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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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The equivalence principle is one of the corner stones of general relativity. Now physicists have used quantum mechanics to show how it fails.

The equivalence principle is one of the more fascinating ideas in modern science. It asserts that gravitational mass and inertial mass are identical. Einstein put it like this: the gravitational force we experience on Earth is identical to the force we would experience were we sitting in a spaceship accelerating at 1g. Newton might have said that the m in F=ma is the same as the ms in F=Gm1m2/r^2.

This seems eminently sensible. And yet it is no more than an assertion. Sure, we can measure the equivalence with ever increasing accuracy but there is nothing to stop us thinking that at some point the relationship will break down. Indeed several modifications to relativity predict that it will.

One important question is what quantum mechanics has to say on the matter. But physicists have so far been unable to use quantum theory as a lever to tease apart the behaviour of inertial and gravitational mass.

All that changes today with the extraordinary work of Endre Kajari at the University of Ulm in Germany and a few buddies. They show how it is possible to create situations in the quantum world in which the effects of inertial and gravitational mass must be different. In fact, they show that these differences can be arbitrarily large.

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