All the world’s a stage,” Shakespeare wrote, and physicists tend to think that way, too. Their stage happens to be space itself, and to them, space sometimes seems like a mere backdrop to the action of the forces and fields that inhabit it. Space, the conventional thinking goes, is not made up of anything at all.
Scientists have begun to question this convention, however. Space—or rather, in the language of general relativity, spacetime—may actually be composed of tiny chunks of information. These chunks, this thinking goes, interact with one another to create spacetime and give rise to its properties, such as the curvature that causes gravity. This notion, if correct, would not just explain spacetime. It could also help physicists achieve a long-sought quantum theory of gravity, which would merge general relativity and quantum mechanics, the two grand theories of the universe that tend not to get along. This line of thinking, dubbed “It from Qubit,” has lately intrigued a growing list of physicists.
The “it” in this case is spacetime, and the qubit (pronounced “cue-bit,” from “quantum bit”) represents the smallest possible amount of information—akin to a computer “bit” but on a quantum scale. The animating idea behind It from Qubit is the notion that the universe is built up from some underlying code and that by cracking this code, physicists will have finally found a way to understand the quantum nature of large-scale events in the cosmos. An early It from Qubit (IfQ) meeting was held in July 2016 at the Perimeter Institute for Theoretical Physics in Ontario. Organizers were expecting about 90 registrants but got so many applications they wound up taking 200 and simultaneously running six remote satellite sessions at other universities. “I think this is one of the most, if not the most, promising avenues of research toward pursuing quantum gravity,” says Netta Engelhardt, a physics professor at the Massachusetts Institute of Technology, who is not officially involved in It from Qubit but who has attended some of its meetings. “It’s just taking off.”
Because the project involves both the science of quantum computers and the study of spacetime and general relativity, it brings together two groups of researchers who do not usually collaborate—quantum information scientists on one hand and high-energy physicists and string theorists on the other. More than five years ago the Simons Foundation, a private organization that supports science and mathematics research, awarded a grant to found the It from Qubit collaboration and finance physicists to study and hold meetings on the subject. Since then, excitement has grown, and successive meetings have drawn in more and more researchers, some official members of the collaboration funded by Simons and many others simply interested in the topic. “This project is addressing very important questions but very difficult questions,” says IfQ collaborator Beni Yoshida, a faculty member at Perimeter. “Collaboration is necessary—it’s not like a single person can solve this problem.”
Even scientists not working on IfQ have taken notice. “If the link with quantum information theory proves as successful as some anticipate, it could very well spark the next revolution in our understanding of space and time,” says string theorist Brian Greene of Columbia University, who is not involved in IfQ. “That’s a big deal and hugely exciting.”
Entangling Spacetime
The notion that spacetime has bits or is “made up” of anything is a departure from the traditional picture according to general relativity. The new view holds that spacetime, rather than being fundamental, might “emerge” via the interactions of qubits. What, exactly, are these bits made of, and what kind of information do they contain? Scientists do not know. Yet intriguingly, that does not seem to bother them. “What matters are the relationships” between the bits more than the bits themselves, says IfQ collaborator Brian Swingle, an assistant professor at the University of Maryland, College Park. “These collective relationships are the source of the richness. Here the crucial thing is not the constituents but the way they organize together.”
The key to this organization may be the strange phenomenon known as quantum entanglement—a weird kind of correlation that can exist between particles, wherein actions performed on one particle can affect the other even when a great distance separates them. “Lately one absolutely fascinating proposal is that the fabric of spacetime is knitted together by the quantum entanglement of whatever the underlying ‘atoms’ of spacetime are,” says Vijay Balasubramanian, a physicist at the University of Pennsylvania who is an IfQ principal investigator. “That’s amazing if true.”
The reasoning behind the idea comes from several earlier discoveries by physicists, such as a 2006 paper by Shinsei Ryu, now at Princeton University, and Tadashi Takayanagi, now at Kyoto University in Japan, showing a connection between entanglement and the geometry of spacetime. Building on that work, in 2013 physicist Juan Maldacena of the Institute for Advanced Study in Princeton, N.J., and Stanford University physicist Leonard Susskind found that if two black holes became entangled, they would create a wormhole—a shortcut in spacetime predicted by general relativity. This discovery (nicknamed ER=EPR, after physicists’ shorthand for wormholes and entanglement, based on the names of the scientists who suggested them) and others like it suggest, surprisingly, that entanglement—which was thought to involve no physical link—can produce structures in spacetime.
