Quantum Mechanics is the study of the very small. It began
in the 1920's and contiues to be the best model of small-scale behavior.
One of the fundamental postulates of quantum mechanics is that all particles
behave in some regard as waves (this discovery was stimulated by Einstein's
work on the photoelectric effect). This
leads to the discovery of the Heisenberg Uncertainty Principle, that one
can never know both the exact position and momentum of any particle. In
fact, particles can only be expressed as probabilities! This view of the
universe is a bit unsettling to many, as it seems to suggest that "reality"
as we know it is, if you look closely enough, nothing.