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SCIENCE AND TECHNOLOGY

Newton discovered it. Einstein complicated it. But nobody really understandsthe force of gravity. Part of the explanation may be that it is not really all here

A matter of gravity

NOT many people think that a smallmagnet is performing a remarkablefeat when it grabs a nail off a table. Nima Ar­kani-Hamed, on the other hand, does. Thenail, he points out, has the entire mass of theearth tugging down on it through gravity, butthis still cannot overcome the force of the

magnet. Why is gravity so miserably weak?This is a question that has puzzled phys­

icists for decades. But two recent papers inPhysical Review Letters, by Lisa Randall ofPrinceton University and Raman Sundrumof Stanford University, suggest an answer.They build on an idea proposed earlier thisyear by Dr Arkani-Hamed, who works at theUniversity of California, Berkeley, and twoof his colleagues: Savas Dimopoulos of Stan­ford, and Gia Dvali, of New York University.Together, all these physicists believethat the reason gravity is such aweak force in the universe is that it

does not actually spend much of itstime here.

Hide and seekIn the traditional way oflooking atthings, gravity is one of four funda­mental forces that hold the universe

together. The other three are thestrong nuclear force, which bindsthe particles in atomic nuclei; theweak nuclear force, which is re­sponsible for some sorts of radioac­tive decay; and electromagnetism.

All three of these other forces

are much more powerful than grav­ity. But it was not always so. Mostphysicists believe that, at the timeofthe BigBang, when the universe be­gan, all four forces were symmetri­cal, and thus of equal strength. Ac­cording to this idea, the differentsorts of sub-atomic particle in theearly universe were also symmetri­cal with each other. Soon, however,the elegant symmetry of everythingwith everything else began to breakdown. The different sorts of particleand force adopted their modern na­tures, and gravity dwindled to apale shadow of its former self.

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This theory would be convincingenough were it not for a rather awkward re­quirement. For gravity to have dwindled as itdid, the starting conditions for the universehad to be exactly what they actually turnedout to be. Even a minuscule deviation in cer­

tain values, such as the mass of an exotic par­ticle called the Higgs boson (which bestowsmass on other, more ordinary particles),would have meant that gravity could nothave weakened as it did. The result would

have been a universe in which stars, planets,human beings and so on could never havecome into existence.

Dr Arkani-Hamed describes these high­ly constrained starting conditions as requir­ing the universe to be like a pencil balancingon its point-possible in theory, bW wildly

improbable in practice. He likens previousattempts to explain the so-called hierarchyproblem (why gravity is so much weakerthan the other three forces) to the creation ofa hand designed to hold the pencil up.

Instead, he and his collaborators pro­pose a different explanation. Rather than cir­cumventing the hierarchy problem, theypropose to abolish it entirely. In their view,the problem does not exist. The weakness ofgravity is an illusion. It actually remains justas strong as it ever was, but not all of itsstrength is exercised in the perceptible uni­verse. Rather-and in contrast to the other

three forces-gravity frequently operates intwo or more extra dimensions beyond thecommonplace four (the three of distance,plus time). And the longer it spends in theseother dimensions, the weaker are its effectsin the dimensions inhabited by people.

Lostin spaceIntroducing extra dimensions to account forgravity's weakness may sound loopy, butthere is a good precedent. One of the best ex­planations of why the universe is the way itis-so-called string theory-requires that theuniverse be, in fact, ten-dimensional.

In string theory, the forces and particlesof which the universe is now com­

posed are actually vibrations of tinystrings made from these ten dimen­sions (six of which are confined tosuch strings, and thus are not per­ceivable in the human-scale world).Some strings have ends. Vibrationsin these correspond to the strong,weak and electromagnetic forces,and particles that interact throughthose forces. Others are closed

loops. Vibrations in these corre­spond to gravity.

Another consequence of stringtheory is that with the addition ofan 11th dimension the universe canbe divided into so-called mem­

branes. These are regions with few­er dimensions than the space sur­rounding them. (A familiar analogymight be with a wall, which is a two­dimensional thing in an otherwisethree-dimensional room.)

Electromagnetism, and also thestrong and weak nuclear forces, areconfined to their mem branes, andthus to this universe. That isbecause

the ends of the strings of which theyare composed tend to "stick" to themembrane in question. But gravita­tional strings have no sticky endsand can wander freely off into DrArkani-Hamed's extra dimensions,

THE ECONOMIST DECEMBER 23RD 1999

SCIENCE AND TECHNOLOGY

where they have no effect on matter stuck tothe membranes of the observable universe.

That is why gravity appears to be so weak.The bad news is that the papers by Dr

Sundrum and Dr Randall suggest that a con­sequence of all this is that the extra dimen­sions into which gravity is wandering mightbe infinitely large. This means that at leastsome of the gravitational energy that entersthem never comes back. The universe, inother words, may be leaking slowly away.

On the other hand, the good news forphysicists is that, if the theory is correct, theloops formed by these extra dimensions, un­like those of standard string theory, will berelatively large. The strings predicted bystring theory are so small and convolutedthat unwrapping them would require ener­gies that have not existed since shortly afterthe Big Bang. (This is the main reason whystring theory remains just a theory.) Dr Arka­ni-Hamed's loops, however, may be observ­able with existing equipment.

