In 2012, physicists showed that this paradox is tightly linked to the nature of the event horizon. They’d known since the 1970s that black holes emit radiation, and that this radiation probably somehow carries the scrambled information about the stuff that fell into the hole. Now they imagined what would happen if an astronaut who was about to cross the horizon of an ancient black hole communicated with someone far away — an observer who had gathered the radiation emitted by the black hole over its lifetime. The result of the thought experiment was puzzling: The astronaut and the faraway observer would end up with two copies of the same information, one recovered over the black hole’s long lifetime and the other from close by. The extra copy is a problem, again spoiling the careful accounting of probabilities that quantum mechanics relies on. Some physicists concluded that something strange must happen just outside the horizon to disrupt the astronaut’s information gathering. 

Short Hair, Long Hair

Attempts to address the information paradox usually add extra detail outside the event horizon, referred to as quantum hair. The researchers who came up with the thought experiment about the astronauts in 2012 suggested that a shell of extremely high-energy particles called a firewall might lie just outside the horizon, breaking the connection between the two observers. Alternatively, the physicist Samir Mathur argues that black holes don’t have a horizon at all. Instead, he says that they are “fuzzballs” — each one a quantum combination, or superposition, of many different configurations of space-time, making the black hole’s edges fuzzy.

Other ideas include “gravastars” that resemble black holes but are surrounded by shells of exotic matter, and so-called regular black holes — reimagined versions of the objects that lack the infinitely dense points in their centers known as singularities.

This zoo of proposals all introduce new effects outside the horizon that should change how a vibrating black hole emits gravitational waves.

The proposed effects generally lie very close to the horizon, perhaps only within 10⁻³³ centimeters — the so-called Planck length. Such close-cropped quantum hair would not be directly observable as a change in the signals from black hole collisions, but it might be visible in other ways. For example, unusual aftereffects called echoes, generated as gravitational waves bounce off a firewall or other structure near the horizon, might appear after an initial signal.

Searches for echoes have so far come up empty. These failed searches don’t rule out the possibility of quantum hair, however, since it’s unclear which kinds of quantum hair should give rise to echoes and which won’t, or how exactly the echoes would appear.

Meanwhile, physicists can also look for “longer” hair — more obvious deviations from Einstein’s theory. There’s less theoretical reason to expect this, but on the other hand, the highly curved space-times near black holes are a new environment for astronomers, and they can’t be sure what they might find. Perhaps space-time curves differently under these conditions than general relativity predicts.

“I think it’s a worthwhile exercise to go and test that,” said Niayesh Afshordi, an astrophysicist at the University of Waterloo in Canada.

Math Meets Data

Since the first detection of colliding black holes by the Laser Interferometer Gravitational-Wave Observatory, or LIGO, in 2015, physicists have been trying to use this data to test Einstein’s theory. The project accelerated after additional observatories — Virgo in Europe and KAGRA in Japan — came online. But a substantial mathematical challenge stood in the way: The black holes that collide are always rotating, which greatly complicates calculations. The mathematician Roy Kerr calculated back in 1963 how rotating black holes behave in the framework of Einstein’s equations. But what if that framework is wrong?

A group of physicists at KU Leuven cracked the problem in 2023. They developed a technique for understanding how fast-spinning black holes would behave if Einstein’s theory were modified.

Then, at a conference later that year, a graduate student in the Leuven group, Simon Maenaut, met Gregorio Carullo, a postdoctoral researcher in Copenhagen at the time who was an expert in analyzing gravitational wave signals. They realized that they could test the Leuven group’s theories against Carullo’s data, and they wasted no time. “We sort of jumped on a free desk and started coding together,” said Carullo, who is now at the University of Birmingham.