Much current debate on this topic could all the

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Unformatted text preview: es always come out in integer multiples • Examples: – – – – Up, charm, top quarks have +2/3 charge Down, strange, bottom quarks have –1/3 charge 2u + 1d (uud) → proton, with +1 charge 2d + 1u (ddu) → neutron, with neutral charge 24 Is There Additional Substructure? • Much current debate on this topic • Could all the particles be different “states” of a more Could basic entity? String theory suggests so. – 11 dimensional Universe!? – Particles correspond to different string vibrational modes – The Elegant Universe, by Brian Greene, describes this view • A difficulty: seems experimentally inaccessible! – “Planck Scale”, 10-35 meters, requires solar-system sized accelerator! 25 What holds the nucleus together? • Electrical charges interact, and like charges repel – Opposites attract, of course • The closer they get, the more protons in the The nucleus should be repelled from each other! nucleus • Something must serve as the glue to hold the Something nucleus together – The “strong” nuclear force: overcomes the electrical “Coulomb” force at short distances – Felt by quarks, not by electrons or their cousins (collectively called leptons) 26 All Forces are Mediated by Exchange Particles • • • • Strong nuclear force – “gluons” (massive) Weak nuclear force – W, Z (massive) Electrical and Magnetic phenomena – photon (massless) Gravity – graviton (massless, although no on...
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