When Physics Breaks: What Black Holes Reveal

There are places in the universe where our physics simply stops working. Not "gets difficult" or "becomes uncertain"—actually breaks down. The equations return infinity. The models collapse. We hit what physicists politely call a singularity, which is just a technical term for "we have no idea what happens here."

Juan Maldacena, a theoretical physicist whose work on quantum gravity has shaped the field for two decades, recently sat down with Curt Jaimungal to discuss exactly where and why our understanding fails. The conversation circles around a deceptively simple question: what is spacetime made of?

The Problem with Fundamental Things

In general relativity, spacetime isn't made of anything. It's the primary object—the stage where physics happens. Maldacena explains: "Spacetime in the theory of general relativity, it's not made out of anything. It's a primary concept. It's the main dynamical object of the theory."

But when you try to apply quantum mechanics to that stage, things get weird. Most of physics describes fields—electromagnetic fields, electron fields, the Higgs field—living in spacetime. We know how to describe those fields quantum mechanically. The field that makes spacetime itself? We only know how to describe it classically. And that's a problem.

Maldacena and his colleagues have been exploring whether spacetime might emerge from something else: quantum degrees of freedom—possibly qubits—that live at the boundaries of spacetime, far away from the interior. It's a radical inversion. Instead of quantum fields living in spacetime, spacetime emerges from quantum information.

This isn't metaphysics. It's an attempt to solve actual, practical breakdowns in our theories.

Where the Equations Fail

There are two places where our current physics demonstrably breaks: the beginning of the universe and the interiors of black holes. These aren't edge cases. They're fundamental features of reality.

"We know of places in the universe where our current understanding of physics breaks," Maldacena says. "The idea is to develop at least some theory that can describe such things and then try to figure out of course in some way that that theory is the correct theory."

The challenge isn't just technical. It's conceptual. In quantum mechanics, you typically have an observer outside the system making measurements. In gravity, everything is inside the system. There's no massless, energy-free observer floating outside the universe taking notes.

"In quantum mechanics we have some observer who's outside the system and in gravity everything is somehow inside the system," Maldacena explains. "We cannot have an observer that has no mass that has you know no energy that measures things from outside."

This creates a deep tension. Quantum mechanics assumes a sequence of measurements, an order of operations. But in general relativity, spacetime can have different geometries, different topologies. The order isn't always clear. Time itself becomes slippery.

Progress in the Dark

Despite these fundamental barriers, there's been real progress. Maldacena discusses recent work by Edward Witten, Geoff Pennington, and collaborators on black hole entropy—specifically, how to calculate it without running into infinities.

Here's the problem they solved: Black holes have entropy proportional to the area of their event horizon. That's a huge number. But they also emit Hawking radiation—thermal radiation that leaks out. If you're an observer approaching the horizon, that radiation looks hotter and hotter. Calculate the entropy contribution naively, and you get infinity.

Physicists knew these infinities had to somehow cancel with the area contribution to give something finite. But the math was messy. The new work provides "a better way to think about black hole entropy in the semiclassical theory," Maldacena notes. "It answers some questions of black hole entropy. Again, it doesn't answer all the questions, but it answers some important questions."

That qualifier—"some important questions"—is characteristic of how frontier physics actually works. Not grand unified theories announced with fanfare, but incremental progress on well-defined problems. The work describes black holes from the outside, for observers who stay safely beyond the event horizon. What happens inside? That's still an open question.

The Greatest Problem

When asked about the biggest unsolved problem regarding black holes, Maldacena doesn't hesitate: the interior.

"The singularity is just the name for things we don't understand," he says. It's not a place inside the black hole in the spatial sense. It's in your future. If you cross the event horizon, you will encounter it. There's no avoiding it—not because you lack skill or speed, but because it's a feature of time itself in that region.

Maldacena offers a vivid comparison: the universe is generally expanding, and we're all fine with that. But in regions where enough matter concentrates, spacetime starts collapsing instead. You get "a small big crunch"—a region where spacetime curvature becomes infinite, the opposite of the Big Bang. It's hidden behind the event horizon, invisible from outside. But if you fall in, you collapse along with everything else.

The fact that curvature becomes extreme suggests quantum effects should dominate there. A complete theory should tell us what actually happens. We don't have that theory yet.

What We Have, What We Don't

The honest assessment: we have theories that can describe aspects of black holes as seen from outside. We can ask precise questions and get answers—at least conceptually. The entropy work, the understanding of Hawking radiation, the holographic principle suggesting spacetime emerges from boundary information—these are real advances.

But the interior? The singularity? The beginning of the universe? Those remain beyond our current framework. Not because physicists aren't smart enough, but because the conceptual tools—the way we think about observers, time, causality, measurement—break down in those regimes.

Maldacena's approach is to be clear about what we know and what we don't. "We don't know whether the theories we have right now of quantum spacetime more complete theories where they are the correct theory or not," he acknowledges. That uncertainty isn't a failure. It's the current edge of knowledge, honestly reported.

The question that haunts this work: is spacetime fundamental, or does it emerge from something else? If it emerges, what are the rules governing that emergence? And what happens in regions where emergence itself breaks down?

Physics has always progressed by finding where the map doesn't match the territory—then redrawing the map. Black holes are offering us that opportunity again. Whether the next map includes qubits at the boundaries of spacetime, or something we haven't imagined yet, depends on which questions we can actually answer.

—Nadia Marchetti, Unexplained Phenomena Correspondent