If the quantum equations are set up independently in these locations in
different gauges, the dynamics of my electron and that in another continent or
on the moon must be consistently accounted for; the results must be independent
of the local choice of gauge. In 1947, in his pioneering work on re-normalizing
QED, American theorist Julian Schwinger proved that for this gauge invariance to
occur there must be some connection linking the various electrons and allowing
us to compare the situation at the different locations. In quantum field theory,
this connection consists of particles. The maths implied that the connection
cares about direction—it is a vector—and the associated particle acts like a
boson because of bosons’ ability to act cooperatively, in this case by building
up the vector field connecting the electrically charged particles. So was born
the concept of the gauge boson, which in the case of electromagnetism is the
familiar photon.
The connection must be able to act over very large distances, and in quantum
field theory this equates to the gauge boson having no mass. In summary,
Schwinger had proved that gauge invariance implies that an electromagnetic force
necessarily occurs between electrically charged bodies, and that this force is
carried by a photon of zero mass. That a photon has no mass and travels through
the void at nature’s speed limit is fundamental to Einstein’s special relativity
theory. However, according to QED the vacuum is not empty because the photon is
immersed in a sea of virtual electrons and positrons, which ensnare it,
interrupting its flight. As QED implies that an electron at rest gains an
infinite energy—or mass—because of these interactions, how does a photon manage
to avoid a similar fate?
By carefully examining the formulae in QED theory, Schwinger concluded that
gauge invariance in QED underpins this phenomenon of the massless photon. This
link between gauge invariance, the existence of a force, the vanishing mass of a
photon, and the ability to make QED viable thanks to re-normalization was a
profound result, which in the course of time would have far-reaching
implications.