“Can we explain the existence and function of black holes with our current understanding of the laws of physics or do they necessitate a new theory?”

Black holes are among the most intriguing and mysterious objects in the universe, and our understanding of their existence and function continues to be a topic of intense scientific study. While our current understanding of the laws of physics, including general relativity and quantum mechanics, can explain many aspects of black hole behavior, there are still some fundamental questions that remain unanswered. In particular, some scientists argue that the extreme conditions found within black holes may necessitate the development of a new theory that goes beyond our current understanding of the laws of physics.

I guess first we should start by quickly defining a black hole, and then dive into the problems scientists are having with them. Black holes are extremely dense regions of space where the gravitational field is so intense that nothing can escape from it, not even light. This occurs because the mass of a black hole has collapsed into a single point, known as a singularity, where the gravitational pull becomes infinitely strong while the singularity itself is infinitely small and dense. The boundary around the black hole, where the escape velocity equals the speed of light, is known as the event horizon. So, what’s our issue? Well, it’s already been said. The singularity is supposed to be impossible, because matter isn’t capable of collapsing into an infinitely small point (according to quantum mechanics). At the infinitely small singularities, the laws of physics appear to break down, creating a scenario where quantum mechanics (which describes the behavior of the extremely small) and general relativity (which describes the behavior of the very large) come into conflict with each other.

Okay, I said “quantum mechanics and general relativity don’t go well together”, but that doesn’t tell you much. General relativity describes the behavior of gravity on large scales, such as those of planets, stars, and galaxies, while quantum mechanics describes the behavior of particles and their interactions on small scales. However, in certain situations, such as at the singularity of a black hole or during the early moments of the universe, these scales can overlap (remember, a singularity’s gravity is big, but the singularity itself is small), and a complete theory is needed to describe these situations accurately. Also, they run into issues because the equations of general relativity predict the existence of singularities, which are infinitely dense and small points, while the equations of quantum mechanics do not allow for such singularities. 

You might be thinking: “Okay, so why can’t we just ignore general relativity in the context of singularities if the singularity is infinitely small”. General relativity is the theory that describes gravity, and it is essential for understanding black holes.

After getting to understand the problem, I looked for the solution. And after some research, let me propose to you: Quantum Gravity. Quantum Gravity is a theoretical framework that attempts to unify the principles of quantum mechanics and general relativity. It aims to provide a complete understanding of the microstructure of spacetime at the Planck scale, where fundamental constants such as the velocity of light, the reduced Planck’s constant, and Newton’s constant come together to form units of mass, length, and time. What quantum gravity is trying to answer is: How can the theory of quantum mechanics be merged with the theory of general relativity / gravitational force and remain correct at (less than) microscopic length scales? 

What is the Planck scale? To put things into perspective, quantum mechanics is used to describe the behavior of particles on both the atomic and the subatomic level. However, on even smaller scales is where the Planck scale lies (10^-35 meters). What’s important about the Planck scale is that this is the scale where the effects of quantum gravity become important. This is also a problem in itself, the Planck scale is so far beyond our current experimental capabilities, it is difficult to test and validate any proposed theories of quantum gravity through standard means. So, why do we need quantum gravity? As mentioned before, general relativity explains gravity on big scales where gravity arises due to the curvature of spacetime caused by the presence of matter and energy. But general relativity breaks down at the quantum level because general relativity treats matter and energy as classical objects that follow classical laws of physics, while quantum mechanics shows that matter and energy can exhibit behaviors that are completely different from classical objects (ex. entanglement, superposition of states, etc.).

Some quick notes to clarify some things: Proposing a theory that allows the description of physics at the center of a black hole does not necessarily include unifying all fundamental interactions into a single mathematical framework (theory of everything), also, the “quantum gravity” theory is not one single theory. There are multiple quantum gravity theories (string theory, loop quantum gravity, etc.)

