What Is Spacetime Really Made Of? Part-II

What Is Spacetime Really Made Of? Part-2

Will we ever know the real nature of space and time?

Yet despite its spookiness, entanglement is a core feature of quantum physics. When any two objects interact in quantum mechanics, they generally become entangled and will stay entangled so long as they remain isolated from the rest of the world—no matter how far apart they may travel. In experiments, physicists have maintained entanglement between particles more than 1,000 kilometers apart and even between particles on the ground and others sent to orbiting satellites. In principle, two entangled particles could sustain their connection on opposite sides of the galaxy or the universe. Distance simply does not seem to matter for entanglement, a puzzle that has troubled many physicists for decades.

But if space is emergent, entanglement’s ability to persist over large distances might not be terribly mysterious—after all, distance is a construct. According to studies of the AdS/CFT correspondence by physicists Shinsei Ryu of Princeton University and Tadashi Takayanagi of Kyoto University, entanglement is what produces distances in the AdS space in the first place. Any two nearby regions of space on the AdS side of the duality correspond to two highly entangled quantum components of the CFT. The more entangled they are, the closer together the regions of space are.

In recent years physicists have come to suspect that this relation might apply to our universe as well. “What is it that holds the space together and keeps it from falling apart into separate subregions? The answer is the entanglement between two parts of space,” Susskind says. “The continuity and the connectivity of space owes its existence to quantum-mechanical entanglement.” Entanglement, then, may undergird the structure of space itself, forming the warp and weft that give rise to the geometry of the world. “If you could somehow destroy the entanglement between two parts [of space], the space would fall apart,” Susskind says. “It would do the opposite of emerging. It would dis-emerge.”

If space is made of entanglement, then the puzzle of quantum gravity seems much easier to solve: instead of trying to account for the warping of space in a quantum way, space itself emerges out of a fundamentally quantum phenomenon. Susskind suspects this is why a theory of quantum gravity has been so difficult to find in the first place. “I think the reason it never worked very well is because it started with a picture of two different things, [general relativity] and quantum mechanics, and put them together,” he says. “And I think the point is really that they’re much too closely related to pull apart and then put back together again. There’s no such thing as gravity without quantum mechanics.”

Yet accounting for emergent space is only half the job. With space and time so intimately linked in relativity, any account of how space emerges must also explain time. “Time must also emerge somehow,” says Mark van Raamsdonk, a physicist at the University of British Columbia and a pioneer in the connection between entanglement and spacetime. “But this is not well understood and is an active area of research.”

Another active area, he says, is using models of emergent spacetime to understand wormholes. Previously many physicists had believed that sending objects through a wormhole was impossible, even in theory. But in the past few years physicists working on the AdS/CFT correspondence and similar models have found new ways to construct wormholes. “We don’t know if we could do that in our universe,” van Raamsdonk says. “But what we now know is that certain kinds of traversable wormholes are theoretically possible.” Two papers—one in 2016 and one in 2018—led to an ongoing flurry of work in the area. But even if traversable wormholes could be built, they would not be much use for space travel. As Susskind points out, “you can’t go through that wormhole faster than it would take for [light] to go the long way around.”

Space to Think

If the string theorists are correct, then space is built from quantum entanglement, and time might be as well. But what would that really mean? How can space be “made of” entanglement between objects unless those objects are themselves somewhere? How can those objects become entangled unless they experience time and change? And what kind of existence could things have without inhabiting a true space and time?

These are questions verging on philosophy—and indeed, philosophers of physics are taking them seriously. “How the hell could spacetime be the kind of thing that could be emergent?” asks Eleanor Knox, a philosopher of physics at King’s College London. Intuitively, she says, that seems impossible. But Knox doesn’t think that is a problem. “Our intuitions are terrible sometimes,” she says. They “evolved on the African savanna interacting with macro objects and macro fluids and biological animals” and tend not to transfer to the world of quantum mechanics. When it comes to quantum gravity, “ ‘Where’s the stuff?’ and ‘Where does it live?’ aren’t the right questions to be asking,” Knox concludes.

