<p>But what if it’s all an illusion, a sort of cosmic frill, or a side effect of something else going on at deeper levels of reality?<br /><br />So says Erik Verlinde, 48, a respected string theorist and professor of physics at the University of Amsterdam, whose contention that gravity is indeed an illusion has caused a continuing ruckus among physicists, or at least among those who profess to understand it. Reversing the logic of 300 years of science, he argued in a recent paper, titled ‘On the Origin of Gravity and the Laws of Newton’, that gravity is a consequence of the venerable laws of thermodynamics, which describe the behaviour of heat and gases.<br />“For me gravity doesn’t exist,” said Dr Verlinde, who was recently in the United States to explain himself. Not that he can’t fall down, but Dr Verlinde is among a number of physicists who say that science has been looking at gravity the wrong way and that there is something more basic, from which gravity ‘emerges,’ the way stock markets emerge from the collective behaviour of individual investors or that elasticity emerges from the mechanics of atoms.<br /><br />Looking at gravity from this angle, they say, could shed light on some of the vexing cosmic issues of the day, like the dark energy, a kind of anti-gravity that seems to be speeding up the expansion of the universe, or the dark matter that is supposedly needed to hold galaxies together.<br /><br />Some of the best physicists in the world say they don’t understand Dr Verlinde’s paper, and many are outright sceptical. But some of those very same physicists say he has provided a fresh perspective on some of the deepest questions in science, namely why space, time and gravity exist at all — even if he has not yet answered them.<br /><br />Dr Verlinde is not an obvious candidate to go off the deep end. He and his brother Herman, a Princeton professor, are celebrated twins known more for their mastery of the mathematics of hard-core string theory than for philosophic flights.<br /><br />Born in Woudenberg, in the Netherlands, in 1962, the brothers got early inspiration from a pair of 1970s television shows about particle physics and black holes. “I was completely captured,” Dr Verlinde recalled. He and his brother obtained PhDs from the University of Utrecht together in 1988 and then went to Princeton, Erik to the Institute for Advanced Study and Herman to the university. After bouncing back and forth across the ocean, they got tenure at Princeton. And, they married and divorced sisters. Erik left Princeton for Amsterdam to be near his children.<br /><br />He made his first big splash as a graduate student when he invented Verlinde Algebra and the Verlinde formula, which are important in string theory, the so-called theory of everything, which posits that the world is made of tiny wriggling strings.<br /><br />You might wonder why a string theorist is interested in Newton’s equations. After all Newton was overturned a century ago by Einstein, who explained gravity as warps in the geometry of space-time, and who some theorists think could be overturned in turn by string theorists.<br /><br />Over the last 30 years gravity has been ‘undressed’, in Dr Verlinde’s words, as a fundamental force.<br /><br />Disrobing gravity<br />This disrobing began in the 1970s with the discovery by Jacob Bekenstein of the Hebrew University of Jerusalem and Stephen Hawking of Cambridge University, among others, of a mysterious connection between black holes and thermodynamics, culminating in Dr Hawking’s discovery in 1974 that when quantum effects are taken into account black holes would glow and eventually explode.<br /><br />Those exploding black holes (at least in theory — none has ever been observed) lit up a new strangeness of nature. Black holes, in effect, are holograms — like the 3-D images you see on bank cards. All the information about what has been lost inside them is encoded on their surfaces. Physicists have been wondering ever since how this ‘holographic principle’ — that we are all maybe just shadows on a distant wall — applies to the universe and where it came from.<br /><br />Lee Smolin, a quantum gravity theorist at the Perimeter Institute for Theoretical Physics, called Dr Jacobson’s paper “one of the most important papers of the last 20 years.”