wormhole

Wormhole Experiment Called Into Question

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Last fall, a team of physicists announced that they had teleported a qubit through a holographic wormhole in a quantum computer. Now another group suggests that’s not quite what happened.

An illustration of a butterfly falling into a wormhole.
A holographic wormhole would scramble information in one place and reassemble it in another. The process is not unlike watching a butterfly being torn apart by a hurricane in Houston, only to see an identical butterfly pop out of a typhoon in Tokyo.Myriam Wares for Quanta Magazine

Introduction

In January 2022, a small team of physicists watched breathlessly as data streamed out of Google’s quantum computer, Sycamore. A sharp peak indicated that their experiment had succeeded. They had mixed one unit of quantum information into what amounted to a wispy cloud of particles and watched it emerge from a linked cloud. It was like seeing an egg scramble itself in one bowl and unscramble itself in another.

In several key ways, the event closely resembled a familiar movie scenario: a spacecraft enters one black hole — apparently going to its doom — only to pop out of another black hole somewhere else entirely. Wormholes, as these theoretical pathways are called, are a quintessentially gravitational phenomenon. There were theoretical reasons to believe that the qubit had traveled through a quantum system behaving exactly like a wormhole — a so-called holographic wormhole — and that’s what the researchers concluded. When it was published in November, the experiment graced the cover of Nature and was widely covered in the media, including in this magazine.

Now another group of physicists has analyzed the result and determined that, while the experiment may have produced something vaguely wormhole-like, it wasn’t really a holographic wormhole in any meaningful sense. In light of the new analysis, independent researchers are coming to doubt that the teleportation experiment has anything to do with gravity after all.

“I feel that the evidence for a gravitational interpretation is weakening,” said John Preskill, a theoretical physicist at the California Institute of Technology who was not involved with either study.

The group did teleport something on the Sycamore chip, however, and they did it in a way that — at least on the surface — looked more wormhole-like than anything produced by earlier experiments. The dispute over how to interpret the experiment springs from rapid developments involving holography, which functions as a sort of mathematical pair of 3D glasses that lets physicists view a quantum system as a gravitational one. Studying wormholes through the gravitational lens has uncovered new ways to teleport quantum information, raising hopes that such quantum experiments might someday go in the other direction and probe quantum gravity in the lab. But the wormhole brouhaha highlights the fact that determining when the holographic lens works — and therefore whether certain aspects of quantum gravity might be accessible on quantum computers — may require greater subtlety than physicists imagined.

When he read the new response, Vincent Su, a physicist at the University of California, Berkeley who studies wormhole-like teleportation and is not involved with either group, wondered, “Is quantum gravity in the lab dead?”

Scrambling Wormholes

Wormholes have long been a fixture of science fiction writers in need of a mechanism for quickly moving their characters across the vastness of space, but the wormholes that appeared in Einstein’s theory of gravity initially seemed extremely improbable, requiring tricky manipulations of space-time that inevitably led to time-travel paradoxes. That changed in 2016, when three physicists — Ping Gao and Daniel Jafferis at Harvard University and Aron Wall, then at the Institute for Advanced Study — found an unexpectedly simple and paradox-free way to prop open a wormhole with a shock wave of negative energy.

Introduction

“It’s quite beautiful. It started the whole thinking in this direction,” said Hrant Gharibyan, a quantum physicist at Caltech. “There’s a narrow window that you can throw stuff from the left universe to the right.”

The foundation of the work was one of the hotter trends in modern physics, holography.

Holography involves the study of profound relationships known as dualities. On their face, dual systems look completely different. They have different parts and play by different rules. But if two systems are dual, every aspect of one system can be precisely related to an element of the other system. Electric fields are dual to magnetic fields, for instance. A major finding in modern physics is that dualities also seem to link certain gravitational systems to quantum systems.

We might consider a collection of interacting particles, for instance, entirely within the framework of quantum theory. Or, as if by popping on a pair of 3D glasses, we might see the collection of particles as a black hole governed by the rules of gravity. Physicists have spent decades developing mathematical “dictionaries” that let them translate quantum elements into gravitational elements and vice versa, effectively putting on and taking off the glasses. They watch how particles, black holes and wormholes transform as one switches between the two perspectives. Calculations that are hard to do from one perspective are often easier from the other. A major hope of the field is to develop the ability to access the still mysterious rules of quantum gravity by studying better-understood quantum theories.

But questions abound as to how far the glasses trick will hold. Does every conceivable quantum theory pop into a gravity theory when viewed holographically? Can physicists understand gravity in our universe by finding its better-behaved quantum twin? No one knows. But many theorists have dedicated their careers to exploring a few well-understood holographic pairs of theories and are constantly searching for new examples.

Gao, Jafferis and Wall had already suggested in 2016 that passing through a wormhole (a gravitational enterprise) might have a quantum interpretation without the 3D glasses: the teleportation of quantum information. A couple of years later, another team made their speculation concrete.

A smiling man in front of a chalk board.
Daniel Jafferis, a theoretical physicist at Harvard University, helped develop the wormhole teleportation protocol. He was also one of the leaders of last year’s wormhole team.Paul Horowitz

In 2019, Gharibyan and his collaborators translated traversable wormholes into quantum language, publishing a step-by-step recipe for a peculiar quantum experiment that showcases the essence of holography. With the 3D glasses on, you see a wormhole. An object enters one black hole, traverses a sort of space-time bridge, and exits the other black hole. Take the glasses off, however, and you see the dual quantum system. Two black holes become two gigantic clouds of particles. The space-time bridge becomes a quantum mechanical link known as entanglement. And the act of traveling through the wormhole becomes an event that appears quite surprising from the quantum perspective: A particle carrying a qubit, a unit of quantum information, enters one cloud and becomes scrambled beyond all recognition. The qubit unscrambles and exits the entangled cloud as another particle — a development as unexpected as watching a butterfly being torn apart by a hurricane in Houston, only to see an identical butterfly pop out of a typhoon in Tokyo.

