This is the weird yet incredibly powerful theory of the subatomic world in which everyday concepts to do with forces and motion no longer apply in the same way. Instead, we need a new kind of mechanics based on what are called "quantum" rules.This idea was first developed in the early 20th century by German physicist Max Planck, who proposed that energy comes in tiny lumps called "quanta." The theory was extended by Albert Einstein, Niels Bohr, Paul Dirac, and Werner Heisenberg, among several others, in the 1920s.
However, despite its tremendous success, quantum mechanics remains shrouded in mystery because, uniquely among scientific theories, no one really knows how or why it works. It makes certain predictions about the microscopic world that go completely against our common sense. For instance, it explains how an atom can exist in more than one place at the same time until we check to see what it is up to. It also says that an electron can spin both clockwise and counterclockwise at the same time until we measure it. These, and many more strange properties, are not created by problems with the theory but are simply-or not so simply, depending on your point of view -how nature behaves down at this scale.by Jim Al-Khalili, 30 Second Theories