Wormholes are a classic science fiction trope in popular media, if only because they provide such a nifty futuristic plot device to avoid the problem of violating the theory of relativity with faster-than-light travel. In reality, they are purely theoretical. Unlike black holes – also once thought to be purely theoretical – no evidence of a true wormhole has ever been found, though they are fascinating from an abstract theoretical physics perspective. You might be forgiven for thinking the status of undiscovered has changed if you only read the headlines this week announcing that physicists has used a quantum computer to create a wormhole, reporting on a new paper published in Nature.
Let’s set the record right away: this is not a bona fide traversable wormhole – that is, a bridge between two regions of spacetime connecting the mouth of one black hole to the other, through which a physical object can pass – in any real, physical feeling. “There is a difference between something that is possible in principle and something that is actually possible,” study co-author Joseph Lykken of Fermilab said at a media briefing this week. “So don’t hold your breath about sending your dog through a wormhole.” But it’s still a pretty smart one, useful experiment in itself that provides tantalizing proof of principle for the kind of quantum-scale physics experiments that could be possible if quantum computers continue to improve.
“It’s not the real thing; it’s not even close to the real thing; it’s hardly even a simulation of something-not-close-to-the-real-thing,” physicist Matt Strassler wrote on his blog. “Could this method one day lead to a simulation of a real wormhole? Maybe in the distant future. Could it lead to the making of a real wormhole? Never. Don’t get me wrong. What they did is pretty cool! But the hype in the press? Wild, spectacularly exaggerated.”
So what is this thing that was “created” in a quantum computer if it’s not a real wormhole? An analogue? A toy model? Co-author Maria Spiropulu of Caltech called it a novel “wormhole teleportation protocolduring 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. Therefore, the authors prefer the term “quantum experimentation” because they could use Google’s Sycamore quantum computer to create a highly entangled quantum system and perform direct measurements of specific key properties. Those properties match theoretical descriptions of the dynamics of a traversable wormhole, but only in a specially simplified theoretical space-time model.
Lyk described it to The New York Times as “the tiniest, nastiest wormhole imaginable.” Even then, 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 understand exactly what was done and why it matters, we have to take a somewhat winding journey through some pretty heady abstract ideas spanning nearly a century.
Rethinking the holographic principle
Let’s start with what is popularly known as the holographic principle. As I have written rather, almost 30 years ago, theoretical physicists introduced the mind-blowing theory, which stated that our three-dimensional universe actually a hologram. The holographic principle started as a proposed solution to the information paradox black hole 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 electrical 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 be. That information is lost.
But problems arise when quantum gravity enters the picture, because under the rules of quantum mechanics, information can never be destroyed. And in quantum mechanics, black holes are incredibly complex objects and so should contain a lot of information. Jacob Bekenstein realized in 1974 that black holes also have entropy. Stephen Hawking tried to prove him wrong, but in the end he proved him right, concluding that black holes must therefore produce some kind of thermal radiation.
Black holes must therefore also have entropy, and Hawking was the first to calculate that entropy. He also introduced the concept of “Hawking radiation”: the black hole will emit a small amount of energy, reducing its mass by a corresponding amount. Over time, the black hole will evaporate. The smaller the black hole, the faster it disappears. But what happens to the information it contains? Is it really destroyed, violating quantum mechanics, or is it somehow preserved in the Hawking radiation?
According to the holographic principle, information about a black hole’s interior could be encoded on its two-dimensional surface (the “boundary”) rather than within its three-dimensional volume (the “bulk”). Leonard Susskind and Gerard ‘t Hooft extended this idea to the whole universe and compared it to a hologram: our three-dimensional universe in all its glory arises from a two-dimensional ‘source code’.
Juan Maldacena then discovered a pivotal duality, technically known as the AdS/CFT Correspondence— representing 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 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 views 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 that conformal field theory (CFT) is called. ), which has no gravity but does quantum entanglement.
It is this notion of duality that explains the wormhole confusion. As noted above, the authors of the Nature paper didn’t create a physical wormhole – they manipulated some entangled quantum particles in ordinary flat spacetime. But that system is believed to have a double description as a wormhole.