Wormholes are a classic trope of science fiction in popular media, if only because they provide such a handy futuristic plot device to avoid the issue of violating relativity with faster-than-light travel. In reality, they are purely theoretical. Unlike black holes—also once thought to be purely theoretical—no evidence for an actual wormhole has ever been found, although they are fascinating from an abstract theoretical physics perceptive. You might be forgiven for thinking that undiscovered status had changed if you only read the headlines this week announcing that physicists had used a quantum computer to make a wormhole, reporting on a new paper published in Nature.
Let’s set the record straight right away: This isn’t a bona fide traversable wormhole—i.e., a bridge between two regions of spacetime connecting the mouth of one black hole to another, through which a physical object can pass—in any real, physical sense. “There’s a difference between something being possible in principle and possible in reality,” co-author Joseph Lykken of Fermilab said during a media briefing this week. “So don’t hold your breath about sending your dog through a wormhole.” But it’s still a pretty clever, nifty experiment in its own right that provides a tantalizing proof of principle to the kinds of quantum-scale physics experiments that might be possible as quantum computers continue to improve.
“It’s not the real thing; it’s not even close to the real thing; it’s barely even a simulation of something-not-close-to-the-real-thing,” physicist Matt Strassler wrote on his blog. “Could this method lead to a simulation of a real wormhole someday? Maybe in the distant future. Could it lead to making a real wormhole? Never. Don’t get me wrong. What they did is pretty cool! But the hype in the press? Wildly, spectacularly overblown.”
So what is this thing that was “created” in a quantum computer if it’s not an actual wormhole? An analog? A toy model? Co-author Maria Spiropulu of Caltech referred to it as a novel “wormhole teleportation protocol” during the briefing. You could call it a simulation, but as Strassler wrote, that’s not quite right either. Physicists have simulated wormholes on classical computers, but no physical system is created in those simulations. That’s why the authors prefer the term “quantum experiment” because they were able to use Google’s Sycamore quantum computer to create a highly entangled quantum system and make direct measurements of specific key properties. Those properties are consistent with theoretical descriptions of a traversable wormhole’s dynamics—but only in a special simplified theoretical model of spacetime.
Lykken described it to The New York Times as “the smallest, crummiest wormhole you can imagine making.” Even then, perhaps a “collection of atoms with certain wormhole-like properties” might be more accurate. What makes this breakthrough so intriguing and potentially significant is how the experiment draws on some of the most influential and exciting recent work in theoretical physics. But to grasp precisely what was done and why it matters, we need to go on a somewhat meandering journey through some pretty heady abstract ideas spanning nearly a century.
APS/Alan Stonebraker
Revisiting the holographic principle
Let’s start with what’s popularly known as the holographic principle. As I’ve written previously, nearly 30 years ago, theoretical physicists introduced the mind-bending theory positing that our three-dimensional universe is actually a hologram. The holographic principle began as a proposed solution to the black hole information paradox in the 1990s. Black holes, as described by general relativity, are simple objects. All you need to describe them mathematically is their mass and their spin, plus their electric charge. So there would be no noticeable change if you threw something into a black hole—nothing that would provide a clue as to what that object might have been. That information is lost.
But problems arise when quantum gravity enters the picture because the rules of quantum mechanics hold that information can never be destroyed. And in quantum mechanics, black holes are incredibly complex objects and thus should contain a great deal of information. Jacob Bekenstein realized in 1974 that black holes also have entropy. Stephen Hawking tried to prove him wrong but wound up proving him right instead, concluding that black holes, therefore, had to produce some kind of thermal radiation.
So black holes must also have entropy, and Hawking was the first to calculate that entropy. He also introduced the notion of “Hawking radiation”: The black hole will emit a tiny bit of energy, decreasing its mass by a corresponding amount. Over time, the black hole will evaporate. The smaller the black hole, the more quickly it disappears. But what then happens to the information it contained? Is it truly destroyed, thereby violating quantum mechanics, or is it somehow preserved in the Hawking radiation?
Per the holographic principle, information about a black hole’s interior could be encoded on its two-dimensional surface area (the “boundary”) rather than within its three-dimensional volume (the “bulk”). Leonard Susskind and Gerard ‘t Hooft extended this notion to the entire universe, likening it to a hologram: our three-dimensional universe in all its glory emerges from a two-dimensional “source code.”
Juan Maldacena next discovered a crucial duality, technically known as the AdS/CFT correspondence—which amounts to a mathematical dictionary that allows physicists to go back and forth between the languages of two theoretical worlds (general relativity and quantum mechanics). Dualities in physics refer to models that appear to be different but can be shown to describe equivalent physics. It’s a bit like how ice, water, and vapor are three different phases of the same chemical substance, except a duality looks at the same phenomenon in two different ways that are inversely related. In the case of AdS/CFT, the duality is between a model of spacetime known as anti-de Sitter space (AdS)—which has constant negative curvature, unlike our own de Sitter universe—and a quantum system called conformal field theory (CFT), which lacks gravity but has quantum entanglement.
It’s this notion of duality that accounts for the wormhole confusion. As noted above, the authors of the Nature paper didn’t make a physical wormhole—they manipulated some entangled quantum particles in ordinary flat spacetime. But that system is conjectured to have a dual description as a wormhole.
