Quantum physics tells us that
at a fundamental level,
the world behaves less like
hard billiard balls
and more like ripples in a pond.
This wave nature of reality
means that what we imagine
as distinct particles of matter
are actually quite fuzzy.
In quantum physics,
this characteristic of reality
is captured in the Heisenberg
uncertainty principle.
The Heisenberg uncertainty
principle plays a big role
in these very very very
small scales.
So at the scale of atoms.
It's a principle that states
that you cannot know
both the position and the 
momentum of a particle
to arbitrary accuracy
both at the same time.
But,
the particle is not the only
thing that can have
a wave nature.
Water waves for example,
radio waves,
any kind of waves that we see
in our world
can actually have a similar
uncertainty relation.
Consider sound waves.
A typical tune is composed of
many overlapping frequencies
but if we play a specific note,
like Middle C,
its frequency falls within
a narrow band.
The width of this band
is a measure
of the uncertainty about its
precise frequency.
The shorter the duration of
a note,
the wider its bandwidth
and greater the uncertainty
in its frequency.
The longer the duration
of the note
the narrower its bandwidth,
and the more precise
its frequency.
If you take a signal
which is this thick, you know,
the width is this,
and the bandwidth is this,
these two widths,
if you multiply them,
is a constant.
So if you narrow this,
this will increase,
if you make this fatter,
this will decrease.
The mathematics is identical
to the Heisenberg
uncertainty principle.
Heisenberg determined similar
relationships for particles
at the quantum level.
For instance,
the uncertainty in position,
written as delta x
multipled by the uncertainty
in momentum,
written as delta p,
is always greater than or equal
to a constant
written as h over four pi.
We can see the Heisenberg
uncertainty principle
at play in the diffraction
of light.
So, what we can do is
we can have a little slit,
and we can point a laser
through the little slit,
and we can slowly, slowly,
slowly, make the size
of the slit smaller and smaller
and smaller.
In the beginning,
there will be a pattern
on the wall
which is basically a blob
of light or a point of light
coming from the laser.
But after a certain time,
when we make it even tinier,
then actually what we see
is that this blob of light
kind of diffracts out into
a large pattern on the wall.
This can be explained by
the Heisenberg uncertainty
principle.
When we are making the size
of the slit smaller,
We're trying to make sure
that we are sending
the beam of light through
a very accurate position size.
This in turn translates into
saying that there will be
an uncertainty in measuring
the momentum of
the light packet,
so this uncertainty
manifests itself
as an angular uncertainty which
falls on the wall
and produces this
diffraction pattern.
Heisenberg extended the
uncertainties
that are evident in all waves
to the realm of 
quantum particles.
We cannot determine
where an electron is
at a particular point in
space time,
and then also determine
what its energy,
or what its momentum is
at the same particular
space-time point.
And this is very 
important because
it tells us about the
stability of matter.
So when we start from classical
electrodynamics,
the problem was that there's the
nucleus at the centre,
and the electron which is
accelerating around
the nucleus at the centre,
and if it continues on this
acceleration than it should go
closer and closer and closer
to the nucleus and
fall into the nucleus
but that does not happen
because we know that
most matter is stable and the
universe is not disintegrating.
Here's where the Heisenberg
uncertainty principle kicks in,
and it says that, if we know
that the electron is going
closer and closer and closer
to the nucleus then we know
its position to great accuracy.
And if we know its position
to great accuracy,
that means uncertainty
in its momentum
is increasing and increasing
and increasing the more
and more closer it comes
to the centre of the nucleus.
And this is not possible because
an electron can only have
so much energy, as we can
calculate from electrodynamics.
And because this is
not possible,
we know that there is an
inherent uncertainty
in where the position of
the electron is.
If quantum mechanics is
correct,
there is no way of measuring
its position or momentum
accurately simultaneously,
it has nothing to do with the
limitations of our instruments.
It's a fuzziness
built into nature.
The Heisenberg uncertainty
principle reflects the
fundamental nature
of reality,
and it is that uncertain nature
that allows for atoms,
chemistry,
and all the solid stuff
of our every day world.
It does give us a new
insight into reality,
it tells us that in reality
things are fuzzy,
but at the same time,
it also opens up a door
to many different ways of
viewing that reality.
It's an extremely rich sort of
doorway into what reality is.
