The Myth of Gravity

When thieves stole Erik Verlinde’s laptop and keys, while he was holidaying in the south of France, they could have had little notion that their crime would lead to a new model for gravity. But forced into taking an extra week’s vacation time, Verlinde began to ponder whether gravity might not be a fundamental force of nature, arising instead from thermodynamics. His ideas could give the controversial loop quantum gravity theory—in which spacetime is made up of quantum threads—a boost, and help explain the accelerated expansion of the universe.

Gravity may be the force that we are most familiar with in everyday life, but physicists do not yet understand its origin. Newton told us that apples fall towards Earth with an acceleration that depends on the Earth’s mass, the apple’s mass, and its distance from the centre of the Earth, while Einstein described gravity by the warping of the fabric of spacetime. But while these theories describe how gravity works, they don’t explain how it arises.

Verlinde, a string theorist at the University of Amsterdam in the Netherlands, believes that the key to understanding gravity is "information." He was inspired by early work on information storage in black holes by Stephen Hawking and Nobel laureate Gerard ’t Hooft. "When I was about fifteen I saw them on television talking about the physics of elementary particles and black holes," says Verlinde. "I knew then that I wanted to work in that area."

The Television Event Horizon

Hawking and ’t Hooft had both worked on the so-called holographic principle, which relates the information content—or entropy—of a black hole to the surface area of its event horizon, the hypothetical sphere around the black hole where gravity becomes so strong even light can’t escape. It’s as if the horizon is a spherical television screen with all the information about the three-dimensional volume within encoded on the pixels on its surface. Verlinde has shown that by combining the holographic principle with the thermodynamic quantities of heat and mechanical work, it’s relatively straightforward to derive Newton’s classical equation of gravity. (See "Decoding Entropic Gravity" for more details.)

The work has been causing a stir amongst physicists. "Verlinde’s paper is remarkable in that we all felt so stupid for not having seen it before," says FQXi’s Lee Smolin of the Perimeter Institute, Ontario. "The mathematics involved is just high school algebra."

It might sound like re-inventing the wheel, but the approach implies that gravity is nothing more than the result of a system maximising its entropy, or disorder. At first glance, this looks like bad news for the quantum gravity crowd. If gravity is an "entropic force," there is no longer a need for physicists to attempt to reconcile general relativity with quantum mechanics, or hunt for the hypothetical graviton (the particle posited to carry the gravitational force just as photons mediate the electromagnetic force), says Paul Frampton, at the University of Tokyo in Japan. Rather, all we need to explain the interactions of particles is the Standard Model of particle physics and entropy. "It means that everyone looking into quantum gravity is misguided," says Frampton.

Quantum Threads

However, not all gravity researchers take that view. Smolin, a long term proponent of loop quantum gravity (LQG), believes that Verlinde’s work is not only compatible with LQG, it could even help to explain how familiar Newtonian gravity might emerge in this picture. According to LQG, spacetime isn’t the smooth fabric that Einstein envisioned; rather, if you zoom down to scales of 10^-33 cm, the fabric turns out to be woven from quantum threads. The key point for Smolin is that the holographic principle is also valid in this framework, allowing him to apply a version of Verlinde’s argument to demonstrate directly for the first time that loop quantum gravity has a limit that yields Newtonian gravity.

Smolin notes that Verlinde’s model is tied to earlier work by FQXi member Ted Jacobson, who had shown in 1995 that Einstein’s equations of general relativity could be derived using thermodynamics and the holographic principle. "The wonderful thing about the arguments of Jacobson and Verlinde is they give a deep reason for why a quantum theory of gravity should yield the phenomena of gravitation," Smolin writes in his recent paper (arXiv:1001.3668v2).

Frampton and colleagues Damien Easson and Nobel Laureate George Smoot have been looking at possible observable consequences of Verlinde’s entropic force. So far, cosmologists have struggled to explain why the expansion of the universe is accelerating using just standard general relativity. Instead, they attribute the acceleration to some mysterious "dark energy." To find a possible alternative to dark energy, Smoot’s team considered a spherical screen that lies on the apparent horizon of the universe, where distant objects recede at the speed of light. As information is sucked out across the horizon, the area of the screen grows, which, according to the holographic principle, increases the entropy of the universe. This gives rise to an entropic force that could explain the acceleration, "derived as a response to various microscopic fundamental forces such as electromagnetism," says Easson, at Arizona State University, Tempe. However, Easson adds that the work is "extremely speculative" at this stage (arXiv:1002.4278v2).

If such derivations of dark energy stand up then Verlinde’s ideas "could in some sense complete general relativity," says physicist Sabine Hossenfelder at the Nordic Institute of Theoretical Physics in Sweden. However, there is still a long way to go before physicists will abandon the notion that gravity is a real force as there are several things that remain vague in Verlinde’s formulation, she adds.

Frampton, however, is convinced that Verlinde is on the right track. "I believe that gravity is entirely explained by increases in entropy; there isn’t a fundamental gravitational interaction," he says. "That’s the bottom line. Is that crazy enough?"