It is certainly true that objects live in places in everyday life. But as Knox and many others point out, that does not mean that space and time have to be fundamental—just that they have to reliably emerge from whatever is fundamental. Consider a liquid, says Christian Wüthrich, a philosopher of physics at the University of Geneva. “Ultimately it’s elementary particles, like electrons and protons and neutrons or, even more fundamental, quarks and leptons. Do quarks and leptons have liquid properties? That just doesn’t make sense, right?... Nevertheless, when these fundamental particles come together in sufficient numbers and show a certain behavior together, collective behavior, then they will act in a way that is like a liquid.”

Space and time, Wüthrich says, could work the same way in string theory and other theories of quantum gravity. Specifically, spacetime might emerge from the materials we usually think of as living in the universe—matter and energy itself. “It’s not [that] we first have space and time and then we add in some matter,” Wüthrich says. “Rather something material may be a necessary condition for there to be space and time. That’s still a very close connection, but it’s just the other way from what you might have thought originally.”

But there are other ways to interpret the latest findings. The AdS/CFT correspondence is often seen as an example of how spacetime might emerge from a quantum system, but that might not actually be what it shows, according to Alyssa Ney, a philosopher of physics at the University of California, Davis. “AdS/CFT gives you this ability to provide a translation manual between facts about the spacetime and facts of the quantum theory,” Ney says. “That’s compatible with the claim that spacetime is emergent, and some quantum theory is fundamental.” But the reverse is also true, she says. The correspondence could mean that quantum theory is emergent and spacetime is fundamental—or that neither is fundamental and that there is some even deeper fundamental theory out there. Emergence is a strong claim to make, Ney says, and she is open to the possibility that it is true. “But at least just looking at AdS/CFT, I’m still not seeing a clear argument for emergence.”

An arguably bigger challenge to the string theory picture of emergent spacetime is hidden in plain sight, right in the name of the AdS/CFT correspondence itself. “We don’t live in anti–de Sitter space,” Susskind says. “We live in something much closer to de Sitter space.” De Sitter space describes an accelerating and expanding universe much like our own. “We haven’t got the vaguest idea how [holography] applies there,” Susskind concludes. Figuring out how to set up this kind of correspondence for a space that more closely resembles the actual universe is one of the most pressing problems for string theorists. “I think we’re going to be able to understand better how to get into a cosmological version of this,” van Raamsdonk says.

Finally, there is the news—or lack thereof—from the latest particle accelerators, which have not found any evidence for the extra particles predicted by supersymmetry, an idea that string theory relies on. Supersymmetry dictates that all known particles would have their own “superpartners,” doubling the number of fundamental particles. But CERN’s Large Hadron Collider near Geneva, designed in part to search for superpartners, has seen no sign of them. “All of the really precise versions of [emergent spacetime] that we have are in supersymmetric theories,” Susskind says. “Once you don’t have supersymmetry, the ability to mathematically follow the equations just evaporates out of your hands.”

Atoms of Spacetime

String theory is not the only idea that suggests spacetime is emergent. String theory has “failed to live up to [its] promise as a way to unite gravity and quantum mechanics,” says Abhay Ashtekar, a physicist at Pennsylvania State University. “The power of string theory now is in providing an extremely rich set of tools, which has been used widely across the whole spectrum of physics.” Ashtekar is one of the original pioneers of the most popular alternative to string theory, known as loop quantum gravity. In loop quantum gravity, space and time are not smooth and continuous the way they are in general relativity—instead they are made of discrete components, what Ashtekar calls “chunks or atoms of spacetime.”

