Introduction to
QED
Quantum Electrodynamics
Introduction
• Created in 1929 by a number of scientists to
describe the interaction of light and matter
• Melding of Maxwell’s theory and quantum
mechanics
• Attempts to describe behavior of electrons
• Paul Dirac made relativistic adjustments
• 1948 – Richard Feynman, Julian Schwinger and
Sin-Itiro Tomonaga calculated the correction due
to light
Significance
• Describes all phenomenon but gravity and
radioactivity
• QED is the theory behind chemistry and
governs properties of chemicals
• Has survived over 50 years of testing
Basics
• 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
Glass Thickness
Arrow Lengths
Adding Arrows
Determining Direction
Partial Reflection
As the glass gets thicker…
Extremes
For varying frequencies:
Light Propagation
• A photon has nearly equal chances of going
on any path.
• Therefore, all the arrows are nearly the
same length.
• This difference is negligible.
Mirrors and Angle of Incidence
Equal Chances
Arrows have equal lengths, but
different directions.
The middle contributes more.
Where the time is least is also where the time for
the nearby paths is nearly the same; that’s where
the little arrows point in nearly the same direction
and add up to a substantial length; that’s where the
probability of a photon reflecting off a mirror is
determined. And that’s why, in approximation, we
can get away with the crude picture of the world
that says light only goes where the time is least.
-Richard Feynman.
Time is least where the angle of incidence equals
the angle of reflection.
The Edges of the Mirror
Cutting out Pieces
Diffraction Grating
Light through Multiple Media
The Lifeguard
Mirage
Light travels in straight lines?
Light does not move in a straight path,
but rather uses a core of nearby space.
(neighboring paths)
Restricting the Paths
(Single Slit Diffraction)
Uncertainty Principle?
• If the paths are too restricted, the light
spreads out.
• There is no need for the uncertainty
principle.
Light traveling through many
paths.
The Focusing Lens
Diagrams:
Feynman, Richard P. QED: The
Strange Theory of Light and Matter.
Princeton University Press.
Princeton, NJ, 1988.
Questions?