Quantum mechanics

Quantum mechanics consists of the revolutionary breakthrough in physics in the 1920s in understanding how participles behave inside atoms. Classical mechanics, as discovered mostly by Isaac Newton, cannot remain true on the very smallest scales.

Classical mechanics would predict that an electron orbits a proton just as planets orbit the sun. Classical electromagnetism would predict that the orbiting electron would emit a time-varying electrical field just as a radio station does. But the electron would lose energy as it emits this radiation, and would orbit closer and closer to the proton, until it collapses into the proton! So a classical model cannot be correct.

Quantum mechanics discovered that an electronic behaves in many ways like a wave rather than as a particle, and the position of that wave is never precisely known until it is observed. Even when it is observed, there is an inherent uncertainty that prevents measuring both the position and the momentum precisely. This is known as the Werner Heisenberg uncertainty principle.

Quantum mechanics forms the basis for all computers and electronic devices today.

For an excellent discussion of quantum mechanics, see: http://www.chemistry.ohio-state.edu/betha/qm/