“Naïvely you’d never guess,” Gharibyan said, “that you could scramble and unscramble very chaotically, and the information comes out.”

But viewed through a holographic lens, the proceedings make perfect sense. The entangled clouds of particles are not a literal wormhole in our universe. But they are dual to a wormhole, meaning that they have a matching behavior for anything a traversable wormhole can do — including transporting a qubit.

This is what the team announced in the November Nature paper. They simulated the behavior of two clouds of entangled particles in a quantum computer and performed a teleportation that captured the essential aspects of traversing a wormhole from the holographic perspective.

But that wasn’t the only way to interpret their experiment.

Not All That Teleports Is Gravity

Over the past few years, researchers made another surprising discovery. Although they had spotted the scrambling teleportation recipe while using the gravitational lens, gravity wasn’t always essential.

Gravity scrambles information in a very particular way. In fact, theorists have argued that black holes must be the most efficient scramblers in nature. But when Gharibyan and his colleagues used clouds of particles that scrambled by different quantum rules than gravity, they realized that the clouds could still teleport by scrambling, albeit less efficiently. And when they looked at the alternative clouds through a holographic lens, they saw nothing — no wormholes.

Gharibyan’s group and another team led by Norman Yao at Berkeley put everything together in a pair of simultaneous papers in 2021. (Yao has since moved to Harvard.)

A black and white photo of a man smiling.
Norman Yao, a physicist at Harvard University, led the team that poked holes in last year’s wormhole paper.Noah Berger for UC Berkeley

Introduction

These papers laid out some of the characteristics that seemed to distinguish gravitational teleportation from teleportation by more vanilla sorts of scrambling. In particular, they identified a feature of all quantum systems known as size winding, which can be linked holographically to the speed of a particle falling through the wormhole. When gravity was responsible for the scrambling, size winding had a particular mathematical property and was said to be “perfect” in the systems they studied. That gave the Nature team a specific signal to hunt for.

“What was predicted in these earlier papers was that size winding is a holographic signature, almost like a smoking gun,” Su said.

More Particles, More Problems

Last spring, while the Nature paper was going through the peer-review process, Su and his collaborators carried out a teleportation-by-scrambling experiment on two quantum computers, one operated by IBM and another by Quantinuum. They called their teleportation demo “wormhole-inspired,” since they knew their quantum model used one of the nongravitational scrambling recipes. At the time, they suspected that an experimental demonstration of true gravitational teleportation would take a decade or longer.

To understand why gravitational teleportation is so tough to pull off, it helps to keep in mind that these quantum computers don’t literally contain clouds of particles that scramble and unscramble information of their own accord. Instead, they contain qubits, which are objects that act like particles (qubits can be made from either literal atoms or artificial ones). When scientists program the computer, they tell it to make quantum changes to the qubits according to an energy equation called a Hamiltonian. The Hamiltonian describes how the qubits change from one moment to the next. Effectively, this equation lets them customize the laws of quantum physics for the qubits. As the computer runs, it carries out a sort of simulation of how real clouds of particles governed by those laws would act.

Here’s the rub: For a definitive showcase of gravitational teleportation, you need big clouds of particles. How big? The bigger the better. The theorists had done all the math in the context of essentially infinitely large clouds. For an experiment, researchers generally agree that 100 particles per cloud would suffice for indisputable wormhole-behavior to emerge.

A gloved hand holding a square wafer.
Last year’s experiment was run on seven qubits of Google’s Sycamore quantum computing chip.Peter Kneffel/dpa/Alamy Live News

Introduction

Yet as the number of particles goes up, the size of the Hamiltonian explodes. If you’re modeling the particles using one of the more tractable models of gravity, called the SYK model, your Hamiltonian must reflect the fact that every member of a group of particles can directly influence every other member. The Hamiltonian for 100 densely linked particles is an equation with a staggering 3,921,225 terms. This is far beyond what today’s quantum computers can simulate with a few dozen qubits. Even if one were willing to settle for a fuzzy wormhole dual to clouds of just 20 particles, the Hamiltonian would go on for an overwhelming 4,845 terms. This hurdle was a key reason why Su’s group thought that a true wormhole simulation was a decade away.

Then last November, a team of researchers led by Jafferis, Joseph Lykken of the Fermi National Accelerator Laboratory and Maria Spiropulu of Caltech surprised the community by announcing that they had run a quantum experiment displaying perfect size winding — the key signature thought to establish the existence of a gravitational dual, and thus a wormhole — using just seven particles. Even more surprising, they were able to stuff the behavior of this seven-particle system into a Hamiltonian with only five terms.

A Holographic Wormhole on a Chip

The core of the group’s work was a novel way of pruning many of those particle-to-particle connections described by the unwieldy SYK Hamiltonian. Numerous physicists have “sparsified” the SYK model for a given cloud size by dropping random terms, finding that simpler versions can keep the holographic properties of the original Hamiltonian.

Instead of deleting connections at random, Jafferis and his collaborators thought to use machine learning to intelligently prune only the connections that don’t affect the cloud’s ability to teleport, a simplification strategy praised by other researchers.

“I thought it was actually very clever,” Gharibyan said. “The sparsification I thought was a very great insight.”

“It was a good idea,” Preskill said.

The researchers took aim at the 10-particle SYK model, which has a Hamiltonian of 210 terms. They simulated teleportation between clouds of 10 particles on a standard computer and designed a machine learning algorithm to simplify the Hamiltonian as much as possible without breaking its capacity to teleport. The algorithm returned an extremely sparse Hamiltonian measuring just five terms that captured teleportation between two seven-particle clouds. (The machine learning algorithm apparently decided that three of the particles weren’t meaningfully contributing to the process.) The equation was simple enough to run on Google’s Sycamore quantum processor, a notable achievement.

