The fabric of space and time is widely believed by physicists to be emergent, stitched out of quantum threads according to an unknown pattern. And for 22 years, they've had a toy model of how emergent space-time can work: a theoretical "universe in a bottle," as its discoverer, Juan Maldacena, has described it. Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences. The space-time filling the region inside the bottle--a continuum that bends and undulates, producing the force called gravity--exactly maps to a network of quantum particles living on the bottle's rigid, gravity-free surface. Maldacena's discovery of this hologram has given physicists a working example of a quantum theory of gravity.

Reprinted with permission from Quanta Magazine's Abstractions blog. The fabric of space and time is widely believed by physicists to be emergent, stitched out of quantum threads according to an unknown pattern. And for 22 years, they've had a toy model of how emergent space-time can work: a theoretical "universe in a bottle," as its discoverer, Juan Maldacena, has described it. The space-time filling the region inside the bottle--a continuum that bends and undulates, producing the force called gravity--exactly maps to a network of quantum particles living on the bottle's rigid, gravity-free surface. Maldacena's discovery of this hologram has given physicists a working example of a quantum theory of gravity.

Theoretical physicists striving to unify quantum mechanics and general relativity into an all-encompassing theory of quantum gravity face what's called the "problem of time." In quantum mechanics, time is universal and absolute; its steady ticks dictate the evolving entanglements between particles. But in general relativity (Albert Einstein's theory of gravity), time is relative and dynamical, a dimension that's inextricably interwoven with directions x, y and z into a four-dimensional "space-time" fabric. The fabric warps under the weight of matter, causing nearby stuff to fall toward it (this is gravity), and slowing the passage of time relative to clocks far away. Or hop in a rocket and use fuel rather than gravity to accelerate through space, and time dilates; you age less than someone who stayed at home.

As experimental proposals go, this one certainly doesn't lack ambition. First, take a black hole. Now make a second black hole that is quantum entangled with it, which means that anything that happens to one of the black holes will seem to have an effect on the other, regardless of how far apart they are. The rest sounds a bit easier, but a lot weirder. As it falls beyond the event horizon -- the point beyond which not even light can escape -- the information is rapidly smeared throughout the black hole and is scrambled seemingly beyond recall.

After decades of research, quantum computers are approaching the scale at which they could outperform their "classical" counterparts on some problems. They will be truly practical, however, only when they implement quantum error correction, which combines many physical quantum bits, or qubits, into a logical qubit that preserves its quantum information even when its constituents are disrupted. Although this task once seemed impossible, theorists have developed multiple techniques for doing so, including "surface codes" that could be implemented in an integrated-circuit-like planar geometry. For ordinary binary data, errors can be corrected, for example, using the majority rule: A desired bit, whether 1 or 0, is first triplicated as 111 or 000. Later, even if one of the three bits has been corrupted, the other two "outvote" it and allow recovery of the original data.