When Spacetime Gets Bendy: How Quantum Magic Gives Gravity Its Spring

Physicists may have traced the pliability of spacetime to its roots—a measure of quantumness known as magic.

In 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 to space, telling it how to curve. This is the essence of general relativity. But there’s a problem embedded in this elegant picture.

The Problem with Gravity

It’s difficult to get space and matter to interact in a way that is consistent with quantum mechanics. Einstein cast gravity as the geometric bending of space and time, a classical (non-quantum) description that works beautifully at large scales.

But general relativity has a fatal flaw. When a star dies and collapses, mass concentrated into an unimaginably dense point—a dimple in the fabric stretches into an infinitely deep depression, “ripping” all the way through. This is a black hole. If a ball reaches a rip, it’s no longer guided by the fabric. We need another theory to understand this and other similarly extreme cases.

The Quantum Leap

In the late 1990s, physicists had a stroke of luck. They learned that if they imagined spacetime as a collection of purely quantum particles, they could describe a black hole in principle with the “rip” in an entirely new way.

Entanglement between particles gives spacetime its structure, building an environment where matter can move. However, these models did not tell us how matter tells space how to curve.

Until now.

Charles Cao at Virginia Tech has recently determined that a quantity of quantum particles could give spacetime its bendiness. He identified a feature of quantum mechanics called the “fabric softener of space”—a measure of quantumness called magic.

A New Perspective on Spacetime

The perspective shifts in physics when encoded information appears on a boundary. The holographic principle says all information about a 3D region of spacetime can be encoded on the region’s 2D surface—meaning it is bounded by that surface boundary. Any location in the space can be reconstructed by measuring all the quantum particles on the boundary. Information is redundantly encoded in all different groups of quantum particles.

Bekenstein and Hawking took the first step in this new direction, discovering that we could interpret a black hole and anything that has fallen into it as a spherical collection of particles—a 3D region of spacetime with particles on the region’s surface. Consider the surface to be 2D, like a globe.

This dual nature of spacetime—the holographic principle—resembles the way a holographic sticker can cram a whole 3D scene onto a flat surface without losing data.

Building Spacetime from Entanglement

Over the last couple of decades, theorists have explored what gives the 3D fabric of space its shape. Entanglement acts as connective tissue. A 3D wormhole, holographically, is equivalent to two entangled sets of particles. Cutting the threads of entanglement, the tunnel connecting the regions gets thinner and thinner. After the final thread, the connection dissolves entirely.

Physicists used quantum error correcting codes to encode space and its matter into simply a lot of quantum particles. These codes are crucial to quantum computing because quantum computers work by manipulating qubits, which can lose their superposition easily due to measurements and therefore the extra information as well. Physicists have worked out ways to protect this delicate information through redundancy—by spreading out the qubit’s information and scrambling it through random unitary dynamics, entanglement spreads, and information can be preserved even if some qubits are lost.

Redundancy is found in holography too. When codes are designed for quantum computing, the same thing as holography already did appears. A single holographic location is not encoded in just one set; rather, it’s spread across many sets due to entanglement.

Where Gravity Comes From

These codes divided entanglement for matter and space, and it was unbridgeable. This left no room for them to interact. A more sophisticated code was needed.

If we tried to create a quantum program that executed a tweaked code (for space to change and evolve) on a quantum computer, we would need to use a particular operation known as a T-gate, which rotates the qubit. Researchers previously thought the key was entanglement—the quantum computer’s advantage over classical computers.

Non-Clifford gates, including the T-gate, introduce a quality that these operations possess as “magic.” The more non-Clifford gates are needed to produce a quantum state, the more “magical” it is.

Magic gives space its springiness.

Anti-de Sitter Space and the Physics of Magic

Particles in anti-de Sitter space are highly magical. And this is the key: magic is inherently connected to gravity. When quantum codes get their magic from non-Clifford gates, they can produce spatial curvature where before there was only flat, gravity-free space.

Non-magical codes produce inert, gravity-free space because they protect encoded information perfectly. Gravity comes from the mixing of encoded information—from the non-Clifford operations that allow entanglement to flow between matter and spacetime itself.

This is still a proof of concept. Cao’s new code is a proof of concept of the general shape that a theory of quantum gravity should take—step 0.5 of 5. But it represents something profound: spacetime’s ability to bend and curve, to respond to the matter within it, might ultimately arise from a fundamentally quantum property—the “magic” required to make a quantum system do more than what classical physics allows.