Physics Lecture 23

# Physics Lecture 23 - a If ΔE≥ mc 2 “create” particle...

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Lecture 23: Forces and Interactions I. Color A. Each quark has a “color” 1. Red (R) 2. Green (G) 3. Blue (B) B. Colorless 1. R + B + G white 2. Abar + Bbar + Gbar anti-white 3. R + Rbar, B + Bbar, G + Gbar no color II. Fundamental Forces A. Gravitational 1. Every object attracts every other object B. Electromagnetic 1. Attraction/repulsion between objects with electric charges or currents C. Strong 1. Force holding nucleus together. Also experienced by all hadrons. D. Weak 1. Postulated to take care of interactions not accounted for by other ones. For instance, neutrinos are found to interact with electrons, even though there is no electric chare and even though neither neutrinos nor electrons feel the strong force. Also a short-range force. III. Feynman Diagrams and virtual particles A. Space-time Diagrams B. Feynman Diagrams 1. photon: messenger/mediator particle 2. messenger particle “virtual” particle a. “blips” in and out of existence 3. Energy-time uncertainty : ΔEΔt ~ ħ
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Unformatted text preview: a. If ΔE≥ mc 2 “create” particle from nothing 4. Virtual particles continuously fired off in all directions a. Blip out of existence b. No effect unless another particle absorbs one 5. Messenger needs at least ΔE ~ mc 2 a. Particle can live for Δt ~ ħ/ΔE ħ/mc 2 for particles with mass b. Range of force c*Δt = cħ/mc 2 i. m = mass of messenger ii. large range = small mass IV. Weak and Strong Interactions A. Weak W + , W-, Z (do not worry about Z unless asked 1. exchange of a W + or W- conserves charge B. Strong Gluons 1. R Rbar gl R Bbar gl R Gbar gl B Rbar gl B Bbar gl B Gbar gl G Rbar gl G Bbar gl G Gbar gl 2. Concept Test a. Color Before = color after B = G + B + Gbar Colorless rule : Every physically realizable particle must have no net color Energy-time uncertainty ΔEΔt ~ ħ Distance = speed x time x t x t...
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