Have you ever heard of
the Heisenberg uncertainty principle? I was told about it
when I was studying A-Level Chemistry, it basically
states that the more you know about the momentum of a
particle the less you know about the position and vice
Why should that be?
Doesn't that strike you as strange? The very act of
determining a particle's position means that you cannot
know it's velocity.
introduced by Heisenberg, which helps clarify
this idea is; to see an electron, and thus
determine it's position, you might use a powerful
light microscope. For the electron to be visible,
at least one photon of light must bounce off of
it, and then pass through the microscope into
your eye. A problem occurs here, as the photon
transfers some unknown amount of its momentum to
the electron. Thus, in the process of finding an
accurately position of the electron, the same
light that allows you to see it changes the
electron's momentum to an undeterminable extent.
|Another way of
looking at this is by considering wavefunctions.
If you know the exact wavelength of a particle,
then you know it's momentum. However a particle
with a known momentum has a totally unpredictable
location. You can see this if you look at the
graph left - the electron is equally likely to be
anywhere along the wave.
The wave function of a
perfectly localised particle would be a sharply spiked
function at the particles position and zero everywhere
else. This can be aproximated by the addition of of
several waves, which interfere constructively in one
place and destructively in all others. However in doing
this you loose information about the momentum of the
particle - there is no well defined wavelength.
As more waves are used in the
supposition (given by numbers on curves) the
location becomes more precise at the expense of
uncertainty in the particles momentum
An infinite number of waves is
needed to construct the wavefunction of a
perfectly localised particle - so there is total
uncertainty in the momentum
Important things to
small value of h in everyday units, this
principle is only significant on the atomic scale.
uncertainties of Dx and Dp arise from the quantum structure
of matter, and are not due to imperfections in
the measurement instruments.
from: Physical Chemistry, Peter Atkins,
6th Ed, OUP (1998)