So says Erik Verlinde, 48, a respected string theorist and professor ofphysics at the University of Amsterdam, whose contention that gravity isindeed an illusion has caused a continuing ruckus among physicists, orat least among those who profess to understand it. Reversing the logicof 300 years of science, he argued in a recent paper, titled “On the Origin of Gravity and theLaws of Newton,” that gravity is a consequence of the venerable lawsof thermodynamics, which describe the behavior of heat and gases.
“For me gravity doesn’t exist,” said Dr. Verlinde, who was recently inthe United States to explain himself. Not that he can’t fall down, butDr. Verlinde is among a number of physicists who say that science hasbeen looking at gravity the wrong way and that there is something morebasic, from which gravity “emerges,” the way stock markets emerge fromthe collective behavior of individual investors or that elasticityemerges from the mechanics of atoms.
Looking at gravity from this angle, they say, could shed light on someof the vexing cosmic issues of the day, like the dark energy, a kind ofanti-gravity that seems to be speeding up the expansion of the universe,or the dark matter that is supposedlyneeded to hold galaxies together.
Dr. Verlinde’s argument turns on something you could call the “bad hairday” theory of gravity.
It goes something like this: your hair frizzles in the heat andhumidity, because there are more ways for your hair to be curled than tobe straight, and nature likes options. So it takes a force to pull hairstraight and eliminate nature’s options. Forget curved space or thespooky attraction at a distance described by Isaac Newton’sequations well enough to let us navigate the rings of Saturn, the forcewe call gravity is simply a byproduct of nature’s propensity tomaximize disorder.
Some of the best physicists in the world say they don’t understand Dr.Verlinde’s paper, and many are outright skeptical. But some of thosevery same physicists say he has provided a fresh perspective on some ofthe deepest questions in science, namely why space, time and gravityexist at all — even if he has not yet answered them.
“Some people have said it can’t be right, others that it’s right and wealready knew it — that it’s right and profound, right and trivial,”Andrew Strominger, a string theorist at Harvardsaid.
“What you have to say,” he went on, “is that it has inspired a lot ofinteresting discussions. It’s just a very interesting collection ofideas that touch on things we most profoundly do not understand aboutour universe. That’s why I liked it.”
Dr. Verlinde is not an obvious candidate to go off the deep end. He andhis brother Herman, a Princeton professor, are celebrated twins known more for their mastery of themathematics of hard-core string theory than for philosophic flights.
Born in Woudenberg, in the Netherlands, in 1962, the brothers got earlyinspiration from a pair of 1970s television shows about particle physicsand black holes. “I was completely captured,” Dr. Verlinde recalled. Heand his brother obtained Ph.D’s from the University of Utrecht togetherin 1988 and then went to Princeton, Erik to the Institute for AdvancedStudy and Herman to the university. After bouncing back and forth acrossthe ocean, they got tenure at Princeton. And, they married and divorcedsisters. Erik left Princeton for Amsterdam to be near his children.
He made his first big splash as a graduate student when he inventedVerlinde Algebra and the Verlinde formula, which are important in stringtheory, the so-called theory of everything, which posits that the worldis made of tiny wriggling strings.
You might wonder why a string theorist is interested in Newton’sequations. After all Newton was overturned a century ago by Einstein,who explained gravity as warps in the geometry of space-time, and whosome theorists think could be overturned in turn by string theorists.
Over the last 30 years gravity has been “undressed,” in Dr. Verlinde’swords, as a fundamental force.
This disrobing began in the 1970s with the discovery by Jacob Bekensteinof the Hebrew University of Jerusalem and StephenHawking of CambridgeUniversity, among others, of a mysterious connection between blackholes and thermodynamics, culminating in Dr. Hawking’s discovery in 1974that when quantum effects are taken into account black holes would glowand eventually explode.
In a provocative calculation in 1995, Ted Jacobson, a theorist from theUniversityof Maryland, showed that given a few of these holographic ideas,Einstein’s equations of general relativity are just a another way ofstating the laws of thermodynamics.
Those exploding black holes (at least in theory — none has ever beenobserved) lit up a new strangeness of nature. Black holes, in effect,are holograms — like the 3-D images you see on bank cards. All theinformation about what has been lost inside them is encoded on theirsurfaces. Physicists have been wondering ever since how this“holographic principle” — that we are Photo:AFLOAT The astrophysicist Stephen Hawking goesweightless in a special jet. all maybe just shadows on adistant wall — applies to the universe and where it came from.
In one striking example of a holographic universe, Juan Maldacena of the Institute for Advanced Study constructed a mathematical model of a“soup can” universe, where what happened inside the can, includinggravity, is encoded in the label on the outside of the can, where therewas no gravity, as well as one less spatial dimension. If dimensionsdon’t matter and gravity doesn’t matter, how real can they be?
