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?