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factor. The predicted neutrino arrival rateat the earth depends approximately uponthe 25 th power of the central temperatureof the sun, T × T ×...T (25 factors of thetemperature T). The agreement that hasbeen achieved (agreement within a factorof three) shows that we have empiricallymeasured the central temperature of thesun to an accuracy of a few percent.Incidentally, if someone had told me in1964 that the number of neutrinos observedfrom the sun would be within a factor ofthree of the predicted value, I would havebeen astonished and delighted.In fact, the agreement betweennormal astronomical observations (usinglight rather than neutrinos) and theoreticalcalculations of solar characteristics ismuch more precise. Studies of the internalstructure of the sun using the solarequivalent of terrestrial seismology (i.e.,observations of solar vibrations) showthat the predictions of the standard solarmodel for the temperatures in the centralregions of the sun are consistent with theobservations to an accuracy of at least0.1%. In this standard model, the currentage of the sun is five billion years, whichis consistent with the minimum estimateof the sun’s age made by nineteenthcenturygeologists and biologists (a fewhundred million years).Given that the theoretical modelsof the sun describe astronomicalobservations accurately, what can explainthe disagreement by a factor of two orthree between the measured and thepredicted solar neutrino rates?B. New PhysicsPhysicists and astronomers were onceagain forced to reexamine their theories.This time, the discrepancy was not betweendifferent estimates of the sun’s age, butrather between predictions based upona widely accepted theory and directmeasurements of particles produced bynuclear burning in the sun’s interior. Thissituation was sometimes referred to asthe Mystery of the Missing Neutrinos or,in language that sounded more scientific,the Solar Neutrino Problem.As early as 1969, two scientistsworking in Russia, Bruno Pontecorvo andVladimir Gribov, proposed that thediscrepancy between standard theoryand the first solar neutrino experimentcould be due to an inadequacy in thetextbook description of particle physics,rather than in the standard solar model.(Incidentally, Pontecorvo was the firstperson to propose using a chlorine detectorto study neutrinos.) Gribov and Pontecorvosuggested that neutrinos suffer from amultiple personality disorder, that theyoscillate back and forth between differentstates or types.According to the suggestion of Gribovand Pontecorvo, neutrinos are producedin the sun in a mixture of individual states,a sort of split personality. The individualstates have different small masses, ratherthan the zero masses attributed to themby standard particle theory. As they travelto the earth from the sun, neutrinososcillate between the easier-to-detectneutrino state and the more difficult-todetectneutrino state. The chlorineexperiment only detects neutrinos in theeasier-to-observe state. If many of theneutrinos arrive at earth in the state thatis difficult to observe, then they are notcounted. It is as if some or many of theneutrinos have vanished, which can explainthe apparent mystery of the missingneutrinos.Building upon this idea, LincolnWolfenstein in 1978 and Stanislav Mikheyevand Alexei Smirnov in 1985 showed thatthe effects of matter on neutrinos movingthrough the sun might increase theoscillation probability of the neutrinosif Nature has chosen to give them massesin a particular range.Neutrinos are also produced by thecollisions of cosmic ray particles with otherparticles in the earth’s atmosphere. In 1998,the Super-Kamiokande team ofexperimentalists announced that they hadobserved oscillations among atmosphericneutrinos. This finding provided indirectsupport for the theoretical suggestion thatsolar neutrinos oscillate among differentstates. Many scientists working in the fieldof solar neutrinos believe that, in retrospect,we have had evidence for oscillations ofsolar neutrinos since 1968.But we do not yet know what causesthe multiple personality disorder of solarneutrinos. The answer to this questionmay provide a clue to physics beyond thecurrent standard models of sub-atomicparticles. Does the change of identityoccur while the neutrinos are travelingto the earth from the sun, as originallyproposed by Gribov and Pontecorvo? Ordoes matter cause solar neutrinos to “flipout”? Experiments are underway in Canada,Italy (three experiments), Japan (twoexperiments), Russia, and the UnitedStates that are attempting to determinethe cause of the oscillations of solarneutrinos, by finding out how much theyweigh and how they transform from onetype to another. Non-zero neutrino massesmay provide a clue to a still undiscoveredrealm of physical theory.VI. Nature: AWonderful MysteryNature has written a wonderful mystery.The plot continually changes, and themost important clues come from seeminglyunrelated investigations. These suddenand drastic changes of scientific sceneappear to be Nature’s way of revealing theunity of all fundamental science.The mystery began in the middle ofthe nineteenth century with the puzzle:How does the sun shine? Almostimmediately, the plot switched to questionsabout how fast natural selection occursand at what rate geological formationsare created. The best theoretical physicsof the nineteenth century gave the wronganswer to all these questions. The firsthint of the correct answer came, at thevery end of the nineteenth century, fromthe discovery of radioactivity withaccidentally darkened photographic plates.The right direction in which to searchfor the detailed solution was revealed bythe 1905 discovery of the special theoryof relativity, by the 1920 measurement ofthe nuclear masses of hydrogen andhelium, and by the 1928 quantummechanical explanation of how chargedparticles get close to each other. Thesecrucial investigations were not directlyrelated to the study of stars.December/ décembre 2000 JRASC225