05_particles - UCSD Physics 10 Into Innerspace An overview...

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Unformatted text preview: UCSD Physics 10 Into Innerspace An overview of the femto-cosmos UCSD Physics 10 The Search for Basic Constituents Traces Back to Greek Times Atomic Hypothesis from Democritus How many "basic" constituents should we expect? Air Fire Earth Water Say the Greeks Spring 2008 2 UCSD Physics 10 Atoms make up the everyday material world Crystalline arrays solids Loose atoms rattling around gases Sloppy arrangements of atoms liquids We're going to skip over intermediate length scales (biology, materials science, chemistry, condensed matter physics...) and go straight to the atomic scale Used to be, we'd say you can't "see" atoms, but now we can! (Atomic Force Microscopy) Spring 2008 3 UCSD Physics 10 Atoms can now be "seen" Image of individual atoms in Mica (from www.di.com, Digital Instruments) see also http://stm2.nrl.navy.mil/how-afm/how-afm.html Spring 2008 4 UCSD Physics 10 Atoms Are Composite Objects Protons (+ electric charge), p Electrons ( electric charge), e Neutrons (no charge), n Proton and Neutron have about the same mass Electron is about 2000 times less massive than proton Electrical Forces produce attraction between electrons and the protons in the nucleus (they are oppositely charged) Spring 2008 5 UCSD Physics 10 Chemical elements are defined by the number of protons in the atom's nucleus Hydrogen: 1 proton & 1 electron proton 10-10 m Cloud of "electron probability" Spring 2008 6 UCSD Physics 10 Carbon has 6 protons Cloud of 6 electrons Nucleus: 6 protons 6 or 7 neutrons note: scale is wrong (nucleus greatly exaggerated) Spring 2008 7 UCSD Physics 10 The physics of atoms and their nuclei is well understood The breakthroughs were made in the 1920's 1930's Quantum Mechanics discrete energy levels Relativity things are different when you're really zipping! Evidence for depth of understanding is all around you Lasers Microwave ovens X-ray imaging Fluorescent lights Nuclear power Quantum Mechanics helps us understand the basic properties of atoms, and explains the grouping in the Periodic Table Spring 2008 8 UCSD Physics 10 The Periodic Table Spring 2008 9 UCSD Physics 10 What about the constituents of atoms? Separate an electron from the atom for study... Cloud of electrons nucleus Spring 2008 10 UCSD Physics 10 Electrons Appear to be Fundamental As far as we can tell, electrons have no ingredients The lack of substructure makes them useful probes for other particles Fling them really hard at nuclei to see if they "hit" anything Electron microscopes Particle accelerators Spring 2008 11 UCSD Physics 10 What about substructure in protons and neutrons? Can whack protons and neutrons with electrons and see what happens.... proton Electron Beam "Deep Inelastic Scattering" experiments indicate the electrons occasionally strike hard nuggets in the proton Quarks! Spring 2008 12 UCSD Physics 10 The Elementary Particles are "Related" Electric charge of electron and proton are equal and opposite, to remarkable accuracy Neutron left alone for 15 minutes will "Betadecay" into e, p , neutrino (very light, chargeless lepton) proton Poof! neutron neutrino Spring 2008 13 electron UCSD Physics 10 But that's not all! Antimatter : Each elementary particle has an "antimatter" counterpart Electron Positron Proton Antiproton Neutron Antineutron etc. - anti-etc. E=mc2 says matter and energy are interchangeable If they find one another major fireworks! Spring 2008 14 UCSD Physics 10 Particle-Antiparticle Dating Service: Particle Colliders Directing beams of particles and antiparticles at each other at ferocious energies can make new stuff It's like reproducing the Big Bang, but at lower energies particle Lots `o stuff antiparticle Spring 2008 15 UCSD Physics 10 Particle Accelerators are Big! Spring 2008 CERN, Switzerland 16 UCSD Physics 10 Short-Lived Matter A veritable zoo of particles (muon, tau, mesons, hadrons....) Confusion in the 1960's things seemed to be getting worse! hundreds of "new" particles observed Recognition in 1970's, 1980's that there was order to all this mess Electron has two short-lived siblings (muon, tau), each has a neutrino cousin (lepton family) Nuclear matter is made up of 6 quarks, arranged as three generations with 2 members each Imagine all the combinations! Spring 2008 17 UCSD Physics 10 The Standard Model of Particle Physics Basic Ingredients are quarks and the electron-like objects (leptons) Fundamental forces are mediated by photon, gluons, W's and Z's (bosons) (Fermilab) Spring 2008 18 UCSD Physics 10 Quark Soup Combinations of quarks make up all the exotic particles cataloged in the 1960s 6 quarks, 6 anti-quarks, grouped in twos and threes Many dozens of combinations, only 1 or 2 stable Charges 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 19 Spring 2008 UCSD Physics 10 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 accelerator! Spring 2008 20 UCSD Physics 10 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 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) Spring 2008 21 UCSD Physics 10 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 one has yet seen one) The range (extent) of the force depends on the mass of the exchange particle Which is why the strong and weak forces are remote from everyday experience, being mediated by massive particles Gravity and electromagnetic forces extend infinitely far (though weaken with increased distance) Spring 2008 22 UCSD Physics 10 Unification of Fundamental Forces Electricity Magnetism Light Beta-decay Neutrinos Protons Neutrons Pions, etc. Earth Gravity Celestial Mech. Spring 2008 1864 Electromagnetism 1971 Electroweak Interaction Weak Interaction 1976 Standard Model 1965 1973 Strong Interaction ? 1916 General Relativity 23 1687 Universal Gravity Spacetime Geom. UCSD Physics 10 Assignments HW 1 due today (Now) turn in at lecture, or into box outside SERF 336 by 3PM Submit Question/Observation TODAY (4/11) by midnight via WebCT hard time cutoff: after this, late submission available for half credit HW 2: due Friday (4/18): Hewitt 11.E.16, 11.E.20, 11.E.32, 11.P.5, 2.E.6, 2.E.11, 2.E.14, 2.E.36, 2.E.38, 3.E.4, 3.E.5, 3.E.6, 3.E.19 Read Hewitt, Chap. 11, pp. 202217 Read Hewitt Chapters 2, 3, 4 for next week Spring 2008 24 ...
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