A cryostat with lots of metal tubes.
Google’s Sycamore quantum processor must be kept just above absolute zero in a cryostat such as this one.Google

“It’s cool that they were able to run something on quantum hardware,” Su said.

The Sycamore experiment confirmed that the Hamiltonian could carry out the teleportation, just as it had been trained to. But what really excited researchers was the fact that this gang of qubits  also displayed perfect size winding — the supposed signature of a gravitational dual. Somehow a toy model of a toy model of a toy model of gravity had managed to maintain the holographic essence of its grandparent model. The researchers appeared to have done the equivalent of boiling down a tornado to a handful of molecules, which, despite being largely unable to interact with each other, still manage to keep the characteristic funnel shape.

“They had actually a pretty nice way to measure the size winding as well,” Gharibyan said. “It was pretty exciting.”

Many in the field were struck by just how simple the toy model was. One group in particular —Yao and his Berkeley colleagues Bryce Kobrin and Thomas Schuster — started to dig into how such a simple model could possibly capture the unspeakable chaos of gravity.

Too Small to Scramble

On February 15, the trio posted the results of their investigation, which involved analyzing the mathematical properties and behavior of the Nature team’s simple Hamiltonian. It has not been peer-reviewed. Their main finding is that the simple model departs from its parent model of gravity in crucial ways. These differences, the group argues, imply that the signals the researchers considered hallmarks of gravity no longer apply, and because of this, the best description of what the Nature team saw is not gravitational teleportation.

The least gravitational thing about the simplified Hamiltonian is that, unlike in the original SYK model, the five terms are “fully commuting,” which means that they don’t have a certain kind of interdependence. Commutativity makes it much easier to simulate the clouds of particles, but it implies that the clouds can’t scramble chaotically. Since chaotic scrambling is considered a defining property of black holes and is an essential ingredient in gravitational teleportation, experts doubt that such a simple Hamiltonian could possibly capture complicated wormhole-like behavior. Put loosely, the system more closely resembles the gentle spiral of draining bathwater than it does the churning turbulence of Class V river rapids.

A blonde woman with glasses in front of a laptop.
Maria Spiropulu, a physicist at the California Institute of Technology, was one of the leaders of last year’s wormhole experiment.Bongani Mlambo for Quanta Magazine

Introduction

The researchers also proposed a nongravitational explanation for the supposed signature of holography, perfect size winding. The five-term Hamiltonian does have it, but so do other random five-term, commuting Hamiltonians that they tested. Moreover, when they tried to bump up the number of particles while keeping the commuting property, the size winding signal should have strengthened. Instead, it disappeared. The physicists reached a conclusion that researchers had not previously grasped because no one had studied such simple models holographically: Many fully commuting, small Hamiltonians seem to have perfect size winding, even though these models don’t have gravitational duals. This finding implies that, in small systems, perfect size winding isn’t a sign of gravity. It’s just a side effect of the system being small.

Both groups declined to comment while they work out their differences through peer-reviewed publications. The Yao group has submitted their analysis to Nature, and the Jafferis, Lykken and Spiropulu group will likely have a chance to respond. But five independent experts familiar with holography consulted for this article agreed that the new analysis seriously challenges the experiment’s gravitational interpretation.

Holographic Dreams

The holographic future may not be here yet. But physicists in the field still believe it’s coming, and they say that they’re learning important lessons from the Sycamore experiment and the ensuing discussion.

First, they expect that showing successful gravitational teleportation won’t be as cut and dry as checking the box of perfect size winding. At the very least, future experiments will also need to prove that their models preserve the chaotic scrambling of gravity and pass other tests, as physicists will want to make sure they’re working with a real Category 5 qubit hurricane and not just a leaf blower. And getting closer to the ideal benchmark of triple-digit numbers of particles on each side will make a more convincing case that the experiment is working with billowing clouds and not questionably thin vapors.

No one expects today’s rudimentary quantum computers to be up to the challenge of the punishingly long Hamiltonians required to simulate the real deal. But now is the time to start chiseling away at them bit by bit, Gharibyan believes, in preparation for the arrival of more capable machines. He expects that some might try machine learning again, this time perhaps rewarding the algorithm when it returns chaotically scrambling, non-commuting Hamiltonians and penalizing it when it doesn’t. Of the resulting models, any that still have perfect size winding and pass other checks will become the benchmark models to drive the development of new quantum hardware.

If quantum computers grow while holographic Hamiltonians shrink, perhaps they will someday meet in the middle. Then physicists will be able to run experiments in the lab that reveal the incalculable behavior of their favorite models of quantum gravity.

“I’m optimistic about where this is going,” Gharibyan said.

Newfound Wormhole Allows Information to Escape Black Holes

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Physicists theorize that a new “traversable” kind of wormhole could resolve a baffling paradox and rescue information that falls into black holes.

Tomáš Müller for Quanta Magazine

In 1985, when Carl Sagan was writing the novel Contact, he needed to quickly transport his protagonist Dr. Ellie Arroway from Earth to the star Vega. He had her enter a black hole and exit light-years away, but he didn’t know if this made any sense. The Cornell University astrophysicist and television star consulted his friend Kip Thorne, a black hole expert at the California Institute of Technology (who won a Nobel Prize earlier this month). Thorne knew that Arroway couldn’t get to Vega via a black hole, which is thought to trap and destroy anything that falls in. But it occurred to him that she might make use of another kind of hole consistent with Albert Einstein’s general theory of relativity: a tunnel or “wormhole” connecting distant locations in space-time.

