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UCSD Physics 10
Into InnerspaceInto Innerspace
An overview of the femto-cosmosAn overview of the femto-cosmos
UCSD Physics 10
Spring 2008 2
The Search for Basic Constituents Traces The Search for Basic Constituents Traces Back to Greek TimesBack to Greek Times
• Atomic Hypothesis from DemocritusAtomic Hypothesis from Democritus
• How many “basic” constituents should we expect?How many “basic” constituents should we expect?– Air
– Fire
– Earth
– Water
Say the Greeks
UCSD Physics 10
Spring 2008 3
Atoms make up the everyday material worldAtoms make up the everyday material world
• Crystalline arrays – solidsCrystalline arrays – solids• Loose atoms rattling around – gasesLoose atoms rattling around – gases• Sloppy arrangements of atoms – liquidsSloppy arrangements of atoms – liquids• We’re going to skip over intermediate length We’re going to skip over intermediate length
scales (biology, materials science, chemistry, scales (biology, materials science, chemistry, condensed matter physics...) and go straight to the condensed matter physics...) and go straight to the atomic scaleatomic scale
• Used to be, we’d say you can’t “see” atoms, but Used to be, we’d say you can’t “see” atoms, but now we can! (Atomic Force Microscopy)now we can! (Atomic Force Microscopy)
UCSD Physics 10
Spring 2008 4
Atoms can now be “seen”Atoms can now be “seen”
Image of individual atoms in Mica (from Image of individual atoms in Mica (from www.di.com, Digital Instruments), Digital Instruments)
see also see also http://stm2.nrl.navy.mil/how-afm/how-afm.html
UCSD Physics 10
Spring 2008 5
Atoms Are Composite ObjectsAtoms Are Composite Objects
• ProtonsProtons (+ electric charge),(+ electric charge), pp• Electrons Electrons (– electric charge),(– electric charge), e e• NeutronsNeutrons (no charge),(no charge), nn• ProtonProton andand NeutronNeutron have about the same masshave about the same mass• Electron Electron is about 2000 times less massive thanis about 2000 times less massive than protonproton• Electrical ForcesElectrical Forces produce attraction between produce attraction between electrons electrons
and theand the protonsprotons in the nucleus (they are oppositely in the nucleus (they are oppositely charged)charged)
UCSD Physics 10
Spring 2008 6
Cloud of “electron probability”
Chemical elements are defined by the number Chemical elements are defined by the number of protons in the atom’s nucleusof protons in the atom’s nucleus
• Hydrogen: 1 proton & 1 electronHydrogen: 1 proton & 1 electron
proton10-10 m
UCSD Physics 10
Spring 2008 7
Carbon has 6 protons Carbon has 6 protons
Nucleus:6 protons
6 or 7 neutrons
Cloud of 6 electrons
note: scale is wrong(nucleus greatly exaggerated)
UCSD Physics 10
Spring 2008 8
The physics of atoms and their nuclei is well The physics of atoms and their nuclei is well understoodunderstood
• The breakthroughs were made in the 1920’s The breakthroughs were made in the 1920’s –– 1930’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 youEvidence 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 Quantum Mechanics helps us understand the basic properties of atoms, and explains the grouping in the properties of atoms, and explains the grouping in the Periodic TablePeriodic Table
UCSD Physics 10
Spring 2008 9
The Periodic TableThe Periodic Table
UCSD Physics 10
Spring 2008 10
What about the constituents of atoms?What about the constituents of atoms?
• Separate an electron from the atom for study...Separate an electron from the atom for study...
nucleus
Cloud of electrons
UCSD Physics 10
Spring 2008 11
Electrons Appear to be Fundamental Electrons Appear to be Fundamental
• As far as we can tell, electrons have no ingredientsAs far as we can tell, electrons have no ingredients
• The lack of substructure makes them useful probes The lack of substructure makes them useful probes
for other particlesfor other particles
– Fling them really hard at nuclei to see if they “hit”
anything
– Electron microscopes
– Particle accelerators
UCSD Physics 10
Spring 2008 12
What about substructure in protons and neutrons?What about substructure in protons and neutrons?
• Can whackCan whack protonsprotons andand neutronsneutrons withwith electrons electrons and see what happens....and see what happens....
“Deep Inelastic Scattering” experiments indicate the electrons occasionally strike hard nuggets in the proton
Quarks!
proton
Electron Beam
UCSD Physics 10
Spring 2008 13
The Elementary Particles are “Related”The Elementary Particles are “Related”
• Electric charge ofElectric charge of electron electron andand protonproton are equal are equal and opposite, to remarkable accuracyand opposite, to remarkable accuracy
• NeutronNeutron left alone for 15 minutes will “Beta-left alone for 15 minutes will “Beta-decay” intodecay” into e, e, pp , , neutrinoneutrino (very light, chargeless (very light, chargeless lepton)lepton)
neutron
proton
neutrino
electronPoof!
UCSD Physics 10
Spring 2008 14
But that’s not all!But that’s not all!
• Antimatter : Each elementary particle has an Antimatter : Each elementary particle has an “antimatter” counterpart“antimatter” counterpart Electron – Positron
Proton – Antiproton
Neutron – Antineutron
etc. anti-etc.
• EE==mcmc22 says matter and energy are interchangeable says matter and energy are interchangeable• If they find one another – major fireworks!If they find one another – major fireworks!
