It is the perfect crime of Nature: without body or traces there is no crime. That’s a black hole. Everything that crossed your event horizon (the point of no return so to speak) disappears forever. Neither matter, nor light, nor even the information about them could escape its immense gravity. However, in the 1970s, Stephen Hawking revolutionized this picture by showing that, from a quantum mechanical point of view, black holes are not completely black, and they emit what is known as Hawking radiation. Thanks to that, they can be taken to the dock if we know how to look for the evidence. But first you have to understand it.
A black hole is like a football field and its event horizon is the sidelines. Every second, on that same line, two balls appear out of nowhere bouncing together: one falls into the field and the other is thrown towards the stands. The public only sees what reaches the stands (Hawking radiation). What about the other one? It falls into the field with ‘negative energy’ (like an anti-ball) that destroys part of the material on the field, causing the entire field to shrink little by little until it disappears. Now comes the question that really worried Hawking: If the black hole disappears, what happens to all the information that had fallen into it?
An international team of physicists, led by Ulf Leonhardt, has recreated in the laboratory a system that imitates the behavior of a black hole and has observed how this information could reappear during the evaporation process. The simulation results, published in Naturedoes not definitively resolve the paradox, but offers one of the most complete experimental tests to date of a phenomenon that until now only existed in theoretical calculations.
The first thing to clarify is that the researchers did not create a real black hole. Instead they built a quantum analogue, a system formed by a chain of ultracold atoms whose interactions reproduce the same equations that describe certain processes near the event horizon. As much as it may sound like gibberish, it is a very common strategy in modern physics.
When a phenomenon is impossible to study directly (because it occurs inside a star, in a black hole or in the first moments of the Universe) scientists design completely different systems that obey the same mathematical laws. If the equations are the same, the physical behavior can be too. Thanks to the Leonhardt team’s simulation, For the first time it has been possible to study the effects of Hawking radiation. And with this we try to answer what happens to the information that fell into a black hole once it disappears.
Quantum physics states that information can never be completely destroyed. But if a black hole ends up disappearing, it would seem that all the information about what it absorbed also disappears with it. This conflict between general relativity and quantum mechanics is known as the information paradox. and has been challenging physicists for half a century. Leonhardt’s team analyzed quantum entanglement, the connection that can exist between two particles even when they are separated, and points out that the information remains there, thanks to entanglement. But you have to learn to look for it.
Black holes remain one of the biggest challenges in physics because they represent the place where the two most proven theories ever built collide.: general relativity and quantum mechanics. Understanding how information behaves when a black hole evaporates could bring scientists closer to a theory capable of unifying both descriptions of the Universe.