While the simplest theoretical wormholes immediately collapse and disappear before anything can get through, Thorne wondered whether it might be possible for an “infinitely advanced” sci-fi civilization to stabilize a wormhole long enough for something or someone to traverse it. He figured out that such a civilization could in fact line the throat of a wormhole with “exotic material” that counteracts its tendency to collapse. The material would possess negative energy, which would deflect radiation and repulse space-time apart from itself. Sagan used the trick in Contact, attributing the invention of the exotic material to an earlier, lost civilization to avoid getting into particulars. Meanwhile, those particulars enthralled Thorne, his students and many other physicists, who spent years exploring traversable wormholes and their theoretical implications. They discovered that these wormholes can serve as time machines, invoking time-travel paradoxes — evidence that exotic material is forbidden in nature.

Now, decades later, a new species of traversable wormhole has emerged, free of exotic material and full of potential for helping physicists resolve a baffling paradox about black holes. This paradox is the very problem that plagued the early draft of Contact and led Thorne to contemplate traversable wormholes in the first place; namely, that things that fall into black holes seem to vanish without a trace. This total erasure of information breaks the rules of quantum mechanics, and it so puzzles experts that in recent years, some have argued that black hole interiors don’t really exist — that space and time strangely end at their horizons.

The flurry of findings started last year with a paper that reported the first traversable wormhole that doesn’t require the insertion of exotic material to stay open. Instead, according to Ping Gao and Daniel Jafferis of Harvard University and Aron Wall of Stanford University, the repulsive negative energy in the wormhole’s throat can be generated from the outside by a special quantum connection between the pair of black holes that form the wormhole’s two mouths. When the black holes are connected in the right way, something tossed into one will shimmy along the wormhole and, following certain events in the outside universe, exit the second. Remarkably, Gao, Jafferis and Wall noticed that their scenario is mathematically equivalent to a process called quantum teleportation, which is key to quantum cryptography and can be demonstrated in laboratory experiments.

John Preskill, a black hole and quantum gravity expert at Caltech, says the new traversable wormhole comes as a surprise, with implications for the black hole information paradox and black hole interiors. “What I really like,” he said, “is that an observer can enter the black hole and then escape to tell about what she saw.” This suggests that black hole interiors really exist, he explained, and that what goes in must come out.

Lucy Reading-Ikkanda/Quanta Magazine

A Cryptic Equation

The new wormhole work began in 2013, when Jafferis attended an intriguing talk at the Strings conference in South Korea. The speaker, Juan Maldacena, a professor of physics at the Institute for Advanced Study in Princeton, New Jersey, had recently concluded, based on various hints and arguments, that “ER = EPR.” That is, wormholes between distant points in space-time, the simplest of which are called Einstein-Rosen or “ER” bridges, are equivalent (albeit in some ill-defined way) to entangled quantum particles, also known as Einstein-Podolsky-Rosen or “EPR” pairs. The ER = EPR conjecture, posed by Maldacena and Leonard Susskind of Stanford, was an attempt to solve the modern incarnation of the infamous black hole information paradox by tying space-time geometry, governed by general relativity, to the instantaneous quantum connections between far-apart particles that Einstein called “spooky action at a distance.”

The paradox has loomed since 1974, when the British physicist Stephen Hawking determined that black holes evaporate — slowly giving off heat in the form of particles now known as “Hawking radiation.” Hawking calculated that this heat is completely random; it contains no information about the black hole’s contents. As the black hole blinks out of existence, so does the universe’s record of everything that went inside. This violates a principle called “unitarity,” the backbone of quantum theory, which holds that as particles interact, information about them is never lost, only scrambled, so that if you reversed the arrow of time in the universe’s quantum evolution, you’d see things unscramble into an exact re-creation of the past.

Almost everyone believes in unitarity, which means information must escape black holes — but how? In the last five years, some theorists, most notably Joseph Polchinski of the University of California, Santa Barbara, have argued that black holes are empty shells with no interiors at all — that Ellie Arroway, upon hitting a black hole’s event horizon, would fizzle on a “firewall” and radiate out again.

Many theorists believe in black hole interiors (and gentler transitions across their horizons), but in order to understand them, they must discover the fate of information that falls inside. This is critical to building a working quantum theory of gravity, the long-sought union of the quantum and space-time descriptions of nature that comes into sharpest relief in black hole interiors, where extreme gravity acts on a quantum scale.

The quantum gravity connection is what drew Maldacena, and later Jafferis, to the ER = EPR idea, and to wormholes. The implied relationship between tunnels in space-time and quantum entanglement posed by ER = EPR resonated with a popular recent belief that space is essentially stitched into existence by quantum entanglement. It seemed that wormholes had a role to play in stitching together space-time and in letting black hole information worm its way out of black holes — but how might this work? When Jafferis heard Maldacena talk about his cryptic equation and the evidence for it, he was aware that a standard ER wormhole is unstable and non-traversable. But he wondered what Maldacena’s duality would mean for a traversable wormhole like the ones Thorne and others played around with decades ago. Three years after the South Korea talk, Jafferis and his collaborators Gao and Wall presented their answer. The work extends the ER = EPR idea by equating, not a standard wormhole and a pair of entangled particles, but a traversable wormhole and quantum teleportation: a protocol discovered in 1993that allows a quantum system to disappear and reappear unscathed somewhere else.

When Maldacena read Gao, Jafferis and Wall’s paper, “I viewed it as a really nice idea, one of these ideas that after someone tells you, it’s obvious,” he said. Maldacena and two collaborators, Douglas Stanfordand Zhenbin Yang, immediately began exploring the new wormhole’s ramifications for the black hole information paradox; their paperappeared in April. Susskind and Ying Zhao of Stanford followed this with a paper about wormhole teleportation in July. The wormhole “gives an interesting geometric picture for how teleportation happens,” Maldacena said. “The message actually goes through the wormhole.”

David Kaplan explores one of the biggest mysteries in physics: the apparent contradiction between general relativity and quantum mechanics.

