Bits interwoven in space-time

However, scientists began to question this convention. Space, or in fact, in the language of general relativity, the space-time, may actually consist of tiny bits of information. These pieces, according to this line of thought, interact with each other and thus create space-time and grow its properties, such as the curvature that generates gravity. This idea, if it is true, will not only be an explanation of the essence of space-time. It may also help physicists arrive at the theory they've been looking for for years, a quantum theory of gravity that would merge general relativity and quantum mechanics, the two grand theories of the universe that tend not to get along. This exciting possibility has recently swept away hundreds of physicists, who meet every three months or so under the auspices of a project named "It from Qubit", or IfQ for short.

The word "It" here represents space-time, and the word Qubit ("qubit", short for quantum bit) represents the smallest possible amount of information - equivalent to a bit (bit) from computer science, but on a quantum scale . The idea that runs through the veins of this project is the thought that the universe is built from some fundamental code, and that if physicists crack that code they will finally find a way to understand the quantum nature of large-scale events occurring in the universe. A recent IfQ conference was held in July 2016 at the Perimeter Institute for Theoretical Physics in Ontario. Organizers had expected 90 registrants, but received so many applications that they ended up admitting 200 people to the conference while also holding six remote sessions at other universities where they watched the debates via satellite. "I think this is one of the most promising research tracks, if not the most promising, towards quantum gravity," she says Netta Engelhardt, a postdoctoral fellow at Princeton University who is not formally involved in IfQ but has attended several conferences. "this is only the beginning."

The project includes two different fields of research, quantum computing and the physics of space-time and general relativity, so it brings together two groups of researchers who usually do not cooperate: quantum information scientists on the one hand and high-energy physicists and string theory theorists on the other. More than a year ago she gave Simmons Foundation, a private organization that supports scientific and mathematical research, a grant that helped establish the IfQ collaboration and provide funding for physicists to research the field and hold conferences on the subject. Since then, the excitement grew and more conferences attracted more and more researchers, some of them official members of the collaboration founded by the Simmons Foundation and many others who were simply interested in the subject. "This project addresses very important, but also very difficult questions," says one of the IfQ partners, Benny Yoshida, a postdoctoral student at the Perimeter Institute. "Cooperation is necessary; This is not a situation where a single person can solve the problem."

The project even captured the attention of scientists who do not work at IfQ. "If the link with quantum information theory turns out to be successful, as some people expect, it could well be the opening shot of the next revolution in our understanding of space and time," says the string theorist. Brian Greene from Columbia University, who is not involved with IfQQ. "This is a serious matter that arouses enormous excitement."

Interweaving space-time

The idea that space-time has bits or is "made of" something is a departure from the traditional picture outlined by general relativity. The new view holds that space-time is not a fundamental reality but "emerging” through the interactions of quantum bits. What exactly are these bits made of, and what kind of information do they contain? The scientists do not know. However, as strange and intriguing as it sounds, it doesn't seem to bother them at all. "What changes are the relationships" between the bits and not the bits themselves, says one of the partners in the IfQ project, Brian Swingle, a postdoctoral fellow at Stanford University. "These relationships, when grouped together, are the source of wealth. The decisive thing here is not the ingredients but the way they are organized together."

The key to this organization may be the strange phenomenon known as Quantum entanglement: as a kind of deceptive correlation that may exist between particles, where actions performed on one particle can affect the other even when a huge distance separates them. "One of the most spectacular proposals that has been put forward recently holds that the fabric of space-time is a plexus held together by a quantum interweaving of the "atoms" standing in the infrastructure of space-time, whatever they may be," says Vijay Balasubramanian, a physicist at the University of Pennsylvania and one of the principal investigators at IfQ. "If that's true, then that's amazing."

The logic that led to this idea comes from some earlier discoveries of physicists, such as for example Article From 2006 byShinsei Rio, who now works at the University of Illinois at Urbana-Champaign, andTadashi Takayanagi, who now works at Kyoto University in Japan, who showed a connection between interweaving and the geometry of space-time. Based on this work, Juan Maldesana from the Institute for Advanced Study in Princeton, New Jersey, and the physicist from Stanford Leonard Susskind They discovered in 20133 that if two black holes become entangled, they form a wormhole: a shortcut through spacetime predicted by the theory of relativity. This discovery (which was given the nickname EP=EPRR, after the abbreviations that physicists use to describe wormholes and entanglement, based on the names of the scientists who proposed them) and other similar discoveries raise the surprising possibility that entanglement, considered something that does not involve any physical connection, can create structures in space-time .

