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A few years ago a team of chemists unboiled an egg. Boiling causes protein molecules in the egg to twist around one another, and a centrifuge can disentangle them to restore the original. The technique is of dubious utility in a kitchen, but it neatly demonstrates the reversibility of physics. Anything in the physical world can run both ways—it's one of the deepest features of the laws of physics, reflecting elemental symmetries of space, time and causality. If you send all the parts of a system into reverse, what was done will be undone. The information required to wind back the clock is always preserved. Of course, undoing a process may be easy in simple systems but is less so in complex ones, which is why the egg unboiler was so nifty.

But there's a troubling exception: black holes. If a massive enough star collapses under its own weight, its gravity intensifies without limit and locks matter in its grip. Jump into one, and there's no going back. Merge two together, and you can't split them apart. A black hole presents an almost completely featureless façade to the universe. Looking at it, you can't tell what fell in. The black hole does not seem to preserve information. This irreversibility, first appreciated by physicist David Finkelstein in 1958, was the earliest inkling of the black hole information paradox—“paradox” because how could reversible laws have irreversible effects? The paradox signaled a deeper disease in physicists' understanding of the world. Scientists have many reasons to seek a grand unified theory of nature, but the information paradox is their most specific motivation, and it has guided their way when they have little else to go on.

At last, more than 60 years after this puzzle began to appear, physicists are seeing hope for a solution. In the year leading up to the pandemic and through the months of lockdown, a coalition of theorists took huge strides to understand the paradox—the most progress in decades, some say. They bolstered the idea that black holes, despite appearances, are reversible, and they dissolved the official paradox. Physical theory is no longer at odds with itself. The work is contentious, though, and by its proponents' own admission, it is at best a starting point for a full explanation of black holes.

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