What is an Amplitude in Quantum Electrodynamics (QED)?

After looking at some Richard Feynman material on QED I'm putting down some thoughts on amplitudes so that (a) I remember and (b) I remember - yes, I did type that twice.

An amplitude can be described as a line on a plane.  The line can be of any particular length and angle on that plane.  We can use an amplitude to get the probability of an event - to do this we simply square the amplitude, i.e.:

   probability = (ampl)^2

QED is concerned with and can be summarised at a very high level by the following 3 points:

• the probability that an electron goes from place to place
• the probability that a photon goes from place to place
• the probability that an electron will 'emit' a photon

The dimension of time is added to the above so that probabilities can be calculated for electrons and photons to be in a certain place at a certain time.

(more to follow - it's 3am after all ..)

My Notes on Why Agile Works..

After watching Martin Fowler's talk in France I decided to condense the important points from the talk in my own word so that they are easy to remember:

1. we have moved from predictive planning (waterfall) to adaptive planning - because most requirements are NOT static, therefore massive amounts of up-front analysis and planning are doomed to failure (developers have known this for years when having to look at unrealistic project plans)
2. the more traditional fixed type of processes can and often do stifle people and stop them working effectively
3. there has been a shift from people following processes to the process following and adapting to the people - it's now a people-driven process
4. we are moving to evolutionary design
5. it's common that organisations decide to follow agile methods to manage the project but the design of the software is not designed for change - it has been noticed that this is particularly a problem with followers of SCRUM
6. feedback loops are central to Agile 
7. Agile methods are much more effective when combined with XP practices such as pair programming, TDD, continuous integration etc.  This approach combines a flexible agile project management methodology with software that has been designed for change - loose coupling of components for example
8. it's very important when starting and Agile approach to first get the quality of the software under control.  Implement unit tests linked to the build and get quick feedback in case of regression test failures.