Poetry of Physics and the Physics of Poetry

Quantum Electrodynamics 221

exchange can be represented pictorially with diagrams first devised by
Richard Feynman. Fig. 22.1 shows the diagram for single photon
exchange.^212 The Poetry of Physics and The Physics of Poetry^

Fig. 22.1 Photon exchange by electrons

Time is increasing in this diagram as we move from left to right. The

lines labeled e 1 , e 2 , e’ 1 and e’ 2 represent electron 1 and 2 before and after

their mutual interaction. At time t 1 , e 1 emits a virtual photon represented

by the wiggly line, which is absorbed by e 2 at time t 2. Note that as a

result of the emission at t 1 , e 1 changes to e’ 1 and as a result of the

absorption at t 2 , e 2 changes to e’ 2. Corrections to this diagram are shown

below in which two photons are exchanged. There are three possible

diagrams of this type shown below in Fig. 22.2.

Fig 22.2 Two photon exchange diagrams

The first two diagrams are equivalent to each other so that there are

only two types of diagrams involving the exchange of two photons. The

contribution of the two-photon exchange diagrams of Fig. 22.2 is a small

correction to the one photon exchange diagram of Fig. 22.1. Each time a

photon is exchanged a factor of e^2 /hc enters the probability of the

exchange taking place. Since e^2 /hc = 1/137 the relative probability of two

photon exchanges to one photon exchange is 1/137. The relative

probability of n photon exchange to one photon exchange is (1/137)n-1.

Each successive order of photon exchange becomes less and less

probable. Nevertheless, these small corrections have been calculated and

have been shown to agree with experiment as pointed out above. In

addition to the diagrams in which two photons are exchanged between

the charged particles, there are other two-photon diagrams in which only

Fig. 22.1 Photon exchange by electrons.

Time is increasing in this diagram as we move from left to right. The
lines labeled e 1 , e 2 , e′ 1 and e′ 2 represent electron 1 and 2 before and
after their mutual interaction. At time t 1 , e 1 emits a virtual photon
represented by the wiggly line, which is absorbed by e 2 at time t 2. Note
that as a result of the emission at t 1 , e 1 changes to e′ 1 and as a result of
the absorption at t 2 , e 2 changes to e′ 2. Corrections to this diagram are
shown below in which two photons are exchanged. There are three
possible diagrams of this type shown below in Fig. 22.2.

212 The Poetry of Physics and The Physics of Poetry

Fig. 22.1 Photon exchange by electrons

Time is increasing in this diagram as we move from left to right. The

lines labeled e 1 , e 2 , e’ 1 and e’ 2 represent electron 1 and 2 before and after

their mutual interaction. At time t 1 , e 1 emits a virtual photon represented

by the wiggly line, which is absorbed by e 2 at time t 2. Note that as a

result of the emission at t 1 , e 1 changes to e’ 1 and as a result of the

absorption at t 2 , e 2 changes to e’ 2. Corrections to this diagram are shown

below in which two photons are exchanged. There are three possible

diagrams of this type shown below in Fig. 22.2.

Fig 22.2 Two photon exchange diagrams

The first two diagrams are equivalent to each other so that there are

only two types of diagrams involving the exchange of two photons. The

contribution of the two-photon exchange diagrams of Fig. 22.2 is a small

correction to the one photon exchange diagram of Fig. 22.1. Each time a

photon is exchanged a factor of e^2 /hc enters the probability of the

exchange taking place. Since e^2 /hc = 1/137 the relative probability of two

photon exchanges to one photon exchange is 1/137. The relative

probability of n photon exchange to one photon exchange is (1/137)n-1.

Each successive order of photon exchange becomes less and less

probable. Nevertheless, these small corrections have been calculated and

have been shown to agree with experiment as pointed out above. In

addition to the diagrams in which two photons are exchanged between

the charged particles, there are other two-photon diagrams in which only

Fig. 22.2 Two photon exchange diagrams.

The first two diagrams are equivalent to each other so that there are
only two types of diagrams involving the exchange of two photons. The
contribution of the two-photon exchange diagrams of Fig. 22.2 is a small
correction to the one photon exchange diagram of Fig. 22.1. Each time a
photon is exchanged a factor of e^2 /hc enters the probability of the
exchange taking place. Since e^2 /hc = 1/137 the relative probability of two
photon exchanges to one photon exchange is 1/137. The relative
probability of n photon exchange to one photon exchange is (1/137)n-1.
Each successive order of photon exchange becomes less and less