Gravity Seems Holographic. What Does That Mean for Reality? | Quanta Magazine

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. In the 1990s, physicist Leonard Susskind conceived of black holes as literal holograms. He determined that you could know the inside merely by measuring the surface. Linda A Cicero/Stanford News Service 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.”