UCSD Physics 10
Spring 2008 15
Particle-Antiparticle Dating Service:Particle-Antiparticle Dating Service: Particle Colliders Particle Colliders
• Directing beams of particles and antiparticles at each Directing beams of particles and antiparticles at each other at ferocious energies can make new stuffother at ferocious energies can make new stuff
• It’s like reproducing the Big Bang, but at lower energiesIt’s like reproducing the Big Bang, but at lower energies
particle
antiparticle
Lots ‘o stuff
UCSD Physics 10
Spring 2008 16
Particle Accelerators are Big!Particle Accelerators are Big!
CERN, Switzerland
UCSD Physics 10
Spring 2008 17
Short-Lived MatterShort-Lived Matter
• A veritable zoo of particles (muon, tau, mesons, A veritable zoo of particles (muon, tau, mesons, hadrons....)hadrons....)
• Confusion in the 1960’s – things seemed to be getting Confusion in the 1960’s – things seemed to be getting worse!worse!– hundreds of “new” particles observed
• Recognition in 1970’s, 1980’s that there was order to all Recognition in 1970’s, 1980’s that there was order to all this messthis 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!
UCSD Physics 10
Spring 2008 18
The Standard Model of Particle PhysicsThe Standard Model of Particle Physics
Basic Basic Ingredients Ingredients are quarks are quarks and the and the electron-like electron-like objects objects (leptons)(leptons)
(Fermilab)
Fundamental forces are mediated by photon, gluons, W’s and Z’s (bosons)
UCSD Physics 10
Spring 2008 19
Quark SoupQuark Soup
• Combinations of quarks make up all the exotic Combinations of quarks make up all the exotic particles cataloged in the 1960sparticles 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: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
UCSD Physics 10
Spring 2008 20
Is There Additional Substructure?Is There Additional Substructure?
• Much current debate on this topicMuch current debate on this topic
• Could all the particles be different “states” of a more Could all the particles be different “states” of a more basic entity? String theory suggests so. 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!A difficulty: seems experimentally inaccessible!– “Planck Scale”, 10-35 meters, requires solar-system sized
accelerator!
UCSD Physics 10
Spring 2008 21
What holds the nucleus together?What holds the nucleus together?
• Electrical charges interact, and like charges repelElectrical charges interact, and like charges repel– Opposites attract, of course
• The closer they get, the more protons in the The closer they get, the more protons in the nucleus should be repelled from each other!nucleus should be repelled from each other!
• Something must serve as the glue to hold the Something must serve as the glue to hold the nucleus together 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)
UCSD Physics 10
Spring 2008 22
All Forces are Mediated by Exchange ParticlesAll Forces are Mediated by Exchange Particles
• Strong nuclear force – “gluons” (massive)Strong nuclear force – “gluons” (massive)• Weak nuclear force – W, Z (massive)Weak nuclear force – W, Z (massive)• Electrical and Magnetic phenomena – photon (massless)Electrical and Magnetic phenomena – photon (massless)• Gravity – graviton (massless, although no one has yet seen Gravity – graviton (massless, although no one has yet seen
one)one)
• The range (extent) of the force depends on the mass of the The range (extent) of the force depends on the mass of the exchange particleexchange 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)
UCSD Physics 10
Spring 2008 23
Unification of Fundamental ForcesUnification of Fundamental ForcesElectricity
Magnetism
Light
Beta-decay
Neutrinos
Protons
Neutrons
Pions, etc.
Earth Gravity
Celestial Mech.
Electromagnetism
Weak Interaction
Strong Interaction
Universal Gravity
Spacetime Geom.
Electroweak Interaction
StandardModel
GeneralRelativity
?
1864
1965
1971
1973
1976
1687 1916
UCSD Physics 10
Fundamental Forces in PhysicsFundamental Forces in Physics
• Gravitational forceGravitational force– Attractive force between objects with mass
– Weakest, long range
• Electromagnetic forceElectromagnetic force– Attractive and repulsive
– Long range, 1039 times stronger than gravity
• Nuclear Weak forceNuclear Weak force– Cause neutrons to decade into a protons
– Range <10-17 m, 1028 times stronger than gravity
• Nuclear Strong forceNuclear Strong force– Hold the nucleus together
– Range <10-15 m, 1041 times stronger than gravity
UCSD Physics 10
• Law of Gravity:Law of Gravity:
– M = mass of one object
– m = mass of second object
– G = 6.67 x 10-11 Nm2/kg2
– r = distance between the objects
Gravitational ForceGravitational Force
UCSD Physics 10
• Coulomb Law:Coulomb Law:
– Strength of EM-force determines how strongly electrons in an atom are held in orbit
Electro-Magnetic ForceElectro-Magnetic Force
proton electron
UCSD Physics 10
Nuclear Weak-forceNuclear Weak-force
• Repelling forceRepelling force that cause that cause betabeta-decay-decay
neutron proton
electron (beta-particle)
The strength of the nuclear weak-force determines how fast neutrons are converted into protons and electrons
anti-neutrino
UCSD Physics 10
• Hold nucleus togetherHold nucleus together by by overcoming the repelling protonsovercoming the repelling protons in nucleus:in nucleus:
– Strength of the nuclear strong-force determines how fast nuclear reactions will proceed
Nuclear Strong-forceNuclear Strong-force
neutron
neutronproton
proton