Entanglement Builds Space-Time. Now “Magic” Gives It Gravity.

In holographic theories, physicists may have traced the pliability of space-time to its quantum roots: a measure of quantumness known as “magic.”

I n 1973, John Archibald Wheeler described the relationship between space and matter in two sentences: “Space acts on matter, telling it how to move. In turn, matter reacts back on space, telling it how to curve.” Wheeler’s words serve as a pithy encapsulation of general relativity, Albert Einstein’s theory of gravity.

Wheeler’s sentences also lay out a challenge that theorists face today: When they build a model of the universe — at least one that works at the quantum level — it’s been difficult to get space and matter to interact in the way that they must.

Einstein cast gravity not as a force but as the geometric bending of space and time. In a popular analogy, the fabric of space-time is like the flat expanse of a mattress, and a massive object like a star is like a bowling ball sitting on top. The weight of the bowling ball compresses the mattress, forming a dimple — matter tells space-time how to curve.

In this analogy, a planet is like a smaller ball. If it rolls close enough to the bowling ball, its path will be altered by the dimple in the mattress — space-time tells matter how to move.

But general relativity has a fatal flaw. When a star dies and collapses, its mass is concentrated into an unimaginably dense point. The dimple in the mattress stretches into a deep depression, one that essentially rips all the way through. Physicists call this arrangement a black hole. If a ball reaches such a rip, it’s no longer guided by the fabric, and the analogy breaks down; scientists need a new theory to understand this and other, similarly extreme situations.

In the late 1990s, physicists had a stroke of luck. They learned that if they imagined space-time as a collection of purely quantum particles, they could in principle describe a black hole — rip and all — in an entirely new way.

Theorists have spent the last few decades trying to understand exactly how a space-time constructed from such quantum particles could work. And they’ve made progress: They’ve found that entanglement between particles gives space-time its structure, building an environment where matter can move — and satisfying the conditions of Wheeler’s first statement. But the origin of Wheeler’s second statement remained mysterious; in their models, matter didn’t tell space how to curve. The bowling ball sat atop the mattress without making a dent.

Until now. Physicists including Charles Cao at Virginia Tech have recently determined how quantum particles could give space-time its bendiness. In a handful of recent works, multiple teams have identified a feature of quantum mechanics that Cao calls “the fabric softener of space.” It’s a measure of quantumness called “magic.”

“Without magic, things are a little too simple,” said John Preskill , a physicist at the California Institute of Technology who contributed to Cao’s newest paper. “And, you know, quantum space-time isn’t quite that simple.”

How To Code a Universe

Perspective shifts abound in physics. For instance, there’s more than one way to look at the motion of a pendulum. You might specify its location using the height and the horizontal displacement of the weight hanging at the end of the string. Or you might use the length of the string and its angle instead. The perspectives are equivalent; simple trigonometric equations take you from one perspective to the other.

Mark Belan/ Quanta Magazine

For 50 years, theorists have been chasing a far more profound perspective shift: a new way, beyond Einstein’s curved space-time, to look at the universe.

In the early 1970s, Jacob Bekenstein and Stephen Hawking took the first step in that direction when they discovered that you could reinterpret a black hole (and anything that had fallen into it) as a spherical collection of particles . In the late 1990s, Juan Maldacena, Edward Witten, and others extended this insight to a whole universe; they described an exotic, static world as a throng of interacting particles, also arranged in a sphere.

In both cases, you could replace the 3D region of space-time with particles on the region’s surface. You could consider the surface to be 2D, like a globe flattened into a paper map. Physicists call this dual nature of space-time the holographic principle, since it resembles the way a holographic sticker can cram a whole 3D scene onto a flat surface without losing data.

Over the last couple of decades, theorists have explored what gives the 3D fabric of space its shape. Entanglement, a quantum property that links particles to one another, seems to serve as space’s connective tissue. Take, for instance, a wormhole, a theoretical bridge connecting two distant regions of space. Holographically, a 3D wormhole is equivalent to two entangled sets of particles. Start snipping the “threads” of entanglement that link one set with the other, and the tunnel connec…