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.
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.
Connecting ER and EPR
Let's go back to the early days of quantum mechanics for a moment. Albert Einstein devised a famous thought experiment in 1935 with Boris Podolsky and Nathan Rosen demonstrating the absurdity of what he dubbed “spooky action at a distance,” also known as the EPR paradox. But he also wrote a second, less well-known paper with Rosen in 1935 demonstrating mathematically that black holes might come in pairs, connected by shortcuts through space—the genesis of what we now call wormholes but originally dubbed “Einstein-Rosen bridges.” (That's what Jane Foster calls wormholes in 2011's Thor, for reasons.)
Fast forward to 2013, when Susskind and Maldacena made a radical proposition for a new duality they dubbed the “ER = EPR” conjecture as a solution to the black hole information paradox. In essence, they argued that wormholes are equivalent to entanglement. Maybe what we think are faraway points in spacetime aren’t that far away after all. Perhaps entanglement creates invisible microscopic wormholes connecting seemingly distant points. In this scenario, a wormhole exists between a black hole and its Hawking radiation, albeit a much more complicated version, with many strands ending on each of the pieces of Hawking radiation. This preserves the information. ER = EPR rests on the as-yet-untested notion that wormholes are the geometric manifestation of quantum entanglement. In other words, spooky action at a distance creates spacetime.
In 2017, Harvard University's Daniel Jafferis (a co-author of the Nature paper), along with Ping Gao and Aron Wall, managed to extend ER = EPR to traversable wormholes, demonstrating another duality: A traversable wormhole is dual to quantum teleportation, which transfers information across space via entanglement. Just two years earlier, another group of physicists had shown that a simple quantum system's dynamics could be equivalent to quantum gravity effects, suggesting that it might be possible to test this duality on quantum processors. It's known as the SYK model after the authors (Sachdev-Ye-Kitaev).
Enter Google’s Sycamore
Okay, so what does all of that have to do with the work described in the new Nature paper? Essentially, the co-authors drew upon those recent breakthroughs—creating something akin to a "baby" SYK model—as a framework for their experiment. You've got quantum entanglement and quantum teleportation on one side of their SYK-like quantum system and gravitational dynamics on the other, with the ER = EPR duality linking the two sides together. The team created an entangled state between the two sides, each with seven Majorana fermions, roughly analogous to a wormhole at t=0. It took seven qubits to encode this.
Next, they evolved the system backward through time, which moved the positions of the left and right "mouths" of what we'll call a "wormhole" for simplicity's sake. Then they took a "reference" qubit and maximally entangled it with a "probe" qubit, bringing the total circuit to nine qubits. The probe qubit was swapped with one of the qubits in the left "mouth," roughly analogous to a particle entering one mouth of a wormhole. As the wormhole began to evolve forward in time, the information carried by the probe qubit was scrambled throughout the entire quantum system.
So far, so good. Next, the team performed a series of quantum operations on the device amounting to an entangling interaction. On the gravitational side of the system, it's equivalent to injecting a shock of negative energy through spacetime. That's significant because it has long been known that wormholes are inherently unstable and would collapse if anything tried to pass through to the other side. You'd need some kind of negative energy to prop it open long enough to achieve that. There is no negative energy in classical physics, but there is in quantum mechanics, most notably in the virtual particle pairs that briefly pop into existence in the vacuum of space and annihilate almost instantaneously. (This vacuum energy is the underlying mechanism for Hawking radiation.)
Granted, there's no known way to produce or control enough negative energy to prop open a macroscale traversable wormhole in reality, which is one reason wormholes remain firmly in the realm of science fiction. But at the small scale of this experiment, the team produced what amounts to a negative energy shockwave that propped the baby "wormhole" open so the probe qubit could pass through; injecting a positive energy shockwave would close it. As the "wormhole" continued to evolve forward in time, the scrambled information from the probe qubit was gradually transferred to the right "mouth" of the system.
The researchers confirmed this informational transfer by measuring the amount of entanglement between the reference qubit and the rightmost qubit in the right "mouth." There was significantly more entanglement in the negative shockwave scenario than in the positive one, indicating that information had been transferred via a mechanism with similar physics to a traversable wormhole.
An incredibly small quantum duck
"It looks like a duck, it walks like a duck, it quacks like a duck," said Lykken. "We have something that, in terms of the properties we looked at, looks like a [traversable] wormhole. There's basically a door that opens for a while and then closes again. The wormhole has its own timescale, and you'd better go through it at the right time."
It's an incredibly small duck, however; per Jafferis, a wormhole the size of a single electron would still have 1045 times the entanglement of their toy model version. In short, the atoms behaved exactly how one would predict they would using traditional 1920s-era quantum mechanics. What's interesting is that we now have a new dual way of thinking about certain specific systems.
Spiropulu recalled that when she first showed Susskind the results. He said, "Of course you should have seen it. I told you so. Since 2015 I told you I'm right." For his part, Jafferis thinks Einstein would have quite liked the team's version of wormhole teleportation for the same reason that sci-fi screenwriters love to use wormholes. One of the great physicist's pet peeves about the concept of entanglement was that information seemed to be transmitted faster than the speed of light, violating causality. Their protocol preserves causality; because the qubit takes a shortcut via a wormhole, it doesn't travel faster than light.
Other physicists not involved with the research have reacted with caution and a healthy dose of skepticism. “If this experiment has brought a wormhole into actual physical existence, then a strong case could be made that you, too, bring a wormhole into actual physical existence every time you sketch one with pen and paper,” Scott Aaronson of the University of Texas in Austin, told The New York Times.
MIT physicist Daniel Harlow echoed that sentiment by emphasizing just how simplified (and hence unrealistic) the underlying model of quantum gravity used for the experiment was. “I’d say that this doesn’t teach us anything about quantum gravity that we didn’t already know,” he said. “On the other hand, I think it is exciting as a technical achievement because if we can’t even do this (and until now we couldn’t), then simulating more interesting quantum gravity theories would certainly be off the table.”
The authors said that this experiment is just the first baby step. In principle, if they had two quantum computers on opposite ends of the Earth—or in a lab at Caltech and a lab at Harvard—an improved version of the technology should be capable of transmitting quantum information from one end to the other. And as quantum computers continue to improve and scientists can do more detailed experiments, the hope is that they will be able to probe the interior of their pseudo-baby wormholes. "But it's not just the wormholes we're talking about here," said Lykken. "We're trying to understand the whole picture of what makes them possible."
DOI: Nature, 2022. 10.1038/s41586-022-05424-3 (About DOIs).
