Q.E.D.(Quantum Electro Dynamics)

Physics 43, SRJC, spring 2008

• Richard P. Feynman (the father of Quantum Electro Dynamics)

Presented by:

• Anthony Disbrow• Kelly Jewett• Scot Spinner

History• The word electrodynamics was originally coined by

André-Marie Ampère in 1822.

• The phrase "quantum physics" was first employed in Johnston's Planck's Universe in Light of Modern Physics.

• Quantum theory began in 1900, when Max Planck assumed that energy is quantized in order to derive a formula predicting the observed frequency dependence of the energy emitted by a black body.

• The concept that light energy comes in small “packets” was first explored by Einstein in his paper on the photoelectric effect.

http://en.wikipedia.org/wiki/Quantum_electrodynamics

More History...• Feynman's mathematical technique, based on his

diagrams, initially seemed very different from the field-theoretic, operator-based approach.

• QED served as a role model for quantum chromodynamics

• Starting in 1975, physicists showed how the nuclear weak force and QED could be unified into the electro weak force

http://en.wikipedia.org/wiki/Quantum_electrodynamics

What is Q.E.D.?• QED mathematically describes all phenomena involving

electrons, positrons, the exchange of photons and the electromagnetic field.

• QED was the first satisfactory quantum description of a physical field and of the creation and annihilation of quantum particles.

http://en.wikipedia.org/wiki/Quantum_electrodynamicshttp://hyperphysics.phy-astr.gsu.edu/Hbase/forces/qed.html

What is Q.E.D.? Con’t

• QED is used to approximate interactions in electrodynamics

• Perturbation theory is used to find an approximate solution to a problem or experiment with no exact solution.

http://en.wikipedia.org/wiki/Perturbation_theory

Use of Perturbation Theory

• Perturbation theory is used to approximate solutions to the Schrödinger equation . Using perturbation theory, we can use the known solutions to simple Hamiltonians such as that of the hydrogen atom, to generate solutions for more complicated systems

http://en.wikipedia.org/wiki/Perturbation_theory_(quantum_mechanics)

Accuracy

• QED can predict the probability of what will happen in an experiment

• It can predict the outcome of an experiment with an error of only 10−12

• QED is one of the most accurate physical theories to date

http://www.britannica.com/EBchecked/topic/486203/quantum-electrodynamics

Feynman Diagrams

http://hyperphysics.phy-astr.gsu.edu/hbase/particles/imgpar/feynm5.gif

http://www.youtube.com

Mathematical exampleMathematically, QED has the structure of an abelian gauge theory, with the symmetry group U(1) as gauge group. The gauge field which mediates the interaction between the charged spin-1/2 fields is the electromagnetic field. The QED Lagrangian for the interaction of electrons and positrons through photons is

                                          where

   are Dirac matrices.   and its Dirac adjoint   are the fields representing electrically charged particles, specifically electron and positron fields represented as Dirac spinors.                      is the gauge covariant derivative, with    the coupling strength (equal to the elementary charge),     the covariant four-potential of the electromagnetic field and                           the electromagnetic field tensor.

http://en.wikipedia.org/wiki/Quantum_electrodynamics