(Photo credit: Stephania Infante)
Natalie Paquette was thinking about how to add an extra dimension. Starting with small circles distributed at each point in space and time, these circles are curved dimensions that can loop back to themselves. Then shrink these circles to make them smaller and tighter, until an unusual transformation occurs: the dimensions no longer look small but become huge, as when you realize that something that is around and looks small is actually distant and huge. but when we try to shrink it to a particular point, a new, huge spatial direction emerges."
Paquette, a theoretical physicist at the University of Washington, is not the only one studying this unusual dimensional shift. A growing number of physicists working with different approaches in different areas of this discipline are increasingly endorsing the profound idea that space and even time may not be fundamental things. Rather, space and time may simply emerge from the structure and behavior of more fundamental components. The deepest questions of reality, such as "where"? "When"? Paquette says, "We take our cue from physics: what we understand by space-time is not the essential thing."
These radical ideas come from a century of recent advances in the search for a quantum theory of gravity. General relativity is the best theory of gravity in physics, a famous theory proposed by Albert Einstein about how matter distorts space and time. Quantum physics, on the other hand, is the best theory in the rest of physics, and it is amazingly accurate when it comes to the nature of matter, energy and subatomic particles. Both theories easily pass the tests that physicists have devised and implemented over the past few centuries. One might think that by combining the two, one could obtain a "theory of everything.
However, the combination of these two theories does not work well. Putting general relativity into the context of quantum physics produces contradictions and unmanageable infinities in calculations. Nature knows how to apply gravity in a quantum context, an application that occurred at the very first moments of the Big Bang, at the center of a black hole, but humans are still struggling to understand how this process is achieved. The problem is partly that the two theories deal with space and time differently; quantum physics considers space and time to be immutable, while general relativity distorts them.
A quantum theory of gravity requires reconciling these views on space and time. One approach is to eliminate the root of the problem, space-time itself, so that it emerges from something more fundamental. In recent years, several different lines of research have shown that at the deepest levels of reality, space and time do not exist in the same way they do in everyday life. In the past decade, these ideas have fundamentally changed the way physicists view black holes. Now, researchers are using these concepts to explain the principles of even more exotic things, such as wormholes, tunnel-like passages between two points far apart in instant air. These successes have led to fervent hopes for deeper breakthroughs. If spacetime was created from nothing, then if we can figure out where it came from and how it came from nothing, we may finally be able to open the door to a theory of everything.
Today, the most popular of the candidate quantum gravity theories among physicists is string theory. According to this theory, strings are the fundamental building blocks of matter and energy, giving rise to the myriad of elementary subatomic particles that can be seen in particle gas pedals around the world. Strings are even associated with gravity - string theory assumes the existence of a gravity-carrying particle, a "graviton", and that this is a necessary consequence of the theory.
But it is difficult for people to understand string theory, which exists in the field of mathematics and has taken physicists and mathematicians decades to explore it. Much of the theory's structure remains undetermined, research explorations are still being planned, and researchers have many gaps to fill. The main trick that plays a role in navigating this new field is mathematical duality, a correspondence between one system and another.
An example of this is the pairing between small and large dimensions at the beginning of this paper. String theory suggests that trying to cram a dimension into a small space will end up with something that is mathematically identical to the world of that dimensional immensity. According to string theory, the two situations are identical - you are free to switch back and forth between one and the other, as well as use techniques from one to understand how the other works. says Paquette: "If you look closely at the basic elements of this theory you will naturally find that ...... you may have gained a new dimension of space."
(Photo credit: Elena Hartley)
A similar pairing relationship led many string theorists to realize that space itself emerged from nothing. The idea began in 1997 when Juan Maldacena, a physicist at the Institute for Advanced Study, discovered a pairing between a widely known form of conformal field theory (CFT) in quantum theory and a special kind of anti-de Sitter space (AdS) in general relativity. space (AdS) in general relativity. The two theories seem to be distinct - the CFT theory does not contain any gravity, while the AdS space contains all the gravitational theories of general relativity. Yet both can be described in the same mathematical way. The discovery of this AdS/CFT duality provides a tangible mathematical link between quantum theory and a universe that contains gravity.
