The Uncertainty Principle
What is the 'Uncertainty Principle'? Also called 'indeterminacy', this principle deals with one of the most popular and certainly the most important aspects of quantum mechanics. Roughly speaking, the uncertainty principle says you cannot assign exact position and momentum (a measure of the motion of a body equal to the product of its mass) of a quantum mechanical system at precisely the same time.
What this is saying is that if you have have found the full position of a quantum particle, the further you are away from finding the momentum. But is this really a principle of quantum mechanics? What does it mean that a quantity is determined only up to some uncertainty? These are just some questions that will be answered.
If you look up 'uncertainty' in the dictionary, you'll undoubtedly find something along the lines of: inaccuracy or a reflection on the quality of experimental methods. This isn't the case regarding the uncertainty principle. Even with perfect instruments and technique, the uncertainty is inherent in the nature of things. By this I mean that the indeterminacy is inevitable because of the nature of the particle.
This is how it works: to see something, (lets say, an electron) we have to fire photons (an electromagnetic energy, a discrete particle having zero mass, no electric charge, and an indefinitely long lifetime) at it; they will bounce of and come back to us, enabling us to 'see' it. If you choose low-frequency photons, with a low energy, they do not impart much momentum to the electron, but they give you a very fuzzy picture, so you have a higher uncertainty in position so that you can have a higher certainty in momentum. On the other hand, if you were to fire very high-energy photons ( x -rays or gammas) at the electron, they would give you a very clear picture of where the electron is (higher certainty in position), but would impart a great deal of momentum to the electron (higher uncertainty in momentum).
This theory was formulated by Werner Heisenberg (1901-1976). Heisenberg was also best known as the founder of quantum mechanics. He is also known for his controversial role as a leader of Germany's nuclear fission research during World War II. He was considered one of the greatest physicists.
"The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa." --Werner Heisenberg, uncertainty paper, 1927.