HEISENBERG'S UNCERTAINTY PRINCIPLE

Heisenberg's uncertainty principle states that the speed (or momentum) and position of a particle at an instant cannot both be known exactly - the more precisely you measure one, the less you can find out about the other. Werner Heisenberg argued that the very act of observing a particle changes it, making precise knowledge impossible. So neither the past nor the future behaviour of any subatomic particle can be predicted with certainty. Determinism is dead.

In 1927, Heisenberg realized that quantum theory contained some strange predictons. It implied that experiments could never be done in complete isolation because the very act of measurement affected the outcome. He expressed this connection in his 'uncertainty principle'-you cannot simultaneously measure both the position and momentum of a subatomic particle (or equivalently its energy at an accurate time). If you know one then the other is always uncertain. You can measure both within certain bounds, but the more tightly these bounds are specified for one, the looser they become for the other. This uncertainty, he argued, was a deep consequence of quantum mechanics - it had nothing to do with a lack of skill or accuracy in measuring.