Lifting off from my introduction of quantum gravity, let’s see how it tackles our question. We can look at a lot of examples, but let’s look at the most famous one: string theory. String theory attempts to unify quantum mechanics and general relativity by describing all fundamental particles as one-dimensional “strings” rather than point-like particles (these strings are smaller than the Planck length). String theory predicts the existence of gravitons, which are hypothetical particles that carry the force of gravity in a manner analogous to the way that photons carry the electromagnetic force. This allows gravity to be described in a quantum mechanical framework, which is not possible in general relativity. In black holes, string theory predicts that black holes are surrounded by a “horizon” where the curvature of spacetime is so extreme that even light cannot escape. According to string theory, this horizon is a two-dimensional surface made up of a vast number of microscopic strings. These strings vibrate in such a way as to emit Hawking radiation, which is a type of radiation predicted by quantum mechanics that causes black holes to slowly evaporate over time. String theory also says the singularity, the point of infinite density at the center of a black hole, is actually a region of extremely high string density. At the Planck scale, the strings become so tightly packed that the curvature of spacetime becomes infinitely large, giving rise to the gravitational attraction that is associated with the singularity. 

Hold on, you might think that string theory literally just ignores general relativity because it says that the singularity is not a point of infinite density but a “extremely high string density”, but the idea that the singularity in a black hole is not a point of infinite density, but rather a point of extremely high string density, is a prediction that arises from the application of string theory to black hole physics. It is not a case of ignoring general relativity, but rather using string theory to provide a more complete and consistent description of the physics involved. We could dive into mathematical equations like wave functions and perturbation theory to prove that string theory works but none of us are up for that right now. Anyway, if I can give just a little more of a satisfying answer, the theory that the singularity is infinitely dense was never proven anyway so it’s not like violating basic laws of physics. While string theory has the potential to provide a unified description of all fundamental forces in nature, including gravity, it remains an area of active research and debate in the physics community. 

Why does it matter? The development of the internet, GPS, and medical imaging technology all arose from fundamental research in physics, this isn’t much different. Also, humans are naturally curious, and developing a theory of quantum gravity would help us understand the nature of the universe at its most fundamental level. It could help us answer questions such as what happened at the very beginning of the universe and how the universe will end.

Observing research on any of the quantum gravity approaches from an external perspective may seem like watching the slow excavation of a tunnel, with infrequent visible progress except for occasional removal of debris. However, once inside, one can witness a bustling hub of activity, with significant advances in understanding the problem made in recent times. Nevertheless, a tunnel is only useful once a breakthrough is made. 

In conclusion, scientists are trying to explain the existence and function of black holes by implementing new theories that connect two theories which conflict with each other in black holes (quantum mechanics and general relativity). Black holes continue to intrigue scientists, and their existence and function are still the subject of intense scientific study. While our current understanding of the laws of physics, including general relativity and quantum mechanics, can explain many aspects of black hole behavior, there are still some fundamental questions that remain unanswered. The singularity, a point of infinite density and zero volume, is a major issue, as it is supposed to be impossible according to quantum mechanics. To describe these situations accurately, a complete theory of quantum gravity is needed. Quantum Gravity aims to provide a complete understanding of the microstructure of spacetime at the Planck scale, where fundamental constants such as the velocity of light, the reduced Planck’s constant, and Newton’s constant come together to form units of mass, length, and time. One such theory that attempts to unify quantum mechanics and general relativity is string theory, which describes all fundamental particles as one-dimensional “strings” rather than point-like particles. While we still have a lot to learn about black holes, the ongoing research into their function and existence is leading us closer to a more complete understanding of the universe.

 Work cited:

Childers, T. (2021, November 2). What Are Black Holes? New Study May Reveal Their True Nature. Popular Mechanics. Retrieved March 31, 2023, from https://www.popularmechanics.com/space/deep-space/a35875454/what-are-black-holes-new-theory/

Wikimedia Foundation. (2023, March 22). Quantum gravity. Wikipedia. Retrieved March 31, 2023, from https://en.wikipedia.org/wiki/Quantum_gravity

Weinstein, S., & Rickles, D. (2019, May 2). Quantum gravity. Stanford Encyclopedia of Philosophy. Retrieved March 31, 2023, from https://plato.stanford.edu/entries/quantum-gravity/

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Videos:

Greene, B. (2019, July 25). What is string theory? YouTube. Retrieved April 3, 2023, from https://www.youtube.com/watch?v=TI6sY0kCPpk&t=146s