Isaac Newton was never entirely happy with his law of universal gravitation. For decades after publishing it in 1687, he sought to understand how, exactly, two objects were able to pull on each other from afar. He and others came up with several mechanical models, in which gravity was not a pull, but a push. For example, space might be filled with unseen particles that bombard the objects on all sides. The object on the left absorbs the particles coming from the left, the one on the right absorbs those coming from the right, and the net effect is to push them together.
Those theories never quite worked, and Albert Einstein eventually provided a deeper explanation of gravity as a distortion of space and time. But Einstein’s account, called general relativity, created its own puzzles, and he himself recognized that it could not be the final word. So the idea that gravity is a collective effect — not a fundamental force, but the outcome of swarm behavior on a finer scale — still compels physicists.
Earlier this year, a team of theoretical physicists put forward what might be considered a modern version of those 17th-century mechanical models. “There’s some kind of gas or some thermal system out there that we can’t see directly,” said Daniel Carney of Lawrence Berkeley National Laboratory, who led the effort. “But it’s randomly interacting with masses in some way, such that on average you see all the normal gravity things that you know about: The Earth orbits the sun, and so forth.”
This project is one of the many ways that physicists have sought to understand gravity, and perhaps the bendy space-time continuum itself, as emergent from deeper, more microscopic physics. Carney’s line of thinking, known as entropic gravity, pegs that deeper physics as essentially just the physics of heat. It says gravity results from the same random jiggling and mixing up of particles — and the attendant rise of entropy, loosely defined as disorder — that governs steam boilers, car engines and refrigerators.
Daniel Carney, a theoretical physicist at Lawrence Berkeley National Laboratory, spearheaded the latest attempt to explain gravity as an entropic force. The Regents of the University of California, Lawrence Berkeley National Laboratory
Attempts at modeling gravity as a consequence of rising entropy have cropped up now and again for several decades. Entropic gravity is very much a minority view. But it’s one that won’t die, and even detractors are loath to dismiss it altogether. The new model has the virtue of being experimentally testable — a rarity when it comes to theories about the mysterious underpinnings of the universal attraction.
A Force Emerges
What makes Einstein’s theory of gravity so remarkable is not just that it works (and does so with sublime mathematical beauty), but that it betrays its own incompleteness. General relativity predicts that stars can collapse to form black holes, and that, at the centers of these objects, gravity becomes infinitely strong. There, the space-time continuum tears open like an overloaded grocery bag, and the theory is unable to say what comes next. Furthermore, general relativity has uncanny parallels to heat physics, even though not a single thermal concept went into its development. It predicts that black holes only grow, never shrink, and only swallow, never disgorge. Such irreversibility is characteristic of the flow of heat. When heat flows, energy takes a more randomized or disordered form; once it does, it is unlikely to reorder itself spontaneously. Entropy quantifies this growth of disorder.
Indeed, when physicists used quantum mechanics to study what happens in the distorted space-time around a black hole, they find that black holes give off energy like any hot body. Because heat is the random motion of particles, these thermal effects suggest to many researchers that black holes, and the space-time continuum in general, actually consist of some kind of particles or other microscopic components.
Following the clues from black holes, physicists have pursued multiple approaches to understanding how space-time emerges from more microscopic components. The leading approach takes off from what’s known as the holographic principle. It says the emergence of space-time works a bit like an ordinary hologram. Just as a hologram evokes a sense of depth from a wavy pattern etched onto a flat surface, patterns in the microscopic components of the universe may give rise to another spatial dimension. This new dimension is curved, so that gravity arises organically.
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