Sir Isaac Newton and his experience with the apple notwithstanding, gravity is something of an unsolved puzzle in physics. Newton’s theory described it as a force that masses exerted on one another. Einstein, in the theory of General Relativity, refined Newton’s view, and described gravity as a “warping” of the fabric of space-time by the presence of mass. But there are still things about gravity that are puzzling. It is by far the weakest of the fundamental physical forces, and it is the one force that does not fit the Standard Model of particle physics. ANd both Newton’s and Einstein’s theories of gravity are descriptive: they tell us how gravity works, but not why it exists in the first place.
According to an article in the New Scientist, a Dutch researcher, Erik Verlinde of the University of Amsterdam, has proposed a new way of looking at gravity that attempts to get at why gravity exists. (Dr. Verlinde’s paper is available as a pre-print [PDF] from the Physics arXiv.) In the paper, Verlinde “argues that gravitational attraction could be the result of the way information about material objects is organised in space”. He starts from a holographic description of space-time, originally developed by Stephen Hawking and Jacob Bekenstein to describe the properties of black holes.
Their work led to the insight that a hypothetical sphere could store all the necessary “bits” of information about the mass within. In the 1990s, [Gerard] ‘t Hooft and Leonard Susskind at Stanford University in California proposed that this framework might apply to the whole universe. Their “holographic principle” has proved useful in many fundamental theories.
Verlinde’s approach involves using the holographic principle to look at what happens when a small mass is moved in the vicinity of a larger mass. He shows that the movement changes the information contained in a hypothetical holographic surface between the two masses; this corresponds to a change in the energy of the system.
Then, using statistics to consider all possible movements of the small mass and the energy changes involved, Verlinde finds movements toward the bigger mass are thermodynamically more likely than others.
Using this as a starting point, and incorporating Einstein’s relationship between matter and energy, Verlinde is able to derive Newton’s law of gravity. And the extension to gravity as described by General Relativity is fairly straightforward.
The approach that Verlinde has described is very elegant, and seems promising, but a great deal of work will be needed to fill in all the blanks of the theory’s implications. As Dr. Verlinde says, it is not really even a theory yet, but more of a framework or paradigm, and “All the hard work comes now.”.
Still, this is fascinating stuff. Beginning with Einstein himself, physicists have been struggling to find a reconciliation between General Relativity and the Standard Model of particle physics. Both are elegant theories that are very successful at predicting experimental results. They’re just not consistent with each other. Finding a way of unifying them would be an enormous breakthrough.
Random Walks 