That, of course, begs the question of whyno one has actually observed the loops al-

Jam tomorrow?

FORmost people, checking the weatherbefore setting off for work in the morn­ing is simply a matter of switching on theradio or television, looking at a thermom­eter, or just sticking a nose out of the win­dow. Determining the extent to which traf­fic congestion will delay that journey is,however, less straightforward. Radio andtelevision reports normally provide only ageneral outline of which roads are runningsmoothly. Looking out of the window willnot help much. And even those broadcast­ers who are able to afford helicopters to spyon the rush-hour cannot easily quantifytheir observations in the ways that meteor­ologists are able to do with rainfall, wind­speed and temperature.

What isneeded, according toJohn Leo­nard of the Georgia Institute of Technologyin Atlanta, is a way of specifying trafficcongestion with greater precision. Hethinks he has one. Conditions on the roads,he suggests, should be expressed in termsof a traffic "temperature".

In Dr Leonard's scheme, the degree ofcongestion for each road leading into a citywould be worked out from data gatheredby roadside cameras and induction-loopsburied in the tarmac, and converted into asingle number. This number-the traffictemperature-would enable drivers toesti­mate how long particular journeys wouldtake before they set ou t.

A traffic temperature of 600, for exam­ple, might correspond to "no delay" along aroute compared with the traffic conditionsnormally experienced on that route. A traf-

THE ECONOMIST DECEMBER 23RD 1999

ready. One possibility is that they are notthere, and that Dr Arkani-Hamed is wrong.Another is that too many extra dimensionsare involved (the more there are, the smallerthe loops will be). But a third is simply thatno one has looked for them before, becauseno one knew they might exist.

And, in truth, they would not be all thateasy to blunder across accidentally. pho­tons-the particles that carry the electro­magnetic force-are stuck to their own par­ticular membrane and so cannot interactwith Dr Arkani-Hamed's new dimensions.

Nor can the more exotic particles responsi­ble for the weak and strong nuclear forces.You have to look using gravity itself. But al­though the other forces have been probedendlessly, nobody has ever tried measuringgravity accurately over short distances.

Now, that is changing. Experiments cur­rently being undertaken at Stanford, andalso by John Price, a physicist at the Univer­sity of Colorado, will measure the force ofgravity over a distance of less than a mil­limetre (the size ofloop expected if there are

fic temperature of 800 might then corre­spond to 20 minutes' delay, a temperatureof 1000 to 40 minutes' delay, and so on.(Though Dr Leonard has yet to decide ex­actly how to define the temperature scalefor traffic, he is keen for the resulting num­bers to fall into a range familiar to Ameri­cans, who stick stubbornly to Fahrenheit.)

For those whose routes take them

along but a single traffic artery, the inter­pretation would be simple. And even.driv­ers whose journeys involve switching be­tween several major roads should rapidlyacquire a feel for the relationship betweenthe various traffic temperatures and the to­tal durations of their trips, and would beable to allow extra time when necessary.

Determining and broadcasting currenttraffic temperatures would, however, be

only two extra gravity-swallowing dimen­sions). The hope is that the strength of thegravitational field across such short distanc­es will be radically different from that expe­rienced between bodies further apart.

If that does not work, there is a secondpossibility-to look for the energy leak intothe extra dimensions using particle accelera­tors. At the moment, data collected for otherpurposes at the Fermi National Laboratory,near Chicago, are being analysed again forsigns of a leak. If there are none, a more pow­erful accelerator may be needed-such asthe Large Hadron Collider that is about to bebuilt at the international CERN laboratory,near Geneva.

If that does not find anything, then DrArkani-Hamed and his colleagues arewrong, and it may be mere fluke tha t the uni­verse had exactly the right starting condi­tions for the emergence of humanity. If theyare right, however, that universe may not bearound for eternity. It is slowly leaking downa multidimensional plughole. .

just the first step. Dr Leonard's website al­ready displays detailed traffic-flow maps,which are updated every five minutes andpresented in colour lurid enough to make aweather-forecaster proud. He plans to ex­tend the analogy between traffic andweather further, by producing detailedtraffic forecasts.

Forecasting the short-term future-thenext hour or so-will be done by combin­ing "real-time" data with a traffic-flowmodel in order to work out how current

congestion will ripple and dispersethrough the road network. Slightly longer­term forecasts-for the following day ortwo-will also draw on historical dataabout how the flow of traffic varies with

the day of the week, the weather, the levelof hotel occupancy, and whether it is aschool day. Such forecasts should helpcommuters to set their alarm clocks much

more accurately.Admittedly, predicting tomorrow's

traffic temperature differs from predictingtomorrow's air tem perature in a crucial re­spect. Weather forecasts do not affect theweather. But if empty roads are predicted,drivers will be keener to use their cars, andthose roads will fill up-an effect that will,itself, have to be incorporated in the fore­casting models.

If traffic forecasting can be made towork, keeping an eye on the traffic tem­perature should help drivers to keep theircool. And if Dr Leonard's idea takes off,traffic temperature could someday be­come a standard measure for congestionthroughout the world-though the scalewould, presumably, have to be rejigged forCelsius-loving Europeans.

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