Professor Dave here, let's talk about
quantum electrodynamics.
After spending some time with Einstein looking
at the nature of space and time, we have
to go back to the quantum realm and pay
some attention to particles. In classical
physics we became aware of field forces
like gravity and electromagnetism. These
are forces whereby a gravitational or
electromagnetic field imparts force onto
some object, which generates an
acceleration in that object, like a rock
falling to the earth, or the attraction
and repulsion between charged particles.
But as quantum theory progressed, it
began to find a way to redefine these
field forces, and show that the idea of a
field becomes obsolete if we are instead
able to explain these forces as the
interaction between certain kinds of
particles. The first complete step in
this direction came from Richard Feynman,
an American physicist and all-around
peculiar man. Building upon prior work by
Dirac and others, Feynman and his
colleagues worked under the assumption
that the electromagnetic force was not
actually mediated by electromagnetic
fields, but rather by interactions
between virtual photons. Let's recall
from our discussion of the Heisenberg
uncertainty principle that tiny
particles of different varieties are
actually able to manifest from nothing
because of the uncertainty associated
with time and energy, and particles that
do this are called
virtual particles, because they do not
exist with the same sense of permanence
as other particles that make up ordinary
matter. But if it were the case that
charged particles like protons and
electrons were able to manifest these
virtual photons, then the attraction and
repulsion between charged particles
could be explained by the exchange in
momentum that occurs during collisions
between virtual photons and regular
particles. No electromagnetic field would
have to be referenced. This means that
classical electrodynamics can be
explained by quanta,
and therefore quantum mechanical
principles, and since light is a subject
of special relativity,
this makes QED a successful merging
between special relativity and quantum
theory. To illustrate the exchange of
virtual particles, Feynman developed a
system for representing quantum
phenomena called Feynman diagrams.
In these surprisingly simplistic
representations, a straight line
signifies an electron, which we know has
both mass and charge. A wavy line
signifies a photon, which has neither
mass nor charge. And when there is a
junction between a straight line and a
wavy line, this means that the electron
has either absorbed or emitted a virtual
photon. These diagrams are mere
reductions of reality. They do not
contain all the information that could
be pertinent to a system of one or more
electrons. But they illustrate in a basic
way that two electrons could approach
one another, and then have their paths
deflected due to the exchange of one or
more virtual photons that carry some
momentum. Given that this is quantum
mechanics, there is a ton of math we are
glossing over, but the main takeaway is
that the electromagnetic force has been
demonstrated to be the result of
particle exchange or quanta, rather than
an electromagnetic field. Quantum field
theories such as QED seek to explain all
four fundamental forces in this regard,
and they have been largely successful, so
let's move forward and see which force
was to be updated next.
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