This article is part of a package in collaboration with Forbes on time capsules, preserving information and communicating with the future. Read more from the report.
Around 38 percent of websites that were on the Internet in 2013 are gone now. Half of Wikipedia pages reference dead links. Information seems to be disappearing all around us, and that’s nothing new. Over geological time, information loss is the norm, not the exception.
Yet according to physics, information is never destroyed. In principle, a burned book is just as readable as the original—if you analyze the ashes of the fire, the smoke and the flames to re-create the incinerated words.
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The “unitarity” principle of quantum mechanics says that the universe is reversible—what’s done can always be undone. If you know everything about the cosmos in the present, then you should be able to rewind it and understand everything about it in the past and predict everything about it in the future. In other words, knowledge is always preserved; destroying information is impossible, according to the fundamental laws of physics.
Or is it? In some special parts of the universe, such as inside black holes, information seems to get lost. The paradox has vexed physicists for half a century, but in the past several years scientists have discovered a path toward solving it. Information, it seems, finds an escape hatch. Many questions remain, though. To answer them, scientists will need to wade into one of the biggest open problems in physics: How does gravity work in the quantum realm? By studying the fate of information, physicists hope to find a path toward a fully realized theory of quantum gravity.
Black holes pose a problem for information because they don’t last forever. They leak particles very slowly in a process called Hawking radiation, discovered by the late physicist Stephen Hawking in 1974. Eventually, they completely evaporate, leaving nothing behind. But if this is the case, then how can we know what fell into them? “It looks like the information is destroyed, unless it gets out,” says physicist Thomas Hartman of Cornell University.
Impermanence itself doesn’t pose an information paradox. Many things in nature don’t last. But those other disappearing things transform into something else or leave behind a residue through which an enterprising physicist could piece together their existence. Black holes, however, famously let nothing go. Their gravity is so strong that even light cannot escape their bounds, defined by a spherical border called an event horizon. The Hawking particles they radiate, which deplete them, do not originate within the event horizon but outside it, which is why they are immune. Hawking radiation represents “energy squeezed out of the vacuum near the event horizon,” wrote physicist Ahmed Almheiri in Scientific American in 2022. But that is also why scientists assumed these particles can’t carry information about the black hole’s interior with them.
Hawking recognized the problem when he calculated the entropy of a black hole. Entropy is a measure of disorder, or uncertainty. “Information is knowledge about a system, and a lack of information is entropy,” says quantum information theorist Hrant Gharibyan, who co-founded the quantum software company BlueQubit. The more information you have about a system, the lower its entropy is. And if you were to know everything about it—exactly where every particle was—the entropy would be zero. But Hawking found that in the final quantum state after a black hole evaporates completely, its entropy isn’t zero. Uh-oh—information is gone.
Since then, physicists have been trying to unravel the puzzle. From the beginning, they hoped to find a way for the Hawking particles to carry a black hole’s information with them. “Once the black hole is completely evaporated, all that’s left is the Hawking radiation, so it has to be there,” says University of California, Santa Cruz, physicist Edgar Shaghoulian. Yet how information could be encoded in the escaping particles wasn’t clear.
Physicists made a breakthrough several years ago when they thought about spacetime itself as a quantum phenomenon. Like particles or any other quantum object, quantum spacetime would not have a fixed, certain geometry—instead it would exist as a combination of different geometries, each with their own probability. Though the most likely shapes would mimic the classical picture of a black hole, some of the outlier probabilities would look much different. In some cases, the insides of black holes would even form wormholes connecting to other black holes. Factoring in the wormholes revealed that, in effect, a region of spacetime within a black hole called an island in some ways exists outside of the hole altogether. And information seems to find a path through such wormholes.
This escape route means that, in principle, physicists should be able to find the data they need to rewind black holes, just as they can reverse any other event in the universe. “You have to do something very, very delicate to access the information, similar to the ashes of the burned message but even worse,” Shaghoulian says. Scientists used this insight to devise a new formula for a back hole’s entropy and revealed that its final state is zero, as should be the case if all the laws of physics are obeyed.
These recent revelations alleviate the black hole information paradox, but many details still need to be worked out. “We’ve made a lot of progress on it, but there are still questions that are not answered,” Hartman says. “I don’t call it solved.”
Black holes’ immense gravity puts them under the purview of Einstein’s general theory of relativity, which first predicted their existence. Yet their minuscule size means they must also obey the rules of quantum mechanics, which lords over the realm of the very small. The opposition of these two incompatible theories is why many black hole problems arise. It’s also why black holes might point the way toward their reconciliation.
“Black holes are great for theoretical physicists,” Hartman says. “A black hole is kind of the simplest, cleanest theoretical system in which to study gravity. And the properties of black holes are sort of properties of spacetime itself.”
Researchers hope to use the puzzles and paradoxes of black holes to elucidate how general relativity fits into quantum physics and even to understand what spacetime is. The fact that black holes store information implies that spacetime itself can store information and possibly that spacetime is information. The fabric of the universe may be made up of tiny, quantized units of information, and the ways those units are correlated with one another—or entangled—might determine the shape of space and time.
Either way, it seems that information is safe in the universe. In a world where life is ephemeral, where no one and nothing can last forever, not even planet Earth, this rule feels comforting. There will always be a record, however inscrutable, of what happened here.
