Editing Quantum electrodynamics (section)

===Probability amplitudes===
[[File:Feynmans QED probability amplitudes.gif|frame|right|Feynman replaces complex numbers with spinning arrows, which start at emission and end at detection of a particle. The sum of all resulting arrows gives a final arrow whose length squared equals the probability of the event. In this diagram, light emitted by the source '''''S''''' can reach the detector at '''''P''''' by bouncing off the mirror (in blue) at various points. Each one of the paths has an arrow associated with it (whose direction changes uniformly with the ''time'' taken for the light to traverse the path). To correctly calculate the total probability for light to reach '''''P''''' starting at '''''S''''', one needs to sum the arrows for ''all'' such paths. The graph below depicts the total time spent to traverse each of the paths above.]]

[[Quantum mechanics]] introduces an important change in the way probabilities are computed. Probabilities are still represented by the usual real numbers we use for probabilities in our everyday world, but probabilities are computed as the [[square modulus]] of [[probability amplitude]]s, which are [[complex number]]s.

Feynman avoids exposing the reader to the mathematics of complex numbers by using a simple but accurate representation of them as arrows on a piece of paper or screen. (These must not be confused with the arrows of Feynman diagrams, which are simplified representations in two dimensions of a relationship between points in three dimensions of space and one of time.) The amplitude arrows are fundamental to the description of the world given by quantum theory. They are related to our everyday ideas of probability by the simple rule that the probability of an event is the ''square'' of the length of the corresponding amplitude arrow. So, for a given process, if two probability amplitudes, '''v''' and '''w''', are involved, the probability of the process will be given either by

:<math>P = |\mathbf{v} + \mathbf{w}|^2</math>

or

:<math>P = |\mathbf{v} \, \mathbf{w}|^2.</math>

The rules as regards adding or multiplying, however, are the same as above. But where you would expect to add or multiply probabilities, instead you add or multiply probability amplitudes that now are complex numbers.

[[File:AdditionComplexes.svg|thumb|right|200px|Addition of probability amplitudes as complex numbers]]
[[File:MultiplicationComplexes.svg|thumb|right|200px|Multiplication of probability amplitudes as complex numbers]]

Addition and multiplication are common operations in the theory of complex numbers and are given in the figures. The sum is found as follows. Let the start of the second arrow be at the end of the first. The sum is then a third arrow that goes directly from the beginning of the first to the end of the second. The product of two arrows is an arrow whose length is the product of the two lengths. The direction of the product is found by adding the angles that each of the two have been turned through relative to a reference direction: that gives the angle that the product is turned relative to the reference direction.

That change, from probabilities to probability amplitudes, complicates the mathematics without changing the basic approach. But that change is still not quite enough because it fails to take into account the fact that both photons and electrons can be polarized, which is to say that their orientations in space and time have to be taken into account. Therefore, ''P''(''A'' to ''B'') consists of 16 complex numbers, or probability amplitude arrows.<ref name=feynbook/>{{rp|120–121}} There are also some minor changes to do with the quantity ''j'', which may have to be rotated by a multiple of 90° for some polarizations, which is only of interest for the detailed bookkeeping.

Associated with the fact that the electron can be polarized is another small necessary detail, which is connected with the fact that an electron is a [[fermion]] and obeys [[Fermi–Dirac statistics]]. The basic rule is that if we have the probability amplitude for a given complex process involving more than one electron, then when we include (as we always must) the complementary Feynman diagram in which we exchange two electron events, the resulting amplitude is the reverse – the negative – of the first. The simplest case would be two electrons starting at ''A'' and ''B'' ending at ''C'' and ''D''. The amplitude would be calculated as the "difference", {{nowrap|''E''(''A'' to ''D'') × ''E''(''B'' to ''C'') − ''E''(''A'' to ''C'') × ''E''(''B'' to ''D'')}}, where we would expect, from our everyday idea of probabilities, that it would be a sum.<ref name=feynbook/>{{rp|112–113}}