Video: David Kaplan explores one of the biggest mysteries in physics: the apparent contradiction between general relativity and quantum mechanics.

Filming by Petr Stepanek. Editing and motion graphics by MK12. Music by Steven Gutheinz.

Diving Into Wormholes

In their paper, “Diving Into Traversable Wormholes,” published in Fortschritte der Physik, Maldacena, Stanford and Yang consider a wormhole of the new kind that connects two black holes: a parent black hole and a daughter one formed from half of the Hawking radiation given off by the parent as it evaporates. The two systems are as entangled as they can be. Here, the fate of the older black hole’s information is clear: It worms its way out of the daughter black hole.

During an interview this month in his tranquil office at the IAS, Maldacena, a reserved Argentinian-American with a track record of influential insights, described his radical musings. On the right side of a chalk-dusty blackboard, Maldacena drew a faint picture of two black holes connected by the new traversable wormhole. On the left, he sketched a quantum teleportation experiment, performed by the famous fictional experimenters Alice and Bob, who are in possession of entangled quantum particles a and b, respectively. Say Alice wants to teleport a qubit q to Bob. She prepares a combined state of q and a,measures that combined state (reducing it to a pair of classical bits, 1 or 0), and sends the result of this measurement to Bob. He can then use this as a key for operating on b in a way that re-creates the state q. Voila, a unit of quantum information has teleported from one place to the other.

Maldacena turned to the right side of the blackboard. “You can do operations with a pair of black holes that are morally equivalent to what I discussed [about quantum teleportation]. And in that picture, this message really goes through the wormhole.”

Juan Maldacena, a professor of physics at the Institute for Advanced Study.

Juan Maldacena, a professor of physics at the Institute for Advanced Study.

Sasha Maslov for Quanta Magazine

Say Alice throws qubit q into black hole A. She then measures a particle of its Hawking radiation, a, and transmits the result of the measurement through the external universe to Bob, who can use this knowledge to operate on b, a Hawking particle coming out of black hole B. Bob’s operation reconstructs q, which appears to pop out of B, a perfect match for the particle that fell into A. This is why some physicists are excited: Gao, Jafferis and Wall’s wormhole allows information to be recovered from black holes. In their paper, they set up their wormhole in a negatively curved space-time geometry that often serves as a useful, if unrealistic, playground for quantum gravity theorists. However, their wormhole idea seems to extend to the real world as long as two black holes are coupled in the right way: “They have to be causally connected and then the nature of the interaction that we took is the simplest thing you can imagine,” Jafferis explained. If you allow the Hawking radiation from one of the black holes to fall into the other, the two black holes become entangled, and the quantum information that falls into one can exit the other.

The quantum-teleportation format precludes using these traversable wormholes as time machines. Anything that goes through the wormhole has to wait for Alice’s message to travel to Bob in the outside universe before it can exit Bob’s black hole, so the wormhole doesn’t offer any superluminal boost that could be exploited for time travel. It seems traversable wormholes might be permitted in nature as long as they offer no speed advantage. “Traversable wormholes are like getting a bank loan,” Gao, Jafferis and Wall wrote in their paper: “You can only get one if you are rich enough not to need it.”

A Naive Octopus

While traversable wormholes won’t revolutionize space travel, according to Preskill the new wormhole discovery provides “a promising resolution” to the black hole firewall question by suggesting that there is no firewall at black hole horizons. Preskill said the discovery rescues “what we call ‘black hole complementarity,’ which means that the interior and exterior of the black hole are not really two different systems but rather two very different, complementary ways of looking at the same system.” If complementarity holds, as is widely assumed, then in passing across a black hole horizon from one realm to the other, Contact’s Ellie Arroway wouldn’t notice anything strange. This seems more likely if, under certain conditions, she could even slide all the way through a Gao-Jafferis-Wall wormhole.

The wormhole also safeguards unitarity — the principle that information is never lost — at least for the entangled black holes being studied. Whatever falls into one black hole eventually exits the other as Hawking radiation, Preskill said, which “can be thought of as in some sense a very scrambled copy of the black hole interior.”

Taking the findings to their logical conclusion, Preskill thinks it ought to be possible (at least for an infinitely advanced civilization) to influence the interior of one of these black holes by manipulating its radiation. This “sounds crazy,” he wrote in an email, but it “might make sense if we can think of the radiation, which is entangled with the black hole — EPR — as being connected to the black hole interior by wormholes — ER. Then tickling the radiation can send a message which can be read from inside the black hole!” He added, “We still have a ways to go, though, before we can flesh out this picture in more detail.”

Indeed, obstacles remain in the quest to generalize the new wormhole findings to a statement about the fate of all quantum information, or the meaning of ER = EPR.

In Maldacena and Susskind’s paper proposing ER = EPR, they included a sketch that’s become known as the “octopus”: a black hole with tentacle-like wormholes leading to distant Hawking particles that have evaporated out of it. The authors explained that the sketch illustrates “the entanglement pattern between the black hole and the Hawking radiation. We expect that this entanglement leads to the interior geometry of the black hole.”

But according to Matt Visser, a mathematician and general-relativity expert at Victoria University of Wellington in New Zealand who has studied wormholes since the 1990s, the most literal reading of the octopus picture doesn’t work. The throats of wormholes formed from single Hawking particles would be so thin that qubits could never fit through. “A traversable wormhole throat is ‘transparent’ only to wave packets with size smaller than the throat radius,” Visser explained. “Big wave packets will simply bounce off any small wormhole throat without crossing to the other side.”

