Gravity Seems Holographic. What Does That Mean for Reality?

In my first months as a physics journalist nearly a decade ago, I kept running into an inscrutable string of characters: AdS/CFT. Thoroughly intimidated, I decided to just ignore it.

But I couldn’t keep my head in the sand for long. I soon learned that those characters are shorthand for a surprising connection between the seemingly inharmonious worlds of gravity and quantum mechanics. And even more bizarrely, this “anti-de Sitter/conformal field theory” correspondence suggests that gravity eliminates the distinction between volume and area. This broader idea is known as the holographic principle, and it now strikes me as the most profound proposal in theoretical physics in the last 30 years.

Theoretical physicists tend to vote with their feet, and AdS/CFT sparked a stampede. The three foundational papers on the topic in the late 1990s have garnered tens of thousands of citations, making them by far the most highly cited theoretical physics works of the digital era. In my interviews with physicists who study holography, they often seem genuinely stunned, and reach for words like “magical” and “miraculous” to describe it. And it doesn’t hurt that holography led to a widely accepted answer to the most famous puzzle in physics: Contrary to what Stephen Hawking argued, black holes are not inescapable prisons.

But even after covering numerous developments in holography and having countless conversations with the physicists involved, I still felt confused. I had heard that holography suggested that gravity and quantum mechanics are one and the same, and that space might be an illusion. I had also heard holography described both as a mathematical fact and as a speculative flight of fancy. So I tried to triangulate these wild ideas and figure out what, exactly, the holographic principle implies about our universe. 

The Evidence

Put a box around any region of space (space-time, really, but I’m going to drop time throughout this essay for ease of visualization, as physicists often do). The holographic principle asserts that no matter what’s going on inside — from gas molecules pinging around to black holes colliding — you can decipher the entire contents of the box just by repeatedly measuring points on the surface.

Pause for a moment to reflect on how outrageous this assertion is. You can’t see into the box at all. Nevertheless, holography says that you can learn exactly what’s happening everywhere in the box without any access to the interior. Observing the surface alone is enough. In this sense, the amount of stuff that fills a box is the same as the amount of paint that covers it. That’s a violation of logic and geometry. It asks us to erase the categorical difference between square meters and cubic meters. It recalls how holographic images appear to have depth despite being flat, except the bird in the hologram is the same as an actual bird.

Bartek Czech, a theorist at Tsinghua University in China, highlights the power of the principle by comparing it to a CT scan of the brain, which uses X-rays to look inside the organ and reconstruct it from hundreds to thousands of cross-sectional images. Holography implies that you can do that — reconstruct every fold, vessel, and neuron in three dimensions — without actually looking inside. Simply photographing the surface of the brain somehow suffices.

Why would anyone entertain such a far-fetched notion? It’s rooted in thought experiments and math, and it appears to trace back to one force: “a miracle of gravity,” Czech said.

Scientists have known for more than a century that gravity is different from the other forces. Imagine a box filled with electric charges, representing one of the other fundamental forces, electromagnetism. The stuff in the box consists of the charges and the electric field they create, which also passes through the outer surface. You will have a problem if you try to infer what arrangement of charges generates the field by looking at the surface alone. Because positive charges neutralize negative charges, different arrangements can look the same. If you observe no field, it could mean there’s no charge inside — or it could mean that the effects of the positive charges are perfectly blocking the effects of the negative charges. From the surface, you can’t tell the difference.

With gravity, mass plays the role of charge. It bends space-time around it, and it is always positive. There is no negative mass, so you can always infer the one real arrangement of stuff inside from the warping of space-time at the surface of your box. “Intuitively, this is why holography is plausible,” said Laurent Freidel, a physicist studying quantum gravity at the Perimeter Institute for Theoretical Physics in Waterloo, Canada.

But holography really starts to bite only after you take the intricate details of quantum mechanics into account. The first clues came in the 1970s, when Jacob Bekenstein and Stephen Hawking calculated the entropy of black holes — typically a measure of how much stuff fits inside an object. They used quantum theory to predict how a black hole would grow as it swallowed particles. Perplexingly, as they imagined adding particles to the black hole, they found that the entropy grew in lock step with the surface area — not the volume, as you would expect.

Leonard Susskind, a physicist at Stanford University, built on their result in the 1990s and proposed that the black hole was literally a hologram, that everything happening inside can be observed from the outside. In some sense, the interior was superfluous. “I thought it was a little bit crazy,” Susskind said, “but I thought it was the least crazy of all the possibilities.” (Gerard ’t Hooft, a Nobel laureate, and Charles Thorn, a physicist at the University of Florida in Gainesville, came to similar conclusions around the same time.)

