GoKawiilIn 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
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