In order to understand how entanglement can create space-time, physicists first need to better understand how entanglement works. This phenomenon looked likeGhost action", if we use the language of Albert Einstein, since he and his colleagues predicted it in 1935, because it involves an immediate connection between particles located at a great distance from each other, so that this seemingly violates the limit according to which nothing can move at a speed higher than the speed of light. Recently, scientists have been studying several different types of interweaving. Normal entanglement involves creating a connection between several particles scattered in space through a single property (such as the spin of the particles). But "ordinary braiding is not enough," says Balasovermanian. "It dawned on me that there are other forms of interweaving that turn out to be relevant to this project, of rebuilding space-time." For example, particles of a certain type can be interwoven in one place with particles of another type in the same place - an interweaving that does not involve space. Also, scientists grapple with the bewildering complexities of entwining larger amounts of particles.

Once the dynamics of the entanglement become clearer, scientists hope to understand how spacetime is formed, just as the microscopic movements of molecules in the air create the intricate patterns of thermodynamics and weather. These are emergent phenomena (Emergence), says Engelhardt. "When you look at something from the outside and from a distance, you discover a different picture that you would not have known would be created due to smaller dynamics. This is one of the most fascinating things about IfQ, because we do not understand the fundamental quantum dynamics from which space-time is formed."

Cosmic holograms

The main goal of all this work is to finally arrive at a theory that describes gravity from a quantum point of view. However, physicists striving to achieve this goal have worked hard in the last hundred years. Einstein himself stubbornly tried to formulate such a theory until the day of his death, without success. IfQ scientists put their trust in an idea known as "The holographic principle,” hoping he would help them.

This principle means that some physical theories are equivalent to simpler theories operating in a universe with fewer dimensions, in the same way that a two-dimensional postcard with a hologram of a unicorn printed on it can contain all the information needed to describe and display the unicorn's three-dimensional figure. Since finding a working theory of quantum gravity is such a difficult task, according to this line of thought physicists could try to discover a sound theory that is easier to work with that works in a universe with less dimensions than our own.

One of the most successful applications of the holographic principle is a discovery known as AdS/CFT adjustment (short for the technical term "anti-de Sitter fit/conformal field theory"), which shows that it is possible to provide a complete description of a black hole through a description of what is happening on its surface. In other words, the physics of its interior, i.e. the three-dimensional "body", perfectly matches the physics of its exterior, i.e. the two-dimensional "boundary". Maldesana discovered this connection in the late 90s, when he was working within the framework of String theory, which is another attempt to formulate a theory of quantum gravity. String theory replaces all the elementary particles of nature with tiny, vibrating strings.

It is possible that the things we perceive as gravity and space-time are nothing but another way of looking at the final product of interweaving.

The AdS/CFT fit might allow physicists to discover a theory equivalent to quantum gravity, achieving all the same goals and capable of describing all the same physics, yet much easier to work with, by neglecting gravity altogether. "Theories that include gravity pile up many difficulties for those who try to give them a quantum description, while theories that do not include gravity are much easier to fully describe," says Balasovermanian. But the question arises: how can a theory that leaves gravity behind claim the crown of a quantum gravity theory? It is possible that the things we perceive as gravity and space-time are nothing but another way of looking at the final product of interweaving. In other words, it is possible that the interlacing somehow encodes the information from the XNUMXD body and turns it into bits stored on the XNUMXD boundary. "It's a very exciting direction," he says.

For 20 years scientists have been discovering that the AdS/CFT fit works, a XNUMXD theory can describe a XNUMXD state, a state known as "duality", but they don't understand why. "We know that both of these theories are dualities, but it's not really clear what makes the dualities work," says Swingle. “One of the outcomes [of IfQ] that you can hope to achieve is a theory that explains how these states of duality arise. This is something that I think may certainly happen and will indeed happen as a result of this cooperation, or at the very least [we will have] serious progress towards it."