Stanford, who co-wrote the recent paper with Maldacena and Yang, acknowledged that this is a problem with the simplest interpretation of the ER = EPR idea, in which each particle of Hawking radiation has its own tentacle-like wormhole. However, a more speculative interpretation of ER = EPR that he and others have in mind does not suffer from this failing. “The idea is that in order to recover the information from the Hawking radiation using this traversable wormhole,” Stanford said, one has to “gather the Hawking radiation together and act on it in a complicated way.” This complicated collective measurement reveals information about the particles that fell in; it has the effect, he said, of “creating a large, traversable wormhole out of the small and unhelpful octopus tentacles. The information would then propagate through this large wormhole.” Maldacena added that, simply put, the theory of quantum gravity might have a new, generalized notion of geometry for which ER equals EPR. “We think quantum gravity should obey this principle,” he said. “We view it more as a guide to the theory.”

In his 1994 popular science book, Black Holes and Time Warps, Kip Thorne celebrated the style of reasoning involved in wormhole research. “No type of thought experiment pushes the laws of physics harder than the type triggered by Carl Sagan’s phone call to me,” he wrote; “thought experiments that ask, ‘What things do the laws of physics permit an infinitely advanced civilization to do, and what things do the laws forbid?’”

Theoretically Passing Through Space And Time In A Wormhole

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Wormholes have nothing to do with earthworms, but are more like space tubes. A wormhole is a theoretical passage through space-time that could help people and things travel huge distances through space in short amounts of time. Albert Einstein and Nathan Rosen proposed this theory in 1935; wormholes are also known as Einstein-Rosen bridges. According to Einstein’s theory of general relativity, they mathematically should exist. But we have never actually observed one.

A wormhole, theoretically speaking, has two mouths connected by a throat that connects two different points in space-time. They may not only connect two points, but some theories about wormholes suggest they may also be able to connect two universes. So could we time travel in a wormhole like plenty of science-fiction movies suggest? Perhaps not, according to Einstein and Rosen’s theory, which states that a wormhole collapses quickly. New theories have emerged that suggest wormholes may stay open longer, but we’re far from having the technology required to find and use them. Learn more about wormholes in the videos below.

How Do Wormholes Actually Work?

This gets heady.

How Scientists Created A Wormhole In A Lab

Did it really work?

Neil deGrasse Tyson Explains Wormholes And Black Holes

What’s the difference?

Through the wormhole: Spacecraft could survive black hole shortcut across the universe

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© NASA
A spacecraft could survive a journey through a wormhole in the center of a black hole and pass into another universe despite its strong tidal forces, according to theoretical physicists.

A new study conducted by Diego Rubiera-Garcia, of Instituto de Astrofísica e Ciências do Espaço (IA) in Portugal, and his team, published in the journal Classical and Quantum Gravity reconsiders Einstein’s theory that black holes destroy everything within their reach.

A wormhole is a theoretical passage through space-time that could create ‘shortcuts’, facilitating journeys across the universe. It was proposed by Albert Einstein and Nathan Rosen with the aid of the theory of general relativity in 1935.

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The hypothetical structure was considered unstable, however, and at risk of collapsing on any particle passing through it at the point of ‘singularity’ – where the forces of gravity are at their most intense, and where time and space effectively end.

However, following their analysis of what causes the destruction, Rubiera-Garcia and his team say the problem is much“subtler”.

“In practical terms, we dropped one assumption that holds in general relativity, but there is no a priori reason for it to hold in extensions of this theory,” Rubiera-Garcia told Phys.org.

They examined various objects as an aggregation of points interconnected by physical or chemical interactions holding them together.

“Each particle of the observer follows a geodesic line determined by the gravitational field. Each geodesic feels a slightly different gravitational force, but the interactions among the constituents of the body could nonetheless sustain the body,” Rubiera-Garcia said.

The study found that the time a light ray takes in making a round trip between any two nearby geodesics is always finite and causal despite the infinite (spatial) stretching caused by the unbounded tidal force.

This suggests that curvature divergences may not be as extreme as traditionally thought, the study states.

“Our observer could get swallowed into the black hole, cross the wormhole throat, exit through the other side, and live to tell it regardless of the existence of infinite tidal forces,” Rubiera-Garcia explained

“The emergence of wormholes and new physics beyond Einstein’s gravity may help solve longstanding puzzles and bring a happy ending to an otherwise horror story,” he concluded.

At the end of last year, NASA announced plans for further wormhole research with the view to carrying out robotic interstellar missions in the future.

One such mission has been realized on the big screen in the blockbuster Interstellar where astronauts search for a new planet by traveling through a wormhole.

 

Chasing Wormholes: The Hunt for Tunnels in Space-Time

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Science fiction literature is full of stories in which tunnels in space-time — known as wormholes — are used for time travel. How much fact lies within the fiction? The answer is, more than you might think. Scientists are looking at ways to use traversable wormholes (if they exist) to travel faster than the speed of light — and even to travel through time itself.

“A traversable wormhole is a hyperspace tunnel, also called a throat, that connects together two remotely distant regions within our universe, or two different universes — if other universes exist — or two different periods in time, as in time travel, or different dimensions of space,” physicist Eric Davis told Space.com by email.

Davis specializes in the field of space-time as a member of the Tau Zero Foundation, where he uses equations from Einstein’s general theory of relativity to think about possible (or impossible) designs for traversable wormholes, warp drives and time machines.

Building a wormhole

Wormholes were first proposed in 1916 by mathematician Ludwig Flamm, who was toying around with equations from Einstein’s theory of general relativity that describe how gravity can curve space-time, which refers to the fabric of physical reality. While these tunnels through space-time are a fascinating theoretical possibility, according to physicist Kip Thorne, a professor emeritus at the California Institute of Technology, scientists have not yet come up with an agreed-upon way that wormholes could form in nature, and no wormholes have ever been detected.

Thorne and some of his colleagues also showed that even if a wormhole appeared, it would likely collapse before an object (or person) could pass through it. To keep the wormhole open long enough to traverse it would require some kind of scaffolding, but normal matter wouldn’t stand up to the job — it would require an “exotic material.”

