Introduction to

QEDQuantum 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?