| A couple of years ago, I was chatting about black holes with Dan Harlow of the Massachusetts Institute of Technology when he made a casual comment that left a deep impression on me. I asked if some new work he'd been doing strengthened the case that space-time was "emergent." Without missing a beat he replied, "Sure, if it needed strengthening."
Harlow isn't the only physicist with serious doubts about what reality is made of. For more than a decade now, Nima Arkani-Hamed of the Institute for Advanced Study has been delivering a polished lecture arguing that space-time is "doomed." Time and again, I've heard theorists in high-energy physics make similar-sounding statements, and I've always been struck by their confidence. We don't have the faintest idea what the next theory of physics will look like, whether it will involve strings, loops, triangles or something entirely new that no one has thought to propose. And yet so many theorists seem rather convinced that whatever it will be, it won't involve space or time.
Why? What does that statement mean? What would it look like to do physics without referring to space or time? I've spent most of this year trying to find out. The results have just been published in "The Unraveling of Space-Time," a massive package that includes articles, videos and interactive animations from me and my colleagues Mark Belan, Emily Buder, Amanda Gefter and Joseph Howlett.
Over the course of more than 40 interviews with nearly 30 physicists, I learned that there are many ways to define emergent space-time. But at the most basic level, "emergent space-time" means that space and time are the outputs of a theory instead of the inputs. A classic analogy is heat. To explain why a teacup cools, scientists of the 1700s put heat into their theory of the world as a substance that repels itself and naturally spreads out. But this "caloric theory" was ultimately replaced by thermodynamics, a theory where a primary input is molecules that buzz around with some energy. As molecules crash into each other, their energy spreads, and we now recognize this process as the origin of heat transfer. Heat is an output — a prediction — of thermodynamics. It is an emergent phenomenon.
Space-time is the ultimate input. If physics is largely about predicting what happens where and when, you need a stage upon which things can happen. Albert Einstein became a household name for revealing that this stage acts like a fabric that bends in ways we experience as gravity. He described in spectacular detail how space-time behaves, much as 19th-century scientists described how heat behaves with caloric theory. The idea that space-time is emergent is the idea that space-time will eventually go the way of heat, water, air and so many other substances before it; we will someday understand it to be the inevitable consequence of the behavior of simpler entities. Call them the "atoms" of space-time.
What's in the Series This week's series explores the mind-bending notion of emergent space-time from a number of angles. There is, of course, the why of it all. This mostly boils down to the strange things that happen when Einstein's theory of space-time collides with quantum mechanics, the theory of the subatomic world. When we combine features from both theories, we see that any experiment that tries to probe reality a little too closely will get thwarted by the appearance of a black hole, an enigma that undermines the familiar picture of space-time in its own way.
For this and other reasons, physicists are pushing to escape our familiar space-time, often referred to as the "bulk," in search of alien environments conducive to new ways of doing physics.
Where else might one do physics, if not in the bulk? A few ideas are being developed, including one that goes by the name of holography. This is roughly the idea that any gravitational system — even the entire universe — can have an alternative description as a collection of quantum particles moving around a flat surface. From these gravity-free surfaces, a bulk world with gravity somehow pops out. It's a remarkable theoretical claim, and over the past few years, holographers have developed a suite of tools that have helped them decode the bulk from the behavior of these surface particles.
Another research program, spearheaded by Arkani-Hamed, has even more ambitious aims — getting both space-time and quantum mechanics as outputs from even more alien inputs. His group has recently developed an entirely new language for making predictions, one that makes no reference to space-time. Instead, it uses only geometric shapes and primitive counting tasks.
Is space-time, at least in its current form, definitely doomed? The idea tortured one of the pioneers of gravitational theory, John Wheeler. And today, the end of space-time is even more widely accepted. Most of the theorists I spoke with struggled to think of colleagues in the quantum gravity community who would defend space-time as a fundamental ingredient of reality. However, some researchers are pursuing alternatives. I spoke at length with Latham Boyle about patterns in particle physics that have led him and his collaborators to the more conservative notion that space-time might come in two "sheets."
The various proposals under development are unlikely to see experimental tests this century, so a conclusive answer doesn't seem near. But if it were someday established that space-time does break down, what would that mean for us?
On a practical level, not much. Einstein's fabric of space-time is so sturdy that little short of a black hole would put a noticeable dent in it. But at a conceptual level, it's hard to imagine a more dramatic rethinking of reality. When Democritus suggested that matter emerges from tiny barbed "atoms" more than 2,000 years ago, he couldn't possibly have foreseen that parts of his proposal would ultimately be realized in the form of quantum theory — a framework asserting that reality is an ocean of overlapping waves of possibility that resolve into fixed objects only in certain situations.
If the void itself emerges from something, that something will be at least as alien. Just as individual molecules don't themselves have a well-defined notion of heat, the base level of reality could lack marquee features of our existence that we take for granted. Places. Times. The ability to influence only nearby objects. The requirement that causes precede effects. Physicists are already finding that these notions seem unlikely to be present in a more precise accounting of the world. They seem to be the approximate outputs of something stranger.
"One of the most spectacular aspects of these new findings is the emergence of causality can only happen in the approximate description," Elliott Gesteau, a quantum gravity researcher at the California Institute of Technology, told me over Zoom earlier this year. If there is gravity, he continued, "which is what we have in our world, then this causal structure is only approximate and must break down."
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