that he and two colleagues believed they had come up with an answer to the paradox. He explained that, according to their theory, information about the black hole does not actually enter the black hole but is stored on the black hole's boundary, in the event horizon. When matter falls into the black hole, it leaves imprints (two-dimensional holograms) as it crosses the event horizon that engender shifts in the position of particles released by Hawking radiation, which he and his team referred to as "supertranslations." These holograms would serve as a record for each particle that had entered the black hole. However, Hawking and his colleagues were still working on producing a paper and detailing the mathematical evidence to support the theory.

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