218 The Poetry of Physics and The Physics of Poetry
enters our eyes and is absorbed by the electrons in our retina. As the
electrons absorb photons, they also absorb energy, which causes them to
move about. The motion of these electrons is converted by the optical
nerve into an electrical impulse, which is transmitted to our brain where
we become aware of the light.
What can we learn from this description of the production and the
detection of light? What I find most interesting is that the moving
electrons in the light filament caused electrons in the retina to move also
through the exchange of light or photons between them. The photons
were the medium that allowed the electrons in the filament to move the
electrons in the retina. In a sense, the photons transmitted a force from
the filament electrons to the electrons of the retina.
Perhaps we can apply this lesson to help explain how the electric
force between static charges is transmitted through the exchange of
virtual photons. At first this seems to violate the principle of the
conservation of energy. If one simply claims that the electric force arises
from the exchange of photons then one must account for the energy
necessary to create the exchanged photon.
In the case of the light produced by the moving electrons in the light
fixture the energy necessary to create the photons came from the de-
excitation of the atoms of the filament of the light fixture. Those photons
that were transmitted to the electrons in the retina transferred their
energy to those retinal electrons providing them with the kinetic energy
of their motion.
In the case of static charges, however, the particles remain at
rest before and after the transmission of the force. The electrostatic
force, however, is independent of the motion of the charges. If the
charges remain static throughout the electrostatic interaction there is no
way to provide the energy for any photon that may be transmitted
between the two charges. This is best seen by considering the time just
prior to the hypothetical exchange of a photon, the time during the
exchange and the time just after the exchange. Just before the exchange
and just after the exchange the total energy of the system is just equal to
the energy of the two static electrons. But during the time of the
transmission of the photon there is, in addition to the energy of the two
static electrons, the energy of the exchanged photon, and hence, during
this time there is a violation of energy conservation.
This would seem to kill our hypothesis. However, if we invoke the
uncertainty principle it is possible to resurrect our scheme. A violation of