“All the world’s a stage…,” Shakespeare wrote, and physicists tend to think that way, too. Space seems like a backdrop to the action of forces and fields that inhabit it but space itself is not made of anything—or is it? Lately scientists have begun to question this conventional thinking and speculate that space—and its extension according to general relativity, spacetime—is actually composed of tiny chunks of information. These chunks might interact to create spacetime and give rise to its properties, such as the concept that curvature in spacetime causes gravity. If so, the idea might not just explain spacetime but might help physicists achieve a long-sought goal: a quantum theory of gravity that can merge general relativity and quantum mechanics, the two grand theories of the universe that tend not to get along. Lately the excitement of this possibility has engrossed hundreds of physicists who have been meeting every three months or so under the banner of a project dubbed “It from Qubit.”
The “it” in this case is spacetime, and the qubit (pronounced “cue-bit,” from “quantum bit”) represents the smallest possible amount of information—a computer “bit” on a quantum scale. The idea suggests the universe is built up from some underlying code, and that by cracking this code, physicists will finally have a way to understand the quantum nature of large-scale events in the cosmos. The most recent It from Qubit (IfQ) meeting was held in July at the Perimeter Institute for Theoretical Physics in Ontario, where organizers were expecting about 90 registrants. Instead, they got so many applications they had to expand to take 200 and simultaneously run five satellite sessions at other universities where scientists could participate remotely. “I think this is one of the most, if not the most, promising avenues of research toward pursuing quantum gravity,” says Netta Engelhardt, a postdoctoral researcher at Princeton University 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 tend to collaborate: quantum information scientists on one hand and high-energy physicists and string theorists on the other. “It marries together two traditionally different fields: how information is stored in quantum things and how information is stored in space and time,” says Vijay Balasubramanian, a physicist at the University of Pennsylvania who is an IfQ principal investigator. About a year 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 postdoctoral researcher at Perimeter. “Collaboration is necessary—it’s not like a single person can solve this problem.” Even scientists outside of the project 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.”
The notion that spacetime has bits or is “made up” of anything is a departure from the traditional picture according to general relativity. According to the new view, spacetime, rather than being fundamental, might “emerge” via the interactions of such bits. What, exactly, are these bits made of and what kind of information do they contain? Scientists do not know. Yet intriguingly, “what matters are the relationships” between the bits more than the bits themselves, says IfQ collaborator Brian Swingle, a postdoc at Stanford University. “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,” Balasubramanian says. “That’s amazing if true.”
This animation of quantum entanglement shows two ions entangled in an experiment performed at the National Institute of Standards and Technology. Credit: NIST
The reasoning behind the idea comes from several earlier discoveries by physicists, such as a 2006 paper by Shinsei Ryu and Tadashi Takayanagi showing a connection between entanglement and the geometry of spacetime. Building on that work, in 2013 Juan Maldacena and 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) 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. Lately scientists have been studying the various kinds of entanglement that can exist. For instance, 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 one could instead entangle multiple particles of a certain kind at one location with particles of a different kind at the same location. “That’s not entanglement in space,” 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—conventional entanglement is not enough.” 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 atoms in the air give rise to the complex patterns of thermodynamics and weather. “This is an emergent phenomenon—when you zoom out of something, you see a different picture that you wouldn’t know comes about because of smaller dynamics,” Engelhardt says. “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.”
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 a century so far—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 2-D postcard with a hologram of a unicorn on it can contain all the information necessary to describe and portray the 3-D shape of the unicorn. Because finding a working theory of quantum gravity is so hard, the 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, found by Maldacena in 1997 within the framework of string theory. String theory, itself an attempt at a theory of quantum gravity, replaces all the fundamental particles of nature with tiny vibrating strings. In the AdS/CFT correspondence Maldacena showed that one can completely describe a black hole purely 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.”
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 that 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 “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 adds.
For the past 20 years scientists have found that the AdS/CFT correspondence works—a 2-D theory can describe a 3-D situation—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 major progress towards that.”
Quantum information theory may be able to help because it turns out that a familiar concept from this field, quantum error–correcting codes, could be at work in the AdS/CFT correspondence. In quantum computers, quantum error–correcting codes are a method scientists devised to help protect information from being lost if the entanglement between any particular bits gets broken. Rather than using single bits to encode information, quantum computers use highly entangled states of multiple bits to stand in for each bit, so that a single error cannot affect the overall bit. “There’s an underlying mathematical structure that seems to be common to the error-correcting codes and AdS/CFT,” says quantum information scientist Dorit Aharonov, an IfQ principal investigator at The Hebrew University of Jerusalem. In computers that redundancy is being used to correct errors, but in AdS/CFT it may be able to encode the bulk physics into an entangled state on the boundary. “It’s very intriguing that you find quantum error–correcting codes inside black holes,” she says. “Why on Earth would that happen? These connections are just fascinating.”
If physicists do eventually understand the how the AdS/CFT correspondence works—and come up with a lower-dimensional theory that stands in for quantum gravity—they are still not home free. The correspondence itself only works in a “toy model” of the universe that is somewhat simplified from the fully realized cosmos we inhabit. “AdS/CFT has a kind of gravity, but it’s not the theory of gravity in an expanding universe like 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 explains.
Some skeptics have questioned how productive IfQ can ever be if it is based on an unrealistic foundation. “That certainly is one very valid criticism: Why are we focusing on this toy model?” Engelhardt says. “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.”
Regardless of whether It from Qubit will ultimately achieve the holy grail of a unified theory, scientists inside and outside the project say the approach is worth trying and is already turning up many new avenues 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 University 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 that can exist, both for purposes of understanding spacetime as well as 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.” For instance, scientists 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 in evaluating whatever theory the researchers find. “A crucial question we would ask—once we actually manage to come up with a detailed-enough physical theory of quantum gravity—is what is the computational power of this model?” Aharonov says. Any physical theory can be thought of as a computational model, its input and output akin to the theory’s initial state and a later state that can be measured—and any computational model has a 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. It’s a way to actually tell whether the theory is sensible or not from a different point of view.”
The project is reminding 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 who has been working on IfQ. “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 PhDs decades ago.”