To understand how entanglement might give rise to spacetime, physicists first must better understand how entanglement works. The phenomenon has seemed “spooky,” in the words of Albert Einstein, ever since he and collaborators predicted it in 1935, because it involves an instantaneous connection between particles at great distances that seems to defy the limitation that nothing can travel faster than the speed of light. Lately scientists have been studying several different kinds of entanglement. Conventional entanglement involves linking a single characteristic (such as a particle’s spin) in multiple particles of the same type spread out in space. But “conventional entanglement is not enough,” Balasubramanian says. “I’ve come to realize that there are other forms of entanglement that turn out to be relevant for this project of reconstructing spacetime.” One could, for instance, entangle particles of a certain kind at one location with particles of a different kind at the same location—an entanglement that does not involve space. Scientists are also tackling the confusing complexities of entangling larger numbers of particles.
Once the dynamics of entanglement are clearer, scientists hope to comprehend how spacetime emerges, just as the microscopic movements of molecules in the air give rise to the complex patterns of thermodynamics and weather. These are emergent phenomena, Engelhardt says. “When you zoom out of something, you see a different picture that you wouldn’t know comes about because of smaller dynamics. This is one of the most fascinating things about It from Qubit because we don’t understand the fundamental quantum dynamics from which spacetime emerges.”
Cosmic Holograms
The major goal of all this work is to finally achieve a theory that describes gravity from a quantum perspective. Yet physicists chasing this goal have been stymied for the past century—Einstein himself pursued such a theory doggedly until his death, with no success. The It from Qubit scientists are banking on an idea known as the holographic principle to help them.
This principle suggests that some physical theories are equivalent to simpler theories that work in a lower-dimensional universe, in the same way that a two-dimensional postcard with a hologram of a unicorn on it can contain all the information necessary to describe and portray the three-dimensional shape of the unicorn. Because finding a working theory of quantum gravity is so hard, this thinking goes, physicists could aim to discover an equivalent, easier-to-work-with theory that operates in a universe with fewer dimensions than ours.
One of the most successful embodiments of the holographic principle is a discovery known as the AdS/CFT correspondence (an acronym for the technical term “anti–de Sitter/conformal field theory correspondence”), which shows one can completely describe a black hole by describing what happens on its surface. In other words, the physics of the inside—the 3-D “bulk”—corresponds perfectly to the physics of the outside—the 2-D “boundary.” Maldacena found this relation in the late 1990s, working within the framework of string theory, which is yet another attempt at a theory of quantum gravity. String theory replaces all the fundamental particles of nature with tiny, vibrating strings.
AdS/CFT might allow physicists to discover a theory that is equivalent to quantum gravity, accomplishes all the same goals and can describe all the same physics but is much easier to work with—by leaving out gravity altogether. “Theories with gravity are very difficult to get quantum descriptions of, whereas theories that don’t have gravity are much easier to describe completely,” Balasubramanian says. But how, one might ask, could a theory that leaves out gravity ever be a theory of so-called quantum gravity? Perhaps what we think of as gravity and spacetime is just another way of looking at the end product of entanglement—in other words, entanglement might somehow encode the information from the 3-D bulk into bits stored on the 2-D boundary. “It’s a very exciting direction,” he says.
For about 20 years scientists have found that the AdS/CFT correspondence works—a 2-D theory can describe a 3-D situation, a setup known as a duality—but they do not fully understand why. “We know these two theories are dual, but it’s not exactly clear what makes the duality work,” Swingle says. “One output [of IfQ] you could hope for is a theory for how these dualities arise. That’s something I think definitely can and will happen as a result of this collaboration, or at least [we will make] major progress toward that.”
