# No, physicists haven’t made a real wormhole. What they did was still great

Wormholes are a classic sci-fi trope in popular media, if only because they provide such a practical futuristic tell-tale device to sidestep the issue of violating relativity with faster-than-light travel. In fact, it is purely theoretical. Unlike black holes – which were also thought to be purely theoretical – no evidence of a real wormhole has been found, although it is fascinating from the point of view of abstract theoretical physics. You might be forgiven for thinking the undiscovered state has changed if you only read this week’s headlines announcing that physicists used a quantum computer to make a wormhole, and wrote about a new paper published in the journal Nature.

Let’s set the record straight: This is not a bona fide wormhole that can be traversed – say, a bridge between two regions of space-time connecting the mouth of one black hole to another, through which a physical object can pass – in any real physical sense. “There is a difference between something being possible in principle and being possible in practice,” Fermilab co-author Joseph Leiken said during a briefing this week. “So don’t hold your breath about sending your dog through a wormhole.” But it’s still a clever, clever experiment in its own right that provides an exciting proof-of-principle for the kinds of physics experiments on the quantum scale that might be possible as quantum computers continue to improve.

“It’s not the real thing; it’s nowhere near the real thing; it’s hardly a simulation of something that isn’t close to reality,” physicist Matt Strassler wrote on his blog. Could this method lead to a real wormhole simulation one day? Maybe in the distant future. Could it lead to a real wormhole? Never.

So what is this thing “created” in a quantum computer if it’s not an actual wormhole? analog? game model? Co-author Maria Spiropolo of Caltech referred to it as the “wormhole teleportation protocol” novel during the briefing. You could call it simulation, but as Strassler writes, that’s not entirely true either. Physicists have simulated wormholes on classical computers, but no physical system has been created in those simulations. This is why the authors prefer the term “quantum experiment” because they were able to use Google Sycamore’s 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 particularly simplified theoretical model of space-time.

Leikin described it to The New York Times as “the smallest and most inhumane wormhole you can imagine.” Even then, perhaps a “cluster of atoms with wormhole-like properties” would be more accurate. What makes this breakthrough intriguing and potentially important is how the experiment benefits from some of the most influential and exciting recent work in theoretical physics. But to understand exactly what was done and why it matters, we need to go on a somewhat meandering journey through some powerful abstractions spanning nearly a century.

## Revisiting the hologram principle

Let’s start with what is popularly known as the holographic principle. As I wrote previously, nearly 30 years ago, theoretical physicists introduced the mind-bending theory positing that our three-dimensional universe is actually a hologram. The principle of holography began as a proposed solution to the black hole information paradox in the 1990s. Black holes, as described by general relativity, are simple things. All you need to describe it mathematically is its mass and spin, as well as its 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 was. This information is missing.

But problems arise when quantum gravity enters the picture because the rules of quantum mechanics state that information can never be destroyed. And in quantum mechanics, black holes are incredibly complex objects, and therefore must 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 ended up being proven right instead, concluding that black holes must be producing some kind of heat radiation.

So black holes must also have entropy, and Hawking was the first to calculate that entropy. He also introduced the idea of ”Hawking radiation”: a black hole would emit a tiny 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 will disappear. But what happens then to the information contained in it? Is it really destroyed, thus violating quantum mechanics, or is it somehow preserved in Hawking radiation?

According to the principle of holography, information about a black hole’s interior can be encoded on its two-dimensional surface area (“boundary”) rather than on its three-dimensional volume (“volume”). Leonard Susskind and Gerard Hooft extended this idea to the entire universe, likening it to a hologram: our three-dimensional universe emerges in all its glory 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 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 steam are three different phases of the same chemical substance, except for dualism that looks at the same phenomenon in two different, inversely related ways. In the case of AdS/CFT, the duality is between a model of space-time known as anti-de Sitter space (AdS) — which has a constant negative curvature, unlike our de Sitter universe — and a quantum system called conformational field theory (CFT), which lacks gravity but Quantum entanglement.

It is the idea of duality that explains the confusion of a wormhole. As noted above, the authors of the Nature paper didn’t make a physical wormhole — they dealt with some entangled quantum particles in ordinary flat space-time. But this system is supposed to have a double description of a wormhole.

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