Quantum physics is physics that takes the
postulate of the quantum as one of its foundations.
It is mostly concerned with sub-atomic particles,
atoms, and molecules, and the radiation that
belongs to them.
The postulate of the quantum is that change
in Nature is through jumps between sub-atomic
stationary states.
There are three principal versions of quantum
physics:
the old quantum theory
quantum mechanics
quantum field theory.
A fourth version, mainly the work of Louis
de Broglie, is less widely studied.
Old quantum theory
The old quantum theory originated in 1900,
with the work of Max Planck.
It accepts the postulate of the quantum, but
does not live by all of its consequences.
A fundamental consequence of the postulate
is that the finest possible observation is
the detection of a single quantum.
The old quantum theory does not recognize
this, and consequently is unable to express
itself in terms of strictly experimentally
based concepts.
For this reason, it is largely obsolete.
Nevertheless, it has a degree of physical
interest, for example in simple studies of
the hydrogen atom, initiated by Niels Bohr.
Quantum mechanics
Quantum mechanics replaced the old quantum
theory, starting in 1925 and 1926, initiated
by Werner Heisenberg and Erwin Schrödinger.
Quantum mechanics fully lives by the postulate
of the quantum.
But it does not give a picture of quantal
processes that is expressible in terms of
ordinary physical space-time and its intrinsic
partner, causality.
This is because quantum mechanics is defined
by a mathematical formalism that is built
on the abstract concept of the wave function.
The wave function has as its domain an abstract
mathematical object called configuration space.
Its range is another abstraction, the so-called
"probability amplitude".
Neither configuration space nor probability
amplitude are intuitively visualizable or
immediately recognizable in ordinary everyday
language or thinking.
In this they differ respectively from ordinary
ideas of physical space-time and of betting
odds.
Consequently, there arise questions of interpretation
of quantum mechanics.
A fundamental ontological category of quantum
mechanics is the quantum state, represented
in the wave function.
An observed fact in quantum mechanics is primarily
that the system of interest has shown itself
to be in some quantum state.
In general, a quantum state is not fully located
in ordinary physical space-time.
Quantum field theory
Quantum field theory lives by the postulate
of the quantum, but in a way that expresses
experimental observations more directly physically
than does quantum mechanics.
Its primary observed facts are quantum detection
rates as functions of ordinary physical space-time,
rather than the somewhat abstract quantum
states of quantum mechanics.
This is expressed in the scattering matrix,
rather than in the more abstract wave function
that characterizes quantum mechanics.
The domain of the field of quantum field theory
is ordinary physical space-time, in contrast
with the abstract configuration space of quantum
mechanics.
The range of the field of quantum field theory
is still rather far from ordinary thinking;
it is distinctly of a quantal nature.
But the interpretational puzzles of quantum
mechanics are eased in quantum field theory
because the domain of the field is ordinary
physical space-time.
One way of expressing the profound difference
between quantum field theory and quantum mechanics
is in saying that quantum field theory is
constructed by second quantization.
Ordinary physical space-time is a fundamental
ontological category of quantum field theory,
in contrast to quantum mechanics, which in
some respects renounces it.
Interpretation
Albert Einstein was famously or infamously
unhappy with quantum mechanics because its
primary domain is configuration space, not
ordinary physical space-time.
Consequently, Einstein was deeply concerned
that quantum mechanics has a serious problem
with causality.
Opposing Einstein in this, Niels Bohr preferred
to renounce the claims of causality.
In order to escape this problem of quantum
mechanics, Einstein tried to develop a field
theory along the lines of general relativity,
including the sub-atomic processes studied
in quantum physics.
He did not succeed in this.
But his concern is significantly addressed
by quantum field theory.
A possible fit between general relativity
and quantum physics is a subject of active
current research.
In some respects quantum field theory is more
physically complete than quantum mechanics,
though merely mathematically, quantum mechanics
may be regarded as complete.
In physical interpretation, quantum field
theory does not suffer from the causality
problem of quantum mechanics.
