For many years, black holes have been one of the most talked about subjects all over the world: their sheer size, as well as their mysterious effects have created a peak in the interest of many scientists, in particular astrophysicists and mathematicians. However, not much is known about black holes, despite many studies that have been and are currently being conducted on them. 

From Theoretical Predictions to Classification

The first definition of a black hole was given in 1783: “Volume of space that possesses so much gravity, nothing can escape its pull, even light”. In 1939, Albert Einstein further predicted that black holes were remnants of massive stars that collapsed under the force of gravity, creating an object with such intense gravitational pull that not even light can escape. It is widely hypothesized that black holes could hold the key to why the universe keeps expanding. However, their existence was not proved until decades later, when physicists detected X-rays that were emitted from black holes feeding and by analyzing gravitational waves from their collisions. Surprisingly to most people, only two images have ever been captured of black holes and only in recent years, making headlines everywhere.

Illustration of a supermassive Black Hole

Black holes are known to form in several ways, the most important ones being: Stellar collapse – a star with a mass greater than about 20 times that of the Sun exhausts its nuclear fuel and collapses; another way is through the accretion of matter – gasses and dust are pulled in by the gravitational force, which greatly increased their density. Other two ways are through a merger – two black holes come close to each other and primordial black holes – formed from the extreme density and pressure of matter in the early universe. Black holes can be categorized in 3 major classes: stellar black holes are formed from the collapse of a single massive star, Intermediate black holes are thought to be formed from the merging of smaller black holes, and supermassive black holes (100 to 100,000 times the mass of our sun), are found at the centers of galaxies. Both images are from supermassive black holes. 

Unlocking the Secrets: Characteristics and Paradoxes

Black holes are known for their numerous astrophysical effects, including their remarkable stability; if disturbed in any way, they will eventually revert to their original shape and size. Additionally, the accretion disk that forms around black holes when they “feed” is a byproduct of matter that heats up and emits radiation as it spirals toward the black hole. Furthermore, the “accretion disk” that forms around black holes when they “feed” is the product of matter that heats up and emits radiation as it spirals into the black hole. In fact, it was through this process that their existence was initially proved (as was seen earlier). Gravitational lensing, on the other hand, is the process that occurs when distortion of light appears from an object such as a star or galaxy which is located behind the black hole. This is caused by the intense gravity of the black hole, and it is used to study its properties as well as the distribution of matter in the universe

Illustration of an accretion disk

However, there are many questions linked to black holes, such as: What happens to matter and the information falling into a black hole? In physics, information is the knowledge or data that can be encoded in physical systems. This is a fundamental concept related to the laws of thermodynamics, quantum mechanics and information theory. Black holes crush everything beyond the ‘event horizon’, while all the rest of the space will attract the bodies toward the black hole. The phenomenon that describes what happens to an object falling into a black hole is ‘spaghettification’, which indicates how a body is stretched out into a thin, long strand due to the difference in gravitational force between its head and tail. On the other hand, it is thought that information is lost because the only thing we can see of a black hole, apart from its gravity, is the radiation it emits, which seems to be random and uncorrelated to the information falling in it. This raises a paradox called the’ information paradox’, in which the information seems lost because it is not detected from outside the black hole.

Illustration of the ‘spaghettification’ effect

The information paradox is a long-standing paradox in theoretical physics, and there are only hypothetical solutions to it up to now. The first solution, which requires a link between quantum physics and the general theory of relativity, which has yet to be discovered, is that it is just encoded in the form of Hawking radiation, so the information is preserved. Another possible solution is the idea that information is lost in the event horizon, and it is simultaneously emitted in the form of a firewall of high-energy particles. Even though the distance from the nearest known black hole is 3,000 light-years, finding a solution to this paradox is of great significance. Considering that some of the most critical areas of physics research are involved, understanding this paradox would mean using nature to advance our understanding of the universe. Moreover, the paradox violates the principle of unitarity (the information cannot be lost) and the principle of causality (any event in the universe has a cause). If the paradox were discovered to be correct, meaning the information is lost inside a black hole, it would cause an immediate necessity to correct our theories of physics. Finally, bearing in mind that a revolution is underway regarding computational power with quantum computers, having detailed and correct quantum theories would help advance these technologies, affecting cryptography, communication, and computing.

A Thriving Topic Driving Investments and Breakthroughs

While the creation of black holes discussed above has already been thoroughly investigated, the exact mechanism of black hole formation is still an active area for research. New theories and models are being proposed to understand this fascinating phenomenon and this field has attracted significant fundings in the past decades. Among the different investments performed we can notably highlight:

  • The creation of large telescopes and observatories, such as the Very Large Telescope (VLT), the Atacama Large Millimeter/submillimeter Array (ALMA), the Hubble Space Telescope and the Chandra X-ray Observatory. The Hubble mission in particular is emblematic of long-term commitment in space research with billions of dollars devoted to maintenance and several upgrades performed to keep the telescope functioning
  • The spendings in theoretical physics research. By combining theory and observation, mathematicians and physicists can see and predict how black holes impact their surroundings. In addition, the research around blackholes also enables us to achieve progress in other domains of science. For instance, black holes have helped prove important milestones in the field of physics such as Einstein’s theory of general relativity.
  • The development of new advanced technologies such as the Laser Interferometer Gravitational-Wave Observatory (LIGO). This advance too was used to detect gravitational waves produced by the collision of two black holes in 2015. The development of the LIGO has a required significant contribution amounting to more than $1 billion from the U.S. National Science Foundation and international partners.
  • The increase of financing dedicated to education through science museums, planetariums, public lectures, and educational materials. One emblematic example of the development of education around astrophysics is the Smithsonian Astrophysical Observatory which proactively reached the public to share findings about black holes and other celestial topics. This approach aims to raise awareness about the importance as well as the complexity of these objects.  

    ALMA telescope in Chile

    In recent years, numerous missions have led to significant improvements and have attracted the attention of the whole scientific community. In 2019, the Event Horizon Telescope (ETH) has marked the history of astrophysics by capturing the first image of a black hole. The most promising research in this boiling area includes:

    • The observation of intermediate mass black holes. More precisely, how these medium-sized bodies impact their environment.
    • The hunt for black hole interactions with other astronomical objects. Among the different interactions we can cite the “disruption” events in which stars are torn apart and release significant amounts of intense light.


    In conclusion, the study of black holes is a fascinating and ongoing area of research that continues to captivate physicists and researchers around the world. These enigmatic objects hold many secrets that have yet to be uncovered, and new discoveries in this field will undoubtedly shape the future of science and technology for generations to come. From the creation and formation of black holes to the information paradox and the effects of their extreme gravity on surrounding matter, there are still many unanswered questions that challenge our current understanding of the universe. However, with the advancements in technology and the continued dedication of researchers and funding, we can be optimistic that we will continue to make progress in unraveling the mysteries of black holes and expanding our knowledge of the cosmos.

    Authors: Francesco Lovati, Elisa D’Errico, Elena Kybett Vinci

    Categories: Articles


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