## What is Quantum Theory?

Quantum theory superceded the classical theory of Newton and Maxwell in the early 20th century. It is still often believed that the effects predicted by quantum theory are subtle and hard-to-measure. In fact, there are numerous easy-to-observe phenomena that classical physics is unable to explain and that quantum theory explains beautifully, such as:

- blackbody radiation
- the stability of atoms
- line spectra of atoms
- radioactive decay
- the photoelectric effect

In
other
words,** classical
physics
fails
to
describe
the
real
world!**

Explaining
these
phenomena
required
the
fundamental
insight
that
on
the
scale
of
atoms **energy
is
quantised**,
i.e.
it
can
only
be
transferred
in
discrete
packets,
called **quanta** (the
Planck
Hypothesis).
For
example,
ordinary
light
comes
in
discrete
packets
of
energy,
called **photons**.

Further development of quantum theory led to the realization that our physical intuition, developed in everyday life, fails significantly at the atomic and subatomic level. One encounters totally new and unfamiliar phenomena such as:

- the uncertainty principle
- wave-particle duality
- anti-particles and anti-matter
- quantum tunnelling
- superconductors
- quantum entanglement

**
The
quantum
world
is
indeed
weird!**

Despite
the
fact
that
quantum
theory
operates
on
the
atomic
level,
it
manifests
itself
in
many
ways
on
the
macroscopic
level,
i.e.
in
everyday
life.
Indeed,
over
the
past
50
years,
quantum
phenomena
have
been
exploited
repeatedly
in
making **technological
advances**,
for
example:

- nuclear reactors
- transistors
- lasers
- photovoltaic cells (solar panels)
- electron microscopes
- Magnetic Resonance Imaging (MRI)
- Charge-Coupled Devices (CCD)

Through
devices
such
as
these
quantum
theory
is
having
an
impact
in
areas
ranging
from
power
generation,
through
information
and
communication
technology,
to
medicine.
In
other
words, **Quantum
phenomena
are
changing
the
world
we
live
in.**