Transcript Q.E.D. (Quantum Electro Dynamics)

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_electrodynamics
http://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/quantumelectrodynamics
Feynman Diagrams
http://hyperphysics.phy-astr.gsu.edu/hbase/particles/imgpar/feynm5.gif
http://www.youtube.com
Mathematical example
Mathematically, 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