QED: Quantum Electrodynamics

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  • Most complex thing I ever learnt. Explain in 7 minutes?Talk features complicateddiagrams and photos of dead physicists
  • Light = electromagnetic spectrumWave-like nature: oscillating electric and magnetic fieldSpeed, frequency, wavelength
  • ‘GCSE view’ of light and matterElectrons as hard particles orbiting a central nucleus
  • Discovered by Heinrich Hertz, 1887Red light won’t cause emission of an electron, blue light will
  • Electron diffraction: wave-particle dualityDiscovered 1929 George Paget ThomsonClinton DavissonLester Germer
  • Erwin Schrödinger, 1926the E=mc2 of chemistryTime-dependent version shown
  • Solutions to the wave equation show that electron trajectories are not sphericalArranged in ‘shells’ spdDonut shape
  • Electronic energy states are quantisedAbsorption and emission spectra (colour)Einstein predicted lasers (stimulated emission)
  • Some things are fundamentally unknowable
  • Thought experiment50:50 chance exists as both alive and dead until it is observed
  • Now we deal with systems in terms of probabilityProbability amplitude: diffraction if two slits, not if one (or a detector at one)Thomas Young (probably), 1803
  • Classic view of light wavefront slowing (army marching through swamp)QED view of light taking every single available route, but probability reinforcing
  • Personal heroDouble Nobel laureateSafe cracking at Los AlamosChallenger disasterBongo playerAmazing humility(auto)biographies
  • Time, space axisWiggly = photonStraight = electron or other massive particle
  • First Feynman diagram 1949Photon travels back in time!
  • Matter and antimatter
  • One of the things QED allows us to calculatePredicted value adjusted by correction terms
  • Despite the strangeness of ideas above, it provides the most remarkable agreement between theory and measurementDifference equivalent to thickness of human hair when measuring the distance from East to West Coast USA
  • Also: Lamb shift, Compton scattering, etc.
  • QED: Quantum Electrodynamics

    1. 1. Quantum Electrodynamics<br />The interaction of light with matter<br />Tristan, Cogapp PechaKucha, August 2010<br />
    2. 2.
    3. 3. Light<br />Matter<br />
    4. 4.
    5. 5.
    6. 6. Schrödinger equation<br />
    7. 7. Electron orbital shells<br />
    8. 8. Energy is quantised<br />
    9. 9. Heisenberg Uncertainty Principle<br />It is impossible to specify simultaneously, with arbitrary precision, both the momentum and the position of a particle.<br />
    10. 10. Schrödinger’s cat<br />
    11. 11. Two-slit experiment<br />
    12. 12. Refraction of light<br />
    13. 13. Richard Feynman<br />
    14. 14. Feynman diagrams<br />
    15. 15. Time travel<br />
    16. 16. Annihilation and disintegration<br />
    17. 17. Magnetic moment of the electron<br />
    18. 18. 99.9999975% accurate<br />USA<br />Human hair<br />
    19. 19. Summary<br />Wave-particle duality<br />Quantised energy levels<br />Uncertainty<br />Time travel<br />Spontaneous creation and destruction<br />… but it works.<br />
    20. 20. Further reading<br />www.roddis.org/qed<br />www.bbc.co.uk/archive/feynman/<br />

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