Juin 2005

Allen measured changes in the penod ofa torsion pendulum, a massive disesuspended from il wire attached ta its centre.Rotating the dise slightly causes the wire rotwist. When it is released. the dise continuesta twirl, first clockwise, then anticlcckwise,with a fixed period. But not during an(lt-~clipse, when Saxi and Allen's pendulumsped up significantly.Needlcss ta say, most physicists do notwant to believe there is sornethiog up withrelativitv. Surelv there must he sorne otherexplanation, th~y say, perhaps an error withthe instruments. Certainly. the jury is out over(5) gravity reallv does go berserk fromlime ta tune. During a soler eclipse in Indiain 1995.D C.Mishra arri M B SRaoofthcJSa National Geophysical Research Institute inHyderabad observed a slight but suddendrop in the srrengïh ofgravit)' as measuredby an extremelv accurate gravimeter.But similar experiments carried out bvFinnish geophvsicists on 22 July 1990,when the cclipsed sun rose above Helsinki.showed nothing of the sort.Flight from reality?According to many physicists, null resultelike thèse praye thar the effects are not reelltoJay Pasachoff an eclipse expert at William...sCollège, Massachusetts, is one orthe sceptics."There are enough fascinating and importantthings to be studied during total solar eclipsesthar il is tao bad people waste ume lookingfor things that aren't therc, (( he says.Thenagain, the eclipse effectsare notthe onlv gravitaticnal anomalies that havesurfaced in the past decades. NASA'sPioneer 10 and Il spacecraf have been1.1oexperiencing a mysrerious deceleration ontheir joumey to the edge ofthe solar system(aee "Pioneering the way"). Maybe there reallyIs a akeleton in Einstein's closet waltlnq to bediscovered and ta revolutionfse physics.Or are there more conventional explanations?Instrument errcrs are very unlikely,says Duif Ali the experiments that sawsomething siqniflcant were carn ed out withextreme care using sensitive instruments.!?sv One suggestion is that increased humanactivity dunng a soler eclipse might producesmall setemc dlsturbances that could affectthe instruments. But, says Duif, that doesn'texplain why scientists observed gravimeteranomalies durtng an ecJipsein March 1997 in avery remote area of north-east China, (6) anexperiment in Belgium found nothing on11 August 1999, when millions of Europeans lefttheir homes ta observe the total solar eclipse.lt10 Other researchers have suggested thatthe observations could be due to atmosphericeffects. When the moon's shadow hitsthe Earth, it produces a cool spot in theatmosphere, which moves with a speed of aPloneerlng the walAocording to physicist Chris Duitof Delft University of Teohnology,themysterious behavicur ofpendulums during sclareclipsesMay berelated toanothergravitational enigme: the Pioneeranomaly. ln 1998, physlcists andengîneers atNASA's JetPropulsionLabcratcry (JPL) in Pasadena,California, dlscovered thattheunmanned space probes Pioneer 10and 11 areslowly veertng offtheirexpected oourse, asifthesolarsystem is tugging a bittoo hardonthe two oraft.Fuel leaks and heat radiationare among theproposedexplanations ofthePioneeranomoly, but (7) "tromolycareful analyses theproblem hasnever been solved. Dur"mg aspecialconference ontheanomaly lastMay in Bremen, Germany, a widevariety of unccnventicnat solutionswere discussed, butnoclearconsensus emerged. Scientists fromJPL and theunlversities ofBremenand Cologne have newproposed aEuropean Space Agenoy mission tostudy the mysterious decelerationin more detail.Some astrcncmers thinkthePioneer anomaly is evidence ofaminorbut importantflaw in thelaws ofgravlty. According taNewton's laws, the strength ofgravity falls withtheinverse squareof distance. But Mordehai MilgromoftheWeizmann Institute of Scienoein Rehovot, Israel, has proposed analternative expIa nation whîch hecallsmodified Newtonian dynamics,ln MONO, theinverse square lawonlyapplies where gravityis strong.Where ît is weak, gravity fadesmoreslowly withdistance (NewSc{entist, 20July2002, P28).Modifying theinverse squarelaw, sorne physicists ctatm, wouldalsc explain themotion of starsandgalaxies without the need toInvoke huge amounts of unseendark matter in the universe. It mayeven pointtheway tc a successfulmerger of general relativity withquantum mecnanics • somethingscientîsts have been unable toaooomplish sofar.Ifthe eclipse effect and thePioneer anomaly both turn outto baconnected with gravity,saysDuif, they could veryweil berelated, andthe same mightevenbetrua for darkmatter. "!t seemsunHkely thattllereared02ens cfunexplained effeots," hesays.