4. Brownian motion Albert Einstein (PhD, 1902-05): suspensions in liquids applied kinetic theory to particles in liquids derived expression for diffusion constant D used expression to estimate size of water molecule used expression to estimate Avogadro’s number N Good agreement with estimates of N by other means Albert Einstein (1905): ‘Brownian motion paper’ derived expression for mean displacement of particle λ statistical mechanics relation between λ , D and t simplified calculation to 1dimension (Δx)2 = 2Dt But Clear prediction that could be tested experimentally Experimental confirmation of Einstein’s formula
many α-particles undeflected many α-particles deflected by very small amounts a few α-particles deflected by angles > 90o a few α-particles deflected bounced back Rutherford backscattering (RBS) Conclude +ve charge of atom concentrated at tiny core mass of atom concentrated at tiny core rest of atom almost empty Analysis nuclear radius ~ 10-14 m nuclear density ~ 1017 kg/m3 Problem with nuclear model: what holds protons together in nucleus? where are the electrons? what gives solids their structure? 3. Atomic spectra light emitted from excited gas comprises a discrete line spectrum each element has its own characteristic spectrum Hydrogen spectrum: described exactly by Rydberg-Ritz formula
Wolfgang Pauli (1936): postulates neutrino massless, chargeless particle (detected in 1956) no p+ + e- + Splitting the ‘atom’ 1932: Cockcroft and Walton (Rutherford group, Cambridge) Experiment Linear accelerator: protons accelerated in electric field to 1 MeV Fired at Lithium atoms: alpha particles detected on zinc screens Explanation Li atoms split into 2 helium nuclei First transmutation of elements by artificial means Verification of E = mc2 (Einstein) Verification of quantum tunnelling (Gamow) Nobel Prize (1951)
Higher fraction of K.E. converted into new particles VII The quark model of particle physics (i) The eightfold way Hundreds of new particles Similar to hundreds of new elements Periodic Table: repeated patterns → elements are composed of atoms (protons etc) Does a similar pattern exist for elementary particles? Gellmann (1961): arranged particles into patterns using group theory The eightfold way Predicts new particle (Ω-); mass 1680 MeV/c2 Detected 1963 (ii) The quark model Gellmann and Zweig (1964): particles contain more fundamental units? quarks: up, down strange fractional charges Q S
Note: Force between hadrons different from force between quarks (v) Three new quarks: charm, truth and beauty a) Glashow and Bjorken: leptons and quarks fundamental many similarities 4 lepton → 4 quarks? Also: electroweak theory: symmetry between quarks and leptons? 1970s: neutral weak currents additional channel of disintegration – new quark with new flavour? 1974: new hadronic resonance at SLAC, Brookhaven New particle – J/ψ : ‘long’lifetime (1976 Nobel prize) J/ψ = meson made of charm (cc) 1976: SLAC and Berkeley: neutral charmed meson D0 (cu) D+ and D- later observed b) 1977 Fermilab: new heavy meson У too heavy to be made of known quarks consists of pair of new quarks (beauty?) new search for b- mesons (bu or bd pairs) Discovered in Cornell storage ring in 1982; confirmation of b-quark c) Does beauty quark have a partner? truth quark? Discovered in 1992 (Tevatron, Fermilab): t →W+ + b P.S. 5th lepton discovered in 1975 (taon → taon neutrino also)
- less infinities - less renormalization - more natural -hints of role for gravity – supergravity 3. Super-gravity attempts to unify all four fundamental interactions Theories of Everything (ToE) Kaluza (1921): ‘unified’ GR and electromagnetism Wrote GR in 5 dimensions, em appears naturally Klein (1927): converted Kaluza theory to quantum field theory Einstein; attemoted to generalize Snag: no new predictions, no evidence Today: super-gravity Super-symmetric version of GR SUSY-GUT with gravity incorporated Snag; not a quantum field theory (better than GR) 3. Superstring theory string theory Veneziano (1968): solution to ‘bootstrap’ model of the hadrons