Quantum information theory may be able to help because a concept from this field called quantum error–correcting codes could also be at work in the AdS/CFT correspondence. Scientists researching quantum computing devised these codes to help protect information from being lost if anything interferes with the entanglement between bits. Quantum computers, rather than encoding information in single bits, use highly entangled states of multiple bits. That way a single error cannot affect the accuracy of any piece of information. Strangely, though, the same mathematics involved in quantum error–correcting codes shows up in the AdS/CFT correspondence. It seems that the arrangement that scientists use to entangle multiple bits together into error-proof networks could also be responsible for encoding the information from the interior of the black hole onto its surface through entanglement. “It’s very intriguing that you find quantum error–correcting codes inside black holes,” says quantum information scientist Dorit Aharonov, an IfQ principal investigator at the Hebrew University of Jerusalem. “Why on earth would that happen? These connections are just fascinating.”
Even if physicists manage to understand how the AdS/CFT correspondence works and thereby devise a lower-dimensional theory that stands in for quantum gravity, they are still not home free. The correspondence itself works only in a “toy model” of the universe that is somewhat simplified from the fully realized cosmos we inhabit. In particular, all the rules of gravity that apply in our real universe are not in play in the streamlined world of the correspondence. “AdS/CFT has a kind of gravity, but it’s not the theory of gravity in an expanding universe like the one we live in,” Swingle says. “It describes a universe as if it was in a bottle—if you shine a light beam, it bounces off the walls of the space. That doesn’t happen in our expanding universe.” This model gives physicists a useful theoretical playground in which to test their ideas, where the simplified picture makes tackling quantum gravity easier. “You can hope it’s a useful way station in the eventual goal of understanding gravity in our own universe,” Swingle says.
If IfQ is based on an unrealistic foundation, some skeptics say, how productive can it ever be? “That certainly is one very valid criticism,” Engelhardt says. “Why are we focusing on this toy model? All of this depends on the validity of the toy model and the idea that in the end the toy model is representative of our universe. I would like to make sure that if we understand the toy model, we understand the real deal.” IfQ researchers are betting that by starting with a simplified picture that is easier to work with, they can eventually add the necessary complexity to apply the theory to the real world.
The Payoff
Despite their doubts, scientists inside and outside the project say the approach is worth trying. It has already turned up new avenues of research to pursue. “I’ve long felt that the relation between quantum information and quantum gravity is of fundamental importance,” says Raphael Bousso, a physicist at the University of California, Berkeley, who is not involved in IfQ but has worked with some of its collaborators. “The connection has deepened over the years, and I’m thrilled that so many outstanding scientists are now working together to confront these questions and see where they lead us.” Stanford theorist Eva Silverstein, who is not part of the collaboration, concurs: “It is clearly worthwhile to develop and apply quantum information to these problems. But to understand the dynamics [of quantum gravity], much more is required, and it is important for the field not to focus too narrowly on a single approach.”
Furthermore, even if the project does not pay off with a theory of quantum gravity, it is still likely to have beneficial offshoots. Bringing the techniques and ideas of string theory and general relativity to bear on questions of quantum information can, for instance, help to better define the different types of entanglement, both for understanding spacetime and for constructing quantum computers. “When you start playing with these tools in a new setting, it’s very likely that it will bring up ideas that are interesting and might be useful in other areas,” Aharonov says. “It looks like people are making progress on questions that have been out there for many, many years, so it’s exciting.” Scientists, for instance, have found that measuring time within wormholes may be possible by thinking of the wormhole as a quantum circuit.
Furthermore, combining quantum information science with string theory may help not just in deriving a theory of quantum gravity but also in evaluating whatever theory the researchers may find. Any physical theory can be thought of as a computer, its input and output akin to the theory’s initial state and a later state that can be measured—and some computers are more powerful than others. Once researchers have deduced a quantum gravity theory, they could ask, what is the theory’s computational power? “If that power is too large, if our quantum gravity model would be able to compute things that we don’t believe can be computed in our world, that would at least raise a question mark on the theory,” Aharonov says. “It’s a way to actually tell whether the theory is sensible or not from a different point of view.”
The project reminds some physicists of the heady days in the past when other big ideas were just getting started. “I became a grad student in 1984, when the so-called first string theory revolution took place,” says Hirosi Ooguri, a physicist at the California Institute of Technology. “That was a very exciting time, when string theory emerged as a leading candidate for a unified theory of all the forces in nature. I do see the current explosion of excitement around this similarly. This is clearly an exciting time for young people in the field as well as those of us who received our Ph.D.s decades ago.
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