I’ve always found this black hole entropy argument compelling, because any patch of space can become a black hole if you put enough mass into it. Despite their reputation for weirdness, black holes are representative examples of space. They just have a way of bringing space’s stranger properties to the fore. So if a black hole is holographic, and any region of space can become a black hole, then, the argument goes, even the room you’re sitting in should be holographic. “It’s completely general,” Susskind said.

This argument has a rock-solid universality, but I’ve also heard physicists describe holography as a speculative idea with an uncertain connection to reality. So I called up Latham Boyle, a physicist at the Higgs Center for Theoretical Physics at the University of Edinburgh, hoping for an alternative view. He did not disappoint.

Boyle doesn’t dispute Bekenstein and Hawking’s black hole findings, but he does question the holographic interpretation. He suspects that the act of putting a surface around a region of space — as happens when a black hole forms — creates two distinct types of entropy. One entropy tells you how many particles can fit inside — and that really does depend on the volume. The existence of the surface gives you a second, “entanglement” entropy. Particles inside share a quantum connection, known as entanglement, with those outside; the bigger the surface, the more entanglement crosses it. The entanglement entropy depends on the area, not the volume. They’re not, Boyle posits, the same thing.

“That seems like a less mystical, more down-to-earth interpretation of what’s going on,” he said.

But it helps holography that there is a second, more conceptually airtight finding behind it: AdS/CFT.

AdS/CFT asks us to imagine a universe that is not like our own, one that curves in such a way that its infinite expanse of space can be pictured as fitting inside a finite snow globe. That might sound like a big ask, but it’s one that mathematicians — and mathematically minded artists such as M.C. Escher — are perfectly comfortable with. This geometry is known as anti-de Sitter (AdS) space.

Other than its peculiar curvature, the interior of the anti-de Sitter snow globe is a lot like our universe, filled with electrons and atoms. More importantly, it also ripples in response to that matter, providing the effect of gravity. The snow globe’s surface, meanwhile, is a universe of its own. It’s also populated with quantum particles, but it’s rigid, so it can’t react to the particles: no gravity. This surface world is ruled exclusively by a type of quantum theory known as a conformal field theory (CFT), where the rules of physics don’t change as you zoom in or out.

The blockbuster trilogy of papers in the late 1990s showed that, mathematically, these two theoretical worlds (the AdS interior and the CFT surface) are the same. This is the AdS/CFT correspondence. As with the black hole entropy argument, the volume and surface are equivalent. But unlike the black hole argument, AdS/CFT is essentially a mathematical fact about gravity and quantum mechanics with no alternative interpretation. Even skeptics find this genuinely surprising. “I don’t know of any mundane way to explain it,” Boyle said.

The undeniable message of AdS/CFT is that, at least in this special snow globe, the rules of gravity and the rules of quantum mechanics are secretly describing the same game — despite the storied antagonism between the two theories. “Far from being opposed, they’re actually intertwined,” said Brian Swingle, a physicist at Brandeis University. “One emerges from the other.”

The correspondence came as a shock. I think of it as akin to discovering a way of converting any checkers move into a valid chess move: Why on Earth would that work? When I ran that picture by Sebastian Mizera, a physicist at Columbia University who studies the mathematical structure of quantum theories, he told me it wasn’t dramatic enough. “That’s a good analogy,” he said, except “it’s more like checkers and basketball.”

But does the holographic nature of the snow globe tell us anything about our reality? On this point, physicists disagree. Skeptics emphasize that our universe is the opposite of a snow globe. The accelerating expansion of the cosmos implies that we live in a space that curves outward, in the opposite direction — a de Sitter space. Because our space does not curve back in on itself, it has no boundary surface where you can project the hologram. So there’s little reason to think that AdS/CFT has anything to do with the real world.

The most dedicated holographers, however, take a ground-level perspective. An ant living deep inside the snow globe can’t easily detect any curvature, and therefore can’t tell the difference between anti-de Sitter and de Sitter space. So perhaps what’s true of one space, they argue, should more or less hold for the other. (And in case you were wondering, we’re the ants.)

While both arguments have merit, I lean toward the holographers. Black holes provide intriguing but circumstantial evidence that all types of space are holographic. And the AdS/CFT correspondence essentially guarantees that anti-de Sitter space — which happens to be the space physicists understand best — is holographic. What are the odds that our universe works in a totally different way? It absolutely could, but I wouldn’t bet on it. I take seriously the possibility that gravity makes every kind of space, including ours, holographic.

And so what would it mean for us to live in a hologram?  

The Meaning(s)

I found that most physicists are hesitant to speculate about the connection between the holographic nature of space and “ontology” — the capital-T truth about what’s real.

“I don’t try to answer that question,” Susskind said. “That’s beyond my pay grade.”

