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
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
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
nucleus should be repelled from each other!
• Something must serve as the glue to hold the
– 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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