These atoms of spacetime are connected in a network, with one- and two-dimensional surfaces joining them together into what practitioners of loop quantum gravity call a spin foam. And despite that foam being limited to two dimensions, it gives rise to our four-dimensional world, with three dimensions of space and one of time. Ashtekar likens it to a piece of clothing. “If you look at your shirt, it looks like a two-dimensional surface,” he says. “If you just take a magnifying glass, you will immediately see that it’s all one-dimensional threads. It’s just that those threads are so densely packed that for all practical purposes, you can think of the shirt as being a two-dimensional surface. So, similarly, the space around us looks like a three-dimensional continuum. But there is really a crisscross by these [atoms of spacetime].”

Although string theory and loop quantum gravity both suggest that spacetime is emergent, the kind of emergence is different in the two theories. String theory suggests that spacetime (or at least space) emerges from the behavior of a seemingly unrelated system, in the form of entanglement. Think of how traffic jams emerge from the collective decisions of individual drivers. The cars are not made of traffic—the cars make the traffic. In loop quantum gravity, on the other hand, the emergence of spacetime is more like a sloping sand dune emerging from the collective motion of sand grains in wind. The smooth familiar spacetime comes from the collective behavior of tiny “grains” of spacetime; like the dunes, the grains are still sand, even though the chunky crystalline grains do not look or act like the undulating dunes.

Despite these differences, both loop quantum gravity and string theory suggest spacetime emerges from some underlying reality. Nor are they the only proposed theories of quantum gravity that point in this direction. Causal set theory, another contender for a theory of quantum gravity, posits that space and time are made of more fundamental components as well. “It’s really striking that for most of the plausible theories of quantum gravity that we have, in some sense their message is, yeah, general relativistic spacetime isn’t in there at the fundamental level,” Knox says. “People get very excited when different theories of quantum gravity agree on at least something.”

The Future of Space at the Edge of Time

Modern physics is a victim of its own success. Because quantum physics and general relativity are both so phenomenally accurate, quantum gravity is needed only to describe extreme situations, when enormous masses are stuffed into unfathomably tiny spaces. Those conditions exist in only a few places in nature, such as the center of a black hole—and notably not in physics laboratories, not even the largest and most powerful ones. It would take a particle accelerator the size of a galaxy to directly test the behavior of nature under conditions where quantum gravity reigns. This lack of direct experimental data is a large part of the reason why scientists’ search for a theory of quantum gravity has been so long.

Faced with the lack of evidence, most physicists have pinned their hopes on the sky. In the earliest moments of the big bang, the entire universe was phenomenally small and dense—a situation that calls for quantum gravity to describe it. And echoes of that era may remain in the sky today. “I think our best bet [for testing quantum gravity] is through cosmology,” Maldacena says. “Maybe something in cosmology that we now think is unpredictable, that maybe can be predicted once we understand the full theory, or some new thing that we didn’t even think about.”

Laboratory experiments may come in handy, however, for testing string theory, at least indirectly. Scientists hope to study the AdS/CFT correspondence not by probing spacetime but by building highly entangled systems of atoms and seeing whether an analogue to spacetime and gravity shows up in their behavior. Such experiments might “have some features of gravity, though, perhaps not all the features,” Maldacena says. “It also depends on exactly what you call gravity.”

Will we ever know the real nature of space and time? The observational data from the skies may not be forthcoming any time soon. The lab experiments could be a bust. And as philosophers know well, questions about the true nature of space and time are very old indeed. What exists “is now all together, one, continuous,” said the philosopher Parmenides 2,500 years ago. “All is full of what is.” Parmenides insisted that time and change were illusions, that everything everywhere was one and the same. His pupil Zeno created famous paradoxes to prove his teacher’s point, purporting to show that motion over any distance was impossible. Their work raised the question of whether time and space are somehow illusory, an unsettling prospect that has haunted Western philosophy for over two millennia.

This article was originally published with the title "The Origins of Space and Time" in Scientific American 326, 2, 26-33 (February 2022)

AUTHOR:Adam Becker is a science writer at Lawrence Berkeley National Laboratory and author of What Is Real?,