Juin 2005

Allen measured changes in the pcricd ofa torsion pendulum, a massive disesuspended fi-cm a wire attached ta its centre.Rotating the dise slightly couses the wire totwist. When it is released, the dise continuesto twirl, flrst clockwise, then anticlockwise,wifh a flxed period. But not during an(trooclipse, when Saxl and Allen's pendulumspcd up significantly.Needless ta say, most physicists do notwant ta believe there is something up withrelatîvity. Surely there must be sorne otherexplanation, they say, perhaps an error withthe instruments. Certainly. the jury is out over(5) gravity reallv does go berserk fromtime to time. During a solar eclipse in Indiain 1995,0. C. MishraandM B. S.Raoofthe1So National Geophysical Research Institute inHyderabad observed a slight but suddendrop in the strengrh ofgravity as measuredbyan extrcmely accuratc gravimeter.But similar experiments carried out byFinnish geophysieists on 22 July 1990,when the eclipsed sun rose abovc Helsinki,showed nothing of the sort.Flight from reality?According to many physicists, nul! resultslike these prove thar the effectsare not real,l60Jay Pasachoff an eclipse expert at William..sCollege, Massachusetts, is one ofthe sceptics."There are enough fascinating and importantthings ta be studied during total solar eclipsesthat it is tao bad people waste time lookingfor things that aren't therc," he says.Then egain. the eclipse effects are notthe only gravitational anomalies thar havesurtaced IIIthe past decades. NASA'sPioneer 10 and Il spacecraft have bcen~ 10experiencing a mysterious deceleration ontheirjoumey to the edge ofthe solar system(eee "Pioneering the way"). Maybe there reallyle a skeleton in Einstein's closetwaiting ta bediscovered and ta revolutionise physics.Or are there more convenüonal explanations?Instrument errors are very unlikely,says Duif. Ali the experlments that sawsomethlnq sîgnificantwere carried out withextreme care using sensitive instruments.l'?J'v One suggestion is tf-ratincreased hurnanactivity durinq a solar eclipse might producesmall seismic disturbances that could affectthe instruments. But, says Duif that doesn'texplain why scientists observed gravîmeteranomalies during an eclipse in March 1997 in avery remote erea of north-east China, {G} anexperiment in Belgium found nothing on11 August 1999, \lVhen millions of Europeens leftthelr homes te observe the total solar eclipse.lQo Other researchers have suggested thatthe observations could be due to atmosphericeftects. When the moon's shadow hitsthe Earth, it produces a cool spot in theatmosphere, which maYes with a speed of aPtoneerfng the wayAccording tophysicist Chris Duifof DelftUniversity of Technology,the mysterious behavlcur ofpendulums during solareclipsesmayberelated to anothergravitational enigma: thePioneeranomaly.ln 1998, physlclsts andengmeers atNASA's JetPropulsionLaboratory (JPL) in Pasadena,Californ la,disoovered thattheunmanned space probes Pioneer 10and11 areslowly veerîng offtheirexpeoted course, asif thesclarsystem is tugging a bittoohardon the two craft.Fuelleaks and heat radiationareamong theproposedexplanations ofthePioneeranornaly, but (7) extrernelycareful analyses the problem hasnever been solved. During a specialconference ontheanomaly lastMay in Bremen, Germany, a wldevarlety of unconventiona! solutionswere discussed, but noclearconsensus emerged. Scientlsts frcmJPL and theuniversltles ofBremenand Cologne have nowproposed aEuropean Space Agency mission tcstudythe mystencus decelerationin moredetafl.Sorne astronomers thinkthePioneer ancmaly is evidence ofaminorbut importantflaw in thelaws of gravity. Accordîng toNewton's laws, the strength ofgravity fallswiththe inverse squareof distance, ButMordehaî MilgromoftheWeizmann Institute of Sciencein Rehovot, Israel, hasproposed analternative expia nation whichhecalls modified Newtonian dynamics,ln MONO, theinverse square lawonlyapplies where gravity is strong.Where it is weak, gravityfadesmoreslowly wlth distance (NewSei,n'st, 20July2002, P281.Modifying theinverse squarelaw,sorne physlcists claim, wouldalso explain the motion ofstarsand galaxies withoutthe need tcinvokehuge amounts of unseendarkmatterin the universe.lt meyeven pointthe waytc a successtutmergerof general relativity withquantum mechanics- somethingscientistshave been unable toacccmpllsh sc far.Iftlle eclipse affect and thePioneer anomaly bothturn outto beconnected wlthgravity,saysDuit, they could very weil berelated, andthe same mightevenbe true for darkmatter. "lt seemsunlikely that there aredozens ofunexplained effects," he says.