Quantum information theory may help because a concept from this field, known as quantum error-correcting codes, may also operate behind the scenes of AdS/CFT matching. Scientists studying quantum computing installed these codes to help prevent information loss in the event that something interferes with the interlacing between the bits. Quantum computers do not encode information in single bits but use states of several bits that are strongly intertwined. Thus, a single error cannot affect the accuracy of any piece of information. However, as strange as it sounds, the same mathematics involved in quantum error correction codes also appears in the AdS/CFT fit. It seems as if the array that scientists use to interlace several bits together and create error-proof networks could also be responsible for encoding the information coming from the inside of a black hole to its surface through interlacing. "It's a very tempting idea to find codes of quantum error correction inside black holes," says the quantum information researcher Dorit Aharonov, principal researcher of IfQ at the Hebrew University of Jerusalem. "Why the hell would this happen? These links are simply fascinating.”

Even if physicists manage to understand how the AdS/CFT fit works and in any case also formulate a theory in low dimensions to take the place of quantum gravity, they still cannot rest on their laurels. The adjustment itself only works in a "toy model" of the universe, a somewhat simplified model, different from the actual cosmos we live in. In particular, all the laws of gravity that apply in our actual universe play no role in the clean world of alignment. "AdS/CFT has a kind of gravity, but it is not the theory of gravity in an expanding universe like the one we live in," says Swingle. "She describes a universe as if it were inside a bottle: if you project a beam of light, it will be reflected from the walls of space. This does not occur in our expanding universe.” This model provides physicists with a useful theoretical playground on which to test their ideas, because the simplistic picture allows quantum gravity to be handled more easily. "One can hope that this is an effective way station on the way towards the final goal, to understand gravity in our universe," says Swingle.

But if IfQ stands on an unrealistic foundation, some skeptics will say, how many fruits will it be able to produce? "This is a perfectly valid criticism," says Engelhardt. "Why are we focusing on this toy model? The whole business depends on the validity of the toy model and the idea that the toy model ultimately represents our universe. I would like to make sure that if we understand the toy model, we understand the real story." IfQ researchers are betting on the possibility that if they start with a simplistic picture that is easier to work with, they will eventually be able to add the complexity necessary to apply the theory to the real world.

the fruits of the effort

Despite the doubts, both the scientists involved in the project and those not involved in it, all say that this approach is worth trying. It has already opened up new paths for researchers to tread. "For a long time I have felt that there is a fundamental importance to the connection between quantum information and quantum gravity," he says Raphael Bosso, a physicist at the University of California, Berkeley, who is not involved in IfQ but worked with several partners on the project. "The connection has deepened over the years, and my heart expands to see that so many extraordinary scientists are now working together to attack these questions and see where they will lead us." Eva Silverstein, a theorist from Stanford, who is not part of this collaboration, continues to follow him: "There is no doubt that quantum information theory should be developed and applied to these problems. But to understand the dynamics [of quantum gravity], much more is required than that, and it is important that this field does not settle for a narrow focus on a single approach.”

Moreover, even if it turns out that the project will not bear the fruit of a quantum gravity theory, it is still likely that we will benefit from its consequences. Using the techniques and ideas of string theory and general relativity to address questions of quantum information may, for example, help better define the different types of entanglement, both to understand space-time and to build quantum computers." When you start playing with these tools in a new context, it is very likely that they will come up with ideas that will be interesting and may be useful in other fields," says Aharonov. "People seem to be making progress on questions that have been unresolved for many years, so that's exciting." For example, scientists have discovered that measuring time inside wormholes might be possible if we think of a wormhole as a quantum electrical circuit.

Moreover, combining quantum information theory with string theory may help not only in deriving a theory of quantum gravity but also in evaluating any theory that researchers may discover. Any physical theory can be thought of as a computer, where the input and output correspond to the initial state of the theory and a later state that can be measured, and some computers are more powerful than others. Once researchers succeed in formulating a quantum theory of gravity, they will be able to ask what is the computational power of the theory? "If this force is too great, if our model of quantum gravity can calculate things that we don't think can be calculated in our world, we will at least have reason to doubt the theory," says Aaronov. "This is a way of actually saying whether the theory is plausible or not, from another point of view."

The project reminds some physicists of the bold days of the past, when other big ideas were just beginning to sprout. "I started my advanced degree in 1984, when progress began that is commonly called the first string theory revolution, says Hirusi Uguri, a physicist at the California Institute of Technology. "These were very exciting days, when string theory emerged and became a leading candidate for a theory that would unify all forces in nature. I really understand the burst of excitement from this comparison. There is no doubt that this is an exciting time for young people in the field and also for us, who received our doctorate degrees decades ago."