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.
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.
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