“Dark energy is one form of naturally occurring exotic matter whose negative pressure produces the gravitationally repulsive force that pushes the space inside our universe outward, thus producing the inflationary expansion of the universe,” Davis said.

Along with dark energy, scientists also know of an exotic material called dark matter, which is five times more prevalent in the universe than regular matter. To date, scientists have been unable to directly detect either dark matter or dark energy, so much about them is still unknown. Scientists can learn about these materials, though, by examining the effect they have on the space around them.

According to Ali Övgün of Eastern Mediterranean University in Cyprus, it’s possible that wormholes could form where dark matter is present, and thus that they could exist in the outer regions of the Milky Way, where dark matter lies, as well as within other galaxies. Övgün is working to prove that wormholes could exist in regions dense with dark matter. He and his colleagues have run simulations that show that wormholes in dense regions of dark matter found in galactic halos would satisfy the physical requirements scientists think the tunnels require.

“But it is only mathematical proof,” Övgün said. “I hope one day it will be possible to also find direct experimental evidence.”

So, what happens to a person or instrument traveling through a wormhole?

“Nothing! The space-time geometry of traversable wormholes requires that there be no nasty, intolerable gravitational tidal forces acting upon the spacecraft or its passengers while they move through the wormhole tunnel,” Davis said. “They go into the throat at their departure location near Earth and get shunted through the tunnel to emerge out the other side near the destination star.”

Because these theoretical tunnels cut through space-time, they would allow travelers to achieve speeds that appear to an outside observer to be faster than light (FTL). However, from the travelers’ points of view, they would never actually outpace the speed of light — it would just seem that way to outside observers because the travelers would be taking a route that’s shorter than they would have taken through ordinary space.

Before scientists could use wormholes, they would first have to find them. To date, wormholes have not been discovered. However, if they exist, locating a tunnel through space-time may not be as difficult as it sounds.

“As it is visualized in the movie ‘Interstellar,’ in the future, there will be some experiments to observe [wormholes] indirectly,” Övgün said.

Based on certain wormhole theories, he compared peering through a wormhole to Alice’s glimpse through the looking glass, in Lewis Carroll’s novel of the same name. The region of space at the far end of the tunnel should stand out from the area around the entrance thanks to distortions that would be similar to the reflection in curved mirrors. Another indication may be the way light is concentrated as it moves through the wormhole tunnel, much as the wind blows through a physical tunnel.

Davis refers to what is seen at the near end of a wormhole as a “rainbow caustic effect.” Such effects could be seen from a distance.

“Astronomers were planning to use telescopes to hunt for these rainbow caustics as a sign of a naturally occurring, or even an alien-made, traversable wormhole,” Davis said. “I never heard if that project got off the ground.”

Wormhole Spaceship

Traveling through time

As part of his study of wormholes, Thorne also proposed a thought experiment in which a wormhole could be used as a time machine. Thought experiments about time travel often run into paradoxes. Perhaps the most famous of these is the grandfather paradox: If an explorer went back in time and killed his or her grandfather, that person could not be born, and would never have gone back in time in the first place. This seems to suggest that backward time travel is impossible, but according to Davis, Thorne’s work opened up a new avenue for scientists to explore.

“An entire cottage industry of theoretical physics was born after that, which led to the development of other space-time techniques that can produce causal, nonparadox time machines,” Davis said.

But although using wormholes for time travel may appeal to fans of fiction (and those who’d like to change their past),  Davis said current theories show that to make a wormhole time machine, one or both ends of the tunnel would need to be accelerated to velocities approaching the speed of light.

“It would be extremely difficult to construct a wormhole time machine,” Davis said. “It’s relatively much simpler to use wormholes for FTL interstellar travel between the stars.”

Other physicists have suggested that using a wormhole to travel through time would cause a massive buildup of energy that would destroy the tunnel just before it could be utilized as a time machine — a process known as quantum back reaction. Nonetheless, it is still fun to dream about the potential.

“Think of all the possibilities of what people could do and the discoveries they could make if they could travel through time,” Davis said. “Their adventures would be very interesting, to say the least.”

Magnetic ‘wormhole’ connecting two regions of space created for the first time

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“Wormholes” are cosmic tunnels that can connect two distant regions of the universe, and have been popularized by the dissemination of theoretical physics and by works of science fiction like Stargate, Star Trek or, more recently, Interstellar. Using present-day technology it would be impossible to create a gravitational wormhole, as the field would have to be manipulated with huge amounts of gravitational energy, which no one yet knows how to generate. In electromagnetism, however, advances in metamaterials and invisibility have allowed researchers to put forward several designs to achieve this.
(Left) 3D diagram of the magnetic wormhole, showing how the magnetic field lines (in red) leaving a magnet on the right pass through the wormhole. (Right) In terms of magnetism the wormhole is undetectable, which means that the magnetic field seems to disappear on the right only to reappear on the left in the form of a magnetic monopole.

“Wormholes” are cosmic tunnels that can connect two distant regions of the universe, and have been popularized by the dissemination of theoretical physics and by works of science fiction like Stargate, Star Trek or, more recently, Interstellar. Using present-day technology it would be impossible to create a gravitational wormhole, as the field would have to be manipulated with huge amounts of gravitational energy, which no one yet knows how to generate. In electromagnetism, however, advances in metamaterials and invisibility have allowed researchers to put forward several designs to achieve this.

Scientists in the Department of Physics at the Universitat Autònoma de Barcelona have designed and created in the laboratory the first experimental ‘wormhole’ that can connect two regions of space magnetically. This consists of a tunnel that transfers the magnetic field from one point to the other while keeping it undetectable — invisible — all the way.

