 
First of all, I point out
that quantum electrodynamics
was first set up in the form of
a quantum theory of Maxwell's
electromagnetic
field by Paul Dirac.
And what he was
doing was providing
the theoretical
framework for what,
in Einstein's work in 1905, was
a rather brilliant idea, which
simply didn't fit in with
the previous classical ideas
at all.
Later only, quantum
electrodynamics
as this quantum theory
of interacting photons
and electrons became the first
very successful quantum field
theory, in the
sense that it made
extremely accurate predictions.
That didn't happen
until around 1950.
And previously it had gone
through a rather difficult
period where people
didn't understand
how to handle infinite
quantities which
occurred in the formalism.
We'll be coming back
to infinite quantities
in the context of
other interactions
later in this series.
 
The other point I would pick
up is the matter of symmetries.
The importance of symmetries
in classical mechanics
has already been referred
to in the first week.
However, that's a
very late development
in classical mechanics.
The connection between
symmetries and conservation
laws didn't really get
properly formalized 'til
the work of Emmy Noether
in the early 20th century.
 
But from the point of
view of these lectures,
the relation between symmetries
and conservation laws
is a very important idea.
 
