Is Gravity Just Entropy Rising? Long-Shot Idea Gets Another Look

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.

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.

Entropic gravity, introduced in a famous 1995 paper by the theoretical physicist Ted Jacobson of the University of Maryland, takes a related but distinct tack. Previously, physicists had started with Einstein’s theory and derived its heatlike consequences. But Jacobson went the other way. He started from the assumption that space-time has thermal properties and used these to derive the equations of general relativity. His work confirmed that there’s something significant about the parallels between gravity and heat.

“He turned black hole thermodynamics on its head,” Carney said. “I’ve been mystified by this result for my entire adult life.”

Apparent Attraction

How might gravitational attraction arise out of more microscopic components? Inspired by Jacobson’s approach, Carney and his co-authors — Manthos Karydas, Thilo Scharnhorst, Roshni Singh and Jacob Taylor — put forward two models.

In the first, space is filled with a crystalline grid of quantum particles, or qubits. Each has an orientation, like a compass needle. These qubits will align themselves with a nearby object that possesses mass and exert a force on that object. “If you put a mass somewhere in the lattice, it causes all of the qubits nearby to get polarized — they all try to go in the same direction,” Carney said.

By reorienting the nearby qubits, a massive object creates a pocket of high order in the grid of otherwise randomly oriented qubits. If you place two masses into the lattice, you create two such pockets of order. High order means low entropy. But the system’s natural tendency is to maximize entropy. So, as the masses realign the qubits and the qubits in turn buffet the masses, the net effect will be to squash the masses closer together to contain the orderliness to a smaller region. It will appear that the two masses are attracting each other gravitationally when in fact the qubits are doing all the work. And just as Newton’s law dictates, the apparent attraction diminishes with the square of the distance between the masses.

The second model does away with the grid. Massive objects still reside within space and are acted upon by qubits, but now those qubits do not occupy any particular location and could in fact be far away. Carney said this feature is intended to capture the nonlocality of Newtonian gravity: Every object in the universe acts on every other object to some degree.

Each qubit in the model is able to store some energy; the amount depends on the distance between the masses. When they are far apart, a qubit’s energy capacity is high, so the total energy of the system can fit in just a few qubits. But if the masses are closer together, the energy capacity of each qubit drops, so the total energy has to be spread over more qubits. The latter situation corresponds to a higher entropy, so the natural tendency of the system is to push the masses together, again in keeping with Newtonian gravity.

Strengths and Weaknesses

Carney cautioned that both models are ad hoc. There’s no independent evidence for these qubits, and he and his colleagues had to fine-tune the strength and direction of the force exerted by them. One might ask whether this is any improvement over taking gravity to be fundamental. “It actually seems to require a peculiar engineered-looking interaction to get this to work,” Carney said.

And what works is just Newton’s law of gravity, not the full apparatus of Einstein’s theory, where gravity is equivalent to the curvature of space-time. For Carney, the models are just a proof of principle — a demonstration that it is at least possible for swarm behavior to explain gravitational attraction — rather than a realistic model for how the universe works. “The ontology of all of this is nebulous,” he said.

Mark Van Raamsdonk, a physicist at the University of British Columbia, is doubtful that the models really represent a proof of principle. A practitioner of holography, the leading approach to emergent space-time, Van Raamsdonk notes that the new entropic models don’t have any of the qualities that make gravity special, such as the fact that you feel no gravitational force when you’re freely falling through space-time. “Their construction doesn’t really have anything to do with gravity,” he said.

Furthermore, the models dwell on the one aspect of gravity that physicists think they already understand. Newton’s law arises naturally out of Einstein’s theory when gravity is comparatively feeble, as it is on Earth. It’s where gravity gets strong, as in black holes, that it gets weird, and the entropic model has nothing to say about that. “The real challenge in gravitational physics is understanding its strong-coupling, strong-field regime,” said Ramy Brustein, a theorist at Ben-Gurion University who said he used to be sympathetic to entropic gravity but has cooled on the idea.

Proponents of entropic gravity respond that physicists shouldn’t be so sure about how gravity behaves when it is weak. If gravity is indeed a collective effect of qubits, the Newtonian force law represents a statistical average, and the moment-by-moment effect will bounce around that average. “You have to go to very weak fields, because then these fluctuations might become observable,” said Erik Verlinde of the University of Amsterdam, who argued for entropic gravity in a much-discussed 2010 paper and has continued to develop the idea.

Testing Entropic Gravity

Carney thinks the main benefit of the new models is that they prompt conceptual questions about gravity and open up new experimental directions.

Suppose a massive body is in a quantum combination, or “superposition,” of being in two different locations. Will its gravitational field likewise be in a superposition, pulling on falling bodies in two different directions? The new entropic-gravity models predict that the qubits will act on the massive body to snap it out of its Schrödinger’s cat–like predicament.

This scenario connects to the much-fretted-over question of wave function collapse — which asks how it is that measuring a quantum system in superposition causes its multiple possible states to become a single definite state. Some physicists have suggested that this collapse is caused by some intrinsic randomness in the universe. These proposals differ in detail from Carney’s but have similar testable consequences. They predict that an isolated quantum system will eventually collapse of its own accord, even if it’s never measured or otherwise affected from without. “The same experimental setups could, in principle, be used to test both,” said Angelo Bassi of the University of Trieste, who has led the effort to perform such experiments, already ruling out some collapse models.

For all his doubts, Van Raamsdonk agrees that the entropic-gravity approach is worth a try. “Since it hasn’t been established that actual gravity in our universe arises holographically, it’s certainly valuable to explore other mechanisms by which gravity might arise,” he said. And if this long-shot theory does work out, physicists will need to update the artist Gerry Mooney’s famous gravity poster, which reads: “Gravity. It isn’t just a good idea. It’s the law.” Perhaps gravity is not, in fact, a law, just a statistical tendency.