Science

No, physicists have not created an actual wormhole. What they did was still pretty cool

No, physicists have not created an actual wormhole. What they did was still pretty cool

Zoom in / Artist’s illustration of a quantum experiment studying the physics of traversing holes.

Wormholes are a classic form of science fiction in popular media, if only because they provide such a convenient futuristic plot device to avoid the problem of breaking relativity with faster-than-light travel. In reality, they are purely theoretical. Unlike black holes – also once considered purely theoretical – no evidence of an actual wormhole has ever been found, although they are fascinating from the point of view of abstract theoretical physics. You could be forgiven for thinking that the undiscovered status had changed if you read the headlines this week announcing that physicists had used quantum computer to create a wormhole, accounting for a new paper published in Nature.

Let’s set the record straight: This is not a bona fide traversable wormhole – ie. 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 being possible in reality,” co-author Joseph Liken of Fermilab said during a media briefing this week. “So don’t hold your breath to send your dog through a wormhole.” But it’s still pretty clever, cool experiment itself, which 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 hardly even a simulation of anything-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 the creation of a real wormhole? Never. Do not get me wrong. What they did is pretty cool! But press advertising? Wildly, spectacularly exaggerated.”

So what is this thing that was “created” in a quantum computer if it is not a real hole? Analog? Toy model? Co-author Maria Spiropoulou of Caltech called it 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 these 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. These properties are consistent with theoretical descriptions of the dynamics of a traversable wormhole—but only in a special simplified theoretical model of spacetime.

Happiness I described it of The New York Times as “the smallest, clumsiest hole 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 builds on some of the most influential and exciting recent developments in theoretical physics. But to understand exactly what was done and why it matters, we need to go on a little meandering journey through some pretty heady abstract ideas spanning nearly a century.

Diagram of the so-called AdS/CFT correspondence (aka the holographic principle) in theoretical physics.
Zoom in / Diagram of the so-called AdS/CFT correspondence (aka the holographic principle) in theoretical physics.

APS/Alan Stonebraker

Revisiting the holographic principle

Let’s start with what is popularly known as the holographic principle. Like I wrote earlier, nearly 30 years ago, theoretical physicists presented the mind-boggling theory that our three-dimensional universe is actually a hologram. The holographic principle started as a proposed solution to black hole information paradox in the 90s. Black holes, as described by general relativity, are simple objects. All you need to describe them mathematically is their mass and spin, plus their electric charge. So there would be no noticeable change if you threw something into a black hole – nothing to give any idea what that object might have been. This information is lost.

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 lot of information. Jacob Bekenstein realized in 1974 that black holes also have entropy. Stephen Hawking tried to prove him wrong, but instead proved him right by concluding that black holes must therefore produce some kind of heat radiation.

So black holes must also have entropy, and Hawking was the first to calculate this entropy. He also introduced the concept of “Hawking radiation”: the black hole will emit a small fraction of the 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 then happens to the information it contains? Is it really destroyed, thereby violating quantum mechanics, or is it somehow preserved in Hawking radiation?

According to the holographic principle, information about the interior of a black hole can be encoded on its two-dimensional surface (“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 arises from a two-dimensional “source code.”

Juan Maldacena then discovered a crucial duality, technically known as AdS/CFT Correspondence— which amounts to a mathematical dictionary that allows physicists to move 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 steam are three different phases of the same chemical substance, except that duality looks at the same phenomenon in two different ways that are inversely proportional. In the case of AdS/CFT, the duality is between a model of spacetime known as anti-de Sitter space (AdS) – which has a 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 wormhole confusion. As noted above, the authors of the Nature paper did not create a physical wormhole—they manipulated some entangled quantum particles in ordinary flat spacetime. But this system is supposed to have a dual description as a wormhole.

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