The Heisenberg uncertainty principle, first recognized in 1927 by Werner Heisenberg,
is a very important physical principle. It says that position and momentum are
complementary. The better we know the position of a particle of matter, the less certain
we are of its momentum. The reason for this relation is the way we measure positions.
In order to locate a particle in space, we have to see it (with our eyes or with an instrument).
This requires shining light on the particle (either visible light or some other
wavelength), and it is this light that disturbs the particle, moves it from its position and
thus changes its momentum. We think of light as consisting of small units, photons, that
don’t have any mass but have momentum. When a photon hits a particle, it gives the
particle a “kick,” which moves it away. The larger the particle, the smaller the effects of
the kick, but in principle, the complementary relation between position and momentum
exists even for macroscopic objects.