It was whilst lying on the beach during a holiday in France last summer that Erik Verlinde, professor of Theoretical Physics at the UvA, had his ‘aha moment'. Four months later, in a lecture delivered at the Spinoza Institute in Utrecht in December 2009, Verlinde introduced his new perspective on gravity. One month after that saw the publication of his article ‘On the Origin of Gravity and the Laws of Newton'.

Verlinde's ideas did not go unnoticed. On the contrary: Verlinde featured in national and international newspapers (including the New York Times; see reference below) and magazines, and was a hot topic on Web forums. His article also captured the attention of fellow scientists around the world. In it, the UvA professor lays the groundwork for a new way of thinking about gravity. Whereas Isaac Newton only told us that the apple fell from the tree, Verlinde has taken the first step towards finding out why.

‘Newton and Einstein developed theories capable of describing gravity. For years, scientists took these ideas as their starting point. But Newton and Einstein don't give us a clue as to what the processes underlying gravity might be. What I've done is to lay the basis for looking at gravity from a new angle. My approach to gravity is to describe the underlying processes in the way you normally would in thermodynamics', explains Verlinde.

## Information as a building block

An expert in string theory, Verlinde bases his approach on the idea that information about space and matter - and matter's position in space - is what forms the building blocks of gravity. We can imagine that information as bits - zeros and ones - distributed over the surface of a sphere with matter inside. What is important is not the information itself, but how much of it there is. It would appear that whenever another mass approaches, the amount of information changes. Since it takes energy to effect that change, the process results in a force. How strong that force is depends on the magnitude of the energy change accompanying the small shifts in the masses. Working out those shifts is fairly straightforward: just divide the energy - which is equal to the mass, according to Einstein - equally amongst all the bits occupying the surface. A few more simple calculations and you end up with the familiar law of gravity as posited by Newton.

For his theory, Verlinde looks beyond gravity. In fact, he has set his sights on developing a ‘theory of everything'. He even wants to re-examine string theory. ‘What I want is to develop a new starting point for string theory. String theory is based on the idea that the four fundamental forces in physics (strong and weak nuclear force, electromagnetic force and gravity) each relate to a specific particle that corresponds to a vibrating string. In the case of gravity, that would be a graviton. But, in fact, most string theoreticians have already recognised that we'll have to let go of this notion of vibrating strings eventually and re-examine the origins of those fundamental forces. So I've started back at the beginning and am trying to draw conclusions from there.'

## From idea to bona fide theory

Verlinde acknowledges that his idea still has a long way to go before it can hope to call itself a bona fide theory. ‘At the beginning of the twentieth century, Einstein had a brain wave about gravity, but it was not until many years later that he had built that idea up into his General Theory of Relativity. With quantum mechanics it's the same story. Planck took the first step in an article in 1900, but the theory wasn't fully developed until 1926. That's how I want people to treat my ideas about gravity. This article lays the groundwork. How long it will ultimately take for it to develop into a theory is impossible for me to say. See it as a long-term process.'Since publishing his article, Verlinde's approach to science has changed. He has thrown off the yoke, as it were, of existing knowledge. ‘I've learned to pay closer attention to my intuition. In string theory, mathematics plays an essential role. But in recent years there has been an increasing tendency to assume that articles which don't include complex mathematical formulas aren't any good. Thinking otherwise was simply ‘not done'. In explaining my own notion of gravity, I therefore consciously chose to use very basic mathematical formulas. Doing that emphasises the underlying physics.'

Verlinde even went so far as to keep the formulas he used in his article as simple as possible - an approach also encouraged by Robbert Dijkgraaf. ‘These are formulas you learn in secondary school. Their beauty lies in their very simplicity. But it was precisely that raw purity that I was after.'

Meanwhile, the UvA professor is working on his second article. Questions that the first article purposefully left unanswered will be resolved in those to come, thus slowly but steadily fleshing out a theory. Amongst the questions addressed in his second article is whether his theory might point to the existence of other forces. He also plans to establish whether his idea can coexist with quantum mechanics.

## Varied response

Verlinde's article has elicited a flood of reactions, both positive and critical. One of his biggest supporters is University Professor and President of the Royal Netherlands Academy of Arts and Sciences (KNAW) Robbert Dijkgraaf. Nobel prize winner Gerard 't Hooft has also expressed interest in Verlinde's ideas.

The responses have also included more than 95 follow-up articles published by scientists around the world. Verlinde himself couldn't be happier with the flurry of debate surrounding his idea, although he is not always equally enthusiastic about their quality. ‘Only a handful of scientists display a real grasp of what my article is about. The majority of articles that we've read continue to apply known concepts, like that of gravity. Clearly, many scientists are not yet prepared to admit the possibility of looking at such concepts from a different angle.'