A New View of Gravity

Entropy and information may be crucial concepts for explaining roots of familiar force

Explaining gravity to a small child is simple: All you have to say is, what goes up must come down.

Until the kid asks why.

What can you say? It’s just the way things work. All masses attract each other. Maybe to bright middle schoolers you could explain that spacetime is warped by mass. Or, to high schoolers, you could say that without gravity, the laws of physics would differ for people moving at changing velocities. Yet all those increasingly sophisticated answers merely invite another “Why.” As Sir Isaac Newton himself replied in response to similar questions, “hypotheses non fingo.” Which roughly translates as “I don’t have a clue.”

That such a simple question, about so common a phenomenon, has defied a direct answer for centuries might explain why the physics world has been atwitter lately over a novel attempt to resolve the riddle. A flurry of recent papers have examined this new idea, which mixes principles from string theory and black hole physics with basic old-fashioned thermo­dynamics. If this notion is right, gravity turns out to be a special sort of entropy, a result of the same physics that drives matter to give up its organization and order as it succumbs to the laws of probability. Toss in a dash of quantum mechanics and a pinch of information theory, and the universe emerges, governed on a grand scale by pretty much the same principles underlying the elastic pull of a rubber band.

While similar ideas have been suggested before, nobody has expressed the gravity-as-entropy story as intriguingly as theorist Erik Verlinde of the University of Amsterdam in an online paper (arXiv.org/abs/1001.0785v1) that appeared in January. Titled simply “On the origin of gravity and the laws of Newton,” Verlinde’s paper cooks up a mathematical pièce de résistance connecting gravity to thermodynamics. His ingredients include the law of entropy, the physics of black holes and some speculative conjectures on how space stores information about the matter and energy within it. His recipe replicates Newton’s law of gravitational attraction, and then with some additional mathematical seasoning he arrives at Einstein’s general relativity, the modern and undefeated champion of gravity theories. Verlinde’s analysis indicates that gravity emerges from physical dynamics analogous to basic thermodynamic processes. “Using only … concepts like energy, entropy and temperature,” he writes, “Newton’s laws appear naturally and practically unavoidably.”

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