| One of the most influential recent ideas in theoretical physics came from a paper that has racked up an eye-popping 27,000 citations and counting.
The paper concerns the most famous mystery in theoretical physics: the fundamental origin of the gravitational force. The idea is that you can understand gravity, which shows up in our universe as curves in the fabric of space and time, without knowing the exact rules of gravity and space-time. The better-understood rules of quantum theory implicitly contain all of gravity's secrets.
To get a sense of what this means, consider two more familiar sets of rules: those of chess and checkers. Imagine if every checkers game were secretly equivalent to a chess game. (Maybe two checker pieces side by side can represent a pawn, while a trio arranged in a certain way corresponds to a rook, and so on.) Knowing this, you could watch a checkers game and translate it into an equivalent chess game, and vice versa.
The rules of chess are not actually hidden in the rules of checkers. Imagine the wild conspiracy it would take for that to be true! But that's roughly the sort of conspiracy that the Argentinian American physicist Juan Maldacena stumbled upon in his 1997 paper. He discovered that events involving a type of space-time called anti-de Sitter (AdS) space are equivalent to events involving particles moving through quantum fields in a rigid environment devoid of gravity. As in the chess and checkers analogy, any event could be interpreted in two equivalent ways. What looks like a black hole in AdS space, for instance, corresponds to a hot, soupy plasma of particles in the quantum field theory. The latter is a type called a conformal field theory (CFT), so Maldacena's discovery is known as the AdS/CFT correspondence.
AdS/CFT thrilled many theoretical physicists by suggesting that they could indirectly play a game they didn't fully understand (gravity) by playing a game they knew better (quantum field theory). They have spent nearly three decades enthusiastically working out the "dictionary" that tells them how to translate pieces and moves on one side into the corresponding pieces and moves on the other.
There are caveats. Critically, the shape of AdS space-time is in some sense the opposite of the shape of the space-time we live in. That leads physicists to wonder what lessons — if any — learned from the AdS/CFT correspondence will apply to our world. Some physicists hope that all types of space-time work the same way, and that some as-yet-undiscovered quantum theory will let them do an end run around gravity in our world too. In the meantime, they're using AdS/CFT as something of a cheat sheet. It points them toward promising answers and techniques that work in AdS space, which they can then try to transfer to more realistic contexts.
What's New and Noteworthy
The AdS/CFT correspondence didn't come out of nowhere. It's the most concrete example of a more general property of gravity that physicists had mulled over for years, known as the holographic principle. The AdS space-time is like a volume, while the corresponding quantum particles of the CFT live on the surface surrounding that volume. Thus, the quantum game has one dimension less than the gravity game, as if the checkers game were hiding not normal chess but Star Trek's 3D version of chess. This situation resembles a hologram, where a 3D image projects from a 2D surface. Early hints that gravity and space-time may have this holographic nature sprang from Stephen Hawking's work on black holes in the 1970s.
Holography has recently come full circle, helping to clarify a mystery Hawking grappled with his whole life: What happens to stuff that falls into a black hole? Does it survive in a hopelessly scrambled form, like a burnt book transformed into heat, smoke and light? Or is it literally erased from the universe? Adherents to AdS/CFT have long argued that the contents of a black hole in AdS space must survive, since the equivalent, quantum side of the correspondence has no way of erasing information. A few years ago, a string of breakthroughs took insights from AdS/CFT and generalized them, finding that information does indeed survive black holes in realistic space-times too. And more recently, physicists have used holographic tools to inch closer to understanding what happens deep inside black holes.
Holography, in the form of AdS/CFT and in other guises, has spread to other areas of physics. Anytime you have a quantum system, you can relate it to a gravitational system, which may offer insight. In this way, physicists have used holography to try to understand exotic materials called strange metals. Going in the other direction, experimentalists are prodding collections of quantum particles that they hope might someday answer questions about an equivalent space-time. But all of these efforts are warm-up problems to the ultimate prize: an accounting of our universe, and how it was born. Some groups have taken baby steps toward a holographic model of real space-time, but the future of that effort remains uncertain. | 
