The Black Hole Information Paradox: The Clash between Physics’s Greatest Theories

Credit: Aeon

Imagine taking a notebook, writing a message, and tossing it into a black hole: what would happen to it?

Black holes, one of space’s greatest mysteries, are extremely dense regions of matter where gravity is so strong that nothing, including light, can escape. Formed when colossal stars run out of fuel, causing them to collapse due to their own gravity, black holes cannot be directly observed due to their inability to emit/absorb light. Despite this, their effects on surrounding matter confirm these celestial bodies’ existence (Engelhardt, 2023). Einstein’s 1915 theory of general relativity predicted the existence of black holes by proving how mass and energy curve spacetime, a four-dimensional structure which combines the dimensions of space and time (Doeleman, n.d.). In this framework, gravity is not a force but the curvature itself and an extremely compact mass can warp spacetime to where gravity is strong enough to form “black holes”. Furthermore, his theory hypothesised a “singularity” at the centre of the black hole (Engelhardt, 2023) - a point of infinite gravity and density. Shortly after, Schwarzschild derived solutions to Einstein’s equations, showing how spacetime is distorted around spherical objects like a planet or star (Levy, 2021). Upon closer inspection, his solution revealed the “event horizon”, the boundary where escape velocity equals the speed of light. This essentially created a “point of no return”, where anything surpassing this boundary would inevitably fall towards the singularity.

However, in 1974, cosmologist Stephen Hawking proposed black holes seemingly emit thermal (heat) radiation due to quantum effects at the event horizon. Employing a “semi-classical” approach combining modern quantum mechanics and Einstein’s general relativity, he stated that virtual particle pairs, which spontaneously appear and disappear in quantum fields, can arise naturally in the event horizon and split; one particle with positive energy escapes the black hole (deemed as ‘Hawking Radiation’) whilst the other with negative energy disappears into it, reducing the blackhole’s mass (Gregerson, 2025). This theory challenged the notion that nothing can be emitted within a black hole’s vicinity, and it suggests that black holes can evaporate due to its gradual loss of mass. 

Despite such a novel discovery, this theory posed a significant problem: Hawking radiation is random and thermal, only carrying certain information, such as mass, charge, or momentum. Once a black hole evaporates, the information about the matter that fell into it is lost and impossible to regain, even with all knowledge of motion in the universe (Engelhardt, 2023). This juxtaposes quantum physics's principle of unitarity, where all information is conserved and cannot be lost. For example, according to the laws of classical mechanics, burning a book into ashes and smoke, even if it seems like information is lost, could be theoretically reconstructed with enough understanding of every particle. Contrarily, Hawking contends that if that book were consumed by a black hole, the information would be permanently destroyed and cannot be restored. Thus, the Black Hole Information Paradox emerges: where does this evaporated information go? 

Multiple theories have arisen to explain this supposed loss of energy. In 2012, scientists Almheiri, Marolf, Polchinski, and Sully proposed the Firewall hypothesis, suggesting that an observer falling into a black hole may encounter a “firewall”, a region of high energy at the event horizon which incinerates information, allowing it to escape and be carried away through Hawking radiation. By theorising that the “firewall” breaks the connection between the inside and outside, as well as prevents information from being lost to the black hole, their theory preserves the quantum unitarity principle (Almheiri et al., 2013; Firewall (Physics) - Wikipedia, n.d.). However, the concept clashes with general relativity, which insists no event occurs at the event horizon. Moreover, some claim that the hypothesis is based upon assumptions, stating that there is no experimental evidence. In defence of their hypothesis, the original publishers put forth “An Apologia for Firewall”, claiming that this destructive “firewall” is overall difficult to avoid to keep quantum mechanics and known physics.

Other attempts to resolve this paradox resulted in the creation of Fuzzballs and the Holographic Principle, which both can explain how information is stored in black holes rather than destroyed. For instance, the Holographic Principle illustrates how information is not tied to an object’s 3D volume, but rather its two-dimensional surface (Sutter, 2023). When applied to black holes, it suggests that information is stored on the surface of the event horizon. 