The researchers used metamaterials and metasurfaces to build the tunnel experimentally, so that the magnetic field from a source, such as a magnet or a an electromagnet, appears at the other end of the ‘wormhole’ as an isolated magnetic monopole. This result is strange enough in itself, as magnetic monopoles — magnets with only one pole, whether north or south — do not exist in nature. The overall effect is that of a magnetic field that appears to travel from one point to another through a dimension that lies outside the conventional three dimensions.

The ‘wormhole’ in this experiment is a sphere made of different layers: an external layer with a ferromagnetic surface, a second inner layer, made of superconducting material, and a ferromagnetic sheet rolled into a cylinder that crosses the sphere from one end to the other. The sphere is made in such a way as to be magnetically undetectable — invisible, in magnetic field terms — from the exterior.

The magnetic wormhole is an analogy of gravitational ones, as it “changes the topology of space, as if the inner region has been magnetically erased from space,” explains Àlvar Sánchez, the lead researcher.

These same researchers had already built a magnetic fibre in 2014: a device capable of transporting the magnetic field from one end to the other. This fibre was, however, detectable magnetically. The wormhole developed now, though, is a completely three-dimensional device that is undetectable by any magnetic field.

This means a step forward towards possible applications in which magnetic fields are used: in medicine for example. This technology could, for example, increase patients’ comfort by distancing them from the detectors when having MRI scans in hospital, or allow MRI images of different parts of the body to be obtained simultaneously.

What is a Wormhole?

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A wormhole is a theoretical passage through space-time that could create shortcuts for long journeys across the universe. Wormholes are predicted by the theory of general relativity. But be wary: wormholes bring with them the dangers of sudden collapse, high radiation and dangerous contact with exotic matter.

Wormhole theory

In 1935, physicists Albert Einstein and Nathan Rosen used the theory of general relativity to propose the existence of “bridges” through space-time. These paths, called Einstein-Rosen bridges or wormholes, connect two different points in space-time, theoretically creating a shortcut that could reduce travel time and distance.

Wormholes contain two mouths, with a throat connecting the two. The mouths would most likely be spheroidal. The throat might be a straight stretch, but it could also wind around, taking a longer path than a more conventional route might require.

Einstein’s theory of general relativity mathematically predicts the existence of wormholes, but none have been discovered to date. A negative mass wormhole might be spotted by the way its gravity affects light that passes by.

Certain solutions of general relativity allow for the existence of wormholes where the mouth of each is a black hole. However, a naturally occurring black hole, formed by the collapse of a dying star, does not by itself create a wormhole.

Through the wormhole

Science fiction is filled with tales of traveling through wormholes. But the reality of such travel is more complicated, and not just because we’ve yet to spot one.

The first problem is size. Primordial wormholes are predicted to exist on microscopic levels, about 10–33 centimeters. However, as the universe expands, it is possible that some may have been stretched to larger sizes.

Another problem comes from stability. The predicted Einstein-Rosen wormholes would be useless for travel because they collapse quickly. But more recent research found that a wormhole containing “exotic” matter could stay open and unchanging for longer periods of time.

Exotic matter, which should not be confused with dark matter or antimatter, contains negative energy density and a large negative pressure. Such matter has only been seen in the behavior of certain vacuum states as part of quantum field theory.

If a wormhole contained sufficient exotic matter, whether naturally occurring or artificially added, it could theoretically be used as a method of sending information or travelers through space.

Wormholes may not only connect two separate regions within the universe, they could also connect two different universes. Similarly, some scientists have conjectured that if one mouth of a wormhole is moved in a specific manner, it could allow for time travel. However, British cosmologist Stephen Hawking has argued that such use is not possible. [Weird Science: Wormholes Make the Best Time Machines]

“A wormhole is not really a means of going back in time, it’s a short cut, so that something that was far away is much closer,” NASA’s Eric Christian wrote.

Although adding exotic matter to a wormhole might stabilize it to the point that human passengers could travel safely through it, there is still the possibility that the addition of “regular” matter would be sufficient to destabilize the portal.

Today’s technology is insufficient to enlarge or stabilize wormholes, even if they could be found. However, scientists continue to explore the concept as a method of space travel with the hope that technology will eventually be able to utilize them.

GALACTIC WORMHOLE MAY EXIST IN MILKY WAY

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A Galactic wormhole, a space-time tunnel, could exist in the middle of the Milky Way and we could travel through it.

Video: The (hypothetical) wormhole proposed by Kuefettig, Salucci et al connecting the center with a very far position of our Galaxy when one passes through its throat.

Paolo Salucci, an astrophysicist of the International School for Advanced Studies (SISSA) of Trieste, has published an alternative theory about dark matter the center of our galaxy.

He believes that dark matter at the center of the Milky Way, could sustain a giant wormhole that we could travel through, a doorway to another galaxy.

Based on the latest evidence and theories our galaxy could be a huge wormhole (or space-time tunnel, have you seen “Interstellar?”) and, if that were true, it would be “stable and navigable”. This is the hypothesis put forward in a study published in Annals of Physics and conducted with the participation of SISSA in Trieste. The paper, the result of a collaboration between Indian, Italian and North American researchers, prompts scientists to re-think dark matter more accurately.

Professor Paulo Salucci, said:

“If we combine the map of the dark matter in the Milky Way with the most recent Big Bang model to explain the universe and we hypothesise the existence of space-time tunnels, what we get is that our galaxy could really contain one of these tunnels, and that the tunnel could even be the size of the galaxy itself.

But there’s more. We could even travel through this tunnel, since, based on our calculations, it could be navigable. Just like the one we’ve all seen in the recent film ‘Interstellar.”

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What we tried to do in our study was to solve the very equation that the astrophysicist ‘Murph’ was working on,” said Prof Salucci. “Clearly we did it long before the film came out.”

Watch the video.URL: https://www.youtube.com/watch?v=u_b7Sjatlmg