Lee Smolin, a quantum gravity theorist at the Perimeter Institute for Theoretical Physics, called Dr. Jacobson’s paper “one of the mostimportant papers of the last 20 years.”
But it received little attention at first, said Thanu Padmanabhan of theInter-University Center for Astronomy and Astrophysics in Pune, India,who has taken up the subject of “emergent gravity” in several papersover the last few years. Dr. Padmanabhan said that the connection tothermodynamics went deeper that just Einstein’s equations to othertheories of gravity.“Gravity,”he said recently in a talk at the Perimeter Institute, “is thethermodynamic limit of the statistical mechanics of “atoms ofspace-time.”
Dr. Verlinde said he had read Dr. Jacobson’s paper many times over theyears but that nobody seemed to have gotten the message. People werestill talking about gravity as a fundamental force. “Clearly we have totake these analogies seriously, but somehow no one does,” he complained.
His paper, posted to the physics archive in January, resembles Dr.Jacobson’s in many ways, but Dr. Verlinde bristles when people say hehas added nothing new to Dr. Jacobson’s analysis. What is new, he said,is the idea that differences in entropy can be the driving mechanismbehind gravity, that gravity is, as he puts it an “entropic force.”
That inspiration came to him courtesy of a thief.
As he was about to go home from a vacation in the south of France lastsummer, a thief broke into his room and stole his laptop, his keys, hispassport, everything. “I had to stay a week longer,” he said, “I gotthis idea.”
Up the beach, his brother got a series of e-mail messages first sayingthat he had to stay longer, then that he had a new idea and finally, onthe third day, that he knew how to derive Newton’s laws from firstprinciples, at which point Herman recalled thinking, “What’s going onhere? What has he been drinking?”
When they talked the next day it all made more sense, at least toHerman. “It’s interesting,” Herman said, “how having to change plans canlead to different thoughts.”
Think of the universe as a box of scrabble letters. There is only oneway to have the letters arranged to spell out the Gettysburg Address,but an astronomical number of ways to have them spell nonsense. Shakethe box and it will tend toward nonsense, disorder will increase andinformation will be lost as the letters shuffle toward their mostprobable configurations. Could this be gravity?
As a metaphor for how this would work, Dr. Verlinde used the example of apolymer — 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.
“This is not the basis of a theory,” Dr. Verlinde explained. “I don’tpretend this to be a theory. People should read the words I am sayingopposed to the details of equations.”
Dr. Padmanabhan said that he could see little difference between Dr.Verlinde’s and Dr. Jacobson’s papers and that the new element of anentropic force lacked mathematical rigor. “I doubt whether these ideaswill stand the test of time,” he wrote in an e-mail message from India.Dr. Jacobson said he couldn’t make sense of it.
John Schwarz of the California Institute of Technology, one of thefathers of string theory, said the paper was “very provocative.” Dr.Smolin called it, “very interesting and also very incomplete.”
At a workshop in Texas in the spring, Raphael Bousso of the University of California, Berkeley, was asked tolead a discussion on the paper.
“The end result was that everyone else didn’t understand it either,including people who initially thought that did make some sense tothem,” he said in an e-mail message.
“In any case, Erik’s paper has drawn attention to what is genuinely adeep and important question, and that’s a good thing,” Dr. Bousso wenton, “I just don’t think we know any better how this actually works afterErik’s paper. There are a lot of follow-up papers, but unlike Erik,they don’t even understand the problem.”
The Verlinde brothers are now trying to recast these ideas in moretechnical terms of string theory, and Erik has been on the road a bit,traveling in May to the Perimeter Institute and Stony Brook University on Long Island, stumpingfor the end of gravity. Michael Douglas, a professor at Stony Brook,described Dr. Verlinde’s work as “a set of ideas that resonates with thecommunity, adding, “everyone is waiting to see if this can be made moreprecise.”
Until then the jury of Dr. Verlinde’s peers will still be out.
Over lunch in New York, Dr. Verlinde ruminated over his experiences ofthe last six months. He said he had simply surrendered to his intuition.“When this idea came to me, I was really excited and euphoric even,”Dr. Verlinde said. “It’s not often you get a chance to say something newabout Newton’s laws. I don’t see immediately that I am wrong. That’senough to go ahead.”
He said friends had encouraged him to stick his neck out and that he hadno regrets. “If I am proven wrong, something has been learned anyway.Ignoring it would have been the worst thing.”
The next day Dr. Verlinde gave a more technical talk to a bunch ofphysicists in the city. He recalled that someone had told him the otherday that the unfolding story of gravity was like the emperor’s newclothes.
“We’ve known for a long time gravity doesn’t exist,” Dr. Verlinde said,“It’s time to yell it.”
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