Curiously, in the AdS/CFT dyad, the AdS space has one more dimension than the quantum CFT. But physicists just love this mismatch because it is a special case of another dyad conceived a few years ago, a dyad known as the holographic principle and proposed by physicist Gerard't Hooft of Utrecht University (Netherlands) and Stanford University, and Leonard Susskind, a physicist at Stanford University. Based on some unique characteristics of black holes, Hooft and Susskind conjecture that the properties of a region of space may be completely "encoded" by its boundaries. In other words, the two-dimensional surface of a black hole would contain all the information needed to describe its three-dimensional interior, much like a hologram. "I know a lot of people think we're crazy, they think two good physicists have learned the hard way," Susskind said. Susskind said.
Similarly, in the AdS/CFT dyad, the four-dimensional CFT encodes all the contents of the five-dimensional AdS space associated with it. In this system, the entire spacetime region is built up from the interactions between the components of the quantum system in conformal field theory. maldacena likens this process to reading a novel. He says, "If you tell a story in a book, the characters in the book are doing something, but there is only one line of text in the book. What the characters are doing is inferred from that one line of text in the book. The characters in the book are like AdS. The line of text is like CFT."
But where does the space in AdS come from? If this space emerged from nothing, where did it come from? The answer is a special and strange quantum interaction in the CFT: entanglement, a long-range connection between objects that instantly connects their behavior in a statistically impossible way. Einstein was also troubled by entanglement, which he called "ghostly overdistance interactions".
Will we know the true nature of space and time?
Despite its creepiness, entanglement remains a central feature of quantum physics. In quantum mechanics, any two objects interacting with each other usually become entangled, no matter how far apart they are, and they stay entangled as long as they are isolated from the rest of the world. In experiments, physicists have observed quantum entanglement between particles more than 1,000 kilometers apart, and even between particles on the ground and other particles sent to orbiting satellites. Theoretically, two entangled particles could maintain their entanglement at opposite ends of a galaxy or universe. Distance does not seem to matter for entanglement, a puzzle that has plagued many physicists for decades.
However, if space is emergent, the ability of entanglement to persist to long distances may not be particularly mysterious; after all, distance is a spatial structure. Studies of AdS/CFT dyads by Princeton University physicist Shinsei Ryu and Kyoto University physicist Tadashi Takayanagi have shown that the primary cause of distance in AdS space is entanglement. At the AdS end of the dyad, any two neighboring spatial regions correspond to two highly entangled quantum components at the CFT end. The more they are entangled, the closer the region of space is.
In recent years, physicists have speculated that this relationship may also apply to the universe. "What holds space together and prevents it from splitting into different subregions is the entanglement between the two parts of space." Susskind says, "The existence of continuity and connectivity in space is attributed to quantum entanglement." As a result, entanglement may underpin the structure of space, forming the meridians and latitudes of the world's geometry. He adds, "If you could somehow destroy the entanglement between the two parts of space, that space would collapse and, as opposed to emerge, space would disappear."
The puzzle of quantum gravity is easier to solve if space is made of entanglement: space itself arises from fundamental quantum phenomena, rather than trying to explain spatial distortions in quantum terms. susskind conjectures that this is why it was so difficult to build a theory of quantum gravity in the first place. I think the approach of combining general relativity and quantum mechanics didn't work because it started with two different things, general relativity and quantum mechanics, and then put them together," he says. I think the key is that they are so closely related that they cannot be separated and then recombined. Without quantum mechanics, there would be no gravity."
However, explaining the emergence of space is only half the job. Since space and time are closely linked in relativity, any explanation of the emergence of space must also explain how time emerges. Mark van Raamsdonk, a physicist from the University of British Columbia who pioneered the connection between entanglement and space-time, says, "Time must also emerge in some way, but this is not well understood, and it is an active area of research."
Mark van Raamsdonk says another active area of research is the use of models of spacetime emergence to understand wormholes. Many physicists previously believed that transporting objects through wormholes was impossible, even in theory. But in the past few years, physicists working on the AdS/CFT dyad and similar models have discovered new ways to build wormholes. "We don't know if we can do this in the universe, but what we do know now is that certain traversable wormholes are theoretically feasible," van Raamsdonk said. An article published in 2016 and another in 2018 sparked an ongoing series of work in the field. But even if traversable wormholes could be constructed, they wouldn't be of much use for space travel. As Susskind points out, "the speed of travel through a wormhole cannot exceed the speed of light."
