Back in 2010, I posted a couple of articles about a new approach to gravity, proposed by Dr. Erik Verlinde of the University of Amsterdam, which attempts to explain why gravity exists. That there is a question about this may come as something of a surprise; but both Newton’s theory of gravity, which is good enough to send a spacecraft to Mars, and Einstein’s theory of General Relativity, which models gravity as a “warping” of four-dimensional space-time, are basically descriptive. Gravity still presents some puzzles. It is by far the weakest of the four fundamental forces (the others are electromagnetic force, and the strong and weak nuclear forces); gravity is also the one force that is not consistent with the standard model of particle physics. So far, no one has been able to reconcile General Relativity with quantum mechanics, even though both theories are very successful at predicting physical phenomena, General Relativity on the very large scale, and quantum mechanics on the very small. Dr. Verlinde’s proposal suggested that gravity might be an emergent property caused by the increasing entropy of the universe.
He [Verlinde]suggested that gravity is merely a manifestation of entropy in the Universe, which always increases according to the second law of thermodynamics. This causes matter to distribute itself in a way that maximises entropy. And the effect of this redistribution looks like a force which we call gravity.
A major attraction of this idea is that it might provide a means of reconciling quantum mechanics with General Relativity. Now, according to an article in the “Physics arXiv” blog at Technology Review, Archil Kobakhidze at The University of Melbourne in Australia has argued that existing experimental data are not consistent with Verlinde’s emergent gravity hypothesis. He argues that, because emergent gravity requires that each particle be acted on by a large number of other particles, it has particular implications for the force experienced by each particle, and that those implications differ from those resulting from the traditional view of gravity.
In other words, the emergent and traditional views of gravity make different predictions about the gravitational force a quantum particle ought to experience. And that opens the way for an experimental test.
As it happens, several recent experiments have measured the gravitational force on neutrons, and Dr. Kobakhidze says that these measurements show that the emergent hypothesis is wrong.
“Experiments on gravitational bound states of neutrons unambiguously disprove the entropic origin of gravitation,” he says.
Kobakhidze’s paper, which is quite technical, is available on the arXiv.org site: [abstract, with link to PDF download].
At least for the present, gravity seems set to remain more than a bit mysterious.