Uncertainty principle

The Uncertainty Principle established that one cannot simultaneously know the exact position and momentum of a subatomic particle. Planck's constant represented a fundamental graininess to the universe, and its discovery implied fundamental limits to measurement and explains various phenomena in quantum mechanics.
 Werner Heisenberg Germany 1927 Uncertainty principle

The Uncertainty Principle established that one cannot simultaneously know the exact position and momentum of a subatomic particle. Planck's constant represented a fundamental graininess to the universe, and its discovery implied fundamental limits to measurement and explains various phenomena in quantum mechanics.