(Photo credit: Stephania Infante)
Space to think
If string theorists are correct and space is built up from quantum entanglement, then time may also be built up from quantum entanglement. But what does this mean exactly? How does space "consist" of entanglement between objects? Aren't these objects themselves somewhere? How are these objects entangled if they do not experience time and change? How can things exist if they do not exist in a real space and time?
These are closer to philosophical questions - in fact, physical philosophers are taking them seriously. "How can spacetime be an emergent thing?" Eleanor Knox, a physical philosopher at King's College London, asks. She says it seems intuitively improbable. But Knox doesn't think that's the problem. She says, "Our intuitions are sometimes bad, and those intuitions evolved on the African savanna, interacting with macroscopic objects, macroscopic fluids and biological animals, and tend not to transfer to the world of quantum mechanics." Referring to quantum gravity, Knox concludes, "Where do these things exist?" "Where does it live?" Neither is the right question.
In everyday life, objects do exist in some places. But as Knox and many others have pointed out, that doesn't mean that space and time have to be fundamental, just that they have to emerge reliably from fundamental things. Christian Wüthrich, a physicist at the University of Geneva, says, "In the case of liquid, for example, it is ultimately made up of elementary particles, such as electrons, protons and neutrons, and even more fundamentally, quarks and leptons. But do quarks and leptons have liquid properties? That doesn't make sense at all, does it? However, when these elementary particles come together in large enough numbers, they exhibit a certain behavior, a collective behavior, and they behave like liquids."
Wüthrich says space and time can work in the same way in string theory and other quantum theories of gravity. Specifically, spacetime could come from things we normally think of as existing in the universe - matter and energy itself. He adds, "It's not as if we had space and time first, and then added some matter. Rather, some kind of matter may be necessary for space and time to emerge. It's still a very close connection, but it's the exact opposite of what was originally thought."
There are other ways to explain the latest findings. According to UC Davis physics philosopher Alyssa Ney, the AdS/CFT pairing is often seen as an example of spacetime emerging from a quantum system, but that may not be its true meaning. says Ney, "The AdS/CFT pairing can provide a translation manual between the facts of spacetime and the facts of quantum theory. This is consistent with the claim that spacetime emergence is fundamental to some quantum theories." But the converse is also true, she says, and the correspondence could mean that quantum theory is emergent and spacetime is fundamental - or that neither is fundamental, but that there are also some deeper fundamental things. She also said emergence is a compelling argument, and she is open to the possibility of it. "But still don't see a clear argument in favor of spacetime emergence, at least if you look only at the AdS/CFT dyad."
The bigger challenge to string theory is hidden in the obvious, in the name of the AdS/CFT dyad. susskind says: "We don't live in anti-desit space, we live in a place closer to desit space." Desiderate space describes an accelerating and expanding universe, much like our own. He concludes, "There's no way we can apply holographic theory to it yet." Figuring out how to establish this duality for a space closer to the real universe is one of the most pressing problems for string theorists. van Raamsdonk says, "I think we will be able to understand better afterwards how to apply this duality to a cosmological version of the theory."
Finally, the latest particle gas pedals have not found any evidence for the additional particles predicted by supersymmetry, on which string theory depends. Supersymmetry requires that all known particles have their own "supersymmetric partners," which doubles the number of elementary particles. But CERN's Large Hadron Collider near Geneva has found no sign of these particles. All the really precise versions of spacetime emergence that we have are based on supersymmetric theory," Susskind said. Once supersymmetry doesn't exist, there's no way to continue to calculate the equations mathematically."
Atoms of Space and Time
String theory is not the only one that agrees with spacetime emergence. Abhay Ashtekar, a physicist at Penn State University, is one of the pioneers of one of the most popular alternative theories to string theory, the lap-quantum theory of gravity. He argues that string theory has failed to deliver on its promise of combining gravity and quantum mechanics. String theory now has the advantage of providing an extremely rich set of tools that have been widely used in the field of physics. In the lap theory of quantum gravity, space and time are not smooth and continuous as in general relativity - instead, they are made up of discrete components, which Ashtekar calls "blocks or atoms of space-time".