<br />But it received little attention at first, said Thanu Padmanabhan of the Inter-University Centre for Astronomy and Astrophysics in Pune, India, who has taken up the subject of ‘emergent gravity’ in several papers over the last few years. Dr Padmanabhan said that the connection to thermodynamics went deeper that just Einstein’s equations to other theories of gravity. “Gravity,” he said recently in a talk at the Perimeter Institute, “is the thermodynamic limit of the statistical mechanics of “atoms of space-time.”<br /><br />Dr Verlinde said he had read Dr Jacobson’s paper many times over the years but that nobody seemed to have gotten the message. People were still talking about gravity as a fundamental force. “Clearly we have to take these analogies seriously, but somehow no one does,” he complained.<br /><br />His paper, posted to the physics archive in January, resembles Dr Jacobson’s in many ways, but Dr Verlinde bristles when people say he has added nothing new to Dr Jacobson’s analysis. What is new, he said, is the idea that differences in entropy can be the driving mechanism behind gravity, that gravity is, as he puts it an ‘entropic force’.<br />As a metaphor for how this would work, Dr Verlinde used the example of a polymer — a strand of DNA, say, a noodle or a hair — curling up. “It took me two months to understand polymers,” he said. The resulting paper, as Dr Verlinde himself admits, is a little vague.<br /><br />“This is not the basis of a theory,” Dr Verlinde explained. “I don’t pretend this to be a theory. People should read the words I am saying opposed to the details of equations.”<br />Dr Padmanabhan said that he could see little difference between Dr Verlinde’s and Dr Jacobson’s papers and that the new element of an entropic force lacked mathematical rigor. “I doubt whether these ideas will stand the test of time,” he wrote in an e-mail message from India. Dr Jacobson said he couldn’t make sense of it.<br /><br />The Verlinde brothers are now trying to recast these ideas in more technical terms of string theory, and Erik has been on the road a bit, travelling in May to the Perimeter Institute and Stony Brook University on Long Island, stumping for the end of gravity. Michael Douglas, a professor at Stony Brook, described Dr Verlinde’s work as “a set of ideas that resonates with the community”, adding, “everyone is waiting to see if this can be made more precise.”<br /><br />Until then the jury of Dr Verlinde’s peers will still be out.<br /></p>
<p>But what if it’s all an illusion, a sort of cosmic frill, or a side effect of something else going on at deeper levels of reality?<br /><br />So says Erik Verlinde, 48, a respected string theorist and professor of physics at the University of Amsterdam, whose contention that gravity is indeed an illusion has caused a continuing ruckus among physicists, or at least among those who profess to understand it. Reversing the logic of 300 years of science, he argued in a recent paper, titled ‘On the Origin of Gravity and the Laws of Newton’, that gravity is a consequence of the venerable laws of thermodynamics, which describe the behaviour of heat and gases.<br />“For me gravity doesn’t exist,” said Dr Verlinde, who was recently in the United States to explain himself. Not that he can’t fall down, but Dr Verlinde is among a number of physicists who say that science has been looking at gravity the wrong way and that there is something more basic, from which gravity ‘emerges,’ the way stock markets emerge from the collective behaviour of individual investors or that elasticity emerges from the mechanics of atoms.<br /><br />Looking at gravity from this angle, they say, could shed light on some of the vexing cosmic issues of the day, like the dark energy, a kind of anti-gravity that seems to be speeding up the expansion of the universe, or the dark matter that is supposedly needed to hold galaxies together.<br /><br />Some of the best physicists in the world say they don’t understand Dr Verlinde’s paper, and many are outright sceptical. But some of those very same physicists say he has provided a fresh perspective on some of the deepest questions in science, namely why space, time and gravity exist at all — even if he has not yet answered them.<br /><br />Dr Verlinde is not an obvious candidate to go off the deep end. He and his brother Herman, a Princeton professor, are celebrated twins known more for their mastery of the mathematics of hard-core string theory than for philosophic flights.