If it were possible to measure the
exact position and velocity of every particle in the universe at a certain time, then by applying
the basic laws of Physics we should be able to predict their state at any future time - for
example, we could say how much rain would fall on Taunton on the third of January in the year
2500!

The uncertainty principle of Heisenberg states that it is actually impossible to
make such a measurement

In fact, if DE, Dx and Dp represent the smallest
uncertainty with which the energy, position and momentum of a particle can be measured and
Dt the smallest uncertainty in the time of measurement,
then:

where h is the Planck constant (6.6x10^{-34} Js).

An idea of the
development of this equation can be seen in the following simplified idea of electron diffraction.
If an electron passes through a slit and is diffracted through an angle q then sinq = l/Dy

Also tanq = q^{c} = Dv_{y}/v_{o}.

Therefore: Dv_{y}/v_{o} = l/Dy and so Dv_{y}Dy = lv_{o}

But l = h/mv and therefore Dv_{y}Dy = h/m and so

DpDy = h (which is close to the equation of Heisenberg!)

If Dy decreases then Dv_{y} increases – so if the
electron gets through the slit its position will be known within Dy.

The concept of the detached observer of classical Physics is
a myth, since a completely isolated universe cannot be observed. An observer must always be
part of an experiment - otherwise there is no experiment.

The 'true' nature of a particle,
or at least the modern view of it, is that it has both wave and particle properties described by the
wave mechanical model of Schroedinger. This model is complex but simply states that we
cannot fix the position of a particle (as in classical Physics) but can only predict the probability of
it being at a given point at a certain time. It could therefore be anywhere, but is in fact more
likely to be at one place than another. The square of the amplitude of the wave associated with
a particle at a given point in space and time is proportional to the probability of the particle being
at that point.

Further consideration of this subject is beyond the scope of this text, but it
is hoped that this very brief introduction has been sufficient to demonstrate the uncertainty of
Physics on a nuclear level. Remember that all the world around us is composed of waves, even
ourselves.

As you read this, there is only a probability that you are there at
all!