Wormholes are a classic science fiction trope in popular media, if only because they provide such a handy futuristic plot device for avoiding the problem of relativity violation with longer travel. fast as light. In reality, they are purely theoretical. Unlike black holes – also considered purely theoretical – no evidence of a real wormhole has ever been found, although they are fascinating from an abstract theoretical physical point of view. You could be forgiven for thinking the undiscovered status had changed if you only read the headlines this week announcing that physicists used a quantum computer to create a wormhole, reporting a new paper in Nature.

Let’s set the record straight right away: it is not a real traversable wormhole, that is to say a bridge between two regions of space-time connecting the mouth from one black hole to another, through which a physical object can pass, in any real physical environment. meaning. “There is a difference between something that is possible in principle and possible in reality,” said co-author Joseph Lykken of Fermilab during a press briefing this week. “So don’t hold your breath about sending your dog through a wormhole.” But it’s still a pretty clever and nifty experiment on its own that provides a tantalizing proof-of-principle for 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 a simulation of something that isn’t close to the real thing,” physicist Matt Strassler wrote on his blog. “Could this method lead to a simulation of a real wormhole one day? Perhaps in the distant future. Could it lead to the creation of a real wormhole? Never. Don’t get me wrong. What they did is pretty cool, but the hype in the press is extravagant, spectacularly exaggerated.

So what is this thing that was “created” in a quantum computer if it isn’t an actual wormhole? An analog? A toy model? Caltech co-author Maria Spiropulu called it a new “wormhole teleportation protocol” during the briefing. You could call it a simulation, but as Strassler wrote, that’s not quite true either. Physicists have simulated wormholes on classical computers, but no physical system is created in these simulations. This is 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. These properties are consistent with theoretical descriptions of the dynamics of a traversable wormhole, but only in a special simplified theoretical model of spacetime.

Lykken described it to The New York Times as “the tiniest, seediest wormhole you could 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 builds 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 have to take a somewhat meandering journey through some pretty heady abstract ideas spanning almost a century.

## Revisiting the holographic principle

Let’s start with what is commonly called the holographic principle. As I’ve written before, nearly 30 years ago theoretical physicists introduced the mind-bending theory 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 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 have been. This information is lost.

But problems arise when quantum gravity comes into play because the rules of quantum mechanics state that information can never be destroyed. And in quantum mechanics, black holes are incredibly complex objects and should therefore contain a large amount of information. Jacob Bekenstein realized in 1974 that black holes also have entropy. Stephen Hawking tried to prove him wrong but ended up proving 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 this 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 faster it disappears. But what then happens to the information it contains? Is it really destroyed, thus violating quantum mechanics, or is it somehow preserved in Hawking radiation?

According to the holographic principle, information about the interior of a black hole could be encoded on its two-dimensional surface (the “boundary”) rather than in its three-dimensional volume (the “mass”). 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 then discovered a crucial duality, technically known as the AdS/CFT correspondence, which amounts to a mathematical dictionary allowing physicists to go back and forth between the languages of two theoretical worlds (general relativity and quantum mechanics). . Dualities in physics refer to patterns that look different but can be shown to describe equivalent physics. It’s a bit like ice, water and steam which are three different phases of the same chemical substance, except that a duality looks at the same phenomenon in two different ways which are inversely related. In the case of AdS/CFT, the duality is between a space-time pattern 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 is this notion of duality that explains the confusion of wormholes. As noted above, the authors of the Nature paper did not create a physical wormhole – they manipulated entangled quantum particles in ordinary flat spacetime. But this system is supposed to have a double description as a wormhole.

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