Note on the Pure State of Old Black Holes in Some Models of Asymptotically Flat Quantum Gravity

Black holes have long haunted the imagination of physicists, not just for their terrifying gravity, but for a deep contradiction at the heart of how the universe works. On one side stands the theory of general relativity, which describes gravity as the smooth bending of space and time. On the other stands quantum mechanics, the rulebook for the tiny world of atoms and particles, which insists that information can never be truly lost. When a black hole forms and eventually evaporates, these two theories seem to clash. If a black hole swallows matter and then disappears, what happens to the information about that matter? Does it vanish, breaking the laws of quantum mechanics, or does it escape, perhaps in a way that destroys the smooth fabric of space-time right at the edge of the hole? This edge, known as the event horizon, has become the stage for a fierce debate. Some physicists argue that the horizon is a place of calm, where an observer would feel nothing unusual. Others, facing the information puzzle, have proposed a "firewall"—a searing wall of energy that would incinerate anything trying to cross the threshold. The question is not just about black holes; it is about whether our current understanding of reality, specifically the idea that space is filled with smooth fields of energy, is actually correct.

In a recent paper, theoretical physicist Tom Banks takes a hard look at this problem by stepping away from the complex mathematics of black holes in curved space and examining them through the lens of simpler, more fundamental models. He investigates two specific frameworks where gravity emerges from the behavior of quantum particles, rather than being a fundamental force itself. One model is a variation of a theory involving strings in two dimensions, while the other comes from a theory of eleven-dimensional supergravity. In both cases, the researchers do not start with a black hole and try to fit it into their equations. Instead, they build the black hole from the ground up, using the tools of ordinary quantum mechanics to assemble a collection of particles into a stable, high-energy state that behaves exactly like a black hole. These states are not smooth, empty voids with a mysterious surface; they are complex, chaotic bound states, much like a tightly packed cluster of particles held together by their own interactions.

When Banks analyzes these constructed black holes, he finds that the famous "firewall" paradox simply does not exist. The confusion arises because physicists have been trying to describe the inside of these objects using the language of quantum field theory, which assumes that space is filled with smooth, continuous fields. In the models Banks studies, this assumption is wrong. The particles that make up the black hole are not spread out in a smooth field near the horizon; they are discrete constituents of a single, giant quantum object. As the black hole evaporates, it does so not by emitting particles from a mysterious surface, but by slowly releasing individual pieces of itself, much like a drop of water evaporating from a puddle. These released particles are entangled with the remaining cluster, meaning their quantum states are linked, but this link changes rapidly and chaotically as the cluster evolves.

The paper argues that the idea of a "firewall" is a misunderstanding caused by looking at the wrong kind of mathematics. In these models, there is no sudden, violent break in the laws of physics at the edge of the black hole. Instead, the transition from the black hole to the outside world is a matter of perspective. To an observer far away, the black hole looks like a thermal object radiating heat. To the observer inside the complex quantum system, it is just a collection of particles interacting in a chaotic dance. The "horizon" is not a physical barrier where space-time tears apart; it is merely a boundary where the description of the system shifts from a collection of bound particles to free-moving radiation. The entanglement between the radiation and the remaining black hole is real, but it is not the kind of static, infinite connection that leads to paradoxes. It is a dynamic, fleeting link that fades as the system scrambles its information.

Banks concludes that the standard picture of a black hole, with its smooth event horizon and the associated divergences in energy, is an illusion created by applying the wrong tools to the problem. The models he studies show that black holes are simply complex quantum states that happen to have the same size, energy, and entropy as the black holes predicted by Einstein's equations. They do not possess a special, singular surface where the laws of physics break down. The resolution to the information paradox, therefore, is not to invent a firewall or a new mechanism to save information, but to accept that the quantum field theory description of the space near a black hole is fundamentally incorrect. The universe does not need a dramatic, violent solution to the problem of black holes; it simply requires us to stop assuming that space is smooth and continuous in the places where we have never been able to look. The black hole is not a monster with a secret, nor is it a puzzle with a hidden trapdoor. It is a complex, evolving system that behaves exactly as quantum mechanics demands, without the need for any exotic new physics at its edge.