These atoms of space-time are interconnected in a network of one- and two-dimensional surfaces that link them together to form what researchers of quantum gravity theory call a spin bubble. Although the bubble is confined to two dimensions, it gives rise to our four-dimensional world, including three-dimensional space and one-dimensional time. ashtekar compares it to a shirt, "In the case of a shirt, it looks like a two-dimensional plane, but if you look at it with a magnifying glass, you immediately see that it's all one-dimensional threads. It's just that these lines are so dense that you can think of the shirt as a two-dimensional plane. Likewise, the space around us looks like a three-dimensional continuum, but there is actually a real crisscrossing of these atoms of space-time".
Although both string theory and loop quantum gravity theory consider spacetime as emergent, the two theories describe emergence in different ways. String theory suggests that spacetime (or at least space) originates as the behavior of a seemingly unrelated system, i.e., emergence in the form of entanglement. For example, the collective behavior of individual drivers causes traffic jams. These cars are not made up of traffic jams, which are caused by cars. In the other theory, in the circle quantum theory of gravity, the emergence of spacetime is more like a tilted dune created by the collective motion of sand grains in the wind. The familiar smooth spacetime is produced by the collective behavior of tiny "particles" in spacetime; like a sand dune, these thick crystalline particles in spacetime are still sand, but they do not look or behave like an undulating dune.
Despite these differences, both quantum gravity theory and string theory suggest that spacetime comes from some more fundamental reality. They are also not the only quantum theories of gravity that point in this direction. The theory of causal sets is another candidate for a quantum theory of gravity. Knox says, "What's really surprising is that in a sense, for most existing theories of quantum gravity, the message they send is that yes, general relativistic spacetime is not at the fundamental level. It can be very exciting when different quantum theories of gravity agree on at least some things."
Temporal boundaries of space
Modern physics is a victim of its own success. Both quantum physics and general relativity are so precise in describing their respective phenomena that quantum gravity needs to describe only the most extreme cases, i.e., huge masses crammed into unfathomably tiny spaces. These extreme cases exist in only a few places in nature, such as the center of a black hole - a situation that even the largest and most advanced labs in physics laboratories cannot create. It would take a particle gas pedal the size of a galaxy to directly test the natural behavior under conditions dominated by quantum gravity. The main reason scientists have searched for a theory of quantum gravity for so long is the lack of direct experimental data.
Because of the lack of evidence, most physicists have pinned their hopes on the sky. In the early days of the Big Bang, the entire universe was very tiny and dense - a situation that requires quantum gravity to describe. Today, echoes of the Big Bang may still linger in the sky," Maldacena says. "I think the best way to verify quantum gravity is through cosmology. There may be things in cosmology that we think are unpredictable now that can be predicted after we understand the full theory or learn about something new that we never thought of."
However, it might also become possible to test string theory at least indirectly in the laboratory. Scientists hope to study the AdS/CFT pair not by probing cosmic spacetime, but by building highly entangled atomic systems and seeing if they behave similarly to spacetime and gravity. such an experiment might exhibit some gravitational features, but probably not all of them, Maldacena said. It also depends on how gravity is defined.
Will we ever know the true nature of space and time? Observational data from the sky may not be published soon. Experiments in the laboratory may fail. Philosophers know that the question of the true nature of space and time is very old indeed. 2500 years ago, the philosopher Parmenides said that all existence "converges into a continuous whole, containing all 'is'." He insisted that time and change are illusions and that everything is the same everywhere in the world. His student Zeno proposed some famous paradoxes to support his view, claiming that motion at any distance is impossible. Their work raised the question of whether time and space are somehow illusory, a question behind which disturbing hints have plagued Western philosophy for more than two thousand years.
Wüthrich says, "In fact, the ancient Greeks asked the question, 'What is space?' 'What is time?' 'What is change?' The fact that we are still asking these questions today means that they are questions worth asking. It is by thinking about these questions that we learn a lot about physics."
Introduced from: Scientific American