Quantum Physics

30.6 Positrons and Other Antiparticles 985TABLE 30.1Particle InteractionsInteraction Relative Mediating(Force) Strength a Range of Force Field ParticleStrong 1 Short (1 fm) GluonElectromagnetic 10 2 Long (1/r 2 ) PhotonWeak 10 6 Short (10 3 fm) W and Z bosonsGravitational 10 43 Long (1/r 2 ) Gravitona For two quarks separated by 3 10 17 m.particles is negligible. The gravitational force is by far the weakest of all the fundamentalforces.Modern physics often describes the forces between particles in terms of theactions of field particles or quanta. In the case of the familiar electromagneticinteraction, the field particles are photons. In the language of modern physics, theelectromagnetic force is mediated (carried) by photons, which are the quanta of theelectromagnetic field. Likewise, the strong force is mediated by field particlescalled gluons, the weak force is mediated by particles called the W and Z bosons,and the gravitational force is thought to be mediated by quanta of the gravitationalfield called gravitons. All of these field quanta have been detected except forthe graviton, which may never be found directly because of the weakness of thegravitational field. These interactions, their ranges, and their relative strengths aresummarized in Table 30.1.30.6 POSITRONS AND OTHER ANTIPARTICLESIn the 1920s, the theoretical physicist Paul Adrien Maurice Dirac (1902–1984)developed a version of quantum mechanics that incorporated special relativity.Dirac’s theory successfully explained the origin of the electron’s spin and its magneticmoment. But it had one major problem: its relativistic wave equationrequired solutions corresponding to negative energy states even for free electrons,and if negative energy states existed, we would expect a normal free electron in astate of positive energy to make a rapid transition to one of these lower states,emitting a photon in the process. Normal electrons would not exist and we wouldbe left with a universe of photons and electrons locked up in negative energystates.Dirac circumvented this difficulty by postulating that all negative energy statesare normally filled. The electrons that occupy the negative energy states are said tobe in the “Dirac sea” and are not directly observable when all negative energystates are filled. However, if one of these negative energy states is vacant, leaving ahole in the sea of filled states, the hole can react to external forces and thereforecan be observed as the electron’s positive antiparticle. The general and profoundimplication of Dirac’s theory is that for every particle, there is an antiparticle withthe same mass as the particle, but the opposite charge. For example, the electron’santiparticle, the positron, has a mass of 0.511 MeV/c 2 and a positive charge of1.6 10 19 C. As noted in Chapter 29, we usually designate an antiparticle with abar over the symbol for the particle. For example, p denotes the antiproton andthe antineutrino. In this book, the notation e is preferred for the positron.The positron was discovered by Carl Anderson in 1932, and in 1936 he wasawarded the Nobel prize for his achievement. Anderson discovered the positronwhile examining tracks created by electron-like particles of positive charge in acloud chamber. (These early experiments used cosmic rays—mostly energeticprotons passing through interstellar space—to initiate high-energy reactionson the order of several GeV.) In order to discriminate between positive andnegative charges, the cloud chamber was placed in a magnetic field, causingmoving charges to follow curved paths. Anderson noted that some of theCourtesy AIP Emilio Segre Visual ArchivesPAUL ADRIEN MAURICE DIRAC(1902 – 1984)Dirac was instrumental in the understandingof antimatter and in the unification ofquantum mechanics and relativity. Hemade numerous contributions to thedevelopment of quantum physics andcosmology, and won the Nobel Prize forphysics in 1933.