# Introduction of Concepts

In the previous review of the ‘*Pre-War’* developments of quantum
mechanics, we saw that there were aspects of the theory that had begun
to invoke much philosophical debate about the true nature of reality,
at least, at the quantum level. While this debate has still not really
been resolved, see *Quantum Philosophy*, we probably need to initially
review some of the mathematical and scientific developments prior to
any further reflection of the philosophical issues. However, even when
the context is confined to the particle models, the discussion of quantum
physics can quickly become pre-occupied with the perception of many
paradoxes, which are often generalised in terms of ‘*quantum weirdness’.
*As such, it is all too easy to lose sight of any ‘*objectivity*’
in such esoteric discussions, before coming to any real understanding
the basic concepts. Therefore, this section will initially try to focus
on what was possibly the most important aspect of quantum physics in
the initial post-war years, i.e. the development of the sub-atomic particle
model.

In many respects, the foundations of post-war quantum physics is
based on many ideas contained within a description called ‘*quantum
field theory (QFT*)’. The totality of QFT is really a specialist
subject in its own right, but in terms of this general introduction,
it will be described in terms of an overarching theory, which in-turn
encompasses many other theories, which might be said to substantiate
the standard particle model. In this context, quantum field theory
sought to answer questions as to how and why an atom radiates light
in the form of a photon, i.e. when an electron transitions between orbital
energy states. Einstein himself called this process ‘*spontaneous
emission’* as early as 1916, but had no way to calculate its rate
as this issue required the development of a relativistic quantum theory
of electromagnetic fields, i.e. a quantum theory of light. Of course,
quantum mechanics is also a theory of matter and so quantum field theory
must encompass not only the original idea of electromagnetic fields,
but also the notion of quantum fields, which were to be subsequently
proposed.

*How would quantum fields come to underpin a particle model?
*

Based on the original pioneering work of Dirac, a new approach to
a quantum field theory called Quantum Electro-Dynamics (QED) was developed,
in 1947, by Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga.
In essence, this variant of QED would side-step the known problem of
infinities via a process called ‘*renormalization*’ in order to
generate finite results. Despite certain reservations in this methodology,
QED was shown to provide predictions of the interaction strength between
an electron and a magnetic field that were experimentally confirmed
to a precision of two parts in 10^{12}. However, in the process,
QED also initiated many other speculative ideas concerning the nature
of empty space, i.e. the vacuum, which some initially considered to
be ridiculous. For example, it was suggested that empty space is not
empty, but rather filled with small, fluctuating electromagnetic fields,
which were said to explain Einstein’s idea of spontaneous emission.

*So did experimental data support this approach? *

In the pre-war era, quantum mechanics was able to make accurate predictions
within the realm of low energy physics. However, in the post-era, experiments
started to involve energy levels that increasingly required relativistic
effects to be taken into consideration. In this context, QFT was seen
as a better methodology in which the original ideas of quantum mechanics
could be reconciled with special relativity, which might then help explain
the existence of two fundamental classes of particles, i.e. *
fermions
and bosons*. However, the original scope of QED was primarily
focused on a class of particles called leptons, i.e. electrons and photons,
as it did not seem to adequately describe another class of particles
called hadrons, which included protons and neutrons plus a more complex
sub-class called mesons. Subsequently, a new theory called *Quantum
Chromo-Dynamics (QCD) *would be developed in order to explain these
different classes of particles. In QED, the force between charged particles,
such as the electron, is said to be *‘mediated’* by photons; while
QCD expands on this description to explain how the force between quarks
is ‘*mediated*’ by gluons. While there were obvious similarities
of approach, QED and QCD were distinct theories that might be said to
sit under the umbrella of QFT, which in combination started to provide
the foundations of the ‘*standard model’*. In this context, QFT
started to explain the plethora of elementary particles being discovered
due to the advances in experimental physics.

*
Were there any other important theories at this time?
*

We should also introduce the idea of the electro-weak theory, which
started to be outlined in the early 1970’s. This theory would help unify
the interaction of the electromagnetic and weak nuclear forces, which
had initially appeared quite distinct. For example, the weak force only
acts across distances smaller than the atomic nucleus, while the electromagnetic
force can extend to great distances, e.g. the light from the stars,
which when described as a radiation weakens with the square of distance.
More importantly, the strength of these two fundamental interactions
between two protons reveals that the weak force is some 10 million times
weaker than the electromagnetic force and therefore it was a major discovery
that these two forces are but different facets of a single, more-fundamental
electroweak force. As such, the electroweak theory developed out of
the attempts to produce a self-consistent ‘*gauge theory’* for
the weak force, which would be analogous to quantum electrodynamics
(QED). The discovery of the W and Z bosons, in 1983, provided the experimental
verification of particles that had previously been predicted by the electro-weak
theory. So, the photon that accounted for the electromagnetic interaction
was now complemented by the W and Z bosons in order to unify both the
electromagnetic and weak interactions.

*So is the particle model now a complete theory? *

Today, the quest to understand the ultimate nature of matter is an
on-going goal of research, which has been compounded by some very speculative
assumptions in the field of *cosmology*. In this context, normal
matter particles that dominate the current periodic table only account
for 4% of the matter of the universe, while the remaining 96% is described
in the form of *dark matter* (23%) and *
dark energy* (73%).
Unfortunately, at this time, there is no definitive description of either
dark matter or dark energy within the standard particle model. As such,
theoretical physics may still have to come up with some fairly radical
ideas in the 21^{st} century.