This strikes me as a prudent response, one that stays true to the ultimate goal of physics, which is not, as I am often tempted to think, to explain what is real. Rather, physicists seek to identify a few simple concepts, expressed in mathematical relationships, that make reliable predictions in many different situations. Gravity is a powerful concept because it holds for falling apples, sloshing tides, and orbiting planets. Holography is another step in that tradition, an equivalence between area and volume that holds at least for certain spaces.

“Physicists build models,” Czech said. And it’s exciting that holographic models are even possible to build.

But I craved something more intuitive, less prudent. I wanted to know what holography would mean for us if we lived in anti-de Sitter space (which we don’t), or if physicists developed a holographic theory of de Sitter space (which they haven’t). When I framed the question in that way, Vijay Balasubramanian, a physicist who studies holography at the University of Pennsylvania, gamely laid out a short menu of possibilities.

If our universe ultimately has just one nature (as opposed to multiple equivalent natures, which Balasubramanian said is possible), then there are three options: The quantum surface is the real thing, the gravitational volume is the real thing, or something else is the real thing.

The first interpretation — the surface is real — is the most popular among physicists who spend their time studying AdS/CFT. They suspect that the space we experience is as illusory as water. If you look closely enough at the smooth, clear liquid, it resolves into ricocheting molecules — the “real thing.” Similarly, if you were to look at our universe closely enough, you’d find that it’s emptier than it seems. In this scenario, we would resemble characters in a video game. The apparently bulky buildings and trees of the 3D game world around us would actually be pixels flickering on a flat screen.

“We are fooled into thinking that there is more stuff in the universe than there actually is,” said Charles Cao, a theorist at Virginia Tech. You can “compress all of the three-dimensional world into two dimensions.”

This perspective abounds in the research program called “it from qubit,” which posits that the space around us (“it”) is made up of quantum units of information (qubits). These qubits would make up the true fabric of our reality in the same way that screen pixels make up the physical reality of the video game characters.

The profound implication of this interpretation is that it flips the normal relationship between distance and influence, said Ning Bao, who studies holography at Northeastern University. We typically imagine that two things don’t influence each other because space separates them: Flares from alien stars are far away, and that’s why they don’t knock out power on Earth. But it from qubit suggests we have it backward. Perhaps space seems to separate two things precisely because they don’t influence each other. Consider a video game sun passing behind a video game tree. The sun pixels touch the tree pixels directly, yet the tree does not burst into flames. This is because the sun pixels are independent of the tree pixels. Their independence is what makes the sun “far” from the tree.

The correspondence goes both ways, however. The holographic principle puts the two pictures of the world on equal footing. So why can’t the gravitational volume be the real thing? Holographers shy away from this interpretation because they don’t have a full quantum handle on space — even anti-de Sitter space. But that’s just our ignorance, Balasubramanian said. Some direct quantum theory of space and matter, such as string theory, must exist, and that could be the fundamental description.

If that were the case, the 3D video game world would be the real one, and it would merely seem as if it were made of 2D pixels. Holography would be a mathematical coincidence. In this scenario, “the reality is you’ve got all [three] of these dimensions. It just so happens that they have some [holographic] description,” Balasubramanian said.

And then there’s door number three, the nuclear option: Neither the interior volume nor the surface area is real. Both gravity and quantum mechanics are rough drafts of a sharper, truer, completely unknown theory. At the risk of stretching the video game analogy, you could argue that neither the video game world nor the screen pixels are “real,” and that both are just reflections of the complicated ways that electrons physically flow through the game console and television. In this case, holography tells you how the pixels of the screen relate to the objects of the game world, but it has nothing to do with the nature of the electrons. “The actual theory is something else,” Balasubramanian said.

At this point, I subscribe to a more extreme variation of the it from qubit interpretation — mostly just following the rumble of the stampede. I’d bet that the qubits are the real things, but that they don’t live on anything as familiar as a flat screen.

Physicists have tried to stretch the AdS/CFT correspondence to fit de Sitter space — which has no obvious screen — for decades, with limited success. In recent years, they’ve started to get more creative. Susskind and other teams have made progress on holographic de Sitter models that differ radically from AdS/CFT. Instead of squashing a volume into an area, these universes seem to cram all the dimensions into a lone quantum point. I imagine a bunch of quantum pixels all coexisting in one spot, rather than spreading across a screen. That might be hard to visualize, but we’re already accepting the idea of dropping one dimension of space. Why should tossing the others be so different?

Balasubramanian suspects that even this kind of radical model doesn’t go far enough. Einstein’s theory fused space with time, and so if the three dimensions of space emerge from a spaceless point, then time should emerge from something timeless. Somehow, we and everything we experience exist within an unblinking dot of no size. Physicists are nowhere close to constructing a functional theory of this form, much less finding hard evidence that our universe works this way. But to paraphrase Niels Bohr, during an earlier era when physicists were seeking the next big thing, this sort of theory strikes me as just radical enough — and just simple enough — to be right.