<br /><br />Born in Woudenberg, in the Netherlands, in 1962, the brothers got early inspiration from a pair of 1970s television shows about particle physics and black holes. “I was completely captured,” Dr Verlinde recalled. He and his brother obtained PhDs from the University of Utrecht together in 1988 and then went to Princeton, Erik to the Institute for Advanced Study and Herman to the university. After bouncing back and forth across the ocean, they got tenure at Princeton. And, they married and divorced sisters. Erik left Princeton for Amsterdam to be near his children.<br /><br />He made his first big splash as a graduate student when he invented Verlinde Algebra and the Verlinde formula, which are important in string theory, the so-called theory of everything, which posits that the world is made of tiny wriggling strings.<br /><br />You might wonder why a string theorist is interested in Newton’s equations. After all Newton was overturned a century ago by Einstein, who explained gravity as warps in the geometry of space-time, and who some theorists think could be overturned in turn by string theorists.<br /><br />Over the last 30 years gravity has been ‘undressed’, in Dr Verlinde’s words, as a fundamental force.<br /><br />Disrobing gravity<br />This disrobing began in the 1970s with the discovery by Jacob Bekenstein of the Hebrew University of Jerusalem and Stephen Hawking of Cambridge University, among others, of a mysterious connection between black holes and thermodynamics, culminating in Dr Hawking’s discovery in 1974 that when quantum effects are taken into account black holes would glow and eventually explode.<br /><br />Those exploding black holes (at least in theory — none has ever been observed) lit up a new strangeness of nature. Black holes, in effect, are holograms — like the 3-D images you see on bank cards. All the information about what has been lost inside them is encoded on their surfaces. Physicists have been wondering ever since how this ‘holographic principle’ — that we are all maybe just shadows on a distant wall — applies to the universe and where it came from.<br /><br />Lee Smolin, a quantum gravity theorist at the Perimeter Institute for Theoretical Physics, called Dr Jacobson’s paper “one of the most important papers of the last 20 years.”<br />But it received little attention at first, said Thanu Padmanabhan of the Inter-University Centre for Astronomy and Astrophysics in Pune, India, who has taken up the subject of ‘emergent gravity’ in several papers over the last few years. Dr Padmanabhan said that the connection to thermodynamics went deeper that just Einstein’s equations to other theories of gravity. “Gravity,” he said recently in a talk at the Perimeter Institute, “is the thermodynamic limit of the statistical mechanics of “atoms of space-time.”<br /><br />Dr Verlinde said he had read Dr Jacobson’s paper many times over the years but that nobody seemed to have gotten the message. People were still talking about gravity as a fundamental force. “Clearly we have to take these analogies seriously, but somehow no one does,” he complained.<br /><br />His paper, posted to the physics archive in January, resembles Dr Jacobson’s in many ways, but Dr Verlinde bristles when people say he has added nothing new to Dr Jacobson’s analysis. What is new, he said, is the idea that differences in entropy can be the driving mechanism behind gravity, that gravity is, as he puts it an ‘entropic force’.<br />As a metaphor for how this would work, Dr Verlinde used the example of a polymer — a strand of DNA, say, a noodle or a hair — curling up. “It took me two months to understand polymers,” he said. The resulting paper, as Dr Verlinde himself admits, is a little vague.<br /><br />“This is not the basis of a theory,” Dr Verlinde explained. “I don’t pretend this to be a theory. People should read the words I am saying opposed to the details of equations.”<br />Dr Padmanabhan said that he could see little difference between Dr Verlinde’s and Dr Jacobson’s papers and that the new element of an entropic force lacked mathematical rigor. “I doubt whether these ideas will stand the test of time,” he wrote in an e-mail message from India. Dr Jacobson said he couldn’t make sense of it.<br /><br />The Verlinde brothers are now trying to recast these ideas in more technical terms of string theory, and Erik has been on the road a bit, travelling in May to the Perimeter Institute and Stony Brook University on Long Island, stumping for the end of gravity. Michael Douglas, a professor at Stony Brook, described Dr Verlinde’s work as “a set of ideas that resonates with the community”, adding, “everyone is waiting to see if this can be made more precise.”<br /><br />Until then the jury of Dr Verlinde’s peers will still be out.<br /></p>