Rod Serling knew all about dimensions.

His Twilight Zone was a dimension of imagination, a dimension of sight and sound and mind, a dimension as vast as space and timeless as infinity. It was all very clear except for the space and time part, the dimensions of real life. Serling never explained them.

Of course, ever since Einstein, scientists have also been scratching their heads about how to make sense of space and time. Before then, almost everybody thought Isaac Newton had figured it all out. Time “flows equably without relation to anything external,” he declared. Absolute space is also its own thing, “always similar and immovable.” Nothing to see there. Events of physical reality performed independently on a neutral stage where actors strutted and fretted without influencing the rest of the theater.

But Einstein’s theories turned Newton’s absolute space and time into a relativistic mash-up — his equations suggested a merged space-time, a new sort of arena in which the players altered the space of the playing field. It was a physics game changer. No longer did space and time provide a featureless backdrop for matter and energy. Formerly independent and uniform, space and time became inseparable and variable. And as Einstein showed in his general theory of relativity, matter and energy warped the space-time surrounding them. That simple (hah!) truth explained gravity. Newton’s apparent force of attraction became an illusion perpetrated by space-time geometry. It was the shape of space-time that dictated the motion of massive bodies, a symmetric justice since massive bodies determined space-time’s shape.

Verification of Einstein’s space-time revolution came a century ago, when an eclipse expedition confirmed his general theory’s prime prediction (a precise amount of bending of light passing near the edge of a massive body, in this case the Sun). But space-time remained mysterious. Since it was something rather than nothing, it was natural to wonder where it came from.

Now a new revolution is on the verge of answering that question, based on insights from the other great physics surprise of the last century: quantum mechanics. Today’s revolution offers the potential for yet another rewrite of space-time’s résumé, with the bonus of perhaps explaining why quantum mechanics seems so weird.

“Space-time and gravity must ultimately emerge from something else,” writes physicist Brian Swingle in the 2018 *Annual Review of Condensed Matter Physics*. Otherwise it’s hard to see how Einstein’s gravity and the math of quantum mechanics can reconcile their longstanding incompatibility. Einstein’s view of gravity as the manifestation of space-time geometry has been enormously successful. But so also has been quantum mechanics, which describes the machinations of matter and energy on the atomic scale with unerring accuracy. Attempts to find coherent math that accommodates quantum weirdness with geometric gravity, though, have met formidable technical and conceptual roadblocks.

At least that has long been so for attempts to understand ordinary space-time. But clues to a possible path to progress have emerged from the theoretical study of alternate space-time geometries, thinkable in principle but with unusual properties. One such alternate, known as anti de Sitter space, is weirdly curved and tends to collapse on itself, rather than expanding as the universe we live in does. It wouldn’t be a nice place to live. But as a laboratory for studying theories of quantum gravity, it has a lot to offer. “Quantum gravity is sufficiently rich and confusing that even toy universes can shed enormous light on the physics,” writes Swingle, of the University of Maryland.

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