Introduction to
QED
Quantum Electrodynamics
Part II
Recap
• Describes what happens, not why
• Light behaves like particles, not waves
• Only probability can be calculated
• Little arrows (“probability amplitudes”)
General Principle of Quantum
Theory
The probability that a particular event
occurs is the square of a final arrow
(probability amplitude) that is found by
drawing an arrow for each way the event
could happen, and then combining (adding)
the arrows.
Compound Events
• Events that can be broken down into a
series of steps, or events that consist of a
number of things happening independently
• Multiplication of probability amplitudes
• “turning and shrinking”
Partial Reflection of Multiple
Surfaces
Multiplying Arrows
• Expressed as a transformation of the unit
line
Successive Transformations
• Transformations can be applied in
succession
• Work just like the rules for numbers
Successive Transformations
Reflection by a Single Surface
Transmission by a Single Surface
Reflection from the Back Surface
Reflection by Two Surfaces
• Average of 8% (4% each for front and back
surfaces)
• Fluctuates between 0% and 16%, depending
on the thickness of the glass
• The rest are transmitted
Transmission by Two Surfaces
Making the Sum Probabilities
100%
• When reflection is 0%, sum is 92%
• When reflection is 16%, sum is 108%
• Compensate by considering all possibilities
Other Possibilities
100% Probability
More Reflection Possibilities
Independent Events
• If one of the ways a particular event can
happen depends on a number of things
happening independently, the amplitude for
this way is calculated by multiplying the
arrows of the independent things.
Simultaneous Transmission and
Absorption
Another Possibility
Coming Soon…
• Electron interactions
• Feynman Diagrams
Diagrams:
Feynman, Richard P. QED: The
Strange Theory of Light and Matter.
Princeton University Press.
Princeton, NJ, 1988.
Questions?