30.16 Problems and Perspectives 1001Radiant energy density per wavelength interval (eV/m 3 /m)10 810 610 410 210 010 –2PenziasandWilson0.01 0.1 1 10 100Wavelength (cm)Figure 30.16 Theoretical blackbody (browncurve) and measured radiation spectra (bluepoints) of the Big Bang. Most of the data werecollected from the Cosmic Background Explorer(COBE) satellite. The datum of Wilson andPenzias is indicated.(b. 1945) at the Lawrence Berkeley Laboratory found that the background was notperfectly uniform, but instead contained irregularities corresponding to temperaturevariations of 0.000 3 K. It is these small variations that provided nucleationsites for the formation of the galaxies and other objects we now see in the sky.30.16 PROBLEMS AND PERSPECTIVESWhile particle physicists have been exploring the realm of the very small, cosmologistshave been exploring cosmic history back to the first microsecond of the BigBang. Observation of the events that occur when two particles collide in an acceleratoris essential in reconstructing the early moments in cosmic history. Perhapsthe key to understanding the early Universe is first to understand the world ofelementary particles. Cosmologists and particle physicists find that they have manycommon goals and are joining efforts to study the physical world at its most fundamentallevel.Our understanding of physics at short and long distances is far from complete.Particle physics is faced with many questions: why is there so little antimatter in theUniverse? Do neutrinos have a small mass, and if so, how much do they contributeto the “dark matter” holding the universe together gravitationally? How can weunderstand the latest astronomical measurements, which show that the expansionof the universe is accelerating and that there may be a kind of “antigravity force”acting between widely separated galaxies? Is it possible to unify the strong andelectroweak theories in a logical and consistent manner? Why do quarks and leptonsform three similar but distinct families? Are muons the same as electrons(apart from their different masses), or do they have subtle differences that havenot been detected? Why are some particles charged and others neutral? Why doquarks carry a fractional charge? What determines the masses of the fundamentalparticles? The questions go on and on. Because of the rapid advances and new discoveriesin the related fields of particle physics and cosmology, by the time youread this book some of these questions may have been resolved and others mayhave emerged.An important question that remains is whether leptons and quarks have a substructure.If they do, one could envision an infinite number of deeper structurelevels. However, if leptons and quarks are indeed the ultimate constituents of matter,as physicists today tend to believe, we should be able to construct a final theoryof the structure of matter, as Einstein dreamed of doing. In the view of many physicists,the end of the road is in sight, but how long it will take to reach that goal isanyone’s guess.
1002 Chapter 30 Nuclear Energy and Elementary ParticlesSUMMARYTake a practice test by logging into<strong>Physics</strong>Now at www.cp7e.com and clicking on the Pre-Test link for this chapter.30.1 Nuclear Fission &30.2 Nuclear ReactorsIn nuclear fission, the total mass of the products is alwaysless than the original mass of the reactants. Nuclearfission occurs when a heavy nucleus splits, or fissions,into two smaller nuclei. The lost mass is transformedinto energy, electromagnetic radiation, and the kineticenergy of daughter particles.A nuclear reactor is a system designed to maintain aself-sustaining chain reaction. Nuclear reactors usingcontrolled fission events are currently being used togenerate electric power. A useful parameter for describingthe level of reactor operation is the reproductionconstant K, which is the average number ofneutrons from each fission event that will cause anotherevent. A self-sustaining reaction is achievedwhen K 1.30.3 Nuclear FusionIn nuclear fusion, two light nuclei combine to forma heavier nucleus. This type of nuclear reactionoccurs in the Sun, assisted by a quantum tunnelingprocess that helps particles get through the Coulombbarrier.Controlled fusion events offer the hope of plentifulsupplies of energy in the future. The nuclear fusion reactoris considered by many scientists to be the ultimateenergy source because its fuel is water. Lawson’s criterionstates that a fusion reactor will provide a net outputpower if the product of the plasma ion density n andthe plasma confinement time satisfies the followingrelationships:n 10 14 s/cm 3 Deuterium–tritium interaction [30.5]n 10 16 s/cm 3 Deuterium–deuterium interaction30.5 The Fundamental Forces ofNatureThere are four fundamental forces of nature: the strong(hadronic), electromagnetic, weak, and gravitationalforces. The strong force is the force between nucleonsthat keeps the nucleus together. The weak force is responsiblefor beta decay. The electromagnetic and weakforces are now considered to be manifestations of a singleforce called the electroweak force.Every fundamental interaction is said to be mediatedby the exchange of field particles. The electromagneticinteraction is mediated by the photon, the weak interactionby the W and Z 0 bosons, the gravitationalinteraction by gravitons, and the strong interaction bygluons.30.6 Positrons and OtherAntiparticlesAn antiparticle and a particle have the same mass, butopposite charge, and may also have other propertieswith opposite values, such as lepton number and baryonnumber. It is possible to produce particle–antiparticlepairs in nuclear reactions if the available energy isgreater than 2mc 2 , where m is the mass of the particle(or antiparticle).30.8 Classification of ParticlesParticles other than photons are classified as hadronsor leptons. Hadrons interact primarily through thestrong force. They have size and structure and henceare not elementary particles. There are two types ofhadrons: baryons and mesons. Mesons have a baryonnumber of zero and have either zero or integer spin.Baryons, which generally are the most massive particles,have nonzero baryon numbers and spins of 1/2or 3/2. The neutron and proton are examples ofbaryons.Leptons have no known structure, down to the limitsof current resolution (about 10 19 m). Leptons interactonly through the weak and electromagnetic forces.There are six leptons: the electron, e ; the muon, ;the tau, ; and their associated neutrinos, e , ,and .30.9 Conservation Laws &30.10 Strange Particles andStrangenessIn all reactions and decays, quantities such as energy,linear momentum, angular momentum, electric charge,baryon number, and lepton number are strictly conserved.Certain particles have properties called strangenessand charm. These unusual properties are conservedonly in those reactions and decays that occur viathe strong force.
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29.3 Radioactivity 947INTERACTIVE E
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South poleEarth’s geographic, 626
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CreditsPhotographsThis page constit
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PEDAGOGICAL USE OF COLORDisplacemen
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PHYSICAL CONSTANTSQuantity Symbol V