Introduction to
QED
Quantum Electrodynamics
Part III
Double Slit Diffraction
Interference occurs and probability is between
0% and 4%
When Detectors are Added
Interference disappears and probability is
always 2%
When Detectors are Fallible
Electron – Photon Interactions
• Electrons behave like photons.
- On a large scale, they appear to move in
straight lines.
- On a small scale, they can move
anywhere.
- Interference becomes important.
Electron – Photon Interactions
Three basic actions:
- A photon goes from place to place.
- An electron goes from place to place.
- An electron emits or absorbs a photon.
Objects Moving from Place to Place
Light’s Probability of Movement
• Probability is dependent on only the
distance the photon must travel and the time
it takes to do so (difference of squares).
• Photon moves at c when distance and time
components are equal (at a 45 angle).
• Photon’s highest probability occurs at speed
c.
Light Diagrams
Electron Diagrams
Coupling
• An electron has a specific probability to
emit or absorb a photon.
• Since probability for either to happen is
equal, they will be regarded as the same
event and be referred to as “coupling.”
• The probability of coupling is the constant
“j” (junction number), which is related to
the electron’s charge. j  -0.1
Coupling
Simultaneous Electron
Movements
More Possibilities
Photons Moving at Multiple
Speeds
Two Photons Exchanged
Diminishing Contribution
• The more couplings that are required, the
more times the probability must be
multiplied by j.
• Since j is less than one, the probability of
occurrence decreases with every required
coupling.
Photon Scattering
• Defined as when an electron absorbs and
emits a photon, but not necessarily in that
order
• A photon can disintegrate into an electron positron pair.
• An electron and positron can annihilate and
become a photon or two.
Photon Scattering
Electrons in an Atom
• Electrons are kept in “orbit” by exchanging
photons with the protons in the nucleus.
• Scattering of photons by electrons in atoms
is the cause of numerous optical
phenomena.
Hydrogen Atom
Partial Reflection
Partial Transmission
Index of Refraction
• The additional turning of the probability amplitude
causes the photon to appear to be moving more
slowly through the material.
• The more opaque the material, the more turning
occurs and the more slowly the light appears to
travel.
• The amount of turning by the final arrow caused
by the electrons in a material is called the “index
of refraction.”
Stimulated Emission
• Photons tend to get into the same state.
• The chance that an atom emits a photon is
enhanced if some photons are already
present.
• Principle used in lasers.
Two Photons Moving
When the Points Converge
Electrons and the Exclusion
Principle
Magnetic Moment of an Electron
• Represents the response of an electron to an
external magnetic field
• The number changes over time as more
possibilities for an electron to absorb a
photon are calculated.
An Alternative Possibility
Dirac’s Basic Diagram
More Complex Possibilities
Diagrams:
Feynman, Richard P. QED: The
Strange Theory of Light and Matter.
Princeton University Press.
Princeton, NJ, 1988.
Questions?