Probability amplitude arrows

1. Jan 17, 2012

AlchemistK

I just saw a few of Feynman's lectures on QED: Reflection and transmission, and he describes the concept of probability amplitude arrows, and their representation as vectors, from what I interpreted, the length of the arrow is proportional to the probability of an event, and its angle is dependent on the time.
In the part where he describes the first law of reflection, the arrows form two spirals. (This is perhaps not the best way to describe it, but the important thing is that it forms a spiral)
Now to form these spirals, the length of each successive arrow should be less than the previous one, but since we do not know the probability of these events (which is what we have to find) and hence the lengths, how can he draw them as spirals?

2. Jan 17, 2012

dlgoff

http://en.wikipedia.org/wiki/Quantum_electrodynamics#Probability_amplitudes

3. Jan 18, 2012

AlchemistK

Yes I did refer the Wikipedia page before coming here, but other than its use like complex numbers, I could not find the answer to my question, which is based on the video. How do we know the length of those arrows?

4. Jan 18, 2012