# Quantum Physics and Gravity Meet in Newest QuICS Fellow’s Research

Two landmark achievements of 20th century physics remain stubbornly isolated, despite decades of attempts by scientists to bring them together.

On their own, they’ve been wildly successful. General relativity—Einstein’s grand theory of gravity—fused space and time into a single entity. It birthed the global positioning system and forever changed our conception of the cosmos. Quantum physics, the theory that governs the microscopic realm, powered the engines of the digital revolution, and it’s lighting the way to a new paradigm in computing.

But the two fields have largely been marooned on parallel tracks, rarely intersecting because they seem to describe such disparate domains of reality. “When electricity flows through a circuit, gravity is there,” says Brian Swingle, an assistant professor of physics at the University of Maryland and the newest fellow of the Joint Center for Quantum Information and Computer Science (QuICS). “We just don’t usually need it to describe the physics, so we ignore it.”

That might be fine for the physics of electrical circuits, Swingle says, but scientists expect a unified picture to emerge for the highest-energy, densest material in the universe—a picture in which quantum physics and gravity contribute on equal footing. Swingle, who arrived at UMD this past summer, is part of a vanguard of physicists exploring the connections between these two fields. His work blends quantum information and condensed matter physics with a pinch of gravity, and it’s unearthing curious connections between some of the most eye-catching phenomena that modern physics has on offer—things like quantum entanglement and black holes.

He hopes to expand on this work at Maryland and foster more collaboration between experts in all of these fields. “I think UMD has a lot of great resources, and there’s an opportunity now for new connections to be forged,” Swingle says. “That’s definitely one of the things that really excites me.”

Swingle’s interest in physics was kindled as a teenager, when a set of mysterious symbols captured his imagination. “Senior year of high school I somehow got interested in Maxwell’s equations,” he says, recounting his first contact with the four equations that are the distillation all of the 19th century’s knowledge about electricity and magnetism. “There was this mysterious picture with upside down triangles, which looked like gobbledygook, and it seemed really interesting to me. I’ve sort of been hooked ever since.”

He pursued a physics degree as an undergraduate at Georgia Tech in Atlanta, where he got an early start in research, writing software to simulate the behavior of many interacting particles and spending a summer working as a research assistant at the University of Washington.

He arrived at MIT for graduate school in 2005 with a loose plan to study condensed matter physics. But some early projects didn’t work out. “I thought about switching to neuroscience,” Swingle says. “I was thinking of working on birdsong. I had a general interest in information networks and higher organization.”

But his neuroscience career also stalled, and eventually he ended up back in condensed matter physics working with Xiao-Gang Wen, a luminary in the field. Wen asked Swingle to look into a problem involving quantum entanglement, the curiously strong connection that two quantum objects can share, and something clicked. Swingle became fascinated with entanglement, calculating the entanglement properties of a variety of quantum systems. “I was looking for some kind of picture,” he says, “but I didn’t really know what it was.”

That all changed when he took a class in string theory, the theoretical effort to recast all of modern physics in the language of tiny vibrating strings. String theory made one of the earliest attempts to shoehorn gravity into quantum physics, and over the course of several decades string theorists discovered some interesting relationships between the two. One such connection was a duality between quantum physics playing out in a particular universe and a theory of gravity in a universe with one extra dimension.

Swingle explains the gist of that relationship using a round table. Imagine that the table’s edge—a circle that wraps its perimeter—is a one-dimensional universe with a bunch of interacting quantum systems. It turns out that picking a particular quantum state for this encompassing ring limits what can happen in the interior—the tabletop itself—and vice versa. The truly strange thing is that the tabletop ends up endowed with gravity, but it’s now a theory of gravity in two dimensions. “The question you can kind of ask is ‘Where does the extra dimension come from?’” Swingle says.

It’s a lot of abstract math that need not have much to do with our universe, but Swingle discovered a particular way in which this abstraction becomes real. “I started to see these connections,” Swingle says. “It was starting to gel in an interesting way.”

Tensor networks—graphical webs that can represent the complicated states of interacting quantum systems—provided the key ingredient. Drawing the tensor network of a quantum system living on the edge of a table turned out to be a picture of a theory of gravity inside, a discrete snapshot of the fabric of spacetime on the tabletop. It was a simple theory of quantum gravity, albeit one that does not describe our universe. “Quantum gravity in any spacetime is sufficiently mysterious and sufficiently poorly understood that it's worth understanding even a toy case,” Swingle says.

Since his pioneering result, Swingle has continued to develop the relationship between quantum entanglement—depicted via hierarchical webs of tensor networks—and geometry, and has recently been thinking about the role complexity and computation play in all of it. A recent paper with several collaborators demonstrated a precise mathematical connection between the complexity of a quantum state and the geometry of its dual theory of gravity. It’s part of a body of work that is continuing to shift physicists’ perspectives on quantum gravity. “The old slogan used to be that entanglement was the fabric of spacetime,” Swingle says. “Now maybe it’s more general. Maybe now we think of spacetime itself as a computational history of some process—the picture of the quantum circuit that prepares the quantum state, something like that.”

Now, as an assistant professor at UMD, Swingle hopes to continue research along these lines, perhaps bringing more quantum information tools into the fray. He is also a co-principal investigator for the “It from Qubit” collaboration launched by the Simons Foundation. The name is a play on “it from bit,” a phrase coined by physicist John Archibald Wheeler that underscored the significance of information to the bedrock upon which reality sits. “Of course, information is quantum mechanical, since the world is quantum mechanical,” Swingle says. “Somehow that’s an important ingredient in the story—the quantum-ness is really important.”

Swingle encourages any students interested in learning more about his research to contact him directly or stop by his weekly group meeting, which is typically held on Thursdays at 5 p.m. in PSC 3150.

*—Story by Chris Cesare*

QuICS is a partnership between the University of Maryland and the National Institute of Standards and Technology. It is one of the centers and labs in the University of Maryland Institute for Advanced Computer Studies.