Additionally, classical mechanics assumes a deterministic universe where information predicts the past and the future. The concept of information being lost breaks this idea (Engelhardt, 2023). Developments in quantum mechanics and string theory in the 2000s showed how information is not lost after all, unlike Hawking’s hypothesis. 

Recently, in 2023, a paper published by multiple scientists, including Almheiri, Marolf, and Engelhardt, showed substantial progress in resolving Hawking’s Black Hole Paradox by accounting for a variable Hawking did not: quantum gravity (Appel, 2023), a theory that describes gravity in terms of quantum mechanics. In their paper, they state how quantum gravity essentially modifies Hawking radiation so it is not perfectly thermal anymore, meaning that it can encode information, allowing it to escape. As a pivotal article, their work has provided a powerful resolution to one of science’s most well-known paradoxes.

Contrarily, in a deeper sense, the several questions that have branched from it are considered not to be fully resolved yet. As Engelhardt wrote herself in MIT’s Physics Annual Magazine (2023), her resolution posed a question: why does quantum gravity modify the equations for a black hole, but not other bodies such as the sun or planets? Additionally, quantum gravity, the variable used for the paradox’s resolution, remains without a consistent and complete theory.

Truly understanding quantum gravity, and thus the Black Hole Information Paradox, could redefine our understanding of information, space, and time, potentially leading towards a complete theory that conclusively unifies Einstein’s theory of relativity and quantum mechanics. If this progress is made, new groundbreaking technological advancements may eventually emerge from a newfound understanding of quantum physics, and our world will shift towards a deeper appreciation for our universe as a whole. 

References

Almheiri, A., Marolf, D., Polchinski, J., & Sully, J. (2013, February 11). Black holes: complementarity or firewalls? J. High Energ. Phys., 2013(62(2013)), 27. https://doi.org/10.1007/JHEP02(2013)062

Appel, J. (2023, May 5). Scientists Say They've Finally Solved Stephen Hawking's Black Hole Paradox. Popular Mechanics. Retrieved July 18, 2025, from https://www.popularmechanics.com/space/deep-space/a43519907/black-hole-paradox-stephen-hawking-finally-solved/

Doeleman, S. (n.d.). Einstein's Theory of Gravitation | Centre for Astrophysics | Harvard & Smithsonian. Center for Astrophysics | Harvard & Smithsonian. Retrieved July 16, 2025, from https://pweb.cfa.harvard.edu/research/science-field/einsteins-theory-gravitation

Engelhardt, N. (2023). THE BLACK HOLE INFORMATION PARADOX A RESOLUTION ON THE HORIZON. MIT Physics. Retrieved July 16, 2025, from https://physics.mit.edu/wp-content/uploads/2023/09/PhysicsAtMIT_2023_Engelhardt_Feature.pdf

Firewall (physics) - Wikipedia. (n.d.). Wikipedia, the free encyclopedia. Retrieved July 18, 2025, from https://en.wikipedia.org/wiki/Firewall_(physics)

Gregerson, E. (2025, July 3). Hawking radiation | Black Holes, Quantum Mechanics, Particle Physics. Britannica. Retrieved July 17, 2025, from https://www.britannica.com/science/Hawking-radiation

Levy, A., & Knowable Magazine. (2021, January 14). How black holes morphed from theory to reality. Astronomy.com. Retrieved July 17, 2025, from https://www.astronomy.com/science/how-black-holes-morphed-from-theory-to-reality/

Musser, G. (2020, October 29). The Most Famous Paradox in Physics Nears Its End. Quanta Magazine. Retrieved July 16, 2025, from https://www.quantamagazine.org/the-most-famous-paradox-in-physics-nears-its-end-20201029/

Sutter, P. M. (2023, December 19). The holographic secret of black holes. Phys.org. Retrieved July 18, 2025, from https://phys.